132 J- Alegria, J. Leybaert, B. Charlier and C. Hage

Campbell (Eds.), Hearing by eye: The psychology of lipreading. London: Lawrence Erlbaum Ass.

Rosen, S. M., Fourcin, A. J., & Moore, B. G J. (1981). Voice pitch as an aid to lipreading. Nature, 291,150-152.

Spoehr, K. T., & Corin, W. S. (1978). The stimulus suffix as a memory code phenomenon. Memory & Cognition, 6,583-589.

Summerfield, Q. (1987). Some preliminaries to a comprehensive account of audio­visual speech perception. In B. Dodd & R. Campbell (Eds.), Hearing by eye: The psychology of lipreading. London: Lawrence Erlbaum Ass.

Summerfield, Q. (1991). Visual perception of phonetic gestures. In I. G. Mattingly & M. Studdert-Kennedy (Eds). Modularity and the motor theory of speech perception. Hillsdale, New Jersey: Lawrence Erlbaum Ass.

Sumby, W., & Pollack, I. (1954). Visual contributions to speech visibility in noise. Journal of the Acoustic Society of America, 26,212-215.

Swisher, L. (1976). The language performance of the oral deaf. In Studies in neurolinguistics Vol 2. New York: Academic Press.

Taeschner, T., Devescovi, A., & Volterra, V. (1988). Affixes and function words in written language of deaf children. Applied psycholinguistics, 9,385-401.

Werker, J. F., & Tees, R. G (1984). Cross-language speech perception: Evidence for perceptual reorganization during the first year of life. Infant Behavior and Development, 7,49-63.

Analytic Approaches to Human Cognition J. Alegria, D. Holender, J. Junça de Morais and M. Radeau (cds.) © 1992 Elsevier Science Publishers B.V. All rights reserved. 133

CHAPTER 8

Conjunction Errors as a Tool for the Study of Perceptual Processing

Régine Kolinsky* Laboratoire de Psychologie Expérimentale Université Libre de Bruxelles Brussels - Belgium

ABSTRACT

Illusory conjunction studies emerged within the context of Treisman's feature integration theory of attention. Later on, conjunction-like errors were also systematically examined in the domains of both written word recognition and speech processing. While the relation between these different kinds of integration errors to both Treisman's original account of visual, non-linguistic illusory conjunctions, and integration errors in speech production remains uncertain, it is argued that conjunction error studies may provide a useful tool in exploring the nature of early processing codes.

In 1985, when I was still working on my Ph.D. dissertation, I remember very well listening to a series of lessons given by Paul Bertelson about "Attention, Automaticity, and Effort". Among the many phenomena Paul was speaking about, the one which seemed to me particularly mysterious and counter­intuitive was the conjunction error phenomenon.

The systematic study of conjunction errors emerged as a consequence of Anne Treisman's feature integration theory of attention (Treisman & Gelade, 1980). This theory states that simple visual characteristics are represented separately in preattentive perceptual codes, and thus may be detected in parallel across the visual field. For example, when the task is to detect a red circle among green circles and blue squares, the red property pops-out of the display. By contrast, the detection of conjunctions of attributes would require an attentional, serial scanning of the visual display. For example, the task of detecting a red circle among red squares and green circles requires serial processing. In this case, the target only differs from the distractors in the way form and colour are combined. Focal attention must be directed to each discontinuity detected in the visual field at a time, allowing those attributes which co-occur in the same attention fixation to be conjoined into the correct, unitary object.

* The author is Research Associate of the Bcleian National Fund for Scientific Research IF.N.R.S).

134 R. Kolinsky

Another way to state Treisman's theory is to say that the aim of serial processing is to avoid conjunction errors. One counter-intuitive prediction is thus that, when attention is distracted or overloaded in a conjunctive search task, random couplings of the correctly extracted simple attributes may occur. For example, one may erroneously experience the presence of a red circle when presented with very rapid tachistoscopic displays including red squares and green circles. Treisman and Schmidt (1982) called these integration errors illusory conjunaions. These errors are not just simple attribute misperceptions. Indeed, the number of false detections of the target is much higher in the case described above, which is called the experimental situation, than in control situations, where one attribute of the target is missing in the display, either its colour (e.g., on trials including green circles and blue squares), or its form (e.g., on trials including red squares and green triangles) (Kolinsky, 1989). Some of the false detections observed on experimental trials are probably due to simple attribute misperceptions. But the exceeding number of false alarms, in comparison to control trials, can only be explained by conjunction errors, and therefore they represent an estimation of the true illusory conjunctions rate.

Illusory conjunctions may constitute a useful tool to study the nature of perceptual processing codes. Indeed, one major difficulty in exploring early codes lies in the possibility that they may never be directly accessed or experienced as such. Thus, any attempt to tap them directly through introspection or instrumental responses may be misleading, and some indirect evidence is needed (Fodor, 1983; Marcel, 1983; Treisman, 1979). Indirect evidence of preattentive analysis is provided, for instance, by the occurrence of illusory conjunctions. If attributes can be wrongly combined, they must have been separately registered as independent entities at some, presumably early, stage of processing. Moreover, illusory conjunctions are observed in tasks in which the observer is not explicitly asked to analyze the component properties of the stimuli.

However, the attribution of specific kinds of data to processing levels is often speculative (e.g., Treisman, 1986a). Thus, we need a set of criteria to distinguish between the phenomena tapping early processing and those tapping later, more cognitive abilities.

There are at least two possible ways to test the idea that illusory conjunctions tap early processing codes. One way is to contrast the effects of learning, and more precisely, the consequences of schooling, on illusory conjunctions on the one hand, and on more direct measures of visual performance on the other hand. Another way is to see whether or not illusory conjunctions are affected by high-level contextual knowledge and observers' expectancies. These two approaches will be examined in the two following sections (l.a. and l.b.). In the two last sections of this paper, I will examine conjunction-like errors in language processing.

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1. ILLUSORY CONJUNCTIONS AND PROCESSING STAGES

LA. SCHOOLING EFFECTS

Treisman (1986a) distinguished structural, built-in processes of analysis, which she assumed to be relatively independent of development, and analytic strategies that would be set up through learning. Under this assumption, if illusory conjunctions do actually reflect preattentive, unconscious analytical codes, they should occur as frequently in young children and unsophisticated (i.e., illiterate, unschooled) adults as in highly educated adults, in spite of the fact that both children and unsophisticated adults are unable to perform conscious, explicit analysis of some visual stimuli. In other words, one should observe intra-subject dissociations between the kind of analytical codes that are revealed by illusory conjunctions and the kind of analytical codes that are revealed by more usual measures of visual performance.

Several studies of my Ph.D. • work, which was supervised by Paul Bertelson, were aimed at comparing young children (both preschooled and schooled ones), unschooled illiterate adults, age-matched schooled literate adults, and university students for both the occurrence of illusory conjunctions and direct measures of visual performance. Note that all the experiments reported here which were aimed at examining the occurrence of illusory conjunctions used both a target detection procedure and very short exposure durations. The use of target detection allows to compute d' scores on the basis of both false detection and correct target detection scores. The d' is presumably a better indicator of the occurrence of conjunction errors than any score based on false detections alone, since it is a response bias-free index of discriminability (e.g.. Green & Swets, 1966). The d' was calculated independently for experimental and control trials, for each subject. If conjunction errors result, at least partially, from a loss of discriminability, mean d' should be lower for experimental than for control trials (Kolinsky, 1988, 1989; see also a similar reasoning in Treisman & Souther, 1986).

Except for the processing of parts of shapes, there is no strong developmental effect in the occurrence of illusory conjunctions (Kolinsky, 1989). More importantly for the present purpose, when there is some developmental effect, as it is the case with segments of shapes, it depends only on age, not on the specific instruction provided by school. Separability, as reflected by illusory conjunctions, is as developed in unschooled, illiterate adults as in schooled, sophisticated ones (Kolinsky, 1988). This pattern of results contrasts sharply with the one observed on all tasks requiring explicit visual analysis of the stimuli. For example, both young children and unschooled adults were very poor at finding a three-segments part embedded within a six-segments figure (Brito Mendes, Kolinsky, & Morais, 1988; Kolinsky, Morais, & Brito Mendes, 1990; Kolinsky, Morais, Gary, & Content, 1987). Both groups of subjects were also poorer than adults classifying stimuli according to a prespecified dimension, like in Garner's

136 R. Kolinsky

(1974; Garner & Felfoldy, 1970) filtering tasks (Kolinsky, 1988; Kolinsky, Verhaeghe, Grimm-Cabral, & Morais, 1989). On the basis of these results, we may conclude that illusory conjunctions do indeed tap a different, probably earlier, processing stage than the other, more direct measures of performance.

Lb. EFFECTS OF HIGH-LEVEL CONTEXTUAL KNOWLEDGE AND OBSERVERS' EXPECTANCIES ON ILLUSORY CONJUNCTIONS

The absence of schooling effects, while suggesting that illusory conjunctions arise at a rather early processing stage, does not allow one to conclude that the phenomenon taps processing systems that may be assimilated to what Pyiyshyn (1980; 1981) and Fodor (1983) called modules. To approach this problem, one must refer to the list these authors proposed to be characteristics of early, modular systems, among others impenetrability by high-level contextual knowledge as well as by observers' expectancies.

Treisman (1986b) manipulated the labels she gave to the stimuli presented to the observers. For example, one group of subjects was told that a display included "an orange carrot, a blue lake and a black tire". For another group of subjects, the same display was described as "an orange triangle, a blue ellipse and a black ring". For the two groups of subjects, most of the stimuli were "natural" pairings of colours and forms. The subjects made more mistakes in associating colours with shapes when they expected arbitrary pairings of colours and shapes than when they expected pictures of familiar objects. Thus, prior knowledge and expectations help one to use attention efficiently in conjoining features. The interesting result was observed on a third group of subjects, who were occasionally given the wrong combinations (e.g., a green triangle, an orange ellipse and a black tire) when they were expecting most stimuli to be natural objects. Subjects were not more likely to erroneously report, say, an orange carrot if orange was present elsewhere in the display than if orange was absent. Thus, prior knowledge and expectations seem not to induce illusory exchanges of properties to make abnormal objects become normal again. This latter result implies that illusory conjunctions are formed at a level of processing that is not affected by prior knowledge and expectations.

My own studies, which were run in collaboration with Paul Bertelson (Kolinsky, 1988), show that the occurrence of illusory conjunctions seems to be insensitive to both strategies and context. Using a detection task in which the target was a tilted dollar sign and experimental stimuli right-angle triangles and tilted subjects (a material which was designed by Treisman & Paterson, 1984, Exp. 4), I manipulated both the probability of presentation of the real target and the presence of the real target as a potential context for illusory conjunctions. In a first experiment, three probability conditions were used. The target was either never presented, or presented on either 30% or 50% of the trials. In a second study, it appeared on either 14% or 62% of the trials. Moreover, in spite of the fact that target-present trials included only one real target, the observers were asked to report the number of perceived targets on

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each trial. Moreover, they had to make confidence judgments on the presence of the target, on both target-present and target-absent trials.

The main result of these experiments was that the probability of target occurrence did not affect the rates of illusory conjunctions. Moreover, the observers frequently reported to have seen more than one target on target-present trials. The interesting point is that illusory conjunctions were neither enhanced nor hampered by the presence of the real target in the same visual display, since they were as frequent on target-present trials than on target-absent trials. This is particularly remarkable if one takes into account that subjects introspectively reported some phenomenal difference between the two kinds of percepts: They were less confident in their yes response when it was driven by an illusory target than when it was driven by a real target. What the results show is that this impression did not help them to repress the occurrence of the illusion.

Nonetheless, some experimental effects reported in the literature seem at first sight to challenge the view that illusory conjunctions actually tap early, modular processes. For example, Virzi and Egeth (1984) argued against the idea that illusory conjunctions reflect early processes by showing that they may imply high-level, semantic codes, as it is the case for example when observers report the word BIG in black ink or the word RED in blue ink when presented with the words BIG in red ink, BLACK in blue ink and WIDE in green ink. However, Virzi and Egeth's effects may arise at a rather late level of processing. As a matter of fact, these semantic effects disappear when the design reduces the importance of memory factors, for example by using target detection rather than a free recall task (Kolinsky, 1988).

The conclusion of this very brief survey of the work on visual illusory conjunctions is twofold. First, illusory conjunctions can indeed be a powerful exploratory tool of early perceptual codes. Second, as for many other psychological effects, severe experimental precautions are needed. Without appropriate methodological guarantees, conjunction errors arising from ad-hoc reconstructive mnemonic processes may be erroneously taken as reflecting early, perceptual codes.

2. ILLUSORY CONJUNCTIONS AND LANGUAGE PROCESSING

The success of Treisman's predictions within the visual domain has been persuading psychologists to adapt the illusory conjunction phenomenon to the study of other issues. Sometimes such adaptations seem promising, but sometimes they just add confusion to already confused issues. In the next two sections, I will discuss the study of conjunction-like errors in both written word (2.a.) and speech (2.b.) processing.

138 R. Kolinsky

2.0. CONJUNCTION ERRORS IN WRITTEN WORD PROCESSING

Interactions among items in multiword visual displays were incidentally reported by Woodworth (1938), Allport (1977), Shallice and McGill (1978), and Shallice and Warrington (1977), and were referred to later on as letter migration errors (Mozer, 1983). They are exchanges of one letter between sequences of several letters. For example, when presented rapidly with the words LINE and LACE, observers sometimes report seeing either the word LICE or the word LANE. These migration errors are much more numerous than the intrusion of letters that were not actually presented, like reporting the word LIVE in the above example. The occurrence of such errors may be a useful way to assess the relevance of possible access codes, like letters and syllables.

The letter migration phenomenon received several interpretations. According to Treisman and Souther (1986), it is related to illusory conjunctions. Like these errors, letter migrations are supposed to result from the exchange of features that were separately extracted at an early, preattentive stage. In two other models, proposed by McClelland and Mozer (1986; McClelland, 1985; 1986; Mozer, 1983; 1987), letter migrations are supposed to result from interactions between units at the word level of representation. The predictions of these two kinds of models differ regarding the influence of lexicality on the phenomenon of letter migration phenomenon, thus making a controversy to emerge on this point.

Having tried myself to shed some light on this controversial issue, I also looked for letter migrations. However, letter migrations only occurred in a detection task when the initial consonant of the target (e.g., BAC) corresponded to the initial consonant of the left sequence (e.g., BAR - SAC). They did not occur when the reverse presentation order was used (e.g., SAC - BAR). Similarly, in a free recall task, the conjunction errors consisted almost exclusively of migrations of a left-presented letter into the right presented item (Kolinsky & Martin, 1990). Thus, a directionality effect was observed that is consistent with the direction of reading.

This directionality effect had never been reported in the literature, even though other directionality effects were mentioned. For example, in a task where subjects had to indicate the nature of either a letter or a string of letters whose position was cued during the interval between the stimulus onset and the response, McClelland and Mozer (1986, Exp. 1 & 2; see also Mozer, 1983. Exp. 1, 2, & 4) observed an opposite effect: Right-to-left migrations were sightly more numerous than left-to-right ones, at least for familiar strings. The authors related this observation to the advantage usually obtained for words presented to the right of fixation, that is, to a preattentive right visual field advantage (McConkie, 1979; Rayner, Weli, & Pollatsek, 1980). In agreement with this interpretation, the directionality effect described formerly by Mozer (1983) varied as a function of presentation time: It was only observed with rather short exposure durations (Mozer, 1983. Exp. 1 & 4).

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This right-to-left direction modulates the letter migration phenomenon, but its influence is certainly much less strong than it is the case of the directionality effect observed in our experiments. The two directionality effects thus seem to have different origins.

In order to eliminate our directionality effect, we decided to run again the detection experiment, but using this time pairs of letter strings presented one on the top of the other. Under these conditions, there were no migrations at all, neither top-down nor bottom-up. Thus, it seems that, at least under the experimental conditions we used, the occurrence of letter migrations crucially depend on a left-to-right scanning, probably driven by our reading habits. Even though eye movements were prevented by using short exposure durations (around 150 ms), an attentional scanning was possible, and probably yielded an experimental artefact, namely the migration phenomenon itself ! The problem is that many of the published studies either used the same range of exposure durations as we did (e.g., Mozer, 1983), or did not mention where the migration response comes from (e.g., Treisman & Souther, 1986). Other experiments, like McClelland and Mozer's (1986) ones, used recall tasks, in which mnemonic ad-hoc reconstructive processes could have played some role, and in which there could be no control for response biases.

Letter migrations between items are not the only phenomenon one can think of in trying to adapt the illusory conjunction technique to written language processing. Studying visual, non-linguistic illusory conjunctions, Prinzmetal (1981; Prinzmetal & Keysar, 1989) found that these are more likely to occur within a perceptual group than between perceptual groups, when perceptual groups are defined by Gestalt properties like similarity and good continuation. Inspired by Prinzmetal's (1981) idea, several authors hypothesized that within-word illusory conjunaions of colours and letter forms might be affected by linguistic structures like syllabic, orthographic or morphological units, or by pattern of bigram frequencies (Millis, 1986; Prinzmetal, 1990; Prinzmetal, Hoffman, & Vest, 1991; Prinzmetal & Keyzar, 1989; Prinzmetal & Millis-Wright, 1984; Prinzmetal, Treiman, & Rho, 1986; Rapp, 1992; Seidenberg, 1987). The results observed with this technique are promising. However, as far as I know, no effort has been made until now to integrate this line of research with the one on between-words letter migrations. As noted by Treisman and Souther (1986), differences in attention deployment conditions between the two kinds of situations make comparisons between the two phenomena difficult. Intra-word migration studies contrasted attention to one letter string with attention to its component letters, while the inter-words migration studies contrasted attention to component letters with attention to a whole, say two-items, display.

According to Mozer (1983), between-words letter migrations are in any case different in nature from illusory conjunaions. First, letter migrations seem to involve mainly copies of letters rather than letter exchanges (e.g., perceiving LiNE - L\cE when presented with LINE - LACE, rather than

140 R. Kolinsky

perceiving LANE - LICE). As noted by Mozer (1983), although the feature-integration theory does not explicitly rule out the formation of copies, such a possibility does not seem parsimonious to the theory. Moreover, illusory conjunctions seldom involve copies; features generally leave no trace of ghostly replica behind them when they move to a different location (Treisman & Schmidt, 1982, Exp. 1). Second, decreasing similarity between words reduces the number of letter migrations: For example, CAPE - CONE, having similar non tested letters (c and E), led to more letter migrations than CAPE -MONK, which have the same tested letters as in the previous pair but different nontested letters (Mozer, 1983; McClelland & Mozer, 1986; Shallice & McGill, 1978). Treisman and Schmidt (1982, Exp. 1 & 4), on the other hand, observed no effect of similarity on illusory conjunctions. Because migration of one letter in a word is dependent on the identities of the other letters, migrating letters do not behave as separable features of a word in the sense of feature-integration theory. However, Fang and Wu (1989), discussing this point, argued that what seems to be a surround-similarity effect in letter migrations may be better characterized as an effect of perceptual distinctiveness: The AN string in SAND and LANE, for example, might have enhanced the perceptual distinctiveness, and hence the separability, of the letters S, L, D, and E, resulting in a higher migration rate.

Besides, it should be noted that the copy effect observed by Mozer (1983) seems to indicate that letter migrations are of different nature than spoonerisms, which involve, in speech production, the interchange of phonemes of two nearby or adjacent words, for example transforming KARN DOOR into ï>ARN BORE (e.g., MacKay, 1970). Indeed, copies are foreign to both illusory conjunctions and spoonerisms, the latter being, by definition, exchanges (see data by Baars & Motley, 1976).

2. b. CONJUNCTION ERRORS IN SPEECH PROCESSING

It was suggested by Studdert-Kennedy (1981) that the occurrence of errors which, like spoonerisms, arise at the level of the phoneme, might indicate that this unit plays some role in speech production. In the last years, our group1

has been concerned with the possible occurrence of speech migration-like recognition errors. The objective underlying this investigation is to compare the relative contribution of different sublexical properties to the representations that mediate the mapping between the acoustic signal and the mental lexicon.

There are at least two examples of migration-like error which have been studied in speech perception. One is the so-called phonetic feature blending effect. Studdert-Kennedy and Shankweiler (1970) already reported that blending errors are responsible for the feature-sharing advantage observed in the recognition of dichotic speech stimuli, that is, when two different stimuli

M. Cluytcns. R. Kolinsky. J. Morais, and M. Radeau.

Conjunction Errors and Perceptual Processing 141

are presented simultaneously, each to one ear. Dichotic cv syllables that differ in both features of the stop consonant (place of articulation and voicing) receive fewer correct responses than syllables that contrast only in one single feature. This is because incorrect combination of voicing and place of articulation can occur between the double-contrast items, leading for example /ba-ta/ to be heard as /da-pa/. Later on, several researchers showed that these conjunction errors are more numerous than anomalous stop responses that differ in place (in the above example, /ga/ or /ka/). The phonetic features blending effect seems to arise at a level where acoustic information is evaluated by reference to phonetic categories, that is, at a relatively early processing level (Cutting, 1976; Day, 1968; Halwes, 1969; Pisoni & McNabb, 1974; Tartter & Blumstein, 1981).

Another phenomenon of unit migration in the recognition of speech stimuli is the so-called phonological fusion. In this case, the simultaneously presentation of say the words LACK to one ear and BACK to the other ear may lead to the illusory word BLACK. This effect suggests that listeners segment the speech input into phonemes. However, phonological fusions may arise rather late in the recognition process. They are not only highly sensitive to the lexical status of the potential migration error (Cutting, 1975), but also to spelling factors. As a matter of fact, in literate people, but not in illiterates, the phenomenon is inhibited when orthography does not fit phonology, as it is the case when the two Portuguese words /HJ7 and /fiJ7 lead to the word which is written FELTZ but pronounced /fit// (Morais, Castro, & Kolinsky, 1991).

The occurrence of phonetic feature blendings, while suggesting that some features may contribute to the intermediate representations of speech, does not exclude the possibility that other linguistic properties also play some role. In particular, two linguistic units have often been proposed as being the perceptual building blocks making up the intermediate representations: the phoneme (Pisoni & Luce, 1987) and the syllable (Mehler, 1981; Segui, 1984). We thus decided to develop an experimental paradigm where the aptitude of the different properties to be represented separately in the intermediate representations of speech can be directly compared (see Kolinsky & Morais, submitted, for a more detailed description of this paradigm).

In our former experiment using this paradigm, the distribution of information between two dichotic stimuli was experimentally manipulated in order to test the relevance of different word constituents as units of spoken word recognition: phonetic features (voicing), phonemes (stop consonants), and syllables. We have chosen sets of pseudoword pairs so that all the corresponding trials share the same potential migration response. So, the pairs within each of these sets only differ in terms of the particular property whose possible migration is under examination. For instance, the French words BUOV (/bi3u/) or COTON (/loto/) might be illusorily perceived by combining either voicing when using stimuli like /pJ3u/-/gotô/, or first consonants with /W3U/-/botcV as stimuli, or syllables, in /bïtô7-/ko3u/. Potential migration responses

142 R. Kolinsky

could be either words, as in the above example, or pseudo-words (e.g., /t»3u/ and /kitö/). We presented all these pairs, which we call experimental, as well as control pairs where the crucial attribute is missing, in a target-detection task. For example, let us take the target /bi3u/. In the control trial of the consonant condition, the pseudoword /W311/, which is the one perceptually closer to the target, was maintained, and the pseudoword làotol, which lacks the labial feature, replaced the experimental item ibotöl. For the syllable, it is difficult to decide a priori which of the two experimental stimuli is perceptually closer to the target. Thus, in this condition, we presented two different control pairs, sharing with the target either the initial syllable (as in /bitö/vio/o/ for the target Ib'^ul) or the final syllable (as in /detö/-/k03iV).

As for visual illusory conjunctions, it is the extra number of false detections on experimental trials relative to false detections on control trials that provides evidence of migrations errors. Target-present trials (in the above example, for the target fb'^ul, either /bi3u-gDto/ or /bî3u-kotc7) were also used in the detection task. Besides, target-absent trials formed the material of a second experiment, in which we used a free recall task. The whole set of material was constructed from three sets of word target pairs (BUOU - COTON, TISSU - GAMIN and DéLIT - PINCEAU) and three sets of corresponding pseudo-word target pairs. The main phase of each experiment was preceded by a set of training trials, which were constructed from a different list of words and pseudowords. Within each experiment, trials were presented pseudo-randomly.

The stimuli used in both experiments were recorded by a male native speaker in an anechoic room. Using a computerized speech work station, they were then digitalised, normalized and synchronized. Intensity of the two dichotic stimuli of a pair was equated, without affecting intensity differences within each stimulus. The two stimuli were then synchronized at the beginning of the explosion of ci. For second syllable synchronization, the point separating non-identification from identification of the second consonant was assessed by two judges. Then, the time difference between the identification points of the two dichotic stimuli was estimated. The longest initial syllable was compressed by cutting the appropriate number of cycles in the most stable part of the vowel, and the shortest initial syllable was expanded by multiplying as much as necessary one cycle of the initial vowel, again at its most stable part. Finally, the quality of each item was checked.

The stimuli were played back through headphones, using a digital recorder. Each type of trial was presented twice, counterbalancing for ear assignment. Headphones assignment to the ears was also counterbalanced within subjects.

Let us examine first the results of the free recall experiment (Morais, Kolinsky & Cluytens, in preparation). We compared the number of migration errors (like responding /bi3u/ when presented with /ki3u/-/bDtô/) to the number of intrusions (like /di3u/ in the above example). We observed significant

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migration rates, estimated as the proportion of migration errors relative to the sum of migrations and intrusions for both phonetic features and syllables, but not for first consonants. Moreover, syllabic migrations were far more frequent than the other migration types. Comparisons of migration probabilities also showed that syllables migrate as whole units, and not as simultaneous migrations of first consonant and first vowel.

The detection task will be described here in detail. It was run on 24 native French speaking university students. It included 216 trials, and was preceded by 68 training trials.

The subjects were given a booklet in which the targets were listed. They were told that, on each trial, they would hear two speech stimuli which could be meaningful or meaningless, and asked to pay attention to both ears in order to decide whether the target indicated in the booklet for each particular trial had been presented or not.

Migration rates were estimated as the number of false detections made on experimental trials divided by the number of false detections made on both experimental and control trials. The pattern of results is similar to the one observed in the free recall situation. Consonantal migrations were marginally significant («33 = 2.154, p<.05, and r23

= L946, p<.05, for word and pseudo-word targets, respectively), whereas migration rates for both voicing (r23 = 5.062, p<.0001, and t^ = 3.08, p<.003, for word and pseudo-word targets, respectively) and syllables (t^ = 5.496, p<.0001 and r23

= 5.409, p<.0001, for word and pseudo-word targets, respectively) were highly significant. The analysis of variance, which was run on arcsine values of migration rates (see Figure 1), showed a highly significant main effect of speech attribute (F(2-46) = 13.159, p < .0005). Neither lexical status of the target nor the interaction of this lexical factor with speech attributes were significant (F< 1). There were significantly more syllabic than either consonantal (F(i-23) = 28.87, p<.0005 ) or voicing (F(i-23) = 9.64, p<.01) migrations.

The detection task was designed in such a way that we were unable to compute d' scores, thus to control for response bias. In fact, there were no target-present trials corresponding to each unit condition. However, more recently we ran several other experiments in which we included target-present trials corresponding to each unit condition (Kolinsky & Morais, in press; submitted). The results observed on d' fully confirmed the previous observations.

This first set of results is, we believe, very encouraging. Particularly encouraging is the fact that our results are consistent with effects observed in other situations, like the syllabic effect observed by Mehler, Dommergues, Frauenfelder, and Segui (1981). Using a segment-monitoring task, these authors found that detection times were faster when the target corresponded to the first syllable of the carrier word (like BA in BALANCE and BAL in BALCON)

144 R. Kolinsky

Figure 1

=5 apcsino o! 0.5 VOICING FIRST CONSONANT SYOABi-E

Mean arcsine values of migration rates in each speech property condition, separately for word targets (-•- )and pseudo-word targets (..A...).

than when it does not fit the syllabic structure of the carrier word (like BAL in BALANCE and BA in BALCON).

Syllables thus appear much more plausible candidates as units of speech processing than consonantal phonemes, at least stop consonants. However, these conclusions might be restricted to a language like French, in which timing is syllable-based and syllable boundaries are clear. Indeed, previous work using the segment-detection task suggests that speech segmentation may depend on the phonological characteristics of the listener's native language (Cutler, Méhler, Norris, & Segui, 1986).

Contrary to segment detection tasks, however, in our situation both stimuli and targets are at the word structure level, and segmentation is indirectly inferred from the pattern of recognition errors. Thus, our situation does not include any metaphonological judgment that could distort what happens at the recognition stage (Morais, 1985; Morais, Bertelson, Cary, & Alegria, 1986). The possibility of language-dependent processes of word recognition may thus be fruitfully investigated by mean of our illusory word detection task (see Kolinsky & Morais, submitted).

However, we should also test the assumption that speech conjunction errors arise quite early in the processing route. As a matter of fact, this idea is supported by the fact that we did not observe any difference in phonetic feature blending rates between Portuguese literate and illiterate adults (Morais, Castro, Scliar-Cabral, Kolinsky, & Content, 1987). This result contrasts sharply with the fact that, in the very same study, we did observe a literacy

Conjunction Errors and Perceptual Processing 145

effect on both the phonological fusion phenomenon we already mentioned (Morais et al., 1991) and the proportion of what we called segmental errors. Literate people made a greater proportion of recognition errors limited to the initial consonant than illiterates, whereas the reverse pattern of results was observed for more global (i.e., syllabic) errors. This result was observed even when overall performance was equated between the groups of subjects. Thus, literate people, being aware of phonemes, may focus more on the phonemic constituents of speech than illiterates, at least when a language presenting vocalic reduction like Portuguese has to be processed in difficult listening conditions. This effect shows that some attentional strategies may affect speech recognition processes.

We still do not know whether all kinds of speech migration errors will be unaffected by literacy, as it seems to be the case for phonetic feature migrations. According to Anne Treisman's original proposal regarding visual illusory conjunctions, speech migrations would reflect early, preattentive processing, that is, the mandatory operations of a speech module. But the comparison of either written letter migrations or speech migrations with visual illusory conjunctions might well be a loose analogy. What is more important and promising, however, is the fact that the study of conjunction errors seems to provide an interesting tool for examining the nature of perceptual processing codes.

In this chapter, I have tried to show how the young Ph.D. researcher I was some years ago, fascinated by visual illusory conjunctions, is now, by a subtle game of sometimes loose analogies, trying to study speech perception using our illusory word detection task. I am very proud to be one of the latest pupils of Paul. The hope I have is, of course, that Paul will also be proud of our present work, which would never have occurred without his support.

ACKNOWLEDGEMENTS

Support for the reported work was obtained from the Human Frontier Science Program (project Processing consequences of contrasting language phonologies), from the Belgian Fonds de la Recherche Fondamentale Collective (F.R.F.C., Convention n° 2.4562.86), from the Belgian Services de la Programmation de la Politique Scientifique (S.P.P.S., Impulsion à l'intelligence articielle: la connaissance lexicale, acquisition, représentation et accès, Convention n°. RFO/AI/23), from the Belgian Action de la Recherche Concertée (A.R.C., Convention n°. 91/96-148), and from the Belgian F.N.R.S. -Loterie Nationale (convention n° 8.4527.90). Special thanks are due to M. Cluytens for preparation of tapes, to N. Martin for data treatment, and to J. Morais for constant advice.

146 R. Kolinsky

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