differential attention effects on dichotic listening · pdf filelas pruebas dicóticas...

J Am Acad Audiol 16:205–218 (2005)

*University of Connecticut, Storrs, Connecticut; †Currently at the University of Kentucky College of Medicine; ‡University of Massachusetts Amherst, Amherst, Massachusetts

Jennifer B. Shinn, Department of Surgery, Otolaryngology-Head and Neck Surgery, University of Kentucky College of Medicine,800 Rose Street, C236, Lexington, Kentucky 40536-0293; Phone: 859-257-3390, ext. 82173; Fax: 859-257-5096

This paper was presented at the American Academy of Audiology convention, April 3, 2004, in Salt Lake City, Utah.

Differential Attention Effects on DichoticListening

Jennifer B. Shinn*†Jane A. Baran‡Deborah W. Moncrieff*Frank E. Musiek*

Abstract

The role of attention in the differentiation of auditory processing disorders fromattention deficit disorders is gaining considerable interest in both the clinicaland research arenas. It has been well established that when attention isdirected to one ear or the other on traditional dichotic tests, performance canbe altered. However, preliminary studies in our laboratory have shown thatdichotic fusion paradigms are resistant to shifts in ear performance associatedwith changes in attention. The purpose of this study was to assess theperformance of normal listeners on a dichotic consonant-vowel and a dichoticrhyme (fusion) test. Both test procedures were administered to 20 youngadults in three different listening conditions (free recall, attention directed tothe left ear, and attention directed to the right ear). Results from this studysupported the hypothesis that dichotic rhyme tests are resistant to alterationsin the laterality of attention and have implications for the development of testparadigms that can be used to segregate attention from pure auditory deficitsin the clinical domain.

Key Words: Attention, auditory processing, consonant vowel, dichoticconsonant-vowel, dichotic listening, ear advantage

Abbreviations: ADHD = attention deficit-hyperactivity disorder; APD = auditoryprocessing disorder; CV = consonant vowel; DCV = dichotic consonant-vowel;DL = directed left; DR = directed right; DRT = dichotic rhyme test; FR = freerecall; LEA = left ear advantage; NEA = no ear advantage; REA = right earadvantage; SRT = speech recognition threshold

Sumario

El papel de la atención al diferenciar entre un trastorno de procesamiento auditivoy un trastorno de deficiencia de la atención está ganando considerable interéstanto en el campo clínico como en el de la investigación. Se ha establecidoclaramente que cuando se dirige la atención a uno u otro oído durante laspruebas dicóticas tradicionales, el desempeño puede alterarse. Sin embargo,los estudios preliminares en nuestro laboratorio han mostrado que losparadigmas de fusión dicótica son resistentes a cambios en el desempeñoauditivo asociados con cambios en la atención. El propósito de este estudiofue evaluar el desempeño de sujetos normo-oyentes en una prueba dicóticade consonante-vocal y una de rima dicótica. Ambos pruebas fueronadministradas a 20 adultos jóvenes en tres diferentes condiciones de escucha(audición libre, con atención dirigida al oído izquierdo, y con atención dirigidaal oído derecho). Los resultados de este estudio apoyan la hipótesis de quelas pruebas dicóticas de rima son resistentes a las alteraciones en la lateralidad

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The concept of dichotic listening was firstintroduced by Broadbent in 1954.Dichotic listening occurs when different

auditory stimuli are presented to each earsimultaneously. It has been used historically toassess hemispheric dominance andlateralization, as well as hemisphericasymmetries (Kimura, 1961a, 1961b, 1967;Zattore, 1989), with diminished scores on thesetypes of listening tasks suggesting auditoryand/or cognitive dysfunction or pathology(Kimura, 1961a, 1961b). Kimura is credited asthe first researcher to investigate the associationbetween lateralization of speech and itsrelationship to the right ear advantage (REA).Her early work has been the foundation forthe widely accepted theory that in man, thecontralateral (or crossed) auditory pathwayhas more neural connections than the ipsilateralpathway and is considered the dominantpathway. On dichotic listening tasks, individualswill generally show an ear advantage in the earcontralateral to the hemisphere dominant forlanguage. For most individuals this will resultin an REA, which is believed to be the result ofthe left hemisphere’s dominance for languageand the auditory perception of speech stimuli(Kimura, 1967).

Although most individuals will tend todemonstrate an REA, this ear advantage is byno means a universal finding. The variabilityin performance on dichotic tasks with respectto ear advantage has been extensively studiedin the past (Pizzamiglio et al, 1974; Ryan andMcNeil, 1974; Blumstein et al, 1975; Speaksand Niccum, 1977). Results of these studieshave demonstrated considerable variabilityin both the numbers of individualsdemonstrating REAs, the size or extent of theear advantage, and the stability of the REA

across test sessions (i.e., test-retest correlation).One potential explanation for the differencesnoted in these measures is that attentioneffects may be operating to bias responses onseveral of these tests.

A number of studies have demonstratedthat some dichotic tests are vulnerable toshifts in attentional focus (Keith et al, 1985;Hugdahl and Andersson, 1986; Asbjørnsenand Hugdahl, 1995; Asbjørnsen and Bryden,1996; Wiens and Emmerich, 1999; Hugdahl etal, 2000, 2001; Vingerhoets and Luppens,2001; Voyer and Flight, 2001; Foster et al,2002; Hugdahl et al, 2003). For example,Asbjørnsen and Hugdahl (1995) assessed theperformance of normal subjects on a dichoticconsonant-vowel (DCV) task in a non-forced(NF) condition (attention divided betweenears) in which the normal subjects wereinstructed to freely repeat what they heard,and two forced conditions in which the subjectswere instructed to direct their attention toeither the left or right ear (i.e., attention forcedright and forced left paradigms). Theseresearchers demonstrated that the size and/orthe direction of the ear advantage could bealtered based on the focus of the subjects’attention in the forced attention conditions.When attention was directed to the right ear,the expected REA was enhanced, whereas areversal of the ear advantage from an REA toa left ear advantage (LEA) was commonlyobserved when the subjects were instructed todirect their attention to the left ear.

Hugdahl and colleagues (2000)subsequently studied the effects of attention bymeasuring brain activation through the use ofa 15O-PET study using dichotic consonantvowels (CVs) or short musical instrumentpassages as the stimuli. The subjects were

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de la atención, y tienen implicaciones en el desarrollo de paradigmas deevaluación que puede usarse, en el campo clínico, para segregar los trastornosde atención de los trastornos auditivos puros.

Palabras Clave: Atención, procesamiento auditivo, audición dicótica, oído conventaja

Abreviaturas: ADHD = trastorno de hiperactividad y deficiencia en la atención;ADP = trastorno de procesamiento auditivo; CV = consonante-vocal; DCV =consonante-vocal dicótica; DL = dirigido a la izquierda; DR = dirigido a la derecha;DRT = prueba dicótica de rima; FR = audición libre; LEA = ventaja del oídoizquierdo; NEA = sin ventaja de oído; REA = ventaja del oído derecho; SRT= umbral de reconocimiento del lenguaje

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evaluated using a paradigm in which theywere directed to detect a target stimulus ineither ear, in the left ear only, or in the rightear only. Their results indicated that whenattention was focused on either the right or leftear, there was a significant decrease in brainactivation bilaterally in certain areas of thetemporal lobes when compared to the activationpatterns observed in the NF condition. Forthe forced attention conditions, brain activationmigrated from the temporal lobes to the rightposterior and inferior superior parietal lobes.These areas have been identified previously asbeing active during attentional tasks (Cabezaand Nyberg, 1997).

Although CVs are often the stimuliutilized in research on dichotic listening, thereare many alternative stimuli that have beenemployed. These include words (Niccum etal, 1981; Wexler and Halwes, 1983; Speaks etal, 1985), digits (Kimura, 1961a, 1961b, 1967;Milner et al, 1968), chords (Efron, 1977; Efronet al, 1979), and sentences (Jerger et al, 1972;Speaks, 1975).

A less frequently used dichotic test is thedichotic rhyme task (DRT). This task was firstintroduced by Wexler and Halwes (1983) andthen later modified by Musiek et al (1989). TheDRT uses temporally aligned consonant-vowel-consonant pairs that vary only in their initialconsonants. Although subjects are presentedtwo words (one word to each ear), the precisealignment of the words, as well as the fact thatthe final vowel-consonant elements in eachpair of words are identical, result in thesubjects perceiving only one word the vastmajority of the time. As a result of these testfeatures, normal right-handed subjects tendto demonstrate test scores that are slightlygreater than 50% in the right ear and slightlyless than 50% in the left ear (Musiek et al,1989). This unique pattern of performance ispresumed to be the result of some type ofdichotic “fusion” of the signals, which occurslow within the central auditory nervoussystem. Such a phenomenon had beendescribed earlier by Repp (1976) as a fusion ofstimulus pairs resulting from a “low-levelcross-correlated mechanism.”

The DRT was originally developed toaddress the limitations of many of thetraditional dichotic tasks, including low test-retest correlations, the presence of an LEA insome right-handed subjects, and high errorrates (Wexler and Halwes, 1983). Wexler andHalwes reported LEA rates in the literature

of 25 to 30% on dichotic speech tests for right-handed subjects. However, in an earlier studyutilizing a sodium amytal procedure toinvestigate brain lateralization, Rasmussenand Milner (1975) reported that 97% of right-handed individuals exhibited left hemispheredominance for language. Therefore, these REAstatistics reflect a possible 22 to 27%discrepancy between results of traditionalbehavioral dichotic testing and truehemispheric language lateralization.

Concerned by these findings, Wexler andHalwes (1983) investigated the utility of theirDRT as an alternative dichotic tool. Usingthis test, these investigators demonstratedan REAin 85% of right-handed subjects versusthe 70 to 75% they had previously reported intheir review of the literature. These results metmore closely the expected validity criterionwith respect to REA in right-handed subjectsas well as left-handed subjects. In addition,their results demonstrated better intrasubjectreliability, and they found that the DRT wasresistant to shifts or changes in attention(Wexler and Halwes, 1985). This latter findingbecomes important particularly whenevaluating individuals with attentionaldisorders since it is unlikely that focus ofattention may result in an artificial earasymmetry on this test.

Zattore’s (1989) work that investigatedspeech lateralization using the carotid sodiumamytal test supports the validity claims ofthe DRT made by Wexler and Halwes (1983,1985). Zattore examined the DRT in 61patients (35 subjects with left hemisphericrepresentation, 4 with right hemisphericrepresentation, and 22 subjects with bilateralrepresentation as determined by the resultsof the carotid sodium amytal test). Thosepatients who had a significant REA on theDRT exhibited left hemispheric speechrepresentation, whereas the reverse was truefor those patients with right hemisphericspeech dominance. Those patients who showedno significant ear asymmetry demonstratedbilateral speech representation. This studyclearly demonstrated the sensitivity of theDRT test to hemispheric dominance for speech.

Asbjørnsen and Bryden (1996) examinedthe effects of biased attention on a dichoticfused word test in comparison to the DCVtest. Although they did not test a free recall(FR) condition, they did study the effects ofattention in forced right and forced leftconditions. Their overall findings indicated

that attention significantly affectedperformance on both the DCV test and thedichotic fusion test, with the effects beingmuch greater on the DCV test than on thedichotic fusion test. These findings wereconsistent with previous attention research onDCVs; however, the finding of significantattention effects on the dichotic fusion testwas contrary to earlier claims made by Wexlerand Halwes (1985), which indicated thatdichotic fusion tasks were resistant to shiftsin attention.

The DRT was also used in studyingdichotic listening performance in split-brainpatients (Musiek et al, 1989). In this study, twosignificant observations were made for thispopulation of patients. The first of these wasthat the subjects in this investigationconsistently demonstrated the “expected” leftear deficit due to the compromise of the normalinterhemispheric pathways, and the secondwas that the subjects not only showed theexpected REA but that the size of thisadvantage was noticeably greater than thatnoted for normal subjects. The results of thisstudy demonstrated that in addition to beingclinically feasible for use with patients withcompromise of the central auditory nervoussystem, this test was highly sensitive inassessing the integrity of interhemispherictransfer of auditory information.

The unique nature of dichotic fusion tasksleads to the hypothesis that this dichotic taskmay be uniquely resistant to the effects ofattention. Moreover, preliminary work byBaran and Musiek (1987) has demonstratedthat the DRT is highly resistant to shifts inattention. Therefore, the purpose of the presentstudy was to expand upon this earlierinvestigation by increasing the number ofsubjects tested and to additionally investigatethe effects of directing attention to one earversus the other on a DCV as well as the DRTtest. Due to overwhelming evidence regardingthe instability of dichotic CVs with respect toattentional demands, the need to comparethese two tests of dichotic listening wasapparent. In addition, the current pressurebeing placed upon professionals todifferentially diagnose attentional deficitsassociated with auditory processing disorders(APD) from those associated with attentiondeficit-hyperactivity disorder (ADHD) rendersthis investigation both appropriate and timely.

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A total of 20 college students recruitedfrom the University of Connecticut participatedin the present study. Subjects ranged in agefrom 20 to 27 years with a mean age of 22.5years (SD ± 2.25). All subjects were right-hand dominant based on results obtained fromthe Annette (1970) handedness questionnaireand were native speakers of English. Allsubjects reported negative histories ofaudiologic, neurological, and otologicinvolvement, and all subjects denied any ofhistory of learning disabilities. In addition,each subject was screened for ADHD using theBrown ADD Scale™ (Brown, 1996) prior toparticipation in this study, and all 20 subjectstested negative for ADHD.

Prior to participating in the experimentalprotocols, each subject underwent an otoscopicand audiological examination. Criteria forinclusion in this study included (1) a normalotoscopic exam; (2) pure-tone air-conductionthresholds of 20 dB HL or better bilaterallywith no significant asymmetries (<10 dBbetween ears at any frequency) for theaudiometric test frequencies between 250 and8000 Hz, including the interoctave frequenciesof 3000 and 6000 Hz; and (3) monaural CVidentification scores of 80% or higher for 30tokens of the experimental test stimuli (pa, ta,ka, ba, da, ga) presented at a 50 dB sensationlevel (SL) re: SRT.

The mean pure-tone averages were 6.8(SD ± 10.5) for the right ear and 6.1 (SD ± 4.3)for the left ear. Mean speech recognitionthresholds (SRT) derived using theDepartment of Veterans Affairs (1998) recordedspondee words were 7.7 (SD ±5.3) and 8.3 (SD± 5.7) for right the left ears, respectively, andall 20 subjects demonstrated monaural CVidentification scores of 80% or higher in boththe right and left ears.

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The CV stimuli used for the DCV testwere the dichotic CVs from the Veterans Affairsand Dartmouth Hitchcock Medical CenterTonal and Speech Materials for AuditoryPerceptual Assessment compact disc (1992)and consisted of the six stop consonants pairedwith the vowel /a/, resulting in /ba/, /da/, /ga/,


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/pa/, /ta/, and /ka/. Each individual stimulusranged in duration from 241 to 284 msec, andthe paired stimuli had essentiallysimultaneous onsets as shown in Figure 1.The six CVs were paired with each other,excluding homogenous pairs (e.g., ka-ka), andthree randomizations of the original list werecreated, with each list consisting of 30counterbalanced pairs (60 words total) (seeWilson and Leigh, 1996, for additional detailsregarding the construction and parametersof the DCV test).

The DRT was composed of monosyllabicconsonant-vowel-consonant (CVC) pairsbeginning with /b/, /d/, /g/, /p/, /t/, and /k/. Therhymed words were temporally aligned sothat their onsets were essentiallysimultaneous, and each stimulus pair differedonly with respect to the initial stop consonants.A total of 15 pairs of rhyming words wereused in the present investigation (Appendix A),with each pair being presented twice in acounterbalanced format such that each earreceived each word once. An example of onepair of stimuli (pen and ten) is provided inFigure 1 (see Wexler and Hawles, 1983, foradditional details regarding the constructionand parameters of the DRT).

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Subjects were seated comfortably in adouble-walled sound-treated booth (IAC), andtesting was completed using matched TDH-50P supra-aural headphones. All stimuli were

played on a compact disc player (Sony XE270)and passed to the headphones through thespeech circuitry of a diagnostic audiometer(Grason Stadler 61). A complete acousticcalibration was conducted prior to the onset,at the midpoint, and at the end of the presentstudy. Test stimuli were presented at 50 dB SLre: SRT, and each subject was given a briefpractice session before being administeredthe actual experimental tasks to ensure thatthe subject understood the task. The practicesession consisted of six sample pairs of boththe DCVs and the DRT test stimuli.

Following completion of the practicesession, subjects were tested on both the DCVand DRT in three different listening conditions:free recall, attention directed right (DR), andattention directed left (DL). In the FRcondition, the subjects were asked to repeatany stimuli they heard in either ear. In the DRcondition, they were asked to ignore the stimulipresented in the left ear and to repeat thestimuli heard in the right ear, and vice versafor the DL condition. The presentation of tests,conditions, and ears was randomized andcounterbalanced using an adapted Latin-square methodology (Wagenaar, 1969). Inorder to control for any potential errors in theexaminer’s interpretation of the subjects’responses on the DCV test (i.e., due to theminimal acoustic differences among the six teststimuli), subjects were asked to repeat thestimuli perceived and to additionally circletheir answers on a response sheet that wasprovided for this purpose. In cases where there

FFiigguurree 11.. Waveforms displaying examples and alignment of the test stimuli from the dichotic CV test (ta andpa) and the dichotic rhyme test (ten and pen).


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were differences between the spoken responsesof a subject and the responses recorded onthe response sheets, the responses recorded onthe answer sheet served as the final responsesfor data analysis purposes. For the DRT, thesubject was asked to repeat the word(s) theyheard. In the event that the examiner wasunsure of a response, the subjects were askedto spell the word perceived or to use the wordin a sentence. Subjects were permitted asmuch time as they needed to respond to thestimuli. Percent correct identification scoreswere calculated for each ear on both testsunder each of the three listening conditions.

Following the completion of testing, thesubject was asked to complete a briefpostevaluation attention questionnaireregarding their experiences during the testsession (see Table 1). The questions on thisquestionnaire were designed to (1) probe thesubjects’ self-evaluations of their ability to

maintain directed attention during the DLand DR tasks and their perceptions regardingthe consistency of their performance during thetesting, and (2) to provide an indication thatthe subjects understood the task and were“directing” their attention as instructed. Inthis way, a lack of a change in ear performanceunder the directed attention tasks could beattributed to some underlying physiologicalmechanisms rather than a lack of “effort” todirect attention.

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Descriptive statistics including means andstandard deviations performance for both

the DCV and DRT are displayed in Figure 2.Results of an analysis for the DRT results acrossthe three conditions revealed mean scores andstandard deviations of 40.5 for the left ear (SD= 9.63) and 53.85 (SD = 10.43) for the right ear.

Table 1. Postevaluation Questionnaire Showing Percent of Subjects Responding on a Five-Point Scale to Each of Nine Questions (1–9), as well as the Percentages of Subjects Indicating to

Which Ear It Was Easier to Focus Attention (question 10)

Not Difficult….............………….Very Difficult

1. How would you rate your overall ability to 55 15 25 5 0attend during today’s evaluation?

2. How difficult was it for you to direct your 10 15 30 35 10attention to one ear on the Dichotic CV test? (ex: ba, da, ga, ka, pa, ta)

3. How difficult was it for you to direct your 5 10 20 40 25attention to one ear on the Dichotic Rhyme Test?(ex: car, pill, toy, page)

Not Often….............…………..............Often

4. At any time did you find yourself not attending 85 5 10 0 0to the Dichotic CV Test? (ex: ba, da, ga, ka, pa, ta)

5. At any time did you find yourself not attending 80 5 10 5 0to the Dichotic Rhyme Test? (ex: car, pill, toy, page)

Not Consistent………….......……Consistent

6. Rate how consistent your attention was 5 0 5 30 60 during the Dichotic CV Test. (ex: ba, da, ga, ka, pa, ta)

7. Rate how consistent your attention was during 5 0 0 40 55the Dichotic Rhyme Test. (ex: car, pill, toy, page)

8. Rate how consistent your attention was when 5 0 15 20 60you were asked to focus on your left ear.

9. Rate how consistent your attention was when 5 0 0 30 65you were asked to focus on your right ear.

10. Which ear could focus your attention the best? Right Left Both Equally55 25 20


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For the DCV test, the means were slightly lowerand the standard deviations slightly greater, thatis, 36.57 (SD = 12.20) for the left ear and 49.01(SD = 11.34) for the right ear.

Statistical analysis was performed usinga two-way repeated measures analysis ofvariance (ANOVA) for each test separately.This was accomplished using a 2 ear (right vs.left) x 3 condition (free recall, directed right,directed left) factorial design. Separateanalyses were completed since the purpose ofthis study was to determine the effects ofattention on each test separately. The analysisfor the DRT indicated a significant main effectfor ear only (F = 53.17, p = 0.000) and nosignificant ear by condition interactions. Thisfinding was not surprising since the mean

percentage scores for the right and left earsremained relatively stable across the testconditions (Figure 2).

The DCV analysis also demonstrated asignificant main effect for ear (F = 40.47, p =0.000). In addition, the analysis revealed astatistically significant two-way interactionbetween ear and condition (F = 14.45, p =0.000), which indicated that performance onthis test was affected by attention. A post hocanalysis on the data was performed todetermine where the significant interactionswere occurring for the DCV test. Table 2demonstrates the significant differences amongthe means for the ear x condition interactions.Results of this analysis indicated that therewas a statistically significant difference for theleft and right ear scores between the FR andDR conditions, as well as the DR and DLconditions. That is to say, as attention wasdirected to the DR condition, there was asignificant increase in the right ear score anda significant decrease in the left ear scorewhen compared to the scores for the right andleft ears in either the FR or DL conditions.

The individual subject data wereadditionally analyzed by (1) calculating thenumber of subjects who demonstrated an REA,an LEA, or no ear advantage (NEA) in eachof the three conditions across the two tests(Table 2), and by (2) determining the degreeof ear advantage for each of the 20 subjects forboth tests across all three conditions (Figure3). For purposes of the present investigation,an ear advantage was defined as any differencein performance between the right and leftears. For the DRT, 14 of the 20 subjectsdemonstrated an REA in the FR condition,with 4 demonstrating an LEA on the order ofapproximately 6% and 2 showing a NEA.When assessed in the DL condition, 4 subjectsdemonstrated the “anticipated” LEA(2 shiftedfrom an REA, 1 from an NEA, and 1 had anLEA in both conditions), while 16 subjectsshowed either an REAor NEA(i.e., 12 subjectsmaintained the REA that they demonstratedin the FR condition; 3 shifted from either anNEA or an LEA to a REA; and 1 shifted froman LEA in the FR condition to an NEA in theDL condition). In the DR condition, 16 of the20 subjects demonstrated an REA or NEA,while 4 showed an LEA. In all, 13 of the 20subjects showed stable ear advantages acrossall three conditions, with 12 of the 13 showingan REA and 1 an LEA. Table 3 demonstratesthe actual number of subjects with earadvantages across the two tests.

FFiigguurree 22.. Mean percentage correct identificationscores for the left ear (dotted lines) and the right ear(solid lines) on the dichotic rhyme and dichotic CVtests for three listening conditions (free recall, directedleft, and directed right). Error bars indicated thestandard error of the mean.

On the DCV test, 12 of the 20 subjectspresented with an REA in the FR condition,with 3 demonstrating an LEA, and 5 an NEA.When assessed in the DL condition, thenumber of subjects demonstrating an LEAincreased by 5 (from 3 to 8), with 4 of theseindividuals shifting from the NEA and 1 fromthe REA category when compared to theresults for the FR condition. In the DRcondition, all 20 subjects demonstrated anREA.

These results, coupled with the datapresented above, indicate that the DRT showsstability in ear advantage and meanidentification scores for normal subjects acrossall three test conditions (FR, DR, DL), whereasthe DCV test demonstrates greatervulnerability to alterations in attention forboth of these measures. When subjects wereasked to direct attention to their right or leftears on the DRT, their ear advantage remainedessentially unchanged, as did theirperformance as a group on percentidentification measures. However, for the DCVtest, when subjects were asked to perform thesame tasks, there was a shift in ear advantagefrom the FR to the DL and DR conditions,which was particularly noticeable in the DRcondition where all 20 subjects showed anREA. In addition, the subjects as a groupdemonstrated an overall increase in their rightear score and a decrease in their left ear scorein the DR condition when compared to theFR condition. In the DL condition, the oppositetrend was noted; that is, the mean scores forthe left ear increased slightly, and the right eardecreased when compared to performance inthe FR condition.

Results from the postevaluationquestionnaire are summarized in Table 1. Theresults are presented as percentages of subjects


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Table 2. Post Hoc Analysis Demonstrating Statistically Significant Differences for the Interaction between Ear x Condition for the DCV Test

FFRR DDLL DDRR

RREE LLEE RREE LLEE RREE LLEE

Mean 48.0 38.3 42.9 41.6 56.2 29.9

RREE 48.0 .131 .017*

LLEE 38.3 .340 .014*

RREE 42.9 <.001

LLEE 41.6 .001*

RREE 56.2

LLEE 29.9

* Significant at the .05 level.

FFiigguurree 33.. Histograms demonstrating the percentdifference between the right and left ears for 20subjects on the dichotic rhyme and dichotic CV testsfor three listening conditions (free recall, directedleft, and directed right). Positive percentages indicatean REA, and negative values indicate an LEA.

FFRR

DDLL

DDRR


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selecting ratings of 1 through 5 on a five-pointscale for nine questions, which were designedto probe the subjects’ perceptions of thedifficulty of the various tasks, their attendingbehaviors during the directed listeningconditions, and the consistency of theirattention during these tasks. In addition, thetable provides an indication of the percentageof subjects who indicated that they were ableto focus attention more easily to the right ear,to the left ear, or to both ears equally. Aninspection of this data revealed that althoughthe subjects did not present with overalldifficulty attending during the experimentaltask, they found the DRT to be the moredifficult test with respect to directing attentionin spite of the fact that on both tests theyreported that they found themselves attendingto the task as directed. Interestingly, althoughnot reflected in their test scores, for both theDCV and DRT all subjects reported that theirattention was consistently focused in thedirected condition. When asked which earthey were best able to focus their attentiontoward, more than half of the subjectsindicated their right ear (55% right, 25% left,and 20% both ears equally).

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Results from this study indicated thatlateralization of attention in normal youngadult subjects influenced the performance ofboth the left and right ears on the DCV test.These attention effects were particularlyevident in the DR condition where all 20subjects demonstrated an REA. In addition,significant differences were noted betweenthe percent correct identification scores forthe two ears when the DR scores were

compared to those obtained in the FR condition(i.e., the mean left ear score decreased andright ear score increased). These findingssupport the results of a number of earlierstudies that have shown that DCV test scorescould be altered solely by having subjectsdirect their attention to either the left or theright ear (Keith et al, 1985; Hugdahl andAndersson, 1986; Asbjørnsen and Hugdahl,1995; Asbjørnsen and Bryden, 1996; Wiensand Emmerich, 1999; Hugdahl et al, 2000;Hugdahl et al, 2001; Vingerhoets and Luppens,2001; Voyer and Flight, 2001; Foster et al,2002; Hugdahl et al, 2003).

In contrast, the results of the analysis forthe DRT revealed little or no effect of directedattentional focus on test performance for eitherear. Few changes in ear advantage wereobserved on this test when the subjects wereasked to direct their attention to one ear versusthe other, and no significant differences werenoted in the mean percent correct scores foreither ear when test performance wascompared across the three test conditions.These findings lend support to earlier findingsthat the DRT was highly resistant to attentioneffects (Wexler and Halwes, 1983, 1985) butis in mild conflict with the findings ofAsbjørnsen and Bryden (1996). These latterinvestigators, who used a dichotic rhyme testand a dichotic CV test to study the effects ofattention on dichotic stimuli, found a small, butsignificant, effect of attention on their dichoticrhyme test. This “attention” effect, however,was noticeably smaller than that noted fortheir dichotic CV stimuli.

The present findings demonstrate thestrength of the DRT utilized in thisinvestigation with respect to its resistance todirected attention, and as such, supports theclaim made by Wexler and Halwes (1985) thatthe DRT is not affected to any significantdegree by shifts in the focus of attention. In the

Table 3. Numbers of Subjects Showing a Right Ear (REA), Left Ear (LEA), or No Ear Advantage (NEA)on the Dichotic Rhyme and Dichotic CV Tests for Three Listening Conditions

TTeesstt CCoonnddiittiioonn RREEAA LLEEAA NNEEAA

Free Recall 14 4 2

Dichotic Rhyme Directed Left 15 4 1

Directed Right 15 2 3

Free Recall 12 3 5

Dichotic CVs Directed Left 11 8 1

Directed Right 20 0 0


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present investigation, the number of subjectswho demonstrated changes in their earadvantage on the DRT was not noticeablydifferent across the three test conditions. Thiscontrasts with the results for the DCV testwhere all 20 subjects produced an REA on theDCV procedure during the DR condition, andseveral showed a shift to an LEA when theirattention was focused on the left ear as in theDL condition. These findings suggest thatclinical results on tests such as the DCV shouldbe interpreted with caution when significantear asymmetries are identified as these eardifferences may be a function of the allocation(or lack thereof) of attention rather than atrue difference in hemispheric processing.

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The question that begs answering giventhe results outlined above is this: what is thebasis for the observed performance differenceson the DCV and DRT when both tests appear,at least on the surface, to have similarconstructions? One possible explanation isthat there are differences in the centralmechanisms underlying the physiologicalprocessing of the auditory events associatedwith these two tests. Unfortunately, there hasbeen little speculation in the literature as tothe exact physiological mechanisms underlyingthe dichotic fusion phenomenon. At this pointin time, one can only hypothesize regarding theneural representations of the auditory eventsunderlying the DRT. Wexler and Halwes (1983)suggest that the similarity of the pitch contoursfor the paired stimuli, as well as otheroverlapping acoustic parameters, cause therhyming words used in their test (as well asin the present investigation) to fuse into asingle auditory percept. In the present study,as well as in the previously reported literature(Musiek et al, 1989), it was observed thatalthough two words were presented duringeach stimulus presentation, the subjectsgenerally perceived only one word and that theperception of this word was typically localizedat or near the midline.

Although it is clearly the “fusion” of the twostimuli into a single auditory percept thatresults in the resistance to alterations inattention on a dichotic task, the questionremains as to why the DRT results in a “fused”auditory percept when such fused images arenot typically observed in the DCV or otherdichotic speech tests. Cutting (1976) proposed

that there are six types of fusion that can occurwithin the auditory system: sound localization,psychoacoustic fusion, spectral fusion,spectral/temporal fusion, phonetic featurefusion, and phonological fusion. The first, andperhaps most basic, form of dichotic fusion issound localization. The mechanisms underlyingsound localization have been extensivelystudied and are well understood (Melcher,1996). Binaural information arriving from thetwo ears is first fused at the level of the superiorolivary complex, which is also the locationwithin the central auditory nervous systemwhere processing for sound localization firstoccurs, as well as fusions for other binauralstimuli (Matzker, 1959). Therefore, it isreasonable to assume that many, if not all, ofthe other types of fusions are also likely tooccur (or at least be initiated) in the brainstem.

Repp (1976) has suggested that dichoticfusion tests such as the DRT may involvemore of a “low-level cross-correlatedmechanism,” whereas traditional dichoticspeech tests are believed to evoke highercortical activity that can be more easilyinfluenced by attention. According to Cutting’s(1976) categorization scheme, the DCVs wouldbe classified as a phonetic feature fusion, andthe words belonging to the DRT would beclassified as phonological fusion. These twofusion categories are distinguished from theother four categories in that they involvepsycholinguistic phenomenon. They are uniquein that (1) they require synchronous onsettimes (within 70 msec), (2) they are onlymoderately sensitive to the effects of intensity,and (3) they suffer significantly when thepresentation mode is changed from dichotic tobinaural (Cutting, 1976). What uniquelydifferentiates the DRT from the DCV is thatthe earlier test utilizes meaningful words,whereas the latter uses nonsense syllables.

Cherry and Sayers (1956) earlier haddescribed binaural fusion mechanisms usinga mathematical model. They suggested thatwith input at both ears, there is a continuouslygenerated cross-correlation process occurringwithin the auditory system that leads to fusionof the signals arriving at the two ears if thecorrect conditions exist. According to theirmodel, the central auditory nervous systemconstantly searches for similar signalcharacteristics between the two ears to forma single percept. One could infer that given thesimilarity in spectral and temporal informationof the rhyme words in the DRT, these stimulus

pairs share more commonalities (i.e., theyhave similar temporal and overlapping spectralfeatures by design—see discussion that follows)than those used in the DCV. Therefore, it islikely that these stimuli will result in a higherincidence of fusion within the central nervoussystem due to the presence of such cross-correlation mechanisms within the brainstem.As a result of the “fusion” of these stimulithat is presumably occurring within the lowbrainstem area, they are less affected byattentional focus, which is mediated at higherlevels within the central nervous system.

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Another important factor that must betaken into account when considering theresults of the present study are the acousticalfeatures of the test stimuli presented to thesubjects. Although there may be differentunderlying physiological mechanismscontributing to the results of the present study,the differential performance effects observedbetween the DRT and DCV may also be afunction of the actual stimulus recordings. Itis well known that for fusion to occur at thelevel of the brainstem, the stimuli beingpresented must be well aligned temporally. Ithas been established that the stimuli usedfor both the DRT and DCV tests demonstrategood temporal alignment. However, perhapsthe DRT stimuli demonstrate better temporalalignment and/or transition contours than theDCV test stimuli.

When theorizing regarding possibledifferences in spectral alignment, an importantpoint to consider is that it is at the superiorolivary complex (SOC) where fusion is likelyto first take place. It is well established thatthe SOC is highly sensitive to interaural timingand intensity difference of less than 1 msec(Irvine, 1992). Therefore, it is possible thatstimuli without exact and precise alignmentmay result in a lack of total fusion. The DRTstimuli are not only precisely aligned for theironsets and offsets, but also for the onset of theirtransitions.

Another possible mechanism that may becontributing to the differences noted in thelevel of fusion for the DCV and DRT stimuli isvariation in the rise-fall times of the onsetsand/or the transitions for the two differenttest stimuli. Perrott and colleagues (1970)demonstrated that the length of the rise-falltime of a stimulus is more important in

determining the precision or amount of fusion.As the signal duration increases, the thresholdfor fusion decreases. Therefore, it is possiblethat acoustic differences in the lengths of therise-fall times of the DCV and DRT stimuli mayalso result in differential amounts of fusion atthe SOC for these two stimulus types.

Those dichotic tests that have more exactalignment between the two stimuli in regardto intensity, spectral information, stimulusonsets and offsets will result in a more completefusion at the brainstem. When this fusionoccurs at the brainstem, the neural code thatis generated will be relayed to higher corticalcenters where only one stimulus is recognized.Under such circumstances, attentionmechanisms will have less influence. One cantheorize that although small, the discrepanciesseen for the alignments of the DCV stimuli maybe subtle enough for the central auditorynervous system to be able to detect bothinteraural time delay (ITD) and intensitydifferences (IID). When the central auditorynervous system is able to detect either ITDs orIIDs, total fusion is not accomplished.

When a true fusion occurs during dichotictesting, the subject will only perceive one word(Repp, 1976). As indicated earlier, for this tooccur, there must be precise alignment betweenthe test stimuli. Further evidence for the DCVshaving a somewhat less precise alignment isthat in the FR condition, many of the subjectsin the present investigation actually perceivedmore than one stimuli during a number of thetest presentations (i.e., they reported twostimuli rather than a single item to a dichoticpresentation). This, however, was a relativelyrare occurrence when the dichotic rhymestimuli were presented.

Finally, the fact that the individual rhymewords in each DRT stimulus pair share asubstantial amount of “common” spectralinformation (i.e., the final VC segments areidentical) may increase the likelihood thatthe cross-correlational mechanisms describedearlier will result in a “complete” or “nearcomplete” fusion of these dichotic stimuli at alevel early within the central auditory nervoussystem. If this fusion does in fact occur at a lowlevel within the central nervous system, thenit is unlikely that mechanisms mediated athigher levels within the central nervous system(e.g., attention) will be able to overcome thefusion that has occurred lower in the systemand once again separate the neural messageinto two distinct neural codes for processing.


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SSuubbjjeeccttiivvee PPeerrffoorrmmaannccee RRaattiinnggss

Another interesting result in this studythat deserves attention is the postevaluationquestionnaire data. Although the subjectsreported that in general they did not find itdifficult to attend during the experimentaltest procedures (question 1) or to focus theirattention to the right or left ear during theexperimental procedures (questions 8 and 9),they did offer responses to other questionsthat would suggest that they did not find thetwo tests to be of equal difficulty whenattempting to focus their attention. Responsesto questions that specifically probed thesubjects’ perceptions regarding the individualtest procedures (DCV: questions 2, 4, 6; DRT:questions 3, 5, 7) indicated that subjectivelythey found the DRT to be a somewhat moredifficult task on which to focus attention in thedirected-attention conditions than the DCV.This would suggest that the DCV may be aneasier perceptual task than the DRT whenattention-directing tasks are required. It isinteresting to note, however, that althoughthe subjects believed they were successful inmaintaining and focusing attention to the teststimuli for both experimental procedures (thedifferences reported above were minor andgenerally indicated that little or no difficultywas being experienced regardless of the testprocedure), their objective performance didnot necessarily match their subjectiveimpressions. This was particularly evidentfor the DRT where little (i.e., small) or nochanges in the subjects’ performance werenoted when the DL and DR results werecompared to the FR results. A possibleexplanation for this discrepancy between thesubjective measures and the objectiveperformance of these subjects may beassociated with Repp’s (1976) suggestionsregarding the underlying mechanism ofdichotic fusion. If fusion for the DRT stimuliis truly mediated at a “low-level” within thecentral nervous system, then it is occurring ata subconscious stage—in which case, thesubjects would (and should) be unaware oftheir true laterality performance.

FFuuttuurree DDiirreeccttiioonnss

Investigations of the effects of attentionon dichotic listening have focused primarily onthe adult population with limited study ofthese effects in the pediatric population(Hugdahl and Andersson, 1986; Hugdahl et al,

2001). Hugdahl and Andersson (1986)demonstrated that adults clearly show theanticipated REA and its subsequent reversalin the DL condition on a DCV task. However,children in their study between the ages of 8and 9 were unable to reverse the “normal”REA to an LEA when asked to direct theirattention to the left ear. More recently,Hugdahl and colleagues (2001) investigatedthe effects of age on dichotic listening andsuggested that there is a developmentalprogression of attentional effects that emergeswith increasing age. Studying a pool of 240subjects ranging in age from 7 to 70 years,these researchers found that young childrenshowed a clear lateralization of performanceon a dichotic test (i.e., an REA) but that theylagged with respect to older subjects in thedegree of REA they demonstrated for theforced right condition. The effect oflateralization has been demonstrated to bestable at approximately six years of age (Berlinet al, 1973; Hynd et al, 1983). This suggeststhat there may be a maturational effect withrespect to the emergence of an individual’sability to direct attention to the point wherean ear reversal can be elicited. However, it hasnot been established at what age childrenshould be able to focus attention to the left earand thus reverse the REA that is typicallynoted even in young children. The question ofwhether or not children demonstrate the sameability to allocate attention as adults has yetto be answered. Further investigation intothe maturational effects of directed attentionon dichotic listening is clearly warranted,especially when one considers the highprevalence of attention disorders in children.It will be important to determine how normalchildren of different ages perform with respectto directed attention on these tests so thatthese data can be used for comparison purposesto assist in the assessment of children withattentional disorders, such as ADHD.

CClliinniiccaall IImmpplliiccaattiioonnss

The findings with respect to the effects ofattention on dichotic listening may havesignificant implications for the assessment ofauditory processing abilities. In the clinicalassessment of central auditory function,binaural integration is generally evaluatedthrough the administration of tests of dichoticlistening—-often through the use of a singledichotic speech measure. Although the DRT isclearly a test of dichotic listening, it may not


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be appropriate to limit one’s assessment of apatient’s binaural integration abilities to thisone single test because of its uniquecharacteristics. Perhaps an alternativeapproach, particularly for cases in whichattentional deficits are highly suspected, wouldbe to present a test, such as the DCVs, in adirected attention paradigm in addition to theDRT. Such an approach may provideinformation to the clinician regarding theattentional allocation abilities (or inabilities)of an individual, which in turn, can assist inthe proper diagnosis.

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APPENDIX A. List of the 15 Dichotic WordPairs Used as Stimuli in the Present

Investigation

ten pen

tear dear

pit kit

pill bill

can pan

goat coat

car bar

cage page

keg peg

cook book

dig pig

toy boy

tail pail

pier beer

pearl curl

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