Neuropsychology1999, Vol. 13, No. 3,404-414

Copyright 1999 by the American Psychological Association, Inc.0894-4105/99/S3.00

Allocation of Attention in Dichotic Listening:Differential Effects on the Detection and Localization of Signals

Merrill HiscockUniversity of Houston

RoxanneInchUniversity of Saskatchewan

Marcel KinsbourneNew School University

In 2 dichotic listening experiments, 96 normal right-handed adults attended selectively to theleft and right ear and divided their attention equally between both ears. Participants listenedfor specified targets and reported the ear of entry. The material consisted of pairs ofconsonant-vowel syllables in Experiment 1 and pairs of rhyming consonant-vowel-consonant words in Experiment 2. Both experiments yielded a right-ear advantage fordetection and for localization. Attention instructions had no effect on detection. However,focusing attention on 1 ear increased the number of targets attributed to that ear whiledecreasing the number of targets attributed to the opposite ear. The dissociation betweendetection and localization indicates that volitional shifts of attention influence late (responseselection) processes rather than early (stimulus identification) processes. Selective-listeningeffects can be accounted for by a 2-stage model in which a fixed input asymmetry is modulatedby a biased selection of responses.

Dichotic listening is a method that presents simultaneousauditory messages, one message to each ear. When thestimulus material is linguistic (words, nonsense syllables,digit names, etc.), the message presented to the right ear ismore likely to be reported than the message presented to theleft ear. The right-ear advantage (REA) has been associatedempirically with clinical evidence of left-sided languagerepresentation (Geffen & Caudrey, 1981; Geffen, Traub, &Stierman, 1978; Kimura, 1961a, 1961b; Strauss, Gaddes, &Wada, 1987; Zatorre, 1989).

As observed by Bryden (1988), the REA has "proved tobe a very robust effect no matter what [procedural] varia-tions were introduced" (p. 3). The robustness of the REAacross studies, especially studies of normal right-handers,raises the possibility that dichotic listening procedures mightbe useful in determining the side of language representationat the level of the individual. However, the ubiquity of theREA at the level of group is seldom mirrored in individualdata. Despite an occasional report of REA in more than 90%of right-handed participants (e.g., Grimshaw, McManus, &Bryden, 1994), the typical frequency of REA among normalright-handers is about 80% (Bryden, 1988). Assuming that95% to 99% of right-handers have left-sided speech

Merrill Hiscock, Department of Psychology, University ofHouston; Roxanne Inch, Department of Psychiatry, University ofSaskatchewan, Saskatoon, Saskatchewan, Canada; Marcel Kins-bourne, Department of Psychology, New School University.

This work was supported in part by a grant from the MedicalResearch Council of Canada. We thank Marilynn Mackay for herassistance.

Correspondence concerning this article should be addressed toMerrill Hiscock, Department of Psychology, University of Hous-ton, Houston, Texas 77204-5341. Electronic mail may be sent tomhiscock@uh.edu.

representation, as suggested by clinical data (Carter, Ho-henegger, & Satz, 1980; Rasmussen & Milner, 1975), an80% frequency of REA is significantly discrepant from theprevalence of the language asymmetry it is thought torepresent. Satz (1977) has used a Bayesian analysis to showthat inferences about anomalous language lateralization arehighly misleading when based on a laterality measure thatsubstantially underestimates the prevalence of left-lateral-ized language in the population.

The inaccuracy of dichotic listening in classifying individu-als as left dominant or right dominant for language might beattributed to several factors, including unreliability of ear-difference scores (Blumstein, Goodglass, & Tartter, 1975;Teng, 1981), use of suboptimal statistical criteria to classifyparticipants (Wexler, Halwes, & Heninger, 1981), andindividual differences in peripheral auditory sensitivity(Borod, Obler, Albert, & Stiefel, 1983). One of the shortcom-ings of commonly used dichotic listening methods is theresearcher's lack of control over the participant's processingand reporting strategies (Bryden, 1978). Although especiallysalient when multiple stimuli are presented to each ear (e.g.,Kimura, 1961a, 1961b), strategy effects remain evidentwhen stimuli are presented in single pairs (Bryden, 1982,1988; Hiscock, Lin, & Kinsbourne, 1996; Spellacy &Blumstein, 1970). Of particular concern are questions abouthow participants allocate attention in dichotic listening andthe effect of selective attention on left- and right-earperformance. These questions lead us back to the origin ofdichotic listening as a method for studying selective atten-tion (Broadbent, 1954, 1958).

Auditory selective attention has been studied in twocontexts. One line of studies has been focused on the degreeto which signals from an unattended channel are processed.The results have led to various attempts to specify the stage

404

ATTENTION IN DICHOTIC LISTENING 405

of processing at which attention has its effect (e.g., J. A.Deutsch & Deutsch, 1963; N. L. Wood & Cowan, 1995).The other line of studies investigates the effect of attendingto one or the other ear on the REA (e.g., Asbjornsen &Hugdahl, 1995).

Interest in selective auditory attention originated in a 1953study by Cherry in which participants shadowed (repeated)prose heard at one ear while disregarding irrelevant stimuliat the other ear. Selective attending was so effective thatparticipants failed to notice transitions at the unattended earunless the transitions entailed a change in a fundamentalacoustic property, such as pitch. Considerable subsequentwork has focused on the degree or level to which task-irrelevant stimuli are processed and recalled (Broadbent,1958; J. A. Deutsch & Deutsch, 1963; Moray, 1959; Treis-man, 1960; N. L. Wood & Cowan, 1995). Recent evidenceindicates that there is some automatic processing of informa-tion from the unattended channel, but unattended stimuli arenot processed to a level that allows awareness. When inputfrom the unattended channel is recalled and reported, itsavailability apparently reflects a shift of attention to thatchannel (N. L. Wood & Cowan, 1995).

The second line of studies has shown that it typically iseasier to attend to speech stimuli from the right ear than fromthe left ear (Treisman & Geffen, 1968). This asymmetry isthought to be a manifestation of the REA that is commonlyobserved in dichotic listening tasks that have a free-reportformat (e.g., Studdert-Kennedy & Shankweiler, 1970).

Although the REA is thought to be determined byneurological factors (see Studdert-Kennedy, 1975), ear asym-metry may be influenced by volitional shifts of attention(Bryden, Munhall, & Allard, 1983; Treisman & Geffen,1968). Under some circumstances, focusing attention on theleft ear in a dichotic listening task will nullify the REA(Andersson & Hugdahl, 1987; Hiscock & Beckie, 1993) oreven produce a left-ear advantage (LEA; Asbjornsen &Hugdahl, 1995; Bryden et al., 1983; Hugdahl & Andersson,1986). Nonetheless, selective attention instructions in dich-otic-listening tasks typically do not have as dramatic aneffect as do similar instructions in shadowing tasks. Whereasthe shadowing literature suggests very limited processing ofstimuli in the unattended channel (N. L. Wood & Cowan,1995), the dichotic-listening literature indicates that unat-tended stimuli, especially unattended stimuli in the right ear,often are processed to the point of being available for verbalreport (Bryden et al., 1983). In children, signals from theunattended right ear may be processed to a degree compa-rable to, or even exceeding, that of stimuli in the attendedleft ear (Hiscock & Beckie, 1993; Hugdahl & Andersson,1986; Obrzut, Boliek, & Obrzut, 1986).

The differential strength of selective attention effects inthe shadowing and dichotic-listening studies may stem fromeither of two salient differences between the respectivemethods. First, the shadowing task probably entails a moreeffective manipulation of attention. Repeating the materialheard on one channel demands a high level of effort(Underwood & Moray, 1971), and the fact that shadowingperformance is observable presumably enhances partici-pants' compliance with the selective attention instructions.

In contrast, as suggested by Mondor and Bryden (1991), theselective attention instructions used in dichotic-listeningstudies may constitute a rather weak means of manipulatingattention. Second, the attended material in the shadowingtask, being of longer duration, may be more likely to capturethe participants' attention. The material to be shadowedtypically consists of connected prose that is presented over aperiod of at least a few minutes, whereas dichotic stimulitypically consist of single pairs of brief sounds or lists of nomore than three or four pairs presented in rapid succession.The stimuli used in dichotic-listening experiments may notprovide the participants with sufficient time in which tofocus attention optimally (see Mondor & Bryden, 1991;Mondor & Zatorre, 1995). Alternatively, the brief durationof the stimuli may allow attention to be switched to theunattended ear before information in that channel hasdecayed from preattentive (echoic) storage (Darwin &Baddeley, 1974).

Few studies have focused on the mechanism or mecha-nisms of attentional effects in dichotic listening. The avail-able evidence is largely restricted to descriptions of howlistening asymmetry is altered by focused attention, andthese descriptions are not entirely consistent across studies(see Mondor, 1994, for a review). The specific consequencesof attention shifts on performance at the attended andunattended ears also vary across studies. Bryden et al.(1983) and Asbjornsen and Hugdahl (1995) reported thatselective attention instructions produce both an increase inthe number of stimuli reported from the attended ear and adecrease in the number of stimuli reported from the unat-tended ear. A study of children by Hiscock and Beckie(1993) supported that conclusion, but only for consonant-vowel (CV) stimuli. When children were asked to detecttargets from lists of dichotic words and they attended toeither ear, the number of hits from the unattended eardecreased without the number of hits from the attended earincreasing. Obrzut et al. (1986) found that selective attentioninstructions had no effect on children's perception of CVstimuli at either ear and had diverse effects on the perceptionof other dichotic stimuli (words, digit names, and melodies).

Thus, attention shifts typically increase the number ofstimuli reported from the attended ear or decrease thenumber of stimuli reported from the unattended ear. Some-times both effects are obtained. Outcomes appear to varyaccording to the nature of the dichotic stimuli and, in someinstances, according to the ear that is being attended (Obrzutet al., 1986). In the absence of analyses based on signal-detection theory, it is impossible to know whether the effectsof selective attention represent changes in sensitivity or indecision criteria.

The primary purpose of this study is to determine moreprecisely the effect of selective attention instructions onadults' perception of dichotic stimuli from two categories:single pairs of CV nonsense syllables and single pairs offused rhyming words. By combining selective attentioninstructions with a detection task, we are able to assess theeffects of selective attention on (a) the sensitivity of signaldetection at each ear, (b) the sensitivity of signal localizationat each ear, and (c) the criterion for responding to signals at

406 HISCOCK, INCH, AND KINSBOURNE

each ear. In addition, by manipulating the time at which theparticipant learns the identity of the target, we can vary thememory load imposed by the task. If the identity of the targetis not known until after the dichotic stimuli have beenpresented (i.e., in the case of a postcued target), a responsemust be made on the basis of a stored representation of theinput. On the other hand, advance knowledge of the target'sidentity (i.e., in the case of a precued target) may allow theparticipant to choose a response without fully processing orstoring all of the input (Kahneman & Treisman, 1984).Accordingly, precuing serves to minimize the effects ofmemory and associated variables such as order of retrievaland differential decay.

Experiment 1

Pairs of CV nonsense syllables were selected as stimulifor Experiment 1. Since 1970, when Studdert-Kennedy andShankweiler reported a strong REA for the initial stopconsonants in dichotic pairs of consonant-vowel-consonant(CVC) syllables, single pairs of CV nonsense syllables havebeen used frequently as stimuli in dichotic-listening studies(Bryden, 1988). The use of single pairs of stimuli serves toreduce or to eliminate many of the input and outputorganization factors that confound the interpretation ofdichotic-listening studies involving lists of two or morestimulus pairs per trial. Because the paired CV stimuli differonly with respect to the initial consonant's voice onset time,place of articulation, or both, the stimuli frequently "fuse"perceptually so that participants report hearing only onesound (Wexler & Halwes, 1985). Nonetheless, as noted byBryden (1982), using single pairs of stimuli does notpreclude attention being allocated in various ways nor doesit prevent right- and left-ear stimuli from being reported indifferent orders.

Method

Participants. Forty-eight undergraduate volunteers (24 women,24 men) were recruited from introductory psychology classes toparticipate in a study of auditory perception. Ages ranged from 18.1to 33.6 years (M = 22.3, SD = 3.4). All participants were right-handed by self-classification, by use of their right hands for writing,and by their scores on the Edinburgh Handedness Inventory(Oldfield, 1971). The mean laterality quotient from the handednessquestionnaire was 77.1 (SD = 21.0) on a scale ranging from —100(extreme left-handedness) to +100 (extreme right-handedness). Allparticipants were native speakers of English. Criteria for exclusionfrom the study were (a) known hearing impairment, (b) history ofseizures or other neurological disorder, and (c) history of speechtherapy.

Materials. The dichotic stimuli were tape-recorded CV non-sense syllables produced by combining each of the six English stopconsonants with the vowel /a/: /ba/, /pa/, /da/, /ta/, /ga/, and /ka/.Dichotic pairs of these syllables were prepared and were recordedat the Kresge Hearing Research Laboratory of the South (NewOrleans, LA). Computer processing of digitized natural speechtokens (male voice) resulted in alignment of consonant onsetswithin 2.5 ms and matching of vowel segment amplitudes within2.5 dB. The duration of each syllable was 296 ms. The tapeconsisted of three random sets of stimuli, each of which contained

30 counterbalanced CV pairs with an intertrial interval of 6 s.Playing the three sets twice yielded a total of 180 test trials. Inaddition, a set of 30 binaural practice trials was available tofamiliarize participants with the material.

Apparatus. Stimuli were presented via a Revox A77 two-channel tape recorder (Willi Studer AG, Regensdorf, Switzerland)and Koss K/6 stereophonic headphones (Koss Corporation, Milwau-kee, WI). The average signal intensity for each channel was set to80 dB (A), as measured using a 1000 Hz steady-state calibrationtone, and the channels were balanced within 1 dB. The ambientnoise level was 32 dB (A) in the sound-attenuated chamber used fortesting.

Procedure. Each participant was tested individually. The par-ticipant first completed the Edinburgh Handedness Inventory and apersonal history questionnaire, which were used to confirm informa-tion that had been obtained via telephone during an initialscreening. The experimenter then described the CV stimuli that theparticipant would be hearing.

Before the first set of test trials, the participant donned theheadphones and listened passively to 30 binaural presentations ofthe CV stimuli. Following these familiarization trials, three blocksof 60 test trials were administered. One block of trials (left) entailedattending selectively to the left ear, another block (right) entailedattending selectively to the right ear, and the remaining block(divided) entailed dividing attention equally between the left andright ears. The order in which the blocks were administered wascounterbalanced completely across participants within each sex.

A target CV syllable, printed on a 10 X 15-cm card, waspresented either 1 s before the onset of the dichotic stimuli (precuecondition) or 1 s after the offset of the dichotic stimuli (postcuecondition). Half of the participants within each sex were assignedrandomly to the precue condition and half were assigned to thepostcue condition. Irrespective of cue condition, the participantwas instructed to report his or her perception of the target bywriting "L" (left), "R" (right), or "N" (neither) onto a formdevised for that purpose. Within each block of 60 trials, the targetarrived at the left ear on 20 trials, at the right ear on 20 trials, and atneither ear on 20 trials. A list of targets was assigned randomly toeach participant from a set of six lists. Within each list, targets weredistributed randomly across possible locations (left ear, right ear,neither ear). The initial position of the headphones (normal vs.reversed) was counterbalanced within each combination of sex, cuecondition, and target list. The headphones were reversed after every30 trials.

Scoring. Three measures of sensitivity and one measure ofresponse bias were computed using procedures similar to thosedescribed previously (Bryden, 1976; Bryden et al., 1983; Hiscock,Hampson, Wong, & Kinsbourne, 1985). Detection performancecan be represented in terms of a single 2 X 2 decision matrix inwhich the target is present or absent and the response is either"yes" or "no." In computing the index of detection sensitivity, aresponse was counted as correct if the target was identified whenactually present, regardless of whether the ear of arrival wasspecified correctly.

Localization performance can be represented in terms of a 3 X 3decision matrix in which the target is present at the left ear, presentat the right ear, or absent from both ears, and the response is "left,""right," or "neither." A response is counted as a localization hitonly if the target is detected and localized correctly. FollowingBryden (1976), we assumed that decisions about left- and right-earstimuli occur along two separate decision axes. Thus, estimates ofsensitivity and bias are based on four cells of the matrix, that is,localization hits for each ear as well as false alarms attributed toeach ear. The index that we refer to as detection-plus-localizationsensitivity reflects the number of localization hits. The index

ATTENTION IN DICHOTIC LISTENING 407

referred to as localization-only sensitivity is based on the ratio oflocalization hits to detection hits. Localization-only sensitivityindicates the likelihood that a target, once detected, will belocalized correctly.

As in previous detection studies with dichotic stimuli, nonpara-metric measures of sensitivity and bias were used (Grier, 1971;Hodos, 1970; McNicol, 1972; Pastore & Scheirer, 1974). The areaunder the receiver-operating characteristics (ROC) curve, p(A),was selected as the measure of sensitivity, and an arcsine transfor-mation was applied to_ p(A) scores before analysis of variance(ANOVA). Higher p(A) values represent greater sensitivity. Ameasure of displacement from the negative diagonal of the ROCcurve, p', was used as the index of bias. We used a computationalformula that accommodates points falling on either side of thediagonal (Grier, 1971, Expression 11). All mean P' values werepositive, which indicates that the points fall below the negativediagonal. Greater magnitudes represent greater displacement fromthe diagonal and thus more stringent decision criteria.

All data analyses were repeated with sex of participant as anadditional factor. Unless otherwise indicated, no significant sexdifferences were obtained.

Results

Raw scores. Table 1 shows the mean proportion ofdetection hits, localization hits, localization errors, and falsealarms (out of a possible 20) for each cue condition, ear, andattention condition.

Detection sensitivity. Arcsine-transformed p(A) scoresfor detection were analyzed in a 2 X 2 X 3 ANOVA with cuecondition (precue or postcue), ear, and attention condition(left, right, or both) as the respective independent variables.There were repeated measures on the second and thirdvariables. The ANOVA yielded no significant findingsexcept a main effect for ear, F(l, 46) = 21.73, p < .0001,which reflected an REA. There was neither a significantmain effect for attention (F < 1), nor an Ear X Attentioninteraction (p > .10). Mean p(A) scores for each ear andattention condition are shown in the upper left panel ofFigure 1.

Detection-plus-localization sensitivity. The Cue X Ear XAttention ANOVA for arcsine-transformed p(A) scoresyielded significant main effects for ear, F(l, 46) = 11.59,

p < .005, and attention, F(2, 92) = 3.99, p < .025. The eareffect indicated an REA, and the attention effect wasattributable primarily to poorer overall performance with leftattention than with right attention, F(l, 46) = 4.40, p < .05.A significant Ear X Attention interaction, F(2, 92) = 25.99,p < .0001, was attributable entirely to the Ear X Left VersusRight Attention component, F(l, 46) = 37.60, p < .0001.Leftward attention yielded an LEA, whereas rightwardattention increased the REA above the magnitude observedin the divided attention condition. There were no significanteffects for time of cuing. The results are shown in the upperright panel of Figure 1. A supplemental ANOVA with sex ofparticipant as a design variable yielded a significant maineffect for sex, F(l, 40) = 4.17, p < .05, which did notinteract with any other variables. Women achieved higherscores than did men (M = 2.14 and 2.04, respectively).

Localization-only sensitivity. Analysis of this index,which removes the effects of differential detection fromlocalization scores, yielded results similar to those describedin the previous paragraph. Again, there were significantmain effects for ear, F(l, 46) = 6.32, p < .025, andattention, F(2, 92) = 4.28, p < .025, and the attention effectwas attributable to the left versus right component, F(l,46) = 5.34, p = .025. The Ear X Attention interaction wassignificant, F(2, 92) = 33.28, p < .001, as was the Ear XLeft Versus Right Attention component, F(l, 46) = 54.16,p < .0001. The pattern of results is shown in the lower leftpanel of Figure 1.

Response bias. The lower right panel of Figure 1 showsthe mean (3' for each attention condition. An ANOVAyielded neither a significant main effect for ear (F < 1), nora significant Ear X Attention interaction (F < 1). Analysisof false-alarm rates also yielded negative results.

Frequency data. The number of participants with anREA, LEA, or no ear advantage (NEA) for the variousdependent variables is shown in Table 2 as a function ofattention condition. (For false alarms, the advantaged ear isthe ear receiving the greater number of responses. For 3', theadvantaged ear is the ear at which the criterion is more lax.)Cochran's Q test (Siegel, 1956) indicated that for the three

Table 1Mean Proportion of Detection Hits, Localization Hits, Localization Errors, and False Alarms for Each Ear and AttentionCondition in Experiment 1

Left Divided Right

Left ear Left earCue

condition

Detection hitsPrecuePostcue

Localization hitsPrecuePostcue

Localization errorsPrecuePostcue

False alarmsPrecuePostcue

M

0.770.78

0.560.51

0.210.27

0.200.21

SD

0.160.15

0.180.15

0.100.13

0.120.11

ivl£XJ

M

0.850.83

0.480.38

0.370.45

0.170.20

IL VIU

SD

0.120.13

0.190.16

0.160.18

0.100.14

AJV^II

M

0.780.82

0.470.51

0.310.32

0.160.25

L t^CU

SD

0.170.12

0.190.18

0.170.18

0.110.12

j.x»fe.

M

0.830.85

0.590.58

0.240.27

0.180.22

SD

0.140.13

0.170.16

0.120.13

0.110.14

M

0.720.79

0.360.45

0.360.35

0.160.26

SD

0.150.13

0.180.19

0.150.10

0.120.11

i^U£l

M

0.850.87

0.660.67

0.190.21

0.200.16

II V^MJ.

SD

0.130.13

0.150.15

0.110.12

0.110.12

408 HISCOCK, INCH, AND KINSBOURNE

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Detection

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. -*- Right Ear

rLeft Divided Right

Attention

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2.2

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1.8

1.6

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Detection plus

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Localization

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Divided Right

Attention

2.6

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Localization Only

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Left Divided Right

Attention

0.4

0.3

0.2

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Response Bias

Right Ear

Left Divided Right

Attention

Figure 1. Mean detection sensitivity, detection-plus-localization sensitivity, localization-onlysensitivity, and response bias for dichotic consonant-vowel nonsense syllables as a function of earand attention condition in Experiment 1. Error bars represent ± 1 SEM. Smaller values of p' indicatestronger response biases (more lax criteria).

localization scores only, the proportion of participantsshowing an REA varied significantly across attention condi-tions (p < .01).

Discussion

Irrespective of whether detection sensitivity, detection-plus-localization sensitivity, or localization-only sensitivityserved as the dependent measure, there was a significantREA of a magnitude within the range typically reported forfree-report and selective-listening experiments with CVstimuli (Asbjornsen & Hugdahl, 1995; Bryden et al., 1983;Hiscock & Stewart, 1984; Studdert-Kennedy & Shank-weiler, 1970).

Directed shifts of attention, which had no measurableeffect on right- or left-ear detection, had a marked effect onlocalization. When participants were instructed to focusattention on one ear, targets arriving at that ear were morelikely to be attributed correctly to the attended ear andtargets arriving at the unattended ear were more likely to bemisattributed to the attended ear. Although these localizationdata indicate a response bias in favor of the attended ear,neither (3' nor the false-alarm rate for either ear changedsignificantly across attention conditions. The conventionalsignal-detection model is inadequate to account for thedecision processes that underlie this pattern of results. Atarget that is above detection threshold tends to be attributed

Table 2Number of Participants Showing a Left-Ear Advantage (LEA), Right-Ear Advantage(REA), and No Ear Advantage (NEA) for Different Measures of Performancein Experiment 1

Left Divided Right

Measure

Detection hits and detection sensitivityLocalization hitsDetection-plus-localization sensitivityLocalization-only sensitivityFalse alarmsResponse bias

LEA

183130332324

REA

261418131623

NEA

430291

LEA

151499

2125

REA

283038381922

NEA

541181

LEA

76

17212121

REA

354131261824

NEA

610193

ATTENTION IN DICHOTIC LISTENING 409

to the attended ear (which constitutes a response bias), butan absent target, when reported erroneously, is as likely to beattributed to the unattended ear as to the attended ear.

Experiment 2

The principal purpose of Experiment 2 was to ascertainwhether the results of Experiment 1 were specific to CVnonsense syllables or whether they could be generalized toanother category of stimulus material. For this purpose wechose the Halwes Fused Dichotic Words Test (FDWT;Halwes, 1991), which consists of single pairs of carefullysynchronized CVC words that differ only in the first stopconsonant. The FDWT was chosen primarily for its similar-ity to the CV syllable task. FDWT words are distinguishablefrom each other only on the basis of the same phonemicfeatures that distinguish CV nonsense syllables. The FDWTyields a frequency of REA that is at least comparable withthat obtained with CV nonsense syllables (Grimshaw et al.,1994; Wexler & Halwes, 1983; Wexler et al., 1981; Zatorre,1989). As in the case of CV syllables, the competing wordsin the FDWT are so well synchronized that participants oftenare not aware that more than one word has been presented(Wexler & Halwes, 1983). In fact, Bryden (1988) hassuggested that shifts of attention would have no effect on earasymmetry for this material.

Despite the similarities between CV syllables and FDWTwords, the respective stimuli differ in meaningfulness aswell as in the absence or presence of a final consonant.Perhaps for these reasons, preliminary evidence indicatesthat ear differences obtained from one test are not highlycorrelated with ear differences obtained from the other test(Wexler & Halwes, 1985). Because relatively strong correla-tions between other dichotic tests that differ slightly fromeach other have been reported (Clark & Spreen, 1983), thelow correlation in Wexler and Halwes' experiment might beattributable to the small sample size (N = 15). Nonetheless,the low correlation provides a reason to suspect that theFWDT is not completely redundant with the CV test used inExperiment 1.

Method

Participants. A second sample of 48 undergraduate volunteers(24 women, 24 men) was obtained from the source described inExperiment 1. Participants met the same selection criteria as inExperiment 1. The age range was 17.1 to 33.0 years (M = 20.2,SD = 3.3). The mean laterality quotient from the Edinburghinventory was 81.3 (SD = 16.7).

Materials. The FDWT (Halwes, 1991) was obtained fromPrecision Neurometrics (New Haven, CT). The stimuli were 15pairs of rhyming CVC words with stop consonants in the initialpositions (e.g., pen-ten, goat-coat, boy-toy). The words consistedof natural speech tokens (male voice) that had been digitized andclosely matched with respect to acoustic properties (Wexler &Halwes, 1983). The members of a pair were matched for wordfrequency and were identical except for the initial consonant. Eachpair of words was presented twice in 30 trials, and the channel onwhich each member of a pair occurred was reversed between thefirst and second presentation of the pair. The intertrial interval wasapproximately 7 s. Three randomized sets of 30 trials were pre-sented twice, resulting in a total of 180 trials. A set of 30 binauralpractice trials was used to familiarize participants with the stimuli.

Apparatus. The equipment was identical to that used inExperiment 1. The average signal intensity for each channel was setto 75 dB (A), as measured using a steady-state calibration tone, andthe channels were balanced within 1 dB. The ambient noise levelwas 29 dB (A) in the sound-attenuated chamber used for testing.

Procedure. Except for the selection of targets, the procedurewas identical to that of Experiment 1. Targets in Experiment 2 wereselected so that each of the 30 words from the FDWT served twiceas a left-ear target and twice as a right-ear target across the entire180 trials. For the 60 trials on which the target was not present inthe dichotic pair, the target was selected so as to rhyme with themembers of the dichotic pair.

Scoring. Data were scored using the methods described inExperiment 1.

Results

Raw scores. Table 3 shows the mean proportion ofdetection hits, localization hits, localization errors, and falsealarms for each cue condition, ear, and attention condition.

Table 3Mean Proportion of Detection Hits, Localization Hits, Localization Errors, and False Alarms for Each Ear and AttentionCondition in Experiment 2

Left Divided Right

Left earCue

condition

Detection hitsPrecuePostcue

Localization hitsPrecuePostcue

Localization errorsPrecuePostcue

False alarmsPrecuePostcue

-l—iV-ll

M

0.800.83

0.460.47

0.340.36

0.180.18

I V^Ul

SD

0.130.16

0.160.14

0.120.13

0.140.14

Axigi:

M

0.880.87

0.460.42

0.420.45

0.130.14

11 WU.J

SD

0.090.13

0.140.12

0.160.12

0.090.11

M

0.830.84

0.410.40

0.420.44

0.170.22

SD

0.130.15

0.140.14

0.160.13

0.130.15

IVlgll

M

0.880.88

0.470.48

0.410.40

0.130.16

Ik IsMl

SD

0.100.12

0.140.13

0.110.10

0.110.14

M

0.780.80

0.350.37

0.430.43

0.160.20

SD

0.140.15

0.130.13

0.130.13

0.130.14

AV*t>"

M

0.860.85

0.540.52

0.320.34

0.160.12

SD

0.120.13

0.130.14

0.110.13

0.100.14

410 HISCOCK, INCH, AND KINSBOURNE

Detection sensitivity. The ANOVA for arcsine-trans-formed p(A) scores for detection yielded only a significantmain effect for ear, F(l, 46) = 15.12, p < .0005. Moreright-ear targets than left-ear targets were detected. Meanp(A) scores for each ear and attention condition are shown inthe upper left panel of Figure 2.

Detection-plus-localization sensitivity. The only statisti-cally significant main effect in the analysis of arcsine-transformed p(A) scores was the ear effect, F(l, 46) =16.25, p < .0005. This reflected an REA. A significant Ear XAttention interaction, F(2, 92) = 9.70, p < .0005, wasattributable entirely to the Ear X Left Versus Right Attentioncomponent, F(l, 46) = 19.51, p = .0001. With leftwardattention, very little asymmetry was observed; with right-ward attention, there was an REA whose magnitude ex-ceeded that of the REA in the divided attention condition.The results are shown in the upper right panel of Figure 2.

Localization-only sensitivity. Results were similar tothose obtained from the analysis of detection-plus-localiza-tion scores. Again, the ANOVA yielded only a significantmain effect for ear, F(l, 46) = 11.93, p = .001, and asignificant Ear X Attention interaction, F(2, 92) = 11.58,p < .0001. The interaction was attributable to the Ear X Leftversus Right Attention component, F(l, 46) = 23.28, p <.0001. These findings are illustrated in the lower left panel ofFigure 2.

Response bias. As shown in the lower right panel ofFigure 2, the mean (3' was smaller for the left ear than for theright ear across attention conditions. This indicated a more

lax criterion for the left ear. An ANOVA confirmed thestatistical significance of the ear difference, F(l, 46) =11.36, p < .005. The ear effect did not interact significantlywith attention condition, F(2, 92) = 1.02, p > .35. Thefalse-alarm data showed a comparable pattern, that is, ahigher false-alarm rate for the left ear than for the right earacross attention conditions, F(l, 46) = 7.44,p < .01.

Frequency data. Table 4 shows the number of partici-pants with an REA, LEA, or NEA for each of the dependentvariables. Only with respect to the three localization indicesdid the proportion of participants showing an REA varysignificantly across attention conditions (p < .01).

Discussion

The results of Experiment 2 resemble closely those ofExperiment 1. There was a significant REA for detectionsensitivity as well as for detection-plus-localization andlocalization-only sensitivity, and the attention instructionsaffected the REA for localization but not for detection. InExperiment 2, unlike Experiment 1, P' indicated a morelenient criterion for assigning targets to the left ear than tothe right ear. Nonetheless, as in Experiment 1, the criterionfor localizing targets did not change as a result of attentioninstructions.

General Discussion

In both experiments, selective attention enhanced targetlocalization without altering the detection of targets. The

Detection Detection Plus Localization

5? 2.4

J7 2.3

1 228 2.2H< 2.1

•*

*— — J— K-. "f— — ..

1 ~— — K

-"" Left Ear~*~ Right Ear

''

^.Hs*.l< 2.2aV.9 2-°S5 1.8

^^T_ -3^^"^

--,

-«" Left Ear *

, -^- Right EarT

Left Divided Right

Attention

Left Divided Right

Attention

2.4Localization Only

1.8-*-- Left Ear

-°- Right Ear

Left Divided Right

Attention

0.6

0.5

0.4

0.3

0.2

0.1

0

Response Bias

•«•- Left Ear

—°- Right Ear

Left Divided Right

Attention

Figure 2. Mean detection sensitivity, detection-plus-localization sensitivity, localization-onlysensitivity, and response bias for Halwes Fused Dichotic Words (Halwes, 1991) as a function of earand attention condition in Experiment 2. Error bars represent ± 1 SEM. Smaller values of p' indicatestronger response biases (more lax criteria).

ATTENTION IN DICHOTIC LISTENING 411

Table 4Number of Participants Showing a Left-Ear Advantage (LEA), Right-Ear Advantage(REA), and No Ear Advantage (NEA) for Different Measures of Performancein Experiment 2

Left Divided Right

Measure

Detection hits and detection sensitivityLocalization hitsDetection-plus-localization sensitivityLocalization-only sensitivityFalse alarmsResponse bias

LEA

112419242325

REA

282025231319

NEA

9441

124

LEA

101014162430

REA

292832311313

NEA

91021

115

LEA

1048

102229

REA

314040371916

NEA

740173

dissociation between detection and localization providessome insight into mechanisms underlying the REA and itsmodification by directed attention. The dissociation can beunderstood in terms of localization errors. Whereas detec-tion hits represent correctly identified targets irrespective ofwhether they are assigned to the correct ear, localization hitsare detection hits from which localization errors have beensubtracted. Because attending to the right ear, for instance,increases the number of correctly localized right-ear targetswithout increasing the number of correctly detected targetsfrom that ear, then participants must be misattributing fewerright-ear targets to the left ear when they attend to the rightthan when they attend to the left.

Attending to the right ear also reduced the number ofcorrectly localized left-ear targets without changing thenumber of correctly detected left-ear targets. This can occuronly if more left-ear targets are misattributed to the right ear.In other words, improved localization of targets at theattended ear reflects a greater number of targets attributed tothat ear irrespective of the ear at which the targets actuallyarrive.

In the absence of corresponding changes in either P' orthe false-alarm rate as attention is manipulated, the localiza-tion results cannot be ascribed to an overall shift in thecriterion used to make localization decisions. Instead, thebias in favor of the attended ear applies only to correctlydetected targets. This phenomenon might reflect differentialconfidence in the detection decision. Being more confidentin their (correct) detection of a target than their (incorrect)detection of a nontarget, participants might attribute thetarget to the attended ear while showing no bias in assigningthe nontarget to an ear. In other words, subjectively strongtargets (i.e., actual targets) usually are attributed to theattended ear, whereas subjectively weaker targets (i.e.,nontargets mistaken for targets) are attributed equally oftento the unattended ear. This explanation could be testedempirically if a confidence-rating procedure were substi-tuted for the binary decision task used in our study (Pastore& Scheirer, 1974).

Insofar as the present findings dissociate what and wheresystems, they resemble D. Deutsch's (1974) "octave illu-sion." This illusion occurs when a sequence of alternatinghigh and low tones is presented to each ear such that thesequence at one ear is out of phase with that at the other ear.Most participants report hearing a single alternating tone

that shifts from ear to ear in synchrony with the pitchchange. Right-handers usually attribute the higher tone tothe right ear and the lower tone to the left ear. If theperception of pitch change is considered to be correct, thenthe perceived localization is erroneous. Conversely, if theperceived spatial switching is taken to be correct, then theperceived alternation of pitches is incorrect. In either case,what and where judgments appear to depend on separatemechanisms.

Attention shifts in dichotic listening typically are thoughtto affect input, or perceptual, processes rather than output, orresponse selection, processes (Asbjornsen & Hugdahl, 1995;Bryden et al., 1983; Treisman & Geffen, 1967). On thecontrary, our data implicate response selection as the processaffected by selective-listening instructions. At least fordichotic CV and fused-word stimuli, selective attentioninstructions alter the ear to which targets are attributed whilehaving no effect on detection. This finding is pertinent toBryden et al.'s (1983) claim that individual differences in earasymmetry for CV syllables stem from differences in "howaccurately [participants] are able to attribute ear of entry to aparticular syllable" (p. 246). In opposition to that claim, ourdata indicate that apparent differences in the accuracy oflocalization actually reflect a particular kind of responsebias.

Even though Bryden et al. (1983) attributed the REA to aninability to localize left-ear signals as accurately as right-earsignals, they thought that the localization asymmetry wassecondary to a perceptual asymmetry—a difference betweenears in signal strength—rather than to an attentional bias.They based this conclusion on the apparent absence of anasymmetric response bias as reflected in false alarmsattributed to the left and right ears. We confirmed theabsence of laterally biased false alarms and strengthened theargument against asymmetric response bias by computingP'. However, it is difficult to understand how differentialleft- and right-ear signal strength could account for thelocalization data in our study. Greater signal strength at theright ear could underlie the overall REA for localization, butwhy should attending to the left ear increase the accuracy ofleft-ear localization while decreasing the accuracy of right-ear localization? This finding cannot be attributed to differ-ential signal strength unless signal strength varies withattention, in which case detection accuracy should also beaffected.

412 HISCOCK, INCH, AND KINSBOURNE

We propose a general explanation of selective-listeningeffects that involves two stages of processing, the first ofwhich is automatic and the second of which is controlled(slow, effortful, capacity limited, participant regulated; e.g.,Hasher & Zacks, 1979; Shiffrin & Schneider, 1977). Thefirst stage, which is reflected in detection accuracy, ischaracterized by a relatively fixed asymmetry favoring theright ear for linguistic stimuli. Whether the basis of thatasymmetry is perceptual or attentional cannot be specified,even after many years of experimentation and debate (cf.Bryden et al., 1983; Kinsbourne, 1975; Studdert-Kennedy,1975).

Mondor and Bryden (1991) have shown that the REA canbe counteracted by a warning signal in the left ear thatautomatically draws attention to the left side. Because thewarning signal must occur as much as 500 ms before theonset of the auditory target to be effective, it apparentlyaffects an early stage of information processing. Thus, thewarning signal (unlike selective attention instructions) mayaffect detection, but one cannot be sure of that in the absenceof information about the impact of the warning signal ondetection accuracy, localization accuracy, and response bias.

Attention shifts made in response to selective-listeninginstructions evidently represent a different phenomenon.Such shifts are volitional (i.e., controlled), they are effectiveirrespective of whether the target's identity is known beforetarget presentation, and they appear to affect late stages ofsignal processing (S. Wood, Hiscock, & Widrig, 1998). If thetarget is detected, it tends to be attributed to the ear towardwhich the participant has oriented. This directional bias issuperimposed on an intrinsic response bias that causes ahigher percentage of targets to be attributed to the right earthan to the left ear in the divided-attention condition.

The proposed model can account for the absence oftime-of-cuing effects in selective listening. For instance,Hiscock and Kinsbourne (1977), in an early selective-listening study, found that the magnitude of the REA in 3-,4-, and 5-year-old children remained constant irrespective ofwhether the attended ear was designated before or afterpresentation of the dichotic stimuli. In contrast, time ofcuing is critically important when attention shifts aresignalled via a tone (e.g., Mondor & Bryden, 1991). Theapparent contradiction can be resolved by postulating thatwarning signals and attention instructions activate differentprocesses. Warning signals, which bring about automaticshifts of attention, must be presented before stimulus onset.Selective attention instructions, which exert their influenceon late (response selection) stages of processing, remaineffective across a wide range of intervals between cue andsignal.

The selective-listening procedure has been proposed as ameans of enhancing the REA and minimizing error variancein dichotic-listening data (Bryden et al., 1983; Hiscock &Kinsbourne, 1980). Yet, after comparing left- and right-earscores from divided-attention and focused-attention condi-tions within seven different experiments, Mondor (1994)failed to find any consistent difference in the magnitude ofthe ear effect. Whereas Mondor and Bryden (1991) regardedverbal instructions as a generally ineffective way of altering

attention, the problem with verbal instructions may be thatthey have their effect at the wrong stage of processing. Theasymmetry that one presumably wants to measure is therelatively fixed input asymmetry, but our findings show thatselective attention instructions have no significant effect onthat asymmetry. Nor did we find that error variance isreduced in the selective attention conditions.

In a conventional selective-listening experiment, theparticipant is told to report what he or she hears at thespecified ear. For CV or FDWT stimuli, there are twopossible targets to be identified on any trial. Consideringhypothetical boundary conditions will illustrate why selec-tive attention seems to have little or no effect on overall earasymmetry. In the first of the two boundary conditions, thedichotic stimuli fuse completely and appear to originate inthe center of the head. The participant hears one target andmust attribute it to one ear despite the lack of localizing cues.On the basis of our findings, it appears that the participantusually will attribute the stimulus to the attended ear. Thisattributional bias will have no effect on ear asymmetry,which depends entirely on which stimulus becomes domi-nant during the initial stage of processing. The bias willincrease both the number of correct responses from theattended ear and the number of intrusion errors from theopposite ear. This presumably is the reason that even adultsfind it difficult to listen selectively for CV stimuli at one ear(e.g., Bryden etal., 1983).

The contrasting boundary condition is represented bystimuli that remain separate and can be assigned readily tothe respective ears of entry. Hearing two signals andknowing the source of each, the participant reports the signalfrom the attended ear and does not report the signal from theunattended ear. These circumstances would, of course, leadto the strongest possible selective attention effect, that is,perfect performance from the attended ear and no intrusionsfrom the unattended ear. There would be no overall asymme-try, however, because the left-ear score during attention tothe left ear would offset the right-ear score during attentionto the right ear. This boundary condition may be approxi-mated when adults are asked to attend selectively to lists ofdigits at one ear (Freides, 1977).

These boundary conditions imply that stable auditorylaterality is most likely to be observed when there are nosalient timing and intensity differences between competingstimuli that would allow controlled (second stage) process-ing to alter the results. The stable REA depends on somerelevant difference between the stimuli, which is processedautomatically, rather than on cues associated independentlywith one or the other stimulus, which may be used forselective reporting. The degree to which an actual selective-listening experiment approaches one of the two boundaryconditions depends primarily on the degree to which thepaired stimuli fuse so that their distinctiveness and localiz-ability are diminished. However, even when the stimuli failto fuse completely, the magnitude of the selective-listeningREA may not differ reliably from that of the free-report REAbecause the asymmetric effect of controlled (second stage)processing is nullified when attention is switched to theopposite ear. Any significant change in ear asymmetry from

ATTENTION IN DICHOTIC LISTENING 413

free-report to selective-listening conditions is more likely tostem from the differential memory load inherent in therespective tasks or from associated variables such as order ofreport and output interference (Bryden, 1962, 1978, 1982).

This study illustrates the potential of signal-detectionmethods to decompose participants' performance, which istypically represented as correct responses and intrusionerrors, into more specific components such as detectionsensitivity, localization sensitivity, and response bias. Con-tinued application of signal-detection theory to dichoticlistening may help to resolve some of the persistentlyintractable issues in the literature (e.g., Studdert-Kennedy,1975).

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Received June 11, 1998Revision received October 5, 1998

Accepted October 26, 1998