form-specific visual priming in the right cerebral...

Journal of Experimental Psychology:Learning, Memory, and Cognition1992. Vol. 18. No. .1,492-508

Copyright 1992 by the American Psychological Association, Inc.O278-7393/92/S3.OO

Form-Specific Visual Priming in the Right Cerebral HemisphereChad J. Marsolek and Stephen M. Kosslyn

Harvard UniversityLarry R. Squire

Veterans Affairs Medical Center, San Diegoand University of California, San Diego

Results of 4 experiments indicate that both within-modality and case-specific visual priming forwords are greater when test stimuli are presented initially to the right cerebral hemisphere (RH).In contrast, neither within-modality nor case-specific explicit memory for words is greater whenstimuli are presented initially to the RH. Priming is measured using word-stem completion, andexplicit memory is measured using word-stem cued recall. In both cases, Ss first rate how muchthey like words, and then word stems are presented briefly to the RH (in the left visual field) orto the left hemisphere (in the right visual field). Results suggest that at least 2 separate systemsencode the visual representations that produce priming. The system that is more effective in theRH is better at representing form-specific information, whereas another system that is not moreeffective in the RH does not distinguish among distinct instances of word forms.

Reading is one of the most extensively studied topics incognitive psychology, and research findings have begun toilluminate the nature of the representations and processesused when reading words. Recently, the technique of positronemission tomography (PET) has been used to discover whichbrain areas are involved in specific phases of the readingprocess. One discovery from PET studies is that visual repre-sentations of word forms (the structure of printed words)apparently reside in a region of the medial extrastriate cortexof the left hemisphere. This area is activated by visual pres-entation of words and pronounceable nonwords, but not byconsonant letter strings or letterlike shapes (Petersen, Fox,Snyder, & Raichle, 1990). In addition, neither auditory pres-entation of words nor semantic processing of words activatesthis area (Petersen, Fox, Posner, Mintun, & Raichle, 1988,1989; Posner, Petersen, Fox, & Raichle, 1988). These andrelated findings support the inference that a distinct word-form system, relatively more effective in the left cerebralhemisphere than the right, encodes the visual structure ofwords.1 We hypothesize that a word form system takes as

This research was supported by a Jacob K. Javits Graduate Fellow-ship awarded to Chad J. Marsolek. The work was also supported byAir Force Office of Scientific Research Contract 91-0100, NationalInstitute of Neurological and Communicative Disorders and StrokeGrant PO1-17778-09, and National Science Foundation Grant BNS90 09619 awarded to Stephen M. Kosslyn. Larry R. Squire wassupported by the Medical Research Service of the Department ofVeterans Affairs, National Institute of Mental Health GrantMH24600, the Office of Naval Research, and the McKnight Foun-dation.

We wish to thank Chris Azorson, Richard Herrnstein, Kris Kirby,Randy O'Reilly, and Jay Rueckl, for valuable discussion and advice,and Miriam Kwalwasser, for collecting the data in Experiment 3. Wealso thank Derek Besner, Tom Carr, Peter Graf, and Keith Raynerfor helpful comments and constructive reviews of an earlier versionof this article.

Correspondence concerning this article should be addressed toChad J. Marsolek. Department of Psychology, Harvard University,33 Kirkland Street, Cambridge, Massachusetts 02138. Electronic mailmay be sent to [email protected].

input a visual pattern and performs the computations neces-sary to yield as output a representation of the word form towhich the pattern refers. The system apparently performsthese computations by recovering regularities in the structureof words, which allows it also to produce representations ofpronounceable nonwords.

In this article, we develop and test the hypothesis that wordforms are not represented and processed in a unitary, undif-ferentiated system. Rather, at least two separate computa-tional systems appear to be used to encode the visual formsof words, one that represents form-specific information andone that represents abstract form information. Form-specificrepresentations preserve specific characteristics of the patternsof lines themselves. Thus, a form-specific system would pro-duce different output representations when a particular wordis presented as input in different typographic fonts and lettercases. Abstract representations simply specify the identity ofthe word or the identity of the letter clusters (such as syllablesand bigrams) and how they are arranged. Accordingly, anabstract word form system would produce the same outputrepresentation for a word when it is presented in differenttypographic fonts and letter cases. The experiments reportedin this article were designed to investigate whether these twotypes of representations are computed in distinct brain sys-tems, with the system that encodes form-specific representa-tions being relatively more effective in the right cerebralhemisphere.

The following line of reasoning led us to hypothesize thattwo distinct systems may be used to represent word forms:When one reads a word, the goal is usually to access itssemantic content. Extracting the meaning does not dependon the particular font or typeface of the letters, except whencapitalized or italicized letters signal something special about

1 When we hypothesize that a particular system is relatively moreeffective in one hemisphere, we do not intend to make a strong claimabout the extent to which that system is lateralized. We leave openthe question of whether one such system exists and operates in onlyone hemisphere or whether two such systems exist, one in eachhemisphere, but one of the two is more effective.

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FORM-SPECIFIC PRIMING 493

the word. If so, then a word form system should produce asingle output for each word, regardless of the way in which itis written. This procedure would allow one to read a wordeven when it is presented in a previously unseen font. Becausethese computations require generalization over visual forminformation (and not, for instance, semantic information),such computations must take place within a system thatencodes visual forms per se. It is likely that an abstract wordform system would perform such generalizations very effi-ciently, given that it must represent visual forms in a mannerthat abstracts away from visual details. Other theorists alsohave argued for the existence of abstract representations ofword forms (e.g., see Adams, 1979; Besner, Coltheart, &Davelaar, 1984; Besner & McCann, 1987; Carr & Brown,1990; Carr, Brown, & Charalambous, 1989; Carr & Pollatsek,1985; Coltheart, 1981; Evett& Humphries, 1981; McClelland& Mozer, 1986; Morton, 1969, 1979; Paap, Newsome,McDonald, & Schvaneveldt, 1982; Paap, Newsome, & Noel,1984; Rayner, McConkie, & Zola, 1980).

In contrast, a brain system must also be available to encodespecific visual word forms. This hypothesis seems reasonableif only because one can recognize a particular person's hand-writing as well as particular examples of fonts. Indeed, onecan visualize very specific word form information (e.g., tryimaging The New York Times as it appears on the front pageof that newspaper). If so, then form-specific visual represen-tations of words must exist. Other theorists also have arguedthat word forms must be processed or represented in a highlyspecific manner (e.g., see Jacoby & Brooks, 1984; Jacoby &Hayman, 1987; Kirsner & Dunn, 1986; Kirsner, Dunn, &Standen, 1987;Kolers&Roediger, 1984;Roediger&Blaxton,1987; Whittlesea, 1990; Whittlesea & Brooks, 1988).

One might be tempted to argue that these two kinds ofword form information can be encoded in a single system.Logically, however, abstract and form-specific representationsseem likely to be computed by separate systems. On the onehand, a system that computes abstract representations of wordforms must provide a single output for different instances ofthe same word. On the other hand, a system that computesform-specific representations of words must produce a differ-ent output for each distinct instance of a word. These twogoals are incompatible, which suggests that separate systemswould perform the computations more efficiently than onesystem (cf. Kosslyn, Chabris, Marsolek, & Koenig, in press;Kosslyn & Koenig, 1992; Rueckl, Cave, & Kosslyn, 1989).

We investigated this hypothesis by taking advantage ofprevious findings about cerebral laterality. Although it haslong been known that the left hemisphere plays a special rolein language and in verbal processing, there is evidence thatvisual representations of verbal material can be perceived andretained better in the right hemisphere than in the left. Someof this evidence comes from divided-visual-field studies, inwhich stimuli are presented to subjects briefly in their left orright visual fields to ensure that visual input reaches onehemisphere before crossing brain commissures to reach theother hemisphere (e.g., Chiarello, 1985; Gibson, Dimond, &Gazzaniga, 1972; Hellige, 1980; Pirozzolo & Rayner, 1977).Other evidence comes from studies of brain-damaged patients(e.g., Coltheart, 1980 [but see also Baynes, 1990; Coltheart,

1985; Patterson & Besner, 1984]; Dennis, 1980; Levy &Trevarthen, 1977; Zaidel, 1978). In addition, there is evidencethat specific properties of many kinds of nonverbal visualinput are processed better in the right hemisphere than in theleft (Bradshaw & Nettleton, 1983; Bryden, 1982; for a ration-ale for this hemispheric specialization, see Kosslyn, 1987;Kosslyn & Koenig, 1992).

At least two divided-visual-field studies suggest that evenverbal material can be perceived in a form-specific manner inthe right hemisphere. In one study, subjects made physicalmatches between two letters (e.g., "AA") faster when thestimuli were presented to the right hemisphere, whereas theydecided more quickly whether physically different letter pairshad the same name (e.g., "Aa") when the stimuli were pre-sented to the left hemisphere (Geffen, Bradshaw, & Nettleton,1972).2 In another study, subjects identified scriptlike lettersmore accurately when the stimuli were presented to the righthemisphere, whereas they identified sans-serif block lettersmore accurately when the stimuli were presented to the lefthemisphere (Bryden & Allard, 1976). Presumably, the script-like letters had to be compared against form-specific represen-tations in memory, whereas the block letters were matchedeasily to more abstract, prototypical representations. Thesestudies indicate that specific visual properties of verbal mate-rial are perceptually processed better in the right hemispherethan in the left.

In this article, we investigated the hypothesis that twodistinct systems encode word forms by measuring direct, orrepetition, priming in the left and right hemispheres. Directpriming refers to memory for previously processed materialthat subsequently facilitates processing of the same material(for reviews see Richardson-Klavehn & Bjork, 1988; Schacter,1987; Shimamura, 1986; Tulving & Schacter, 1990). Forexample, in word-stem completion experiments, subjects areshown a word stem (a group of three letters that can becompleted to form different words by adding a few letters tothe end of it; e.g., "con ") and are asked to complete itusing the first word that comes to mind. The result is thatsubjects complete the stem using a word that was recentlypresented more often than any other word that can form acompletion (Graf, Mandler, & Haden, 1982). Many subjectsclaim to be unaware that the words they produce had appearedpreviously, despite exhibiting normal priming effects (Bowers& Schacter, 1990). Other priming tests include word-fragmentcompletion (e.g., Tulving, Schacter, & Stark, 1982), percep-tual identification (e.g., Winnick & Daniel, 1970), and lexicaldecision (e.g., Forbach, Stanners, & Hochaus, 1974). Direct

2 In this article, when we refer to stimuli presented to one hemi-sphere in a divided-field task, we are identifying which hemisphereinitially received the stimuli (i.e., stimuli presented in the left visualfield were presented initially to the right hemisphere). We do notassume that only one hemisphere has access to this information,simply that the contralateral hemisphere receives the informationbefore the ipsilateral hemisphere and probably also receives higherquality information (see Kosslyn & Koenig, 1992, chap. 9). Thus,subjects should respond more quickly and accurately if the hemi-sphere that processes the input more effectively receives the stimulusinitially.

494 C. MARSOLEK, S. KOSSLYN, AND L. SQUIRE

pruning is an example of implicit memory. This term refersto memory that is revealed when performance on a task isfacilitated by previously encoded information, yet is not influ-enced by deliberate or conscious recollection of previouslyencoded information. In contrast, explicit memory refers tomemory that is revealed when performance on a task requiresdeliberate recollection of a previous learning episode (Graf &Schacter. 1985).

We used priming to investigate the distinction betweenabstract and form-specific word form systems in part becauseat least some aspects of word priming are modality-specific.For example, when a word is initially presented in one mo-dality and the subsequent priming test is given in a differentmodality, priming is reduced (e.g., Graf, Shimamura, &Squire, 1985; Jacoby & Dallas, 1981; Kirsner, Milech, &Standen, 1983; Roediger & Blaxton, 1987). This finding sug-gests that some of the representations that underlie primingare themselves modality specific and hence preserve at leastsome aspects of the perceptual input. In contrast, explicitmemory for words is not modality specific. When a word isinitially presented in one modality and the subsequent explicitmemory test is given in a different modality, memory per-formance is not reduced (e.g., Graf et al, 1985; Kirsner et al.,1983; Roediger & Blaxton, 1987). Therefore, because wordform systems represent modality-specific information aboutwords, investigations of modality-specific word priming areuseful for examining such systems (cf. Schacter, 1990). Itremains unclear, however, whether a single word form systemor two qualitatively different word form systems encode themodality-specific representations that underlie modality-spe-cific word priming.3

In short, if form-specific priming proves to be relativelymore effective in the right hemisphere, but priming that isnot form specific is either more effective in the left hemisphereor equally effective in both hemispheres, then we will haveevidence that two distinct systems must participate in therepresentation of visual word forms. One of these systemswould be a form-specific system that is relatively more effec-tive in the right hemisphere. This form-specific system wouldbe distinct from the word form system that has been investi-gated in PET studies (Petersen et al., 1990), which apparentlycould be activated in the left hemisphere selectively.

Experiment 1

Graf et al. (1985) found that within-modality priming isgreater than cross-modality priming in word-stem comple-tion. That is, more priming occurred when the initially pre-sented material and the test material were presented in thesame modality. Nevertheless, some priming still occurredwhen the initially presented material and the test materialwere presented in different modalities. This result could meanthat priming affects at least two types of systems, one that ismodality specific and one that is not. In this experiment, weexplored this possibility in greater detail.

We used the divided-visual-field paradigm to measure hem-ispheric differences in word-stem completion priming. Fol-lowing Graf et al. (1985), we first asked subjects to indicatewhether they liked or disliked words that were presented one

at a time. One set of words was presented auditorily, and oneset was presented visually. The visually presented words ap-peared in the central visual field. After this initial-exposurephase, word stems were presented briefly in the left or rightvisual fields. Subjects were asked to read the word stems andto report the first word that came to mind that could completeeach stem.

We assessed (a) within-modality priming by measuring thenumber of stems that were completed with words that wereseen in the initial-exposure phase and (b) cross-modalitypriming by measuring the number of stems that were com-pleted with words that were heard in the initial-exposurephase. The visually presented words in the initial-exposurephase and all word stems were always presented in exactly thesame type font and letter case. Thus, if a form-specific systemis relatively more effective in the right hemisphere (i.e., if thecharacteristic processing of the form-specific system is moreevident in the right hemisphere), then within-modality prim-ing should be greater when word stems appear in the leftvisual field. Cross-modality priming should not reflect proc-essing of word forms per se, whether form specific or abstract,and hence a form-specific system in the right hemisphereshould not contribute to this sort of priming. Thus, whenwords are presented visually, we expect greater priming whenword stems are subsequently presented to the right hemispherecompared with when word stems are subsequently presentedto the left hemisphere. In contrast, when words are presentedauditorily, we do not expect greater priming when word stemsare presented to the right hemisphere compared with whenword stems are presented to the left hemisphere.

Method

Subjects. Thirty-two male Harvard University undergraduate andgraduate students volunteered to participate as paid subjects. Allsubjects were right-handed, as assessed by the Edinburgh HandednessInventory (Oldfield, 1971). The mean laterally quotient was 0.77(range: 1.00-0.33). Only right-handed men were tested in the exper-iments reported here because this group typically exhibits greaterhemispheric lateralization of function than women and left-handedmen (Bradshaw & Nettleton, 1983; Bryden, 1982; Springer &Deutsch, 1981). In addition, in this and all of the following experi-ments, all subjects were native English speakers, no subject partici-pated in more than one experiment, and the subjects were not awareof the purposes or predictions of the experiment at the time of testing.

Materials. One hundred English words, ranging from four toeight letters in length, were selected from a pocket dictionary. Thesewords were selected with the constraint that the initial three letters(i.e., the stem) of each word were unique among the set of stimuluswords and could be completed to form at least 10 common English

3 A representation is any structural change that serves to storeinformation. Explicit memory depends on representations that areanalogous to data structures in computers, which can be retrieveddeliberately and brought into short-term memory (and so one isaware of the content of the representation). In contrast, implicitmemory depends on representations that are embedded in specificprocesses. The content of these representations can neither be re-trieved deliberately nor be brought into short-term memory. Never-theless, even implicit memory (and direct priming) must depend onstored representations.


words. The mean frequency of occurrence per million for these wordswas 70 (Kucera & Francis, 1967).

Ten lists of 10 words each were created. Each list was balanced forword frequency, the number of words that could be completed fromthe stem of each word, and the baseline probability of generating thecritical word when presented with its initial three letters (based onseparate tests with normal subjects). Eight lists were designated asexperimental materials, and the other two were used for practice. Tenadditional words with the same characteristics were used as fillerwords. Three filler words were placed at the beginning and 2 at theend of the initially presented exposure lists in order to attenuateprimacy and recency effects.

We presented visual stimuli on a Macintosh Plus computer screenwith a Polaroid CP-50 filter placed over it to reduce glare. The letterswere presented in black against a white background in a 24-point,Helvetica bold font (letter size was approximately 5 x 6 mm). Onlylowercase letters were used in this experiment. A central fixationpoint (2 mm in diameter, subtending 0.23° of visual angle) was usedto indicate the beginning of the visual trials (but not the auditorytrials). In the exposure phase, visually presented words appeared atthe center of the computer screen. In the test phase, word stems werepresented in the left and right visual fields such that the center ofeach three-letter stem was 2.5° (22 mm) from the center of the screen.The inner edge of any word stem was never closer than 1.5° (13 mm)from the center. A 14-mm horizontal line segment appeared after thethird letter of each word stem (e.g., "con "). This segment was notconsidered part of the word stem in the aforementioned measure-ments.

The time required to generate a word when a word stem waspresented was measured with a voice-activated relay. The subjects'eyes were approximately 50 cm from the computer screen.

Procedure. All subjects were tested individually. Each experimen-tal session had two phases: In the exposure phase, subjects read 20visually presented words and heard 20 words read to them by theexperimenter (plus 5 filler words in each modality). The visuallypresented words and auditorily presented words were presented inseparate blocks. To encourage semantic processing of the words, thesubjects rated how much they liked or disliked each word on a 5-point scale (1 = dislike very much: 5 = like very much). The subjectsmade the ratings by circling numbers on 5-point scales printed onrating sheets.

Half of the words were displayed visually on the computer screenand read silently. In this portion of the exposure phase, a trial beganwith the fixation point, which appeared at the center of the screen for500 ms. Then, a blank screen appeared for 500 ms, after which aword appeared centrally for 3 s. After making a rating, the subjectspressed the space bar on the computer keyboard. Each word remainedon the screen exactly 3 s, regardless of whether the space bar waspressed before or after this time elapsed. After the space bar waspressed and the word disappeared from the screen, the next trialbegan after a 1-s interval. The other half of the words were presentedauditorily by the experimenter. In this portion of the exposure phase,the subjects were told to listen carefully to each word so that none ofthe words would have to be repeated. After hearing a word andmaking a rating, the subjects told the experimenter that they wereready for the next word, and the experimenter then read anotherword no less than 2 s thereafter. The average presentation rate inboth cases was about 5 s per word.

Each subject was presented with two target-word lists visually andtwo auditorily. The order of modality was counterbalanced acrosssubjects. The first 3 and the last 2 words presented in each modalitywere always filler words (which were added to the test lists). The setof words presented in each modality was presented twice in successionso that the processing necessary to encode each word was performedtwo different times; presumably, greater changes useful for priming

should result from two presentations of each word than from onlyone. Thus, each subject rated 25 visually presented words twice and25 auditorily presented words twice (20 target words and 5 filler wordsin each modality). For each subject, different pseudorandom wordorders were used for each of the two presentations within eachmodality. These orders were random with the constraint that no morethan three items appeared consecutively that had stems that wouldlater be presented in the same visual field.

The test phase began approximately 6.5 min after the exposurephase ended (a practice session, which included 20 word-stem com-pletion practice trials, was presented during this interval). In the testphase, the subjects were instructed that three-letter word stems wouldappear briefly, slightly to the left or right of the center of the screen.They were told that each of the word stems was the beginning of anEnglish word and that they were to add a few letters to each group tomake a common English word and then immediately speak it aloudinto the microphone. They were instructed to produce any Englishword, but to report the first word that came to mind, excluding propernouns. Subjects were required to produce a word response on eachtrial. In fact, the next trial did not begin until a word response wasreported in the previous trial.

Test trials began with the presentation of a fixation point at thecenter of the screen for 500 ms. Then, after a 500-ms blank screen, aword stem appeared in the left or right visual field for 183 ms,followed again by a blank screen. Subjects were instructed to focustheir attention on the fixation point whenever it appeared. A wordspoken into the microphone terminated each trial. After a 1-s inter-trial interval, the next trial was presented in the same way. Subjectswere told that the computer would record the time it took them tosay the word and that the experimenter would record the words theysaid.

Half of the word stems (the targets) were taken from the four wordlists (40 words) used in the exposure phase. The other half of thestems (baseline stems) were taken from the four word lists (40 words)that were not presented to the subject. Of the two word lists (20words) presented visually during the exposure phase, one list (10words) served as a source of word stems to present in the left visualfield, and the other list (10 words) served as a source of word stemsto present in the right visual field. The same was true for the twoword lists (20 words) presented auditorily during the exposure phase.Therefore, half of the words from each modality were used to obtainstems presented in each visual field. Finally, half of the 40 baselineword stems were presented in the left visual field, and the other halfin the right. Thus, each subject saw 80 word stems: 20 from wordspreviously exposed visually, 20 from words previously exposed au-ditorily, and 40 from words that were not previously exposed. Thevisual field at test, modality of initial exposure, and target status inwhich each list was presented were counterbalanced across subjects.

The trials were presented in a different order for each subject. Eachorder was random with three constraints: No more than three targetor baseline stems appeared consecutively, no more than three stemsappeared consecutively from words that had been previously exposedin the same modality, and no more than three stems appearedconsecutively in the same visual field. So that subjects could becomefamiliarized with the word-stem completion procedure, they per-formed 20 practice word-stem completion trials (half of the stemswere presented in each visual field) during the interval between theexposure and test phases.

The eight experimental word lists were rotated through the fourtarget conditions such that each 10-word list appeared in all condi-tions an equal number of times. The baseline word stems were takenfrom the four unstudied word lists (two lists presented in the leftvisual field at test, the other two in the right), so that the same wordsappeared equally often as target and baseline word stems in eachvisual field.


In summary, 40 words were presented in the exposure phase (20visually and 20 auditorily). Then, 80 word stems were presented(taken from the 40 previously exposed words and the 40 unexposedwords). Half of the word stems were presented in the left visual field,and half in the right visual field. The subjects completed each stemto form the first word that came to mind and responded by speakingthe word aloud.

Results

In this and subsequent experiments, we used a strict crite-rion to determine whether each completed word was one ofthe 80 words from the eight experimental word lists (criticalwords). A completion for a word stem was scored as a criticalword only if the reported word was exactly the same as thecorresponding word on one of the eight experimental wordlists (no plural forms, past tense forms, or other changes wereaccepted). Completion rates for critical words and the timesrequired to produce critical words were submitted to separaterepeated measures analyses of variance (ANOVAs). All factorswere within-subjects variables. In this and all following exper-iments, reported results were significant (p < .05), and resultsnot reported were not significant (p > .10, in all cases). Insome cases, however, we report nonsignificant results (p >.05), if they are of particular theoretical interest.

Completion rates. The left portion of Figure 1 illustratesthe main results from Experiment 1. We analyzed the com-pletion rate data to examine directly the effects of priming.We computed a measure of within-modality and cross-mo-dality priming for each subject, obtaining separate primingscores for left- and right-hemisphere trials. For each type oftrial, we subtracted each subject's completion rate for criticalwords in the no-exposure, baseline condition from his com-

pletion rate for critical words in the visual-exposure condition(within-modality priming) and from his completion rate forcritical words in the auditory-exposure condition (cross-mo-dality priming). The statistical analysis included initial-expo-sure modality (visual or auditory) and test hemisphere (left orright) as factors. The analysis revealed that there was morewithin-modality priming when word stems were presented tothe right hemisphere than to the left hemisphere (direct prim-ing scores of 21.8% vs. 13.1%, respectively), F(\, 62) = 7.94,p < .01, MSC = 148.8, for the simple effect contrast. Incontrast, we found the same amount of cross-modality prim-ing when the stems were presented to the right or left hemi-sphere(12.3% vs. 10.5%, respectively), F< 1.0, for the simpleeffect contrast. The interaction of initial-exposure modalityand test hemisphere was significant, F(\, 31) = 4.58, p < .05,MSC = 78.9. Furthermore, when word stems were presentedto the right hemisphere, there was more within-modalitypriming than cross-modality priming, F{\, 62) = 17.3, p <.001, MSC = 81.3, for the simple effect contrast. When wordstems were presented to the left hemisphere, within-modalityand cross-modality priming did not differ, F( 1, 62) = 1.39, p> .20, MSC = 81.3, for the single effect contrast.

Finally, the analysis revealed that priming was marginallygreater when stems were presented to the right hemisphere(17.0%) than to the left (11.8%), F(l, 31) = 4.01, p < .06,MSe = 218.7, and that priming was greater after visual expo-sure (17.4%) than after auditory exposure (11.4%), F(l, 31)= 13.8, p<.001, MSe = 83.8.

We also analyzed the same data in their original formwithout subtractions. This second analysis was performedwith three types of initial exposure (visual; auditory; and no-exposure, i.e., baseline) and two test hemispheres (left and

Visual Exposure

Auditory Exposure

Baseline

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Hemisphere Hemisphere

Figure I. Results from Experiment 1. (Mean rates for completion of critical words from word stemsand mean response times for completing critical words are presented as a function of type of initialexposure [visual or auditory] and hemisphere of word-stem presentation. Word stems were alwayspresented visually at test, so that word completions that used visually exposed words reflect within-modality priming and word completions that used auditorily exposed words reflect cross-modalitypriming. Word completions that used critical words that were not encountered during initial exposurereflect baseline performance. Error bars indicate standard errors of the mean.)


right). All effects that were significant in the priming scoresanalysis were also significant in this analysis, with one excep-tion.4 We also tested, in all conditions, whether subjectscompleted word stems to form critical words that had beenpreviously exposed more often than critical words that hadnot been previously exposed. For word stems presented to theright hemisphere, both visual exposure (33.1%) and auditoryexposure (23.8%) resulted in a higher percentage of comple-tions using critical words than the no-exposure baseline(11.4%). In addition, for word stems presented to the lefthemisphere, both visual exposure (26.6%) and auditory ex-posure (21.9%) resulted in a higher percentage of completionsusing critical words than the no-exposure baseline (10.9%; p< .001, in all cases).

Response times. We also analyzed the time required toreport word completions using the critical words (Figure 1).We analyzed the response times in the form of priming scores,computed as described in the previous section. Although thepattern of these results resembled the completion rate data,the variability was high, and no effects approached signifi-cance. Indeed, this analysis showed that priming after visualexposure was the same when word stems were presented tothe right or left hemisphere (mean priming measures of 28ms and 22 ms, respectively), F < 1.0. Similarly, there was nodifference in priming when stems were presented to the rightor left hemisphere after auditory exposure (-25 ms and -68ms, respectively), F < 1.0. Finally, there was no interactionbetween modality of initial exposure and test hemisphere, F< 1.0. If this interaction were significant, the correspondinginteraction in completion rates might have been suspect, inthat high rates of completion using the critical words couldhave been produced simply by subjects taking longer togenerate their responses.

Similarly, in an analysis of the original response time data(before subtractions) with three types of initial exposure (vis-ual, auditory, and baseline), none of the main effects orinteractions approached significance. We also tested, in allconditions, whether subjects completed word stems to formcritical words that had been previously exposed faster thancritical words that had not been previously exposed. We foundno evidence of such response-time priming in any condition(all ts < 1.0). The pattern of data in this experiment suggeststhat response times were at floor levels, such that potentialdifferences between conditions could not be detected.

Discussion

As predicted, within-modality priming was greater whenword stems were presented to the right hemisphere. In con-trast, cross-modality priming did not differ depending onwhich hemisphere received the word stem. Because the wordstems in the within-modality condition were presented in thesame typeface and letter case during initial exposure and inthe test trials, information encoded in a form-specific systemduring the exposure phase could have primed performance inthe test trials. If so, a form-specific system is relatively moreeffective in the right hemisphere than in the left. In addition,we replicated earlier findings (Graf et al., 1985) that bothwithin- and cross-modality priming occur in the word-stem

completion task, but within-modality priming is greater thancross-modality priming.

To specify the nature of the form-specific system, we mustdetermine which form of memory was responsible for theresults. We have supposed that influences on word-stem com-pletion reflect a form of implicit memory. Subjects were askedto complete each stem with the first word that came to mind,and they were not told that some of the word stems could becompleted using words previously shown during the exposurephase. However, it seemed possible that a right-hemisphereadvantage for within-modality word-stem presentation mightalso be found if subjects were instructed to complete the wordstems by deliberately recalling the recently presented words.If so, then the results from this experiment may reflect explicitmemory processes. Experiment 2 was conducted to investigatewhether the same pattern of results would be observed whensubjects were instructed to recall words from the initial-exposure phase that could be used to complete the wordstems.

Experiment 2

In Experiment 2, we tested whether the modality-specificrepresentations that underlie within-modality word primingalso influence within-modality explicit memory. We assumedthat amodal—and not modality-specific—representations ofwords largely underlie explicit memory effects, becausechanges in modality from initial exposure to test do notdecrease performance in explicit memory tasks (e.g., Graf etal., 1985; Kirsner et al., 1983; Roediger & Blaxton, 1987).

We hypothesize that associative memory, a system separatefrom word form systems, encodes amodal information that isaccessed in explicit memory tasks. This system requires proc-essing in medial temporal lobe structures for new informationto be stored, and amnesic patients with damage in the medialtemporal lobes have impaired explicit memory abilities (cf.Schacter, 1990; Squire, 1987; Tulving & Schacter, 1990). Incontrast, word form systems encode modality-specific infor-mation about the visual structure of printed words and do soin posterior cortical areas. Even amnesic patients with dam-aged medial temporal lobe structures but intact posteriorcortical structures reveal normal modality-specific word prim-ing effects (Shimamura, 1986). Thus, word form systems inposterior cortex are largely responsible for producing modal-

4 The interaction between type of initial exposure and test hemi-sphere that was significant in the priming scores analysis did notapproach significance in an analysis with three types of initial expo-sure, F(2, 62) = 1.85, p > .15, MSC = 87.9. However, the simpleeffects analyses of the interaction between type of initial exposureand test hemisphere yielded the same pattern of results in this latteranalysis as in the priming scores analysis. Subjects completed moreword stems to form visually exposed words when the stems werepresented to the right hemisphere (33.1%) than to the left (26.6%),F(\, 93) = 6.42, p < .05, MSt = 107.4, for the simple effect contrast.In contrast, subjects completed similar numbers of word stems toform auditorily exposed words when the word stems were presentedto the right hemisphere (23.8%) or to the left (21.9%), F < 1.0 forthe simple effect contrast.


ity-specific word priming effects, but not for producing ex-plicit memory effects.5

In Experiment 2, the initial-exposure procedure was thesame as in Experiment 1. Moreover, in the test phase, wordstems were presented visually in the left or right visual fieldjust as in Experiment 1. Experiment 2 differed from Experi-ment 1 only in that subjects were asked to use words stemsas cues to help them recall words they had seen or heardduring the first phase of the experiment. If information en-coded in word form systems underlies the word primingeffects observed in Experiment 1, and if explicit memoryreadily draws on different systems, such as associative mem-ory, then we would not expect to find a right-hemisphereadvantage for within-modality cued recall.

Method

Subjects. Sixteen male Harvard University undergraduates andaffiliates volunteered to participate as paid subjects. All subjects wereright-handed; the mean laterality quotient was 0.73 (range: 1.00-0.39; Oldfield, 1971).

Materials and procedure. The materials and procedure of thisexperiment were the same as in Experiment 1, with two exceptions.First, the practice trials were eliminated. Second, the instructions inthe test phase were changed such that subjects were asked to recallwords they previously rated in the exposure phase, using the wordstems presented in the left and right visual fields as cues. Specifically,they were instructed to read each word stem and to try to recall aword that they had rated in the like—dislike task that began with thoseletters. Subjects were encouraged to guess when they could not thinkof such a word. They also were told that only some of the word stemscould be completed to form words that they had rated in the exposurephase. Nevertheless, they were required to produce a word responseon each trial. In fact, the next trial did not begin until a word responsewas reported in the previous trial.

Results

Repeated measures ANOVAs were conducted with initial-exposure modality (visual or auditory) and test hemisphere(left or right) as within-subjects independent variables. Inseparate analyses, the percentage of words correctly recalledand the times to recall these words were the dependentvariables.

Accuracy. Figure 2 shows that stems from words presentedvisually during the exposure phase did not cue recall moreoften when they were presented to the right hemisphere(47.5%) than when they were presented to the left hemisphere(51.9%). In fact, the trend was in the opposite direction, butthis difference did not approach significance, F < 1.0, for thesimple effect contrast. Similarly, stems from words presentedauditorily during the exposure phase did not cue recall moreoften when they were presented to the right hemisphere(35.0%) than when they were presented to the left hemisphere(41.9%), F( 1, 30) = 1.50, p > .20, MSC = 252.3, for the simpleeffect contrast. The interaction between initial-exposure mo-dality and test hemisphere did not approach significance, F <1.0.

Furthermore, the analysis revealed that words presentedvisually during the exposure phase (49.7%) were recalled more

frequently than words presented auditorily (38.4%), F{\, 15)= 10.6, p < .01, MSC = 191.7. Averaging across both modal-ities, there was a numerical but nonsignificant advantage forcued recall to be better when word stems were presented tothe left hemisphere (46.9%) compared with the right (41.3%),F(l, 15) = 2.82,/?> .10, MSe= 179.6.

Response times. We performed a similar analysis to ex-amine the times required to correctly recall words (Figure 2).This analysis revealed that subjects did not recall words pre-sented visually during the initial-exposure phase faster whenword stems were presented to the right hemisphere (1,395 ms)than to the left hemisphere (1,522 ms), F(\, 30) = 1.05, p >.30, MS"e = 123,922, for the simple effect contrast. Further-more, there was only a trend for subjects to recall wordspresented auditorily during the initial-exposure phase fasterwhen the word stems were presented to the right hemisphere(1,626 ms) compared with the left hemisphere (1,844 ms),F( 1, 30) = 3.08, p < .09, MS, = 123,922, for the simple effectcontrast. No interaction was evident between modality ofinitial exposure and test hemisphere (F < 1.0).

Finally, the analysis indicated that subjects recalled wordspresented visually during the exposure phase (1,459 ms) fasterthan words presented auditorily (1,735 ms), F(l, 15) = 11.8,p < .01, MSe = 103,309; and averaging across modalities,they recalled words marginally faster when word stems werepresented to the right hemisphere (1,510 ms) than to the lefthemisphere (1,683 ms), F(l, 15) = 4.24, p < .06, MS, =112,677 (note that this trend goes in the opposite direction tothat in the accuracy data, which may suggest a trend towarda speed-accuracy trade-off).

Discussion

The results from Experiment 2 are in sharp contrast withthose from Experiment 1. Subjects did not recall words pre-sented visually during the initial-exposure phase more accu-rately when the stems of these words were presented to theright hemisphere, compared with when the stems were pre-sented to the left hemisphere. In fact, the trend was in theopposite direction, with a small left-hemisphere advantage forboth within-modality and cross-modality cued recall. In ad-dition, subjects did not recall words presented visually duringthe initial-exposure phase significantly faster when their stemswere presented to the right hemisphere.

The dissociation between results from Experiments 1 and2 shows that within-modality word-stem completion perform-ance, but not cued recall, is better when word stems arepresented to the right hemisphere. It appears that representa-

5 Episodic memory representations (in Tulving's [1972] classicsense of the term) are different from form-specific representations ofvisual forms. Episodic memory effects are subserved by associativememory, which requires processing in medial temporal lobe struc-tures of the brain for new information to be encoded. Amnesicpatients with damage to these structures but intact posterior cortexhave impaired episodic memory, but intact modality-specific priming.Thus, the associative memory system represents information in epi-sodic memory, and word form systems represent information thatunderlies modality-specific priming.


Visual Exposure

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Figure 2. Results from Experiment 2. (Mean rates for cued recall from word stems and mean responsetimes for correct recall are presented as a function of type of exposure [visual or auditory] andhemisphere of word-stem presentation. Word stems were presented visually at test, so that recall ofvisually exposed words reflects within-modality cued recall and recall of auditorily exposed wordsreflects cross-modalitv cued recall. Error bars indicate standard errors of the mean.)

tions of visual word forms in the right hemisphere assist inproducing word-stem completion priming more strongly thanrepresentations of visual word forms in the left hemisphere.However, these same representations of word forms in theright hemisphere do not facilitate cued recall.

presented to the right hemisphere in the opposite letter case.A form-specific representation of a word in one letter caseshould not influence priming unless a word stem is subse-quently presented in the same letter case as the one in whichthe word was originally presented.

Experiment 3

This experiment was conducted to test more directlywhether a form-specific system operates more effectively inthe right hemisphere than in the left (i.e., whether the char-acteristic processing of the form-specific system is more evi-dent in the right hemisphere than in the left). If this hypothesisis correct, greater priming should occur when word stems arepresented in the same typographic letter case (upper or lower)used during the visual-exposure phase than when word stemsare presented in the different case. Specifically, such case-specific priming should be greater when word stems arepresented to the right hemisphere than when word stems arepresented to the left hemisphere.

In Experiment 3, all words were presented visually duringthe initial-exposure phase in either all lowercase or all upper-case letters. In the test phase, half of the target-word stemswere presented in the same case in which the correspondingwords had appeared during the initial-exposure phase, andhalf were presented in the other case. We predicted that same-case priming would be greater after right-hemisphere presen-tations than after left-hemisphere presentations, whereas dif-ferent-case priming would not be greater after right-hemi-sphere presentations. In other words, more priming shouldoccur when word stems are presented to the right hemispherethan to the left, but only when the word stems are presentedin the same letter case as that used when their correspondingwords were presented in the initial-exposure phase. In con-trast, more priming should not occur when word stems are

Method

Subjects. Thirty-two male Harvard University undergraduatesvolunteered to participate as paid subjects. All subjects were right-handed; the mean laterality quotient was 0.83 (range: 1.00-0.29;Oldfield, 1971).

Materials and procedure. Experiment 3 was similar to Experi-ments 1 and 2, except that words and word stems could appear ineither all lowercase or all uppercase letters. In the exposure phase, allwords were presented visually at the center of the screen. Subjectssaw four word lists (40 words) twice in succession and rated themeach time as described earlier. Half of the words in each list (20words) appeared in all lowercase letters, and half (20 words) appearedin all uppercase letters. In addition, two lists (20 words) were presentedin the same case in which they would later appear during the testphase, and two lists (20 words) were presented in the opposite case.The first 3 and last 2 words presented during the exposure phase werealways filler words. Thus, subjects rated 45 words twice, each time ina different order. Different orders were used for each subject. Theorders were random, except that (a) no more than 3 words appearedconsecutively that had stems that would later be presented in thesame visual field at test and (b) no more than 3 words appearedconsecutively that had stems that would appear in the same ordifferent letter case at test. The sequence of events for each exposuretrial was the same as that used for visual-exposure presentations inExperiments 1 and 2.

The instructions and the procedure for the test phase were thesame as in Experiment 1. Thus, the test phase began approximately6.5 min after the exposure phase ended (a practice session including20 word-stem completion trials was presented during this time inter-val). For each subject, half of the word stems (40) were from the fourword lists presented in the exposure phase (target stems), and the


other half (40) were from the four word lists not presented (baselinestems). Half of the word stems from each list were presented in theleft visual field, and half in the right visual field. In addition, half ofthe target-word stems presented in each field (20) were in lowercaseletters, and half (20) were in uppercase letters. Thus, word stems fromtwo of the target word lists (20) appeared in the same case as duringthe exposure phase (same-case stems), and word stems from the othertwo lists (20) appeared in the different case (different-case stems).Word stems were presented in four blocks of 20 trials each, with eachblock containing the 10 stems from one of the target lists and the 10stems from one of the baseline word lists. Brief pauses (about 30 seach) intervened between the test blocks. During each interval, theexperimenter set up the next block of test presentations on thecomputer and did not speak, except to tell the subject that the sameinstructions applied to the next set of trials.

The lists were counterbalanced across subjects so that each wordappeared equally often in each case, each stem appeared equally oftenin the same or different case in which its corresponding word appearedduring the exposure phase, and each stem in each condition appearedequally often in each visual field. The word stems were presented ina random order within each block, except that no more than threetarget stems or baseline stems appeared consecutively and no morethan three stems appeared consecutively in the same letter case or inthe same visual field. In all other respects, the sequence of events foreach test trial was the same as in Experiment 1.

Thus, each subject rated 40 words (plus 5 filler words) in theexposure phase and then received 80 word stems in the test phase.Forty of these were target word stems, 20 in the same case as in theexposure phase (10 in the left and 10 in the right visual field) and 20in the opposite case (10 in the left and 10 in the right visual field).The remaining 40 were baseline word stems, 20 in lowercase letters(10 in the left and 10 in the right visual field) and 20 in uppercaseletters (10 in the left and 10 in the right visual field). Finally, tofamiliarize subjects with the word-stem completion procedure, 20practice word stems (half in each letter case and half in each visualfield) were presented during the interval between the exposure andtest phases.

Results

Repeated measures ANOVAs were conducted on critical-word completion rates and on response times for completionsto critical words. All variables were manipulated within sub-jects. The main results are presented in Figure 3. Figure 4shows half of these data, for word stems presented in lowercaseat test, and Figure 5 shows the other half, for word stemspresented in uppercase at test.

Completion rates. To analyze the completion rate data,we examined directly the effects of priming. We computedmeasures of same-case and different-case priming for eachsubject, obtaining separate priming scores for each of the fourconditions created by crossing left- and right-hemisphere trialswith lowercase and uppercase word-stem trials. For each ofthese four types of trials, we subtracted each subject's comple-tion rate for critical words in the no-exposure, baseline con-dition from his completion rate for critical words in the same-case exposure condition (same-case priming) and from hiscompletion rate for critical words in the different-case expo-sure condition (different-case priming). This analysis includedtype of initial exposure (same-case or different-case), testhemisphere (left or right), and letter case at test (lower orupper) as variables. The most important finding was thatsame-case priming was greater when word stems were pre-sented to the right hemisphere (28.1%) than to the left(16.5%), F(\, 62) = 11.0, P < .01, MSe = 392.3, for the simpleeffect contrast. In contrast, this right-hemisphere advantagedid not extend to different-case priming (17.09% and 17.08%for the right and left hemispheres, respectively; F < 1.0, forthe simple effect contrast). The interaction between type ofinitial exposure and test hemisphere was significant, F{\, 31)= 7.40, p < .05, MSe = 290.6. Furthermore, when word stemswere presented to the right hemisphere, same-case priming

Same-Case Exposure

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Figure 4. Results from Experiment 3, showing only the data from test trials in which word stems werepresented in lowercase letters. (Error bars indicate standard errors of the mean.)

was greater than different-case priming, F(\, 62) = 11.2, p <.01, MSC = 342.9, for the simple effect contrast, but whenword stems were presented to the left hemisphere, same-caseand different-case priming did not differ (F < 1.0, for thesimple effect contrast).

Another important result was that the type of initial expo-sure did not interact with the letter case of word stems at test,F(\, 31) = 2.11, p > .15, MSe = 317.9. In addition, the three-way interaction between type of initial exposure, letter caseat test, and test hemisphere did not approach significance (F< 1.0). These results show that case-specific priming in gen-eral, and the right-hemisphere advantage for case-specificpriming in particular, did not depend on whether word stemswere presented in lowercase or uppercase letters. Simple effectcontrasts confirmed the latter conclusion. There was margin-ally more same-case priming when lowercase stems were

presented to the right hemisphere (28.4%) than to the left(19.0%), F(l, 124) = 3.28, p < .08, MS, = 426.4. In addition,there was more same-case priming when uppercase stemswere presented to the right hemisphere (27.8%) than to theleft (13.9%), F(l, 124) = 7.22, p < .01, MSC = 426.4. Incontrast, when word stems were in the different case from thecase used during the exposure phase, priming was similarwhen lowercase or uppercase stems were presented to the rightor left hemisphere, (20.0% vs. 23.5% for lowercase stems, and14.2% vs. 10.6% for uppercase stems), both Fs < 1.0. Thus,case-specific priming in the right hemisphere did not dependon the letter case of the test stimuli.

Finally, the analysis revealed that same-case priming(22.3%) was generally greater than different-case priming(17.1%), F(l, 31) = 4.33, p<.05, MSe = 395.1. Word stemspresented to the right hemisphere (22.6%) produced greater

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Figure 5. Results from Experiment 3. showing only the data from test trials in which word stems werepresented in uppercase letters. (Error bars indicate standard errors of the mean.)


priming than those presented to the left hemisphere (16.8%),F(l, 31) = 4.38, p < .05, MSC = 493.9. In addition, wordstems presented in lowercase letters (22.7%) produced morepriming than those presented in uppercase letters (16.6%),F(\, 31) = 5.61, p < .05, MS, = 428.1.

We also analyzed the same data in their original formwithout subtractions. This second analysis was performedwith three types of initial exposure (same letter case as at test;different letter case as at test; and no-exposure, i.e., baseline),two test hemispheres (left and right), and two letter cases attest (lower and upper). All effects that were significant in thepriming scores analysis were also significant in this analysis.We also tested, in the four main conditions, whether subjectscompleted word stems to form critical words that had beenpreviously exposed more often than critical words that hadnot been previously exposed. For word stems presented to theright hemisphere, both same-case exposure (39.0%) and dif-ferent-case exposure (28.1%) resulted in a higher percentageof completions using critical words than the no-exposurebaseline (11.0%). In addition, for word stems presented to theleft hemisphere, both same-case exposure (26.1 %) and differ-ent-case exposure (26.8%) resulted in a higher percentage ofcompletions using critical words than the no-exposure base-line (9.7%; p < .0001, in all cases).

Response times. We analyzed the time required to reportcompletions using critical words in terms of priming scores,calculated as described in the previous section. No effectsapproached significance in this analysis. Specifically, therewas no greater priming when same-case word stems werepresented to the right hemisphere (86 ms) than to the left (43ms), F < 1.0, for the simple effect contrast. Similarly, therewas no greater priming when different-case word stems werepresented to the right hemisphere (65 ms) than to the left(-26 ms), F(l, 62) = 1.13, p > .25, MS, = 233,544, for thesimple effect contrast. Finally, the interaction between typeof initial exposure and test hemisphere did not approachsignificance, F < 1.0. If this interaction were significant, thecorresponding interaction in completion rates might havebeen suspect, in that high rates of completion using the criticalwords could have been produced simply by subjects takinglonger to generate their responses.

Similarly, in an analysis of the original response time data(before subtractions) with three types of initial exposure(same-case, different-case, and baseline), none of the maineffects or interactions approached significance. We also tested,in the four main conditions, whether subjects completed wordstems to form critical words that had been previously exposedfaster than critical words that had not been previously ex-posed. We found no evidence of such response time primingin any condition (all ts < 1.0). The pattern of data in thisexperiment suggests that response times were at floor levels,such that potential differences between conditions could notbe detected.

Discussion

In Experiment 3, we found a right-hemisphere advantagein word-completion priming when word stems appeared inthe same case in which their corresponding words had ap-

peared during the exposure phase. In contrast, we did notfind a hemispheric difference when word stems appeared inthe opposite case from the one used during the exposurephase. Moreover, the right-hemisphere advantage for same-case priming was evident in a near-significant effect whenword stems were presented in lowercase letters at test and ina significant effect when word stems were presented in upper-case letters at test. Accordingly, the results indicate that aform-specific system operates more effectively in the righthemisphere than in the left.

Experiment 4

Experiment 3 indicated that the right hemisphere is rela-tively more effective than the left at storing information aboutcase-specific word forms. However, we do not know whetherthis information influences explicit or implicit memory. Ex-periment 4 was conducted to determine whether systems inthe right hemisphere draw on explicit memory when com-pleting same-case word stems. We investigated whether thepattern of results found in Experiment 3 would also beobtained when subjects are asked to use word stems as cuesto recall the words presented during the exposure phase. Theresults from Experiments 1 and 2 suggest that the form-specific information supporting priming is embedded in rep-resentations that are tapped by implicit memory and notexplicit memory. However, no evidence is yet available toshow that this is also true for representations of the subtlevisual changes that distinguish letter case.

Subjects in this experiment followed the same procedure asin Experiment 3, except that word-stem cued recall was testedinstead of word-stem completion. For initial exposure, wordsappeared in either all lowercase or all uppercase letters. In thetest phase, subjects were instructed to use visually presentedword stems as cues to help them recall the words that theyhad rated during the exposure phase. Half of these word stemsappeared in the same case as in the exposure phase, and halfappeared in the different case. If information stored in a form-specific system supports implicit memory, and if explicitmemory readily draws on different systems, such as associa-tive memory, then the results from this experiment should bedifferent from those obtained in Experiment 3. Specifically,when explicit memory is tested, the right-hemisphere advan-tage for case-specific priming that was evident in Experiment3 should not occur.

Method

Subjects. Sixteen right-handed male Harvard University summerschool students volunteered to participate as paid subjects. The meanlaterality quotient was 0.69 (range: 1.00-0.33; Oldfield, 1971).

Materials and procedure. The materials and procedure were thesame as in Experiment 3, with two exceptions. First, we eliminatedthe practice trials that preceded the test phase. Second, we changedthe instructions for the test phase from word-stem completion toword-stem cued recall. Specifically, subjects were asked to read eachword stem and then to try to recall a word from the exposure phasethat began with those three letters. They were informed that not everyword stem could be completed to form a previously presented word,and they were encouraged to guess when they could not recall such aword.


Results

As before, we performed separate repeated measures AN-OVAs to examine the proportion of words correctly recalledand the time to recall those words. The independent variableswere type of initial exposure (same-case or different-case), testhemisphere (left or right), and letter case of word stems at test(lower or upper). These were all within-subjects variables.

Accuracy. The main results are shown in Figure 6. Themost important finding was that subjects did not recall morewords when same-case word stems were presented to the righthemisphere (48.8%) than when they were presented to the lefthemisphere (52.5%), F < 1.0, for the simple effect contrast.In fact, the nonsignificant trend was in the opposite direction.Similarly, subjects did not recall more words when different-case stems were presented to the right hemisphere (46.3%)than to the left (37.5%), F{\, 30) = 2.56, p > .10, MSe =478.3, for the simple effect contrast. Subjects did recall morewords when the word stems presented to the left hemispherewere same-case stems than when they were different-casestems, F(\, 30) = 7.25, p < .05, MS, = 496.7, for the simpleeffect contrast. However, they did not recall more words whenthe word stems presented to the right hemisphere were same-case stems than when they were different-case stems, F < 1.0,for the simple effect contrast. The interaction between typeof initial exposure and test hemisphere did not approachsignificance, F(l, 15) = 2.03, p > .15, MSC = 616.7. Finally,same-case cued recall (50.6%) was overall better than differ-ent-case cued recall (41.9%), F(\, 15) = 6.50, p < .05, MS, =376.7.

Response times. Subjects correctly recalled words morequickly when same-case word stems were presented to the lefthemisphere (1,284 ms) than to the right (1,543 ms), F(l, 30)= 4.24, p < .05, MS, = 253,654, for the simple effect contrast.In contrast, subjects recalled words more quickly when differ-ent-case stems were presented to the right hemisphere (1,246

ms) than to the left (1,586 ms), F(l, 30) = 7.30, p < .05, MS,= 253,654, for the simple effect contrast. These results werealso reflected in an interaction between type of initial exposureand test hemisphere, F( 1, 15) = 9.17, p< .01, MS,= 313,443.This pattern of interaction was opposite to the pattern ofinteraction found in completion rate data from Experi-ment 3.

Discussion

The results of Experiment 4 differed markedly from theresults of Experiment 3. In Experiment 3, same-case word-completion priming was greater when word stems were pre-sented to the right hemisphere, compared with the left (Figure3). In contrast, in Experiment 4, recall was not better whensame-case word stems were presented to the right hemisphere,compared with the left (Figure 6). In addition, in Experiment3, word-completion priming was the same when same-caseand different-case word stems were presented to the lefthemisphere (Figure 3). In contrast, in Experiment 4, recallwas better when same-case word stems were presented to theleft hemisphere, compared with when different-case wordstems were presented to the left hemisphere (Figure 6). Fur-thermore, in Experiment 4, subjects actually recalled wordsfaster when same-case word stems were presented to the lefthemisphere, compared with the right hemisphere, whereasthey recalled words faster when different-case stems werepresented to the right hemisphere, compared with the lefthemisphere (Figure 6). These results are also strikingly differ-ent from those of Experiment 3 (Figure 3). Together, theresults from Experiments 3 and 4 indicate that a word formsystem that operates more effectively in the right hemispherethan in the left stores form-specific representations of words.These representations support implicit memory but do noteffectively support explicit memory.

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Figure 6. Results from Experiment 4. (Mean rates for cued recall from word stems and mean responsetimes for correct recall are shown as a function of hemisphere of word-stem presentation and as afunction of whether word stems were presented in the same or different letter case in which theircorresponding words had been presented during the initial-exposure phase. Error bars indicate standarderrors of the mean.)


General Discussion

We hypothesized that the brain can encode visual wordforms in either of two incompatible ways. On the one hand,general, or abstract, word form representations can be en-coded, which include only what is common across differentinstances of a word. Indeed, such an abstract word formsystem would be efficient at encoding form representationsthat generalize to permit recognition of new instances ofwords. On the other hand, we proposed that representationsof specific examples of word forms can also be encoded, aswould be needed for recognition of words written in a specifictypographical font. Because an abstract word form systemand a form-specific system necessarily have different input-output mappings, the computations performed by each sys-tem are incompatible and would best take place in separatesystems.

Consistent with previous findings that form-specific aspectsof words and letters are processed better when they are pre-sented to the right hemisphere, we found that form-specificpriming is relatively more effective in the right hemisphere.This result is interesting in light of the recent PET findingthat a localized region of the left posterior hemisphere isactivated selectively during passive processing of word forms(Petersen et al., 1990). These results together support theinference that at least two separate systems contribute to wordform representation, one of which is form-specific and isrelatively more effective in the right hemisphere.

In our experiments, eye position during the presentation ofword stems was not strictly controlled. This raises the concernthat subjects may have shifted their eyes to the left or rightafter the central fixation point was presented. However, Pos-ner, Nissen, and Ogden (1978) monitored eye position whensubjects were asked to remain fixated on a point in a divided-visual-field experiment and found that subjects made antici-patory eye movements on only about 4% of the trials. Theclear patterns of results we obtained also strongly suggest thatsubjects remained fixated at the center in our experiments. Ifsubjects were not maintaining fixation, we should not havefound significant interactions between the type of initial ex-posure and test hemisphere. Moreover, the fact that the right-hemisphere advantage was selective, occurring only whenpriming was within modality and when the stems were in thesame case as the words shown initially, suggests that subjectsdid not systematically fixate to one side or the other beforestimulus presentation. If they had, the lateral advantageshould have affected all of the conditions that were randomlyintermixed, yet it did not. Perhaps the most natural tendencywould be for subjects to shift attention to the right of thefixation point in this task, as is done during normal reading.However, if they had done so, we should have found greaterpriming when word stems were presented in the "right visualfield," or to the "left hemisphere." Such left-hemisphere ad-vantages were not detected in any of the conditions in thepriming experiments.

It may seem surprising that we did not find more left-hemisphere priming when the word stems appeared in theopposite letter case from the one used during the initial-exposure phase. However, we have not hypothesized that the

abstract word form system is more effective in the left hemi-sphere. Our claim is only that the form-specific system ismore effective in the right hemisphere and that this system isdissociable from the word form system investigated in PETstudies (Petersen et al., 1990). Indeed, the results of ourexperiments suggest that an abstract word form system in theright hemisphere operates as effectively as an abstract wordform system in the left hemisphere.

If abstract word form systems operate equally well in theleft and right hemispheres, it becomes puzzling why PETstudies have found evidence for word-form processing in theleft hemisphere but not in the right (Petersen et al., 1990). Inthese studies, activation evoked by passive processing of con-sonant letter strings and letterlike forms was subtracted fromactivation evoked by passive processing of words. The result-ing difference in activation in left posterior cortex, but not inright posterior cortex, presumably reflects word-form process-ing. These results contrast with other PET studies in whichactivation evoked by staring at a fixation point was subtractedfrom activation evoked by passive word-form processing. Inthis case, the resulting difference in activation was as large—if not larger—in right posterior cortex as in left posteriorcortex (Petersen et al., 1988, 1989; Posner et al., 1988). If theactivation evoked by processing of words was the same inright posterior cortex in all of these PET investigations, thediscrepancy after subtractions must be due to what is beingsubtracted. In particular, if one assumes that abstract andform-specific systems in one hemisphere are implemented inclosely adjacent cortex, then the lack of a difference in right-hemisphere activation would occur if the consonant stringsand letterlike forms, on the one hand, and the words, on theother hand, evoked a similar level of activation in rightposterior cortex. In this case, the subtraction would removethe contribution of the form-specific system from the contri-bution of the abstract system in the right hemisphere, leavingno difference. In contrast, the difference in activation obtainedin the left hemisphere would occur if the consonant stringsand letterlike forms did not activate the abstract representa-tions in left posterior cortex as much as words. And if theform-specific system is less effective or absent in the lefthemisphere, then the contribution of the abstract systemwould be evident after the subtraction.6

6 There is another potential explanation why positron emissiontomography (PET) studies have found evidence for word-form proc-essing in the left hemisphere but not the right (Petersen et al., 1990).Fox and Applegate (1988) showed that unilateral stimulation of thehand produced contralateral activation in the left or right hemisphere,as appropriate, but bilateral stimulation produced much strongeractivation in the right hemisphere. This led Corbetta, Miezin, Dob-meyer, Shulman, and Petersen (1991) to suggest that in the compet-itive situation produced when both hemispheres are activated, onehemisphere predominates. A mechanism such as this might explainthe PET finding that word-form processing is evident in the lefthemisphere, but not the right, when stimuli are presented to bothhemispheres (in the central visual field). Such a mechanism couldaffect the activation evoked by words or the activation evoked byconsonant letter strings and letterlike forms. According to this ac-count, if stimuli were presented directly to the right hemisphere (inthe left visual field), then PET studies would detect word-form


The preceding argument rests on the hypothesis that con-sonant letter strings and letterlike forms can be encoded in aform-specific system but not in an abstract word form system.It seems likely that nonverbal visual patterns and object formsare represented in the same brain systems that represent visualword forms, given that written language is a relatively recentinvention of the human species. Thus, it is unlikely that adistinct brain system exists for encoding only word forms.Instead, when learning to read, one presumably uses represen-tation systems that developed to encode visual patterns andobject forms. Indeed, it may be a misnomer to refer to theabstract and fonriTspecific systems as word form systems atall. Rather, these systems may operate to encode visual pat-terns of all kinds. If so, our results may shed light on themechanisms underlying priming of unfamiliar visual patternsand object forms (e.g., Musen & Treisman, 1990; Schacter,Cooper, & Delaney, 1990). Novel visual forms do not havepreexisting representations in either a form-specific or anabstract visual form system, and the nearest match in anabstract system would likely not be very close. Thus, novelvisual forms might be encoded as new representations in aform-specific representation system.

Additional recent PET findings provide evidence directlyrelevant to these issues (Squire et al, in press). Subjects inthis experiment completed word stems after rating how muchthey liked or disliked a list of words (all words and word stemswere presented in uppercase letters, and hence same-casepriming was investigated). A region of the right posteriorcortex showed less activity during word-stem completionpriming, in comparison to a baseline condition in which noneof the words that were produced had been previously exposed.This region in the right hemisphere was the same area acti-vated by consonant letter strings and letterlike forms in earlierPET studies (Petersen et al., 1990). The results suggest anaccount for priming, namely, that less neural activity is re-quired to process a stimulus for a period of time after it hasbeen presented. The results also show directly that same-modality, same-case word priming is related to changes in theright hemisphere, presumably because word-stem completionpriming is driven by form-specific representations of visualword forms.

A second relevant finding was that cued recall, using same-case word stems as cues, activated the right (but not the left)hippocampal region more than the baseline condition (Squireet al., in press). This finding might seem surprising, given thatwe did not find a right-hemisphere advantage for same-casecued recall in the present experiments. In fact, in Experiment4, we found that same-case cued recall was better than differ-ent-case cued recall when word stems were presented to theleft hemisphere. It is possible that our subjects stored a de-scription of the letter case in which a word appears as anamodal representation in associative memory as well as amodality-specific representation encoded in the form-specific

processing in the right hemisphere. In general, however, we shouldnote that these tasks probably have many components. The differenttechniques used by PET and divided-visual-field investigations maybe differentially sensitive to the contributions of separate processingcomponents.

system. If so, then the letter case in which a word stem appearsat test could provide additional constraints for the process ofretrieving word representations explicitly from associativememory. Given that the left hemisphere is generally moreefficient than the right at processing amodal verbal informa-tion (Bradshaw & Nettleton, 1981), it would not be surprisingif amodal representations of letter case are encoded moreeffectively in the left hemisphere. Such amodal representa-tions of letter case would not be accessed during priming tasksbecause explicit retrieval is not required and subjects can relyon output from modality-specific systems to perform the tasks(cf. Schacter, 1990; Tulving & Schacter, 1990).

The PET finding that same-case cued recall activated theright hippocampal region, and not the left, suggests that inthe conditions of the PET study, subjects attempted to retrieveinformation in modality-specific systems instead of amodalinformation in associative memory. This might have occurredin the PET study, but not in our experiments, because ofthree differences in the methods: In the PET study, only 15words were presented during the initial-exposure phase, all ofthese words were presented in the same letter case in whichword stems were presented during the test phase, and only 3min intervened between exposure and test. In Experiment 4,45 words were presented in the initial-exposure phase, half ofthe 40 nonfiller words in the exposure phase were presentedin the different letter case, and the exposure-test interval wasabout 6 min. In view of these methodological differences, itseems likely that in our experiments, cued recall involvedretrieval of information stored in associative memory, andthat our subjects used a conceptual search process to retrievethe information. In contrast, in the PET study, the cued-recalltest was relatively simple (as witnessed by the fact that thosesubjects were correct 79.2% of the time, compared with 46.3%for subjects in Experiment 4), and the visual form of the wordstem effectively specified the correct response. Thus, the cued-recall test in the PET study may well have involved retrievalof information stored in the form-specific system of the righthemisphere.

In other experiments that are relevant to our results, Raynerand his colleagues used a preview task to investigate the typeof information that survives a saccade during reading (Rayneret al., 1980; Rayner, McConkie, & Ehrlich, 1978). In thistask, subjects stare at a centralized fixation point, and then apreview word or letter string is briefly presented to the left orright of fixation. The subject makes a saccade to the previewstimulus, and during the saccade, the preview stimulus isreplaced by a word (the "base" word) that the subject thennames. When the base word, or a visually similar alternative,is presented as the preview stimulus, naming the base word isfaster than when the preview stimulus is a random letter stringor an asterisk (Rayner et al., 1978). Thus, encoding visualinformation about the preview stimulus can prime naming ofthe base word.

More important, when the base word is presented as thepreview stimulus in the same letter case, subjects do not namethe base word faster than when the letter case changes betweenpreview and test. Rayner et al. (1980) found this result whenthe preview word was presented either to the left or to theright visual field. Although their experiment was not designed

506 C. MARSOLEK, S. KOSSLYN. AND L. SQUIRE

to test form-specific priming in the two hemispheres, theresults are relevant to our hypothesis that a form-specificsystem is relatively more effective in the right hemispherethan the left. Considering only those trials in which thepreview word was presented 1° from fixation, there weretrends for faster naming times when same-case previews werepresented to the right hemisphere than to the left, and forfaster naming times when different-case previews were pre-sented to the left hemisphere than to the right (see Rayner etal.. 1980, Figure 2). Although these trends were not directlyexamined, it is possible that they were not significant becauseonly a relatively small number of subjects were tested.

Given the nature of the preview task, however, the resultsmay not undermine our inferences. In the preview task, thepreview (prime) word is presented in the left or right visualfield for a brief time (less than the time it takes to make asaccade). In contrast, in the priming task we used, the primeword is presented in the central visual field for several seconds,and the test word stem is presented in the left or right visualfield for a brief time. A very brief exposure to a word in thevisual periphery might not allow the encoding of enough finevisual detail to encode fully a form-specific representation ofthe word form. However, it is likely that this type of stimuluspresentation could allow the encoding of enough visual detailto partially match an already primed form-specific represen-tation of the word form. This difference in procedure betweenthe two tasks may help to explain why the preview task maynot yield data completely consistent with our hypothesis,whereas the priming task we used does yield such data.Another possibility is that the form-specific system could besensitive to a kind of forward visual masking. In the previewtask, processing of the preview word is still taking place whenprocessing of the base word begins. In a system that encodesa large amount of fine visual detail for any input stimulus(such as the form-specific system), processing of the previewword may interfere with processing of a subsequent word. Ifabstract word form systems are not very sensitive to this kindof interference, because they do not process fine visual detail,it is likely that only an abstract system in each hemispheremay be useful when naming the base word. In addition, aparallel argument could be made about backward masking.Perhaps the base word terminates encoding of the previewword before a form-specific representation can be computed,but not before an abstract representation can be computed.

The finding that separate word form systems underlie prim-ing illuminates a major issue concerning priming. Word-stemcompletion priming is generally short-lived. Priming effectshave been found to disappear in a matter of minutes or hours(Graf & Mandler, 1984; Shimamura & Squire, 1984; Squire,Shimamura, & Graf, 1987). This time course makes sense ifpriming effects are mediated in large part by a form-specificsystem. Such a word form system presumably encodes rela-tively fine distinctions between different input patterns andwould require a large amount of representational capacity toencode a single input pattern. A system with this characteristicmight be limited in the number of input patterns that couldbe encoded at one time, and interference from subsequentinputs could be expected to disrupt representations of specificword forms. Accordingly, only a limited number of different

representations could be held in such a system at any onetime without new representations adversely affecting the stor-age of old ones. Thus, priming supported by the form-specificsystem should be strongly evident when a relatively smallstimulus set is used and when the interval between initialexposure and test is short.

This article illustrates the advantages of combining hy-potheses derived from computational reasoning with cogni-tive and neuropsychological approaches. We have used hemi-spheric dissociations between two types of memories to inferthe existence of a distinct system that encodes form-specificrepresentations of words. If this system proves to have muchbroader functions, as seems likely, then understanding it inmore detail will place strong constraints on theories of visualinformation processing and memory.

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Received June 14, 1991Revision received October 18, 1991

Accepted October 21, 1991 •

form-specific visual priming in the right cerebral...

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