Divided-Attention and Directed-Attention Listening Modes ... · En una de las tareas de DL, los participantes monitorearon estímulos dicóticos utilizando el modo de atención dividida

J Am Acad Audiol 18:34–53 (2007)

*Texas Auditory Processing Disorder Laboratory, School of Behavioral and Brain Sciences, University of Texas at Dallas

James Jerger, Ph.D., 2612 E. Prairie Creek Dr., Richardson, TX 75080; E-mail: [email protected]

This research was funded by a student investigator award from the American Academy of Audiology. The authors are grateful to the Academy for their continued financial support of student research in audiology and hearing science.

Portions of this paper were presented by the first author at the Frontiers in Hearing Symposium, July 2005, Breckenridge,Colorado.

Divided-Attention and Directed-AttentionListening Modes in Children with DichoticDeficits: An Event-Related Potential Study

Jeffrey Martin*James Jerger*Jyutika Mehta*

Abstract

Dichotic listening (DL) procedures are commonly employed in the evaluationof auditory processing in children. Review of the various clinical tests revealsconsiderable diversity in both the signals employed and their mode ofadministration. The extent to which other non auditory-specific factors influencethe test outcome is often difficult to determine. Individual differences in memory,attention, facility with test stimuli, and report strategy are always of potentialconcern in the interpretation of results.

In the present study, we examined behavioral and electrophysiological(ERP) responses for 20 children during two DL tasks. Two groups of childrenwere evaluated. One group was comprised of children who showed substantialear differences on clinical measures of DL; the other group showed no suchdeficits and served as age-matched controls. In one of the DL tasks, participantsmonitored dichotic stimuli using the divided-attention (unfocused) mode. In theother DL task, a directed-attention (focused) mode was employed. Both tasksinvolved simple “same-different” judgments for real words presented in a basicreference-probe paradigm. We purposefully sought an easy DL task in orderto minimize the number of extra-auditory factors influencing their performance.For control purposes, a diotic procedure involving the same stimuli was alsoincluded.

Results showed that the amplitude of the elicited late-positive component(LPC) was smaller and prolonged in latency for the group of poor listeners ascompared to the control group. This finding occurred only when dichotic stimuliwere presented in the divided-attention mode. When participants directedtheir attention to a single side, or when listening in a diotic mode, the LPC forboth groups was more similar. Group differences in the N400 componentwere apparent for both listening tasks. Results are discussed in relation to aninability of some children to inhibit processing of unattended auditory information.Implications for the clinical administration of dichotic listening tests are alsodiscussed.

Key Words: Auditory event-related potentials, auditory processing disorder,auditory tests, children, dichotic, directed attention, divided attention

Abbreviations: APD = auditory processing disorder; CHAPS = Children’sAuditory Performance Scale; CV = consonant vowel; DL = dichotic listening;EEG = encephalographic activity; ERP = event-related potential; LED = left-ear disadvantage; LPC = late-positive component; REA = right-ear advantage;SSW = Staggered Spondee Word test

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Divided-Attention and Direct-Attention Listening Modes/Martin et al

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The degree and direction of interauralasymmetry on dichotic listening (DL)tests, or, in some cases, the lack

thereof, has received considerable interestin research examining a wide spectrum ofclinical disorders, ranging from dyslexia andattention-deficit disorder to schizophreniaand epilepsy. In the particular case of childrensuspected of having an auditory processingdisorder (APD), there is cause to suggestthat discrepancies between the right- andleft-ear performance scores, generally in theform of a left-ear deficit (LED), may be onesign characterizing the disorder (Morton andSiegel, 1991; Lamm and Epstein, 1994;

Moncrieff and Musiek, 2002; Moncrieff et al,2004). Studies using auditory event-relatedpotentials (ERPs) to explore interauralasymmetry in these children have obtainedsimilar results (Jerger et al, 2002; Moncrieffet al, 2004). Given the profile of the testoutcome and what is known about thematuration and role of the corpus callosumin mediating interhemispheric communication,it has been hypothesized that the LED mayarise from a disruption in the interhemispherictransfer of auditory information (Musiek et al,1984; Jerger et al, 1986; Musiek et al, 1988).

While abnormal interaural asymmetrymay be indicative of APD (Keith, 1984; Bellis,

Sumario

Los procedimientos de Audición Dicótica (DL) son comúnmente empleadosen la evaluación del procesamiento auditivo en niños. La revisión de variaspruebas clínicas revela una diversidad considerable tanto en las señalesempleadas como en su modo de administración. En qué grado otros factoresno específicos de la audición influyen en los resultados de las pruebas es amenudo difícil de determinar. Las diferencias individuales en memoria, atención,facilidad con los estímulos de la prueba y la estrategia de reporte, tienen siempreinfluencia potencial en la interpretación de los resultados.

En el presente estudio, examinamos respuestas conductuales yelectrofisiológicas (ERP) en 20 niños con dos tareas de DL. Se evaluaron dosgrupos de niños. Un grupo estaba constituido por niños que mostrabandiferencias sustanciales entre los oídos en las mediciones clínicas de la DL;el otro grupo no mostró tal déficit y sirvieron como controles pareados por edad.En una de las tareas de DL, los participantes monitorearon estímulos dicóticosutilizando el modo de atención dividida (no concentrada). En la otra tarea deDL, se empleó un modo de atención dirigida (concentrada). Ambas tareasinvolucraron juicios simples de “igual-diferente” para palabras reales presentadasen un paradigma de sondeo de diferencia básica. A propósito, buscamos unatarea de DL fácil, para minimizar el número de factores extra-auditivos queinfluían en el desempeño. Para propósitos de control, se incluyó también unprocedimiento diótico que utilizara los mismos estímulos.

Los resultados muestran que la amplitud del componente positivo tardío(LPC) generado era más pequeña y más prolongada en latencia para elgrupo de oyentes pobres comparado con el grupo control. Este hallazgo tuvolugar sólo cuando se presentaron estímulos dicóticos en el modo de atencióndividida. Cuando los participantes dirigieron su atención a un sólo lado, o cuandoescucharon en un modo diótico, el LPC para ambos grupos fue similar. Lasdiferencias de grupo en el componente N400 fueron aparentes para ambastareas de audición. Se discuten los resultados en relación con la incapacidadde algunos niños de inhibir el procesamiento de información auditiva a la queno se le está prestando atención. Se discuten las implicaciones para laadministración clínicas de pruebas de audición dicótica.

Palabras Clave: Potenciales auditivos relacionados con el evento, trastornosde procesamiento auditivo, pruebas auditivas, niños, dicótico, dirigido, atencióndirigida, atención dividida

Abreviaturas: APD = Trastorno de procesamiento auditivo; CHAPS = EscalaInfantil de Desempeño Auditivo; CV = consonante-vocal; DL = audición dicótica;EEG = actividad encefalográfica; ERP = potencial relacionado con el evento;LED = desventaja del oído izquierdo; LPC = componente positivo tardío; REA= ventaja del oído derecho; SSW = Prueba de Palabras EspondaicasEscalonadas

1996; Chermak and Musiek, 1999; Jerger etal, 2002; Moncrieff et al, 2004), there areboth theoretical and methodological issuesthat complicate a more auditory-specificinterpretation of test results. Of these, nonehas received more focus in the literaturethan the influence of spatial attention oninteraural asymmetry. Much of the earlydichotic research employed a divided-attention or “unfocused” listening mode;listeners repeated back everything heard inboth ears and were free to choose how theydeployed attention (for review of DLprocedures, see Bryden, 1988). Subsequentstudies using more directed-attention or“focused” procedures have revealed that theear differences obtained in the divided-attention mode were susceptible to theinfluence of spatial attentional factors(Bryden et al, 1983; Mondor and Bryden,1991; Asbjornsen and Hugdahl, 1995; Voyerand Flight, 2001). Studies utilizing functionalbrain imaging techniques such as PET(positron emission tomography) and fMRI(functional magnetic resonance imaging)have further revealed that the degree ofactivation even in primary auditory cortex isaffected by the manipulation of attentionalresources (O’Leary et al, 1996; Jäncke et al,1999; Hugdahl et al, 2000).

Though focused-attention procedures wereintroduced to bring volitional shifts in attentionunder experimental control, such instructionsin children may generate an order or primingeffect that alters the magnitude of interauralasymmetry (Hiscock and Chipeur, 1993).Instructions to report only the informationpresented to the right ear may bias responsesin favor of that ear, even when attention isdirected to the left side on subsequent trials.This may be of greater concern when the teststructure incorporates relatively few directed-right and directed-left listening conditions.The most effective method to capture andmaintain attention to the directed side,whether it be through verbal instructions orthe unilateral presentation of a tonal precue,can also influence the size of interauralasymmetry (Voyer and Flight, 2001).

In summary, while dichotic procedureshave proven sensitive to the identification ofcentrally based auditory deficits, complete“control” for non auditory-specific factors,such as attention, on test results has provendifficult. Regardless of which DL tests areused, there is concern that extra-auditory

factors, such as facility with stimulusmaterials, attention, or memory, mayinfluence test performance. Indeed, the extentto which deficits on clinical DL tests areattributable to auditory and/or other factorsstill remains one of the more dauntingchallenges facing clinicians who evaluatechildren for APD.

Perhaps part of the problem arises from theconsiderable diversity observed among clinicalDL tests in the stimuli employed and mode ofadministration. While review of the morecommonly used tests exceeds the scope of thispaper (for review of dichotic paradigms, seeJerger and Martin, 2006), it is only to be expectedthat children can show considerable variabilityin the amount of interaural asymmetry ondifferent DLtests (Moncrieff and Musiek, 2002).This, in itself, complicates a more directcomparison or “cross-check” of the resultsobtained on screening-based procedures, likethe SCAN test (Keith, 2000), with those obtainedon more “diagnostic” DL tests, such as theStaggered Spondee Word test (SSW: Katz etal, 1963), Dichotic CV Test (Berlin et al, 1973),Pediatric Speech Intelligibility Test (Jerger,1987), and Dichotic Digits Test (Musiek, 1983).

The goal of this study was to examine theeffects of listening mode (i.e., divided anddirected attention) on DL performance in agroup of children suspected of having APD.We also attempted to minimize a number ofextra-auditory factors known to influencebehavioral performance (e.g., facility withstimulus materials, memory, order of report,etc.). While such an approach reduces thosecomponents that make auditory processing inthe real environment interesting, itnonetheless might help uncover a commonfactor influencing DL test results in children.Behaviorally, such an approach is difficult.Responses on a simple DL test would likelyreach ceiling levels and preclude anymeaningful processing differences thatpotentially exist between different subjectgroups. Electrophysiological procedures, onthe other hand, are well suited to examine thetiming of information processing in spite of:(1) a lack of an overt verbal response and (2)similarity between groups in behavioralperformances. Since electrophysiologicalmeasures have proven beneficial to ourunderstanding of other auditory disorders, itis reasonable to ask whether such anapproach might provide insight on the natureof DL deficits.

Journal of the American Academy of Audiology/Volume 18, Number 1, 2007

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The auditory event-related potential(ERP) consists of a series of characteristicpositive and negative voltage fluctuations or“components” in electroencephalographicactivity (EEG) in response to an auditorystimulus (Picton, 1992). Earlier appearingcomponents (e.g., N1) are associated primarilywith sensory or exogenous aspects of astimulus encoding while the later components(e.g., P3) reflect subsequent endogenous orcognitive aspects in stimulus evaluation.Depending upon task demands and stimulusparameters, both the early and latecomponents may be present in the ERPwaveform.

Of the more endogenous ERPcomponents, research examining the brainmechanisms underlying different aspects oflanguage processing has revealed at leasttwo main components that are sensitive toexperimental variations in linguistic content.The first component, a negative goingpotential that reaches maximal amplitudeapproximately 200–600 msec post-onset of anauditory or visual stimulus, is typicallyreferred to as the “N400” (Kutas and Hillyard,1984). The second component, characterizedby a positive deflection in the waveformaround 600–1000 msec, is often referred to asthe “P600” (Osterhout and Holcomb, 1992),the “late positive shift” (Hagoort et al, 1993),or, simply, “a late positive component (LPC).”

The N400 potential appears as anincrease in ERP amplitude (i.e., negativity)to semantically incongruous words presentedeither in word pairs or sentences and is ofteninterpreted to reflect involvement of thosebrain mechanisms underlying semanticprocessing (see review by Kutas andFedermeier, 2001). Studies examining N400scalp topography have shown that it reaches,at least in adults, peak amplitude over themidline electrodes positioned over the parietal(PZ) and centroparietal (CPZ) areas. Theneural generators underlying this componentare uncertain, but important contributionsfrom the medial temporal lobe have beensuggested (McCarthy et al, 1995). While mostaccounts link the N400 to various aspects ofsemantic processing, the amplitude andlatency of the N400 can be modulated by ahost of experimental and subject factors,including semantic priming effects (Holcomband Neville, 1990), the context in whichlinguistic decisions are made (Bentin, 1989),allocation of attentional resources (Bentin

et al, 1995), and age (Federmeier et al, 2003). Appreciably fewer studies have examined

the N400 in children. Holcomb et al (1992)examined N400 amplitude and latency tosemantically anomalous sentences in childrenand young adults 5 to 26 years of age. Resultsshowed that N400 amplitude and latencydecreased with advancing age. In addition,the scalp topography of the N400 was foundto be distributed more anteriorally than thatoften observed in adults, possibly implicatinga second distinct component unique tochildren that overlaps the N400 (Holcomb etal, 1992). Whether this second componentreflects maturational changes in attentionalcapture (Holcomb et al, 1985) is uncertain. Arecent study by Atchley et al (2006) extendsearlier research (Hahne and Friederici, 1999;Friederici and Hahne, 2001) characterizingthe N400 in children, with the most salientfeatures being decrease in N400 amplitudeand latency with advancing age and widedistribution in N400 scalp topography foryounger listeners.

The later P600/LPC component appearsas a positive shift in the ERP waveform, withmaximum positivity over centroparietalareas, and is typically observed in responseto violations in grammatical numberagreement and/or other syntactic anomalies(Osterhout and Holcomb, 1992; Münte et al,1997; Atchley et al, 2006). Despite beingelicited in similar experimental paradigmsthat yield the N400, the LPC is somewhatmore controversial in its neurophysiologicalinterpretation. Some have hypothesized thatthe LPC indexes those mental operationsinvolved in the reanalysis of linguistic stimuliupon detection of morpho-syntactic anomalies(Osterhout et al, 1994; Friederici et al, 1996).Therefore, the LPC may be sensitive todistinct aspects in linguistic processing(Ousterhout and Holcomb, 1995; Osterhoutand Nicol, 1999). Juottonen et al (1996)describe the LPC as a reflection of furtherevaluation in stimulus meaning and abilityto integrate verbal information into theconstruct of semantic memory. Othersassociate the LPC with more general cognitiveaspects in information processing, such ascontext updating and working memory (seeCoulson et al, 1998a, 1998b). Thus, the LPCmay be an extension of those mentaloperations that underlie stimuluscategorization abilities, similar to thosebelieved to produce the P3 component. To


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some extent, the interpretation of the LPCoften reflects the experimental paradigm inwhich it is evoked. We associate the LPC asa psychophysiological manifestation oftimeliness in those cognitive or late selectionprocesses (e.g., context and memory updating)involved in, for example, the categorizationof rare and frequent stimulus events.

In this study, we report N400 and LPCdata obtained from children during dichoticlistening (DL). Two groups were tested: Onegroup was comprised of children who showeda substantial left-ear deficit on clinicalmeasures of DL; the other group showed nosuch deficits and served as age-matchedcontrols. The experimental DL tasks requiredlisteners to make simple “same-different”judgments to familiar words. To examine theinfluence of spatial attention on the N400and LPC, both divided-attention and directed-attention listening modes were administered.A third control condition involving dioticlistening using the same stimuli was alsoincluded. We examined the hypothesis that aninability to allocate appropriate attentionalresources may underlie some of the DL deficitsobserved in children evaluated for APD.

METHODS

Subjects and Recruitment

Twenty children between the ages of 8years, 11 months and 13 years, 8 monthsparticipated in the study. Half of theparticipants were referred to our laboratoryfor evaluation in light of their poorperformances on clinical measures of dichoticlistening (DL), such as the SCAN, SSW, orDichotic Digits Test. These children,

hereinafter referred to as the “dichoticdeficits” group (DD group), were recruited toparticipate in the study via brochures locallydistributed to audiologists and speech-language pathologists. Overall, each child inthe DD group was described by parentalreport as having academic difficulties.Particular areas of concern were auditoryattention, following multistep instructions,reading, and delays in age-appropriateexpressive language skills. A second group,comprised of children who showed no historyof listening and academic problems, based ona detailed parent questionnaire, served ascontrol participants. None of the controlparticipants had any prior experience withthe behavioral and electrophysiologicalprocedures employed in the study. Informedconsent was obtained from all participants inaccordance with the University of Texas atDallas’s guidelines. Table 1 summarizes theage and gender distribution of theparticipants in each group.

All participants were right-handed bystandard questionnaire (Annett, 1970) andlearned English as their first language. Allparticipants had pure-tone hearing thresholdson both ears of 20 dB HL or better at octaveintervals over the frequency range 500 to8000 Hz. Interaural threshold differences ateach test frequency never exceeded 10 dB.Tympanograms and acoustic reflexes,screened at 1000 Hz with ipsilateralstimulation, were normal on each ear for allparticipants. None of the participants had anyknown neurological deficit or were takingmedications that could influence the recordingof electroencephalographic activity (EEG).

Listening ability for each participantwas assessed using the Children’s AuditoryPerformance Scale (CHAPS; Smoski et al,


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Table 1. Age and Gender Distribution of the Participants in the Control and Dichotic Deficit Groups

Mean Age (SD) Gender

Control Group (N = 10) 11 years, 0 months (1.48) 7 males, 3 femalesDichotic Deficits Group (N = 10) 10 years, 7 months (1.35) 8 males, 2 females

Note: Standard deviations are indicated in parentheses.

Table 2. Summary of the Children’s Auditory Performance Scale (CHAPS) for the Control and DD Groups

Listening Condition

Noise Quiet Ideal Multiple Auditory Memory Auditory Total Inputs Sequencing Attention Span

Control group .029 .414 .467 .400 .412 .263 .308 DD group -2.086 -1.371 -.633 -.800 -2.137 -.900 -1.467

Note: Items qualifying as “at risk” (i.e., ≤-1.0) are in bold type for emphasis.


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1998). Table 2 summarizes average parentalratings for each group across the differentlistening conditions implemented on theCHAPS. Overall, parental ratings for theDD group were in the “at-risk” range forlearning difficulties (≤-1.0). From Table 2,at-risk ratings were indicated for listeningabilities in noisy and quiet environments,and for auditory memory sequencing. Thetotal average condition score was -1.47. Forcontrol participants, the total average rangecondition score was in the pass range (.31).Ratings across the different listeningconditions for this group were also withinthe pass range.

General Procedures

Participants were tested over twosessions to reduce fatigue. Care was taken toensure that each study session was conductedapproximately at the same time of day. Thesecond session was completed on the nextday for 14 of the participants. The remainingparticipants completed the second sessionwithin a two week period. Each session lastedapproximately two hours. In the first session,following routine audiometric and middle-ear assessment procedures, the CompetingWords and Competing Sentences subtests ofthe SCAN-C were administered to eachparticipant. The Competing Words subtest isa dichotic listening (DL) task that involvesdivided attention with precued direction.Listeners are asked to report both wordsheard but in a preferential order. On the firsthalf of trials, listeners first report the wordpresented to the right ear followed by theword presented to the left ear. On the secondhalf of trials, the words presented to the leftear are repeated first. The CompetingSentences subtest is a dichotic task thatinvolves directed attention. On the first halfof trials, listeners attend and report only thesentence presented to the right ear. On thesecond half of trials, listeners monitor andreport the sentence presented to the left ear.SCAN stimuli were presented throughstandard audiometric headphones at acomfortable listening level (50 dB HL). TheSCAN subtests, which are screeningprocedures, were selected as clinical measuresof DL because (1) inspection of the diagnosticreports furnished to our laboratory revealedthat children in the DD group showed, overall,particular difficulties on these subtests; (2)

the SCAN test is widely used by audiologistsfor screening for APD; and (3) there has beenoverall consistency of the SCAN test inproducing interaural asymmetry in children(Moncrieff and Musiek, 2002). Mostimportantly, since the results on the SCANsubtests formed the basis of groupclassification used in this study, we felt itappropriate to corroborate past diagnosticreports and re-administer the SCAN subteststo all participants. Scoring of each SCANsubtest was performed in accordance with thesupplied manual for the SCAN.

Upon completion of the SCAN subtests,participants were fitted with an elastic capcontaining multiple electrodes used in therecording of auditory ERPs. Following capplacement, participants underwent one oftwo dichotic listening tasks incorporatingeither a divided-attention or directed-attention listening mode. Half of theparticipants completed the divided-attentiontask on the first session; half at the beginningof the second session. Finally, a diotic controltask, incorporating the same stimuli used inthe dichotic tasks, was presented at the endof the second session. Positioning the controltask at the end of the experiment waspurposefully sought in order to evaluatewhether differences between groups on thedichotic tasks (if any) were related to fatigueand/or the selection of linguistic stimuli (i.e.,stimulus effect).

Experimental Tasks

Participants were comfortably seated ina medical examining chair within a sound-treated room. Auditory stimuli were presentedvia three loudspeakers positioned at ear levelaround the participant. Two of theloudspeakers were positioned 1.5 meters oneither side of the participant’s head. The thirdloudspeaker was positioned 2.2 meters directlyin front of the participant. Presentationintensity for auditory stimuli delivered fromthe three loudspeakers was presented at acomfortable listening level, approximately 58dB SPL-A, as determined by a sound levelmeter positioned at the location of theparticipant’s head during testing.

In each of the three experimental tasks,participants were asked to listen to familiarmonosyllabic words. To facilitate similar wordonsets and offsets in dichotic stimuli, 48

consonant-vowel-consonant words (CVCs)incorporating plosives in the initial and finalword position were used (see Table 3). Speechstimuli were recorded in an audiometricsound-treated room by an adult male,monolingual speaker of English. Speech wassampled at a rate of 22,050 Hz with 16-bitamplitude resolution using the Cool Edit Prosoftware (Syntrillium Software Corp). Overallintensities of the words were equated basedon their average root mean squareamplitudes. Word durations were digitallyequated (if necessary) using a selectiveresampling algorithm on the steady stateportion of the vowel. We ensured, however,that no degradation in the percept of theconsonants or vowels in such cases occurred.The average word duration was 500 msec, andranged from 478 msec to 530 msec. Overallpresentation level of linguistic stimuli wasdetermined using speech noise calibrated tothe average intensity of the words whenpresented in the soundfield.

Our purpose was to evaluate DLperformance with experimental proceduresthat minimized the involvement ofnonauditory factors on performance, such asmemory (e.g., order of report) and facilitywith linguistic materials. To this end, alistening task requiring only simplecategorical “same” or “different” judgmentswas employed. For all listening tasks,auditory stimuli were presented in a basicreference-probe paradigm; participants wereasked to judge whether an initial referencestimulus (e.g., a single word) was repeated ina subsequent probe stimulus. Probe stimuliconsisted of either a single word (dioticcondition) or dichotic word pair (dichotic

conditions). A “same” judgment, therefore,occurred when the reference word wasrepeated again in the probe stimulus. A“different” judgment occurred when thereference word was not repeated. Participantsindicated their response following each trialby manually pressing one of two buttons,labeled “yes” and “no” and verticallypositioned in front of them. Participants wereinstructed to press “yes” for same judgmentsand “no” for different judgments. During thediotic task, for example, the participant wouldpress “yes” if the reference and probe wordwere the same (i.e., a physical match) or “no”if the words were different. For the dichotictasks, the participant would press “yes” ifthe reference word was repeated as one of thetwo words in the dichotic probe. In this case,the physical match could occur either on theirleft side (same-left) or right side (same-right).If the reference word was different from bothwords in the dichotic probe, the participantwould respond “no.” Figure 1 shows anexample of a “same” judgment for each ofthe experimental tasks (further describedhereinafter). Instructions were verbally givento each participant prior to each listeningtask along with a brief practice session toensure that they understood the directions.


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Table 3. Linguistic Stimuli Used in theExperimental Tasks

Linguistic Stimuli (N = 48)

Bait Keep Gap PickBeak Kick Gate PigBed Kid Get PipeBeg Kite Gig PutBike Dad Goat TackBook Date Good TakeBoot Deep Got ToadBought Dip Guide TookCake Dock Pack TopCap Dog Pad TubCoat Duck Paid TugCook Dug Peg Type

Figure 1. Schematic of a single trial presented ineach of the three listening tasks. For the twodichotic tasks, only a same-right judgment isshown.


Stimulus onset asynchrony between thereference and probe was held constant at 1.8seconds. The participant’s manual responsetriggered the next trial, following a 2.5 seconddelay. Each dichotic task required overallabout 40 minutes to complete (~3 minutes perlistening block) including generous restbreaks between listening blocks. The diotictask required approximately 20 minutes tocomplete.

DDiicchhoottiicc wwiitthh DDiivviiddeedd AAtttteennttiioonn

In this task, participants were asked tomonitor dichotic stimuli (i.e., probe stimuli)using a divided-attention listening mode.Participants were instructed to attend toboth words constituting the dichotic pair forthe occurrence of the preceding referenceword. The reference word was presenteddiotically (i.e., from the right and leftloudspeakers) to achieve a percept along themidline. A total of 192 trials were presentedover six listening blocks (32 trials each). Halfof the trials (96 trials) incorporated “same”judgments; the other half of the trialsincorporated “different” judgments. Of thetotal number of “same” trials, half (48 trials)incorporated “same-left” judgments; the otherhalf constituted “same-right” judgments.Since a divided-attention mode was employedduring this task, separation of “different”judgments as a function of side (i.e., “different-right” and “different-left”) was not possible.The number of trials was selected on thebasis of the minimum number of trials neededto adequately assess the electrophysiologicalresponse from participants to each stimulusevent. Trials corresponding to each eventtype were pseudorandomly presented withineach listening block, with the exception thatno more than two of the same stimulus eventsoccurred consecutively.

DDiicchhoottiicc wwiitthh DDiirreecctteedd AAtttteennttiioonn

In this task, participants were asked tomonitor dichotic probe stimuli in a directed-attention (focused) mode. Participants wereinstructed to attend only to the wordspresented on a predetermined side (right orleft) and ignore the competing wordspresented contralaterally. To reinforce spatialattention to the directed side, the reference

word preceding each dichotic stimulus waspresented from the loudspeaker on the sameside to which the participant was asked tofocus. A total of 192 trials were presentedover six listening blocks (32 trials each), withparticipants receiving either attend-left orattend-right instructions on half the blocks.The side to be attended was alternated overthe duration of the task, and the side to beattended first was counterbalanced acrossparticipants. As in the divided-attention task,half of the trials in this condition (96) involved“same” judgments; half involved “different”judgments. Of the total number of “same”trials, half (48) incorporated “same-left”judgments; half incorporated “same-right”judgments. The word in the dichotic probeconstituting a “same” judgment was alwayspresented to the attended side. In contrast tothe divided-attention task, “different”judgments corresponding to side of occurrence(e.g., “different-right” and “different-left”)were possible. Each event type waspseudorandomly presented within eachlistening block.

DDiioottiicc CCoonnttrrooll CCoonnddiittiioonn

In the final listening task, participantswere asked to monitor the same words butfrom a spatially neutral position. That is,both the reference and probe stimulusconsisted of single words presented from aloudspeaker facing the participant. Althoughstimuli were presented from a singleloudspeaker, we refer to this condition as“diotic” since both ears receive the samestimulus via the soundfield. Again,participants were asked to decide whether thereference and probe words were the “same”or “different.” A total of 96 trials werepresented over three listening blocks (32trials each). Half of the trials involved “same”judgments (48 trials); half involved “different”judgments. Each event type waspseudorandomly presented within eachlistening block.

Electrophysiological Procedures

ERPs were collected using the Neuroscanelectrophysiologic data acquisition system(Acquire, Compumedics). Continuous EEGactivity was recorded from 30 silver-silver

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chloride electrodes mounted in an elastic cap(Neuroscan) affixed to the scalp according toa modification of the International 10-20system. Electrode impedances were alwaysless than 5 kohms. Eye movements and eyeblinks were monitored via electrodes placedabove and at the outer canthus of the left eye.EEG channels were referenced to linkedmastoid electrodes with a forehead electrodeas ground. Ongoing EEG activity wassampled at 500 Hz, amplified, analog filteredfrom 0.10 to 70 Hz, digitized, and stored foroff-line analysis.

Offline, EEG recordings were visuallyinspected. Movement and irregular ocularartifacts (nonblinks) were omitted fromfurther analysis. Systematic eye movements(blinks) were spatially filtered from thecontinuous EEG recording by performing aspatial singular value decomposition (spatialPCA) on the average eye blink of eachindividual participant (Ille et al, 2002).Following removal of ocular and otherrecording artifacts, individual epochs, rangingfrom a 200 msec prestimulus interval to 1600msec postonset of probe stimuli, were derived.Accepted epochs were separately averagedaccording to stimulus type. Successfullyaveraged ERP waveforms were then linearlydetrended, baseline corrected relative to theprestimulus interval, and digitally low-pass

filtered at 20 Hz (-48 dB/octave). A minimumof 25 acceptable epochs constituted anindividual ERP waveform; the averagenumber of accepted epochs wasapproximately 40 epochs. Thus, wedetermined that approximately 16% of theERP data across the different stimulus eventswas artifact-rejected for each group ofparticipants.

Description of ERP Waveforms

In the present paper, we focus on thelater endogenous waveform components as anindex of timeliness in cognitive processingduring DL. In this regard, we expected theresultant ERP waveform morphology to showtwo distinct, but overlapping, componentsreflecting two task-specific processes: (1) aprocessing negativity (N400) in the 200 to 500msec region indexing semantic analysis ofcloseness of fit (Kutas and Hillyard, 1984)between the reference and probe stimulus,and (2) a subsequent late-positive component(LPC) in the 500 to 1100 msec regionreflecting further cognitive aspects ofstimulus evaluation and responsecategorization. Based on previous dichoticstudies (Jerger et al, 2000, 2002; Martin etal, 2006), we anticipated that thesecomponents, particularly the LPC component,would be maximally distributed over theparietal recording electrodes. As anillustrative example, Figure 2 shows twoERP waveforms from a nine-year-old boy inthe DD group during the diotic listeningcondition. Data were taken from a singleparietal electrode (PZ) and represent a “same”and “different” judgment. From the figure,both types of judgments evoked a robust LPCcomponent over the 500–1100 msec region. Inthe case of “different” judgments, however, aprocessing negativity (i.e., N400) wasobserved over the 200–500 msec regionrelative to that observed for the “same”judgments.

Analysis of ERP Waveforms

To examine the nature of interauralasymmetry in ERP responses, as opposed tointerhemispheric asymmetry, we restrict ouranalyses to a single midline electrodepositioned over the parietal region (PZ).

Figure 2. ERP waveform morphology. Data weretaken from a nine-year-old boy in the DD group dur-ing the diotic condition. The ERP waveforms repre-sent “same” and “different” judgments. The record-ings are from a midline electrode in the parietalregion (PZ). Both the N400 and LPC componentscan be identified.


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Rationale for the selection of this particularelectrode was based on the topography of theLPC maximum response. Figure 3 shows thegrand-averaged topographic maps for theLPC component in response to “same” and“different” judgments obtained during thediotic condition for both groups of listeners.In all cases, the maximum LPC responseoccurs over the midline in the parietal region.

The LPC and N400 components of eachindividual waveform were quantified viameasurement of mean amplitudes and peaklatencies. For the N400 component, meanamplitudes were taken over the 200 to 500msec latency interval; for the LPC component,mean amplitudes were taken over the 500 to1100 msec latency interval. Mean amplitudesrather than peak amplitudes were selectedas more objective measures of the amplitudeof the N400 and LPC components that oftenshow a maximum response that extends overseveral hundred milliseconds. Peak latencieswere obtained by an automated peak

detection algorithm confirmed by visualinspection. For each dichotic task, meanamplitudes and peak latencies for the N400and LPC were analyzed separately in arepeated measures analysis of variance(ANOVA) design with listener groupaffiliation serving as the between-subjectsfactor and type of judgment (same ordifferent) and side of presentation (right orleft) serving as within-subjects factors. For thediotic task, type of judgment served as thesingle within-subjects factor. Due to relativelysmall sample sizes, nonparametric statisticalprocedures were also employed to confirmsignificant group differences observed withANOVA. Mann-Whitney U-tests were used forbetween-group analyses (Siegel, 1956).Statistical significance was evaluated at the0.05 alpha error level.

RESULTS

Clinical Measures of DichoticListening

Figure 4 shows the results of theCompeting Words and Competing Sentencessubtests of the SCAN for both groups. On theCompeting Words subtest, listeners in thecontrol group showed minimal ear differencesin their responses to the dichotic stimuli,irrespective of the word to be repeated first.For this group, only a slight left-eardisadvantage (approximately two words) wasobtained when the words presented to theright ear were repeated first. This interauralasymmetry disappeared when the wordspresented to the left ear were repeated first.For the group of children suspected of havingdichotic listening deficits (DD group), adifferent pattern emerged. This group showeda substantial left-ear deficit that remainedapproximately the same magnitudeirrespective of the side that attention wasinitially focused. When the word to berepeated first was in the right ear, the averageleft-ear deficit was approximately sevenwords. For the left-ear first condition, theaverage left-ear disadvantage wasapproximately six words. On the CompetingSentences subtest, a similar pattern in resultswas observed. The DD group showed asubstantial left-ear deficit (average of sevenitems) whereas the control group exhibitedlittle ear difference.

Figure 3. Grand-averaged topographic maps of theLPC component obtained during the diotic conditionfor both groups of listeners. In all cases, the LPCresponse is maximal across the parietal electrodearray. Differences in the time scales used to gener-ate each topographic map reflect inherent latency dif-ferences in the two types of judgments (i.e., “same”or “different”) made by participants. Amplitude wasadjusted to reflect the maximal LPC response in eachcondition. Location of the PZ electrode is indicatedwith a black circle.

Based on the recruitment proceduresused in this study, differences in behavioralperformance between the two participantgroups on the SCAN subtests were only to beexpected. Nevertheless, we felt it appropriateto statistically confirm group differences onthe SCAN using ANOVA. For both subtests,participant group served as a between-subjects factor. For the Competing Wordssubtest, the initial side to which attention wasfocused (i.e., report right first, report leftfirst) served as a within-participants factor.For the Competing Sentences subtest, thewithin-subjects factor was the side to whichattention was focused (i.e., report right, reportleft). As expected, a significant group x earinteraction was found on both SCAN subtests(Competing Words: F(1,18) = 15.64, p < 0.001;Competing Sentences: F(1,18) = 51.92, p <0.0001). For both subtests, the interactioneffect occurred as a result of the poorperformances by the listeners in the DDgroup to stimuli presented from the left side.Clinically, each of the listeners in the DDgroup would be identified as having“borderline” to “disordered” dichotic listeningfunction, based on either an overall lowperformance score on one of the two subtests(i.e., percentile rank less than 9%) and/orlarge ear differences exceeding 15% of thenormed sample for the Competing Wordssubtest.

Experimental Findings

RReessppoonnssee AAccccuurraaccyy

Table 4 shows the average accuracies forboth groups of listeners on the three


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Figure 4. Results for the SCAN Competing Wordsand Competing Sentences subtests for both groups.Error bars indicate SEM.

Table 4. Average Response Accuracies (in percent) for Both Groups to Each Stimulus Event on theThree Experimental Listening Tasks.

Accuracy (in %)Task Stimulus Event Control Group DD Group

Divided Attention Same-Right 96.32 (.98) 92.05 (1.58)Same-Left 94.26 (.60) 83.97 (2.65)Different (both) 98.53 (.45) 96.73 (.71)

Directed Attention Same-Right 97.09 (1.32) 94.37 (1.56)Same-Left 96.67 (1.36) 95.42 (1.58)Different-Right 97.92 (.62) 96.04 (1.57)Different-Left 98.75 (.56) 97.92 (.62)

Diotic - Control Same 99.58 (.28) 98.13 (.66)Different 99.58 (.28) 98.54 (.70)

Overall Performance: 97.63 94.80

Note: SEM values are shown in parentheses.

experimental listening tasks. Overall, averageaccuracies were excellent for both groups.The average accuracy for the control groupacross all test conditions was 97%, 94% forthe DD group. For the divided-attention task,both groups were overall more accurate to“same-right” judgments as compared to“same-left” judgments, F(1,18) = 16.628, p =0.0007. The DD group also showed a slightdecrease in response accuracy (83%) to “same-left” responses on the divided-attention taskas compared to the control group, F(1,18) =5.834, p = 0.027. Thus, the LED observedfor the DD group on the SCAN subtests wasalso observed on the experimental DL taskincorporating divided attention. Groupdifferences in response accuracy to left-sidedor right-sided stimulus events on the directed-attention procedure as well as the dioticcontrol task failed to reach statisticalsignificance.

EElleeccttrroopphhyyssiioollooggiiccaall RReessuullttss

Figure 5 shows grand-averaged ERPwaveforms for both groups of listeners acrossthe three listening tasks. Several findingsare evident from the waveforms obtainedduring the divided-attention task (left panels).First, irrespective of which side (left or right)constituted a “same” judgment (top-leftpanel), overall larger LPC amplitudes wereobserved for the control group as comparedto the DD group. Second, in the same figure,LPC peak latencies were earlier for thecontrol group (~600–800 msec) as comparedto those obtained for the DD Group(~800–1000 msec). Third, N400 amplitudesto “same” judgments were overall larger forthe DD group as compared to controlparticipants. Finally, for both groups, LPCamplitudes to “same-right” judgments wereslightly larger than those observed for “same-left” judgments.


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Figure 5. Grand-averaged ERP waveforms for both groups of listeners on the three listening tasks. Data aretaken from electrode PZ.

Grand-averaged ERP waveforms on thedivided-attention task corresponding to“different” judgments (bottom-left panel)were more similar between the two groups.Since a side-specific “different” judgment wasnot possible, only a single ERP waveform isshown for each group. In contrast to “same”judgments, “different” judgments for the twogroups are characterized by a robust N400component consistent with a semanticincongruity between the reference and probestimuli. LPC amplitudes to “different”judgments were slightly larger for the controlgroup as compared to the DD group.

The ERP waveforms obtained on thedirected-attention task (middle panels) weremore similar between the groups. As expected,“different” judgments evoked a more robustN400 component as compared to “same”judgments (i.e., a semantic incongruity effect).A slight decrease in LPC amplitude for “same-left” judgments was observed for the DDgroup as compared to the control group.

However, compared to the group differencesobserved for the LPC on the divided-attentiontask, LPC amplitudes and latencies wereoverall more similar between the groupswhen participants were asked to focus theirattention to one side. A robust N400, similarto that expected for “different” judgments, wasalso observed in “same” judgments for the DDgroup.

Grand-averaged ERP waveforms obtainedduring the control task are shown in the rightpanels of Figure 5. Overall, for both “same”and “different” judgments, LPC and N400amplitudes and latencies were similar foreach group. Again, responses to “different”judgments evoked a larger and more robustN400 component than “same” judgments.

Figure 6 summarizes the mean LPCamplitude and peak latency data for the twogroups on each listening task. For the divided-attention condition (left panels), controllisteners showed larger mean LPCamplitudes to “same” judgments than those


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Figure 6. Summary of the LPC mean amplitude and peak latency data for both groups on the three listeningtasks. Data are taken from electrode PZ.

obtained for the DD group, F(1,18) = 4.688, p = 0.044. Control listeners showed earlierLPC peak latencies to “same” judgments ascompared to those obtained for the DD group,F(1,18 )= 12.020, p = 0.003. While LPCamplitudes tended to be larger for “same-right” as compared to “same-left” judgments(see top-left panel in Figure 5), the amountof interaural asymmetry in either group failedto reach statistical significance. Both groupsshowed earlier LPC responses to right-sided“same” judgments as compared to left-sided“same” judgments, F(1,18) = 10.215, p = 0.001.Mann-Whitney U tests also confirmed groupdifferences on the divided-attention task withregard to LPC mean amplitude (p = 0.012)and peak latency (p = 0.009) to “same”judgments. Group differences in LPCmeasures for “different” judgments failed toreach statistical significance via eitherunpaired t-tests or Mann-Whitney U tests.

Analyses of mean LPC amplitude andof peak latency on the directed-attentioncondition (middle panels) revealed only main

effects of type of judgment (same or different).Mean LPC amplitudes were larger for “same”judgments as compared to “different”judgments, F(1,18) = 7.984, p = 0.011. MeanLPC peak latencies were earlier for “same”judgments as compared to “different”judgments, F(1,18) = 16.824, p < 0.001. Wheneach type of judgment was analyzed separately,no significant main effects or group interactionsfor mean LPC amplitude and peak latencywere obtained for this listening condition.

For the diotic control condition (rightpanels), mean LPC amplitudes to “same”judgments were slightly larger for the DD groupthan for the control group, but neither a maineffect of group or judgment type or theirinteraction was significant. Both groups showedearlier LPC peak latencies to “same” judgmentsas compared to “different” judgments on thecontrol listening task, F(1,18) = 22.163, p < 0.001.Overall, on both the directed-attention anddiotic tasks, LPC measures were similar for thetwo groups of listeners.

Figure 7 summarizes N400 mean


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Figure 7. Summary of the N400 mean amplitude and peak latency data for both groups on the three listen-ing tasks. Data are taken from electrode PZ.

amplitudes and peak latencies for both groupson each listening task. For the divided-attention condition (left panels), the DD groupshowed larger N400 amplitudes to “same”judgments as compared to those observedfor the control group. While N400 meanamplitudes corresponding to “same”judgments were not statistically differentbetween the two groups, an analysis of peakN400 amplitudes did, in fact, reveal thatN400 amplitudes were larger overall for theDD group, F(1,18) = 6.062, p = 0.024. Thisgroup difference in N400 amplitude to “same”judgments on the divided-attention task wasalso confirmed by a Mann-Whitney U test (p= 0.005). N400 mean amplitudes for the twogroups were larger overall for left-sided“same” judgments as compared to right-sided“same” judgments, F(1,18) = 6.110, p = 0.024.For “different” judgments on the divided-attention task, N400 amplitudes were similarbetween groups. While N400 peak latencieswere earlier to right-sided “same” judgmentsas compared to left-sided “same” judgments,F(1,18) = 11.884, p = 0.003, no statisticaldifference between the two groups wasobtained for N400 peak latency.

An analysis of N400 mean amplitudes forthe directed-attention condition (top-middlepanel) revealed a main effect of type ofjudgment (same or different), F(1,18) = 19.703,p < 0.001. While the listeners in the DDgroup tended to show larger N400 meanamplitudes to “same” judgments as comparedto control listeners (see waveforms in Figure5), this trend did not reach statisticalsignificance when analyzed via standardparametric procedures (ANOVA, p = 0.169).When the same data were analyzed via theMann-Whitney U test, N400 amplitudes to“same” judgments did in fact differ betweengroups (p = 0.025). While the two statisticalprocedures yielded different outcomes forN400 amplitude, results of the Mann-Whitney U test appear more consistent withthe magnitude of the group differenceobserved in the ERP waveforms for “same”judgments on the directed-attention task.For “different” judgments, N400 meanamplitudes were similar for each group. Noadditional main effects or group interactionsfor N400 latency were obtained.

For the diotic control condition (rightpanels), “different” judgments, again,produced a robust N400 component ascompared to “same” judgments (N400 mean

amplitude: F(1,18) = 22.442, p < 0.001). Noadditional main effects or group interactionsfor N400 mean amplitudes or peak latencieswere observed on this task.

Summary of Results for ExperimentalTasks

The main results of the study can besummarized as follows:

1. Overall behavioral accuracy in allthree experimental tasks wasexcellent for the control and DDgroups (97% and 94%, respectively).Poorer performance (83%) by the DDgroup for “same-left” judgments wasfound only on the divided-attentiontask.

2. On the diotic control task, ERPwaveforms were similar for the twogroups; no significant differenceswere found for either the LPC orN400 components. Thus, on aspatially “neutral” task, the groupswere similar.

3. On the dichotic task incorporatingdivided-attention, robust groupdifferences in the ERP waveformswere found. Control listeners showedlarger LPC mean amplitudes to“same” judgments as compared tothe DD group. LPC peak latencies to“same” judgments were also earlierfor the control group.

4. Both groups showed larger LPCamplitudes for right-sided “same”judgments as compared to left-sided“same” judgments, particularly onthe divided-attention task. This isconsistent with the well known right-ear advantage in DL. The magnitudeof interaural asymmetry in ERPresponses, however, did not differbetween the groups.

5. When listeners were asked to directattention to a specific side (i.e.,directed attention), LPC meanamplitudes and peak latencies weremore similar between groups.

6. Larger N400 amplitudes to “same”judgments were found on bothdichotic tasks for the DD group ascompared to that observed for controllisteners.


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DISCUSSION

Interaural Asymmetry

While computation of a listener’s totalcomposite score on a DL test can bediagnostically useful, it is the degree anddirection of interaural asymmetry that hashistorically been of most interest forassessment of the integrity of the binauralauditory system. In this study, we selected agroup of children (DD group) whodemonstrated prior difficulty on clinical testsof DL. On the SCAN, these children showedsubstantial interaural asymmetry in the formof left-ear deficit (LED). Experimentally, asimilar behavioral profile was found for thesame listeners, but only on the DL taskinvolving divided attention. When the samelisteners focused attention to one side, thedegree of interaural asymmetry in behavioralaccuracy altogether disappeared. It wouldappear that, for this group of children, thebehaviorally observed LED stemmedprimarily from those processing demandsunique to the divided-attention mode.

Electrophysiologically, the direction ofinteraural asymmetry in ERP responses wasconsistent with the expected right-earadvantage (REA) in DL. On the divided-attention task, for example, the two groupsof listeners showed larger LPC amplitudes toright-sided “same” judgments as compared toleft-sided “same” judgments. Interestingly,the degree of interaural asymmetry in ERPmeasures remained similar between the twogroups on all experimental tasks. It isimportant, however, to bear in mind that theexperimental DL procedures employed herewere considerably less demanding than thosetypically encountered by audiologicalpatients. For example, the Competing Wordssubtest of the SCAN is a divided-attentiontask that requires listeners to repeat, in apreferential order, both of the words heard.Those factors known to influence children’sresponses on basic speech understandingmeasures, such as word familiarity, frequencyof usage, age of acquisition, and theirphonemic similarity to other frequentlyencountered words, apply equally to thisprocedure. For the experimental DLprocedures, on the other hand, listeners weregiven opportunity to hear and review thetest items prior to the listening tasks. This

minimized the effects of facility with testmaterials. The utility of a simple “same-different” paradigm required only a singlemanual response, minimizing the demandson memory and report strategy. It wasexpected, therefore, that listeners wouldcompare favorably with regard to behavioralaccuracy on the experimental DL tasks.

We do not mean to imply that the SCANsubtests were ineffective at identifyingchildren, at least to a first screeningapproximation, who show signs of auditoryprocessing weaknesses. Results clearly showthat some children evaluated for APD doexperience pronounced difficulty on thesetests. Rather, our concern is to what extentdo extra-auditory factors, irrespective of theclinical test employed, potentially influencethe outcome of results? Both the behavioraland electrophysiological results on theexperimental tasks suggest that the degreeof interaural asymmetry observed in DL maybe minimized to a greater extent whendemands on memory, report strategy, andfacility with linguistic materials are reduced.

Mode of Dichotic Administration

Consistent with the behavioral results,group differences were most apparent in theelectrophysiological recordings obtainedduring the DL task with divided attention. Asshown in Figures 5 and 6, control participantsshowed larger LPC amplitudes and shorterLPC latencies to “same” judgments ascompared to the DD group. Although themagnitude of interaural asymmetry in theERP waveforms did not differentiate the twogroups, an overall reduction in LPCamplitude and delay in LPC peak latencyfor the DD group suggests that these listenersmay have particular difficulty processingdichotic stimuli in the divided-attention mode.

What is the basis for this processingdisadvantage in the DD group? Given thecognitive demands unique to the divided-attention mode, the most parsimoniousexplanation is that some children may showdifficulty in dividing attention betweenspatially separate sound sources. The sourceof the processing delay, as evidenced in theLPC to “same” judgments, could result fromtwo temporally overlapping processes: (1)awareness of the phonemic and/or semanticcongruity between the reference word and one


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of the words in the dichotic probe (i.e., a“same” judgment), and (2) suppression of thephonemic and semantic information inherentto competing words. It follows that if thelistener can appropriately divide attention toboth auditory channels, recognition of the“same” stimulus (i.e., the repeated referenceword), irrespective of the side of occurrence,can readily be made and the competing wordbecomes of little consequence. If the listeneris unable to allocate attentional resourceseffectively, both words must be analyzed,either phonemically or semantically, to agreater extent. This comes, however, at theexpense of slower speed in processing.

If the processing delay to “same”judgments on the divided-attention taskresults from prolonged semantic analysis ofdichotic stimuli, group differences in theN400 should also be apparent. Inspection ofthe N400 amplitudes to “same” judgments onthis task is consistent with this idea (Figure5). The DD group showed larger N400amplitudes to “same” judgments as comparedto control listeners. Moreover, N400amplitudes for the DD group to “same”judgments are similar to those observed fortheir “different” judgments. Thus, for the DDgroup, larger N400 amplitudes to “same”judgments may reflect semantic intrusion ofthe competing word, which, in itself,corresponds to a pure “different” judgment.

In summary, robust differences in ERPresponses between the two groups were foundon the divided-attention task. From thestandpoint of audiological applications,however, we can identify some potential extra-auditory factors unique to this mode ofdichotic administration (e.g., memory, speedof processing, attention) that complicate theassessment of more auditory-specific factorson results. We have previously argued thatadministration of dichotic listening testsusing both divided- and directed-attentionmodes can provide insight into the relativecontributions of cognitive and auditory-specific factors influencing interauralasymmetry (Jerger and Martin, 2006). Ifthere is truly an auditory-specific deficitcontributing to poor dichotic performance ona divided-attention task, it should be apparentunder any mode of administration. To thisend, we turn our focus to the ERP resultsobtained on the directed-attention task.

The ERPs obtained during the directed-attention task were, overall, more similar

between groups. Similarities in LPCamplitude and latency on this task wouldsuggest that the DD group benefited frominstruction to focus attention to one auditorychannel rather than both. An unexpectedfinding, however, was the persistent groupdifference (via Mann-Whitney U test) in N400amplitude to “same” judgments on this task.Again, the DD group showed larger N400amplitudes as compared to control listeners(Figure 5). It is possible that the semanticanalysis of the competing word, even thoughnominally unattended, influenced the extentto which information on the attended side isprocessed. Since the physical match betweenthe reference stimulus and the word (in thedichotic pair) constituting a “same” judgmentalways occurred on the attended side, it isdifficult to ascertain to what extent listenersin the DD group may have “monitored”information in the unattended channel.Future studies are underway in our laboratoryto explore this possibility. In any event, thedirected-attention procedure proved tominimize the group differences observed forthe LPC on the divided-attention task.

Taken together, we interpret the patternof ERP results obtained for the DD group onboth DL tasks as consistent with a processingdisadvantage primarily in the cognitivedomain. This conclusion does not, however,negate the possibility that auditory-structuralfactors are involved. Indeed, the auditory-structural basis for interaural asymmetryin DL has been well documented over theyears. Results of this study nonethelesssuggest that the magnitude of interauralasymmetry, particularly when assessed withdivided-attention procedures, may emphasizeadditional mechanisms unique to attention.

It could be argued that the groupdifferences observed on the divided-attentiontask resulted from a more generalized slowingin speed of mental processing. If this were thecase, we might also expect to see groupdifferences on a more simple (nondichotic)task. Results on the diotic control procedure,which incorporated the same stimuli usedin the dichotic tasks, were not consistentwith this view. LPC and N400 measures onthis task were similar between each group.It would appear, therefore, the processingdeficit observed for the DD group is uniqueto the dichotic mode.

Finally, group differences in N400amplitude were observed on both dichotic


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tasks. Traditionally, the N400 has been shownto be a sensitive index in the depth ofsemantic analysis for both auditory and visualstimuli. Following this line of reasoning,group differences in N400 amplitude could beinterpreted as evidence for qualitativedifferences in the semantic analysis of thelinguistic stimuli used in this study. However,given the similarities between groups in theN400 on the diotic task, it is difficult to arguethat this result stemmed from a more globalsemantic deficit per se. Rather, we relate thelarger N400 amplitudes for the DD group toan additional processing negativity, whichmay reflect an increase in the cognitive effortnecessary to allocate attentional resourcesappropriately during DL. ERP studiesfocusing on attention in children havereported a similar attentional componentthat may overlap the time course of the N400(Courchesne, 1978; Holcomb et al, 1985).

Clinical Implications

Results show that the mode of DL testadministration is an important factor toconsider when interpreting test results. Inthis study, group differences in ERP measureswere most robust on the dichotic task involvingdivided attention. This could be interpreted asevidence for the exclusive use of divided-attention procedures for clinical assessment ofAPD in children. We, however, suggest theopposite. The extent to which interauralasymmetry on this listening mode isattributable to attentional factors and/orauditory factors is difficult to sort out. Rather,the utility of directed-attention procedures,ones that incorporate the same or similar testmaterials as those presented using dividedattention, may be helpful in separating bothbottom-up and top-down processing biasesunderlying interaural asymmetry in DL.

What accounts for the difference betweenbehavioral performances on divided-attentionversus directed-attention dichotic procedures?Traditionally, the rationale for using directed-attention procedures was to minimize(control) the cognitive demand imposed by thetask and, in turn, allow for a more directassessment of the auditory-structuralcomponent underlying interaural (andhemispheric) processing biases. Accordingly,increases in the magnitude of interauralasymmetry on divided-attention procedures,

as compared to that observed on directed-attention tasks, would reflect involvementof more cognitive (attentional) factors. It isbecoming increasingly clear, however, that themechanisms underlying the directed-attention paradigm may differ in manyrespects from those found with dividedattention. Hugdahl (2003) suggests thatdespite identical instructions to the listener,the directed-left and directed-right tasks tapdifferent mental processes. In particular, apersistent REA on the directed-left taskindicates impairments in executive functionover the more dominant, stimulus-driveneffect characterizing the processing of right-sided inputs. Of course, maturational delayor impairment of the corpus callosum couldproduce a similar behavioral profile.Whatever the mechanisms may be, we feelthat the combined use of divided- anddirected-attention procedures may play auseful role in uncovering the nature of DLdeficits in some children. Unfortunately, toour knowledge, no standardized testprocedure that incorporates both listeningmodes is commercially available toaudiologists.

Concern about the influence of attentionand other extra-auditory factors on DL testresults is not new. In fact, certain procedures,for example, dichotic fusion tests (Repp, 1976;Wexler and Halwes, 1983), have beenproposed over the years to minimize theinfluence of volitional shifts in attention onDL performance. More recently, Shinn et al(2004) compared behavioral performances inyoung adults on a dichotic rhyme (fusion)test (DRT) with the dichotic CV (consonant-vowel) test. They administered each testunder both divided-attention and directed-attention modes. Results indicated that themagnitude of interaural asymmetry acrosstests was more stable using the DRT ascompared to the dichotic CV test. Because thedichotic “fusion” effect is believed to arisefrom lower centers in the central auditorypathway (e.g., brainstem), it may be lesscontaminated by attentional mechanisms.The authors conclude that the DRT proceduremay be of value to audiologists in theseparation of more global processing biases,such as attention, from more auditory-specificfactors. Future research is needed with theprocedure to investigate potential responsebiases in the stimuli used on the DRT and itsrobustness in the evaluation of children.


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Clearly, the effects of attention and otherextra-auditory factors on behavioralperformance are difficult to disentangle.While interaural asymmetry on DL testsremains an important index in the evaluationof the binaural auditory system, the resultsof this study suggest that extra-auditoryfactors, such as attentional capacity, shouldbe carefully considered. Given the likelycomorbidity of APD and other developmentaldisorders, techniques that aim to separateextra-auditory factors on test results areneeded.

Acknowledgments. We recognize Phillip L. Wilson,Beth Bernthal, and Ross J. Roeser of the CallierCenter for Communication Disorders for their valu-able contributions to this work. The technicalassistance of Gary Overson, Gail Tillman, and RenéeWorkings is gratefully acknowledged.

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Documents

Divided-Attention and Directed-Attention Listening Modes ... · En una de las tareas de DL, los participantes monitorearon estímulos dicóticos utilizando el modo de atención dividida