Toward a model of dichotic listening performance

BRAIN AND COGNITION 4, 241-257 (1985)

Toward a Model of Dichotic Listening Performance

M. P. BRYDEN AND JANICE E. MURRAY

University of Waterloo

Dichotic listening performance for different classes of speech sounds was examined under conditions of controlled attention. Consideration of the complex of target item and competing item demonstrated that, in general, targets were more accurately identified when the competing item shared no relevant features with it and less accurately identified when the competing item shared place, voice, or manner with the target item. Nasals as well as stops demonstrated a significant right-ear advantage (REA). False alarm rates were very similar for left and right attentional conditions, whereas intrusions from the right ear while attending to the left were far more common than intrusions from the left while attending to the right. Attention is viewed as serving to select the stimuli that will be reported, but at a late stage, and only after the right ear perceptual advantage has had its effect. A model of dichotic listening performance is proposed in which both the ease of localizing the item and the strength of evidence for the presence of the item are relevant factors. @ 1985 Academic PESS. I~IC.

Although it has been known for 20 years that normal right-handed subjects manifest a right-ear advantage (REA) for dichotically presented speech material (Kimura, 1961), the mechanisms which lead to this REA are not yet well understood. Probably the most generally accepted model is Kimura’s (1967) direct-access model, which holds that input to the right ear has a more direct access to the left hemisphere, through the contralateral projection system, than does input to the left ear. Because of the stronger representation of right ear input at the left auditory cortex, nearby left-hemispheric speech mechanisms are better able to process this information to the level of recognition or identification. In Kimura’s particular model, simultaneous activation of both left- and right-ear path-

This research was supported by a grant from the Natural Sciences and Engineering Research Council of Canada to M.P.B. The data from this experiment and an interpretation of them were presented at the Canadian Psychological Association meetings in Toronto, Ontario (Murray, Brown, Saxby, Tapley, & Bryden, 1981). The authors acknowledge the assistance of Marion Tapley in preparing the dichotic tapes and of Patrick Brown and Lorie Saxby in testing the subjects and scoring the data.

241

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242 BRYDEN AND MURRAY

ways also produces an occlusion or inhibition of the ascending ipsilateral pathways, thus accentuating the difference between left- and right-ear inputs and accounting for the fact that the dichotic REA is usually found to be more robust than corresponding monaural REAs (Bryden, 1969).

As one alternative, Kinsbourne (1973, 1975) has argued for a right-ear attentional bias in dichotic experiments. He suggests that in any verbal dichotic experiment, the nature of the task serves to prime or activate the left (speech) hemisphere, thus making the subject more receptive to stimuli from the right ear. However, a number of experiments have shown that verbal REAs and nonverbal left-ear advantages (LEAS) can be obtained simultaneously (Goodglass & Calderon, 1977; Ley & Bryden, 1982). Such observations would seem to rule out an attentional bias as the sole determinant of the REA. Nevertheless, attentional factors may well contribute to the magnitude of the REA observed in any specific experiment (cf. Bryden, 1978; Bryden, Munhall, & Allard, 1983; MacKain, Studdert- Kennedy, Speiker, & Stern, 1983).

In addition, several authors (e.g., Teng, 1981; Sidtis, 1982; Gregory, Efron, Divenyi, & Yund, 1983) have suggested that REAs can be profoundly influenced by subcortical asymmetries. To the extent that the ascending auditory pathways are not bilaterally symmetric, asymmetries may be introduced that are not stimulus-specific, but will affect the magnitude of the observed REA.

Finally, the size of the REA varies with the type of stimulus material employed. Early dichotic experiments (e.g., Kimura, 1961, 1967) used words and numbers as stimulus material and found large REAs. Later, it became popular to use simple consonant-vowel (CV) syllables, such as /ba/ and /da/, in order to control timing and acoustic parameters more easily (Studdert-Kennedy & Shankweiler, 1970). Such experiments have demonstrated that there are large REAs with stimulus sets varying only in the initial stop consonant, but that there is only a minimal REA with vowels (Spellacy & Blumstein, 1970; Darwin, 1971a) or fricatives (Darwin, 1971b). Hayden, Kirstein, and Singh (1979) have reported fairly large REAs for both stops and nasals, much smaller ones for affricates and liquids, and minimal REAs for continuants. Such observations have led some to suggest that the rapid onset of stop consonants is a necessary condition for triggering specialized left-hemispheric recognition systems (Dwyer, Blumstein, & Ryalls, 1982).

The present study was stimulated by the observation of Hayden et al. (1979) that a significant REA can be obtained for classes of speech sounds other than stops. Further, it appeared that a consideration of the complex of sounds in a dichotic pairing might better provide the necessary framework for interpretation of the REA. In the Hayden et al. study, 21 different consonants were paired with one another in a constant vowel environment, but any one subject heard each dichotic pairing only once, bringing the

DICHOTIC PERFORMANCE 243

reliability of the data into question and making it impossible to assess individual differences. By carefully exploring the nature of the REA for various classes of speech sounds considering the complex of target and competing item and by collecting enough data to make it feasible to investigate individual differences, it was hoped to provide the basis for a general model of dichotic listening performance.

METHOD

Subjects. Sixteen undergraduate students, eight females and eight males, volunteered to serve as subjects. All subjects were native speakers of English and were right-handed as determined by writing hand preference.

The subjects attended two experimental sessions of approximately I hr duration. Stimulus materials. The stimulus sets, each containing four CV syllables, included two

stop sets (/bo, do, po, to/ and /do, go, to, ko/), one fricative set (/00, fo, vo, hoi), one mixed nasal-stop set (/mo, no, bo, do/), and one mixed nasal-fricative set (/mo, no, vo, a/). Each natural speech sound was produced by a female native speaker of English and recorded on a Revox tape recorder (Model A 700). Stimulus intensity was equalized in recording by peak Vu meter monitoring. Stimuli were digitized on a PDPlU40 computer and edited to 300 msec duration. Following digitizing and editing, each stimulus was dichotically paired with competing stimuli from its own set. The syllables in each dichotic pair were aligned for simultaneous consonantal onset and recorded on separate channels of the audio tape.

The original selection of stimulus sets was guided by the distinctive feature classifications presented by Chomsky and Halle (1968), Massaro (1975), and Goldstein (1980). In each set, one pair of consonants was articulated at one locus, and a second pair at a different locus. Within each pair, the two consonants differed either in voicing or in manner of articulation (stop, nasal, or fricative). Thus, in pairing the consonants for dichotic presentation, the items of a pair differed in place of articulation in two pairings, in manner or voicing in two pairings, and in both place and manner or voicing in two pairings. The major exception to this was in Set III, where in/ and ldi are not actually articulated at the same locus, although Goldstein (1980) classifies both as front-center consonants. The selection of /a/ and /8/ for Set II was dictated largely by the authors’ curiosity about this distinction, and was perhaps unwise, for our subjects had great difficulty in making the discrimination. Further, had we used a purely articulatory distinctive feature set (e.g., Massaro, 1975), we would not have chosen the dentals /@I and Id/ but the alveolars is/ and izl as members of our fricative set. We suspect that our selection contributes to the low accuracy on Set II (see Allard & Scott, 1975), but does not alter any of the basic points we wish to make.

For all five stimulus sets, each of the four stimulus items was identified as the target for two blocks of 60 trials, resulting in eight blocks of trials per stimulus set. With each target, one block was presented with instructions to attend to the left ear and one with instructions to attend to the right ear. Each block of trials was randomized separately. Thus, each resulting tape, using one set of stimuli, consisted of 480 trials, or 40 instances of each of the 12 possible pairings. There was a 3-set intertrial interval which permitted subjects to record their response manually

The stimuli were presented using a two-channel Sony tape recorder (Model 540) and Koss headphones (Model PR04AAA). A constant presentation level of 72 2 2db was maintained across subjects. Calibrations of the recorder and headphones were checked prior to each experimental session to ensure balance between the ears.

Procedure. In an ear monitoring procedure, subjects were asked to attend to one specified ear and to ignore items on the other channel. In addition, subjects were instructed to judge whether a designated target was present or not. Prior to the beginning of each 60-trial


TABLE 1 PERCENTAGE OF DIFFERENT RESPONSES IN EACH CONDITION

Attend to left ear Attend to right ear Signif- icant EAs

Set Hits Intrusions F.A. Hits Intrusions F.A. A L R

I stops 77.11 17.60 0.73 90.11 1.60 1.57 1.96* 1 6 v stops 67.81 24.27 2.55 82.60 15.00 1.98 1.40* 2 8 IV Nasal-stops 78.86 17.60 1.30 88.02 10.94 0.89 1.46* 0 5 III Nasal-fricatives 46.77 31.88 6.41 61.36 25.42 6.10 0.90* 0 6 II Fricatives 50.10 33.64 13.23 56.04 26.25 13.02 0.36 0 4

Total 64.25 25.00 4.84 75.63 17.04 4.71 0.83* 1 9

* p < .05.

block, subjects were informed of the target and ear to be attended to. After hearing each dichotic pair, subjects were required to circle “yes” on a response sheet when the target was on the designated channel and to circle “no” when the target was on the unattended channel or there was no target present.

Each subject heard each of the five test tapes once. Over the two sessions subjects completed 120 practice trials and 2400 test trials. Order of stimulus set, target stimulus and attended ear was counterbalanced across subjects.

RESULTS

Table 1 shows the overall results of this study. In this table, the errors have been divided into two categories: those errors which can be termed intrusions, where the target actually appeared in the stimulus, but on the unattended ear; andfalse alarms, trials on which the subject indicated that the target was present, but on which it did not appear at all. As Table 1 shows, the majority of errors were intrusions rather than false alarms. For an initial analysis of laterality effects, the scores were expressed in terms of the laterality index, h proposed by Bryden and Sprott (1981). This index was developed to provide a measure of laterality independent of overall accuracy, and to provide a means of testing data of an individual subject for statistical significance. For the present data, A corresponds to the difference between the detection rate when the subject is attending to the right ear:

In (Hits R) (Correct Rejections R)/(Misses R) (False Positives R) and the corresponding detection rate when the subject is attending to the left ear, where

False Positives R(or L) = Intrusions R(or L) + False Alarms R(or L), and thus A is

In (Hits R) (Correct Rejections R) (Misses L) (False Positives L)/(Misses R) (False Positives R) (Hits L) (Correct Rejections L).

Positive values of X indicate an REA and negative values an LEA.


Using the h index, a significant REA is seen for all stimulus sets except the one containing only fricatives (Set II). However, analysis of variance of the A values reveals no significant effects of set or sex, nor any interaction between these two variables. Thus, although the right ear advantage for fricatives is not significant, it is not significantly smaller than those obtained with the other sets.

Calculation of the intercorrelations between X values for speech targets in different conditions provided measures of reliability of the ear advantages. First, several sounds appear in more than one set. Although the context in which the targets appear varies from set to set, similar laterality effects should be observed for the same target on different occasions. The correlations, presented in Table 2, between h values for targets from different sets provide strong evidence for internal consistency in the laterality effects obtained for all sounds except the fricatives. Second, the overall correlation between A values for Set I stops and Set V stops provides a good measure of consistency of ear advantages over time. The two stop sets, which were presented separately on different days, are highly correlated, r .75 [p(14) < .Ol].

The standard error of X can be calculated as the square root of the sum of the reciprocals of the eight individual entries (Bryden & Sprott, 1981). If the individual h values are tested for significance, with p < .05 as a criterion, 28 of the 64 tests on the sets showing an ear advantage reach significance, with 23 of these being REAs and only 5 being LEAS. Only 4 of the 16 individuals show significant ear effects for the fricatives, with all 4 exhibiting an REA. When the data are combined across all five sets, 9 subjects show significant REAs and 1 shows significant LEA.

Within any one set, the data can be broken down by the specific targets employed or by the particular type of contrast present on any one trial. Examination of the data indicates that it is the nature of the dichotic pair, rather than the specific target, that is the critical determinant of

TABLE 2 INTERCORRELATIONS BETWEEN A VALUES FOR

SPEECH TARGETS IN DIFFERENT CONDITIONS -

Target Sets r

/dOI

/W /to/

ho/ /noI /f%/ /d

I/IV .59

I/V .59 IV/V s9

I/IV .52

I/V .66 III/IV .39 III/IV .67 II/III - .38 II/III - .28


performance. On any given trial, the target and the competing item may differ only in place of articulation [as in /po/-/to/ (Set I) or /mo/-/no/ (Sets III and IV)], only in voice [as in /bo/-/PO/ (Set I)], or only in manner [as in /mo/-/vo/ (Set III)] (single-contrast trials), or in both place and in voice or manner [as with ho/-/to/ (Set I) or /vo/-/no/ (Set III)] (double-contrast trials). Analysis of variance of the A values for the individual contrast types indicates a main effect of type (F(2,28) = 7.08, p < .005), and a sex x type interaction (F(2, 28) = 3.51, p < .05), with no significant set effect nor any interactions with set. Contrasts of the means indicate that the h’s for the single-contrast trials are larger than the h’s for the double-contrast trials. The Bonferroni test indicates that the overall sex x type interaction is attributable to the fact that females show larger h’s than males on the place contrast only. The A values for the various types of contrast are shown for each set in Table 3.

Further examination of Table 1 suggests that most of the asymmetry arises from the intrusions rather than from the false alarms. The false alarm rates are very similar for left and right ear attentional conditions, but intrusions from the right ear while attending to the left ear are far more common than intrusions from the left ear while attending to the right. In fact, the false alarm rates for Sets I, IV, and V are very low, less than 2%. To analyze the false alarms, the false alarms for Sets I, IV, and V were combined and compared with those in Set II and in Set III. Analysis of variance revealed significant effects of sets [F(2, 28) = 27.91, p < .OOl], type of contrast [F(2, 28) = 56.67, p < .OOll, and an interaction of sets x contrast [F(4, 56) = 18.80, p < .OOl], with no main effect of sex of subject nor ear, and no interactions involving either of these variables. In general, far more false alarms occurred with the fricative set (II, 13.13%) than with any other set, with the combined nasal-fricative set (III) yielding a moderate level of false alarms (6.25%), and the other sets giving very low levels. Virtually all the false alarms occurred on trials on which one of the target features appeared on one

TABLE 3 A VALUES FOR VARIOUS CONTRASTS

Features differing

Set Voice/manner Place Both

I 2.29 1.61 0.94 v 1.25 2.44 1 .os IV 1.10 1.65 1.00 III 1.34 1.07 0.61 II -0.70 -0.50 0.12

Average 1.35 1.46 0.75


channel and the other appeared on the opposite channel. Thus, if the target was /bo/, false alarms would be made to the /do/-/PO/ stimulus complex, but not to /do/-/to/, where the correct place of articulation feature appears in neither item, nor to /PO/-/to/, where the correct voicing feature does not occur. The significant set x contrast type interaction results largely from the fact that the error rate was fairly high for all contrasts in Set II. The critical point, however, is that there were no interactions involving ear: the false alarm rate for the left ear is not significantly different from that for the right ear. This means that the asymmetries in the data can be attributed to asymmetries in the hit and intrusion rates.

Table 4 shows hit and intrusion rates for contrasts between different types of consonants, averaged across all five sets. In general, this table indicates that intrusions from the right ear to the left are far more common than intrusions from the left ear to the right. While these data are not amenable to an analysis in terms of A, some estimate of the relative REA for different contrasts can be made using the laterality index suggested by Marshall, Caplan, and Holmes (1975) to compare relative hit rates when attending to the two ears. Values of this index are given in the last column of Table 4. These values indicate that both stops and nasals give fairly robust REAs no matter what type of material they are paired with, while fricatives give small REAs.

The preceding paragraphs suggest that it is necessary to consider the complex of target item and competing item as the stimulus, rather than focusing on the target alone, to understand performance on this particular dichotic listening task. The accuracy with which a particular target is detected varies as a function of the competing item, and the likelihood of a false alarm varies as a function of the stimulus complex. In general, targets are most accurately identified when the competing item shares no relevant features with it, and somewhat less accurately identified when

TABLE 4 HITS AND INTRUSIONS FOR COMBINATIONS OF DIFFERENT TYPES OF CONSONANT

Target

stops

L R No. of

Competition examples H I %H H I %H REA

stops 26 59.58 15.85 79 69.04 8.54 89 31 Nasals 4 67.25 11.50 85 70.00 7.00 91 25

Nasals stops 4 59.75 12.75 82 71.25 10.25 87 16 Nasals 4 48.25 29.25 62 65.25 19.50 77 26 Fricatives 4 34.75 18.75 65 48.50 13.25 79 25

Fricatives Nasals 4 33.75 24.25 58 42.25 21.25 67 12 Fricatives 14 40.57 27.64 59 45.57 22.21 67 11


the competing item shares place, voice, or manner with the target item. Conversely, false positives are greatest when the target item appears on the unattended channel and the attended item also possesses one of the target features, but are rather less likely if the attended item does not possess one of the target features. When the target item is not physically present at all, false alarms are moderately high when both target features are available in the complex, but negligible when one of the target features is missing.

In order to examine this phenomenon more completely, the data for each of the five sets were divided into six categories, depending on the nature of the particular stimulus complex present on each trial. In this analysis, trials of the type TO are ones in which the target is on the attended ear, and the item on the unattended ear shares no relevant features with it. Type OT trials are the reverse of this, where the target appears on the unattended ear. Trials labeled Tl are those on which the target appears on the attended ear, and the competing item shares a relevant place, voicing, or manner feature with it. As before type IT refers to the reverse pattern, with the target on the unattended ear. Trials of the type 2F are ones in which the target does not actually occur, but where the two distinguishing features are both present, one at each ear. Finally, type 1F refers to trials on which one of the relevant features does not appear at all in the stimulus complex. The proportion of trials on which a “yes” response was made for each of these categories is shown for each set in Table 5.

The pattern of results shown in Table 5 suggests that two factors are important in determining the decision that a particular target is present on the attended channel. One is the strength of the evidence for the target being present at all, while the other is the evidence that the target is localized to the correct or attended ear. The pattern of results suggests that a “yes” response is made if there is reasonable evidence that the target is both present and correctly localized, but that strong evidence

TABLE 5 PERCENTAGE “YES” REWJNSES TO DIFFERENT TARGET TYPES

I V IV III Stimulus

type L R L R L R L R

TO 79.06 89.06 75.63 83.13 82.18 88.75 43.13 56.25 Tl 77.03 90.63 63.75 82.34 77.19 87.66 48.59 63.91 1T 22.19 8.28 31.25 20.16 19.69 12.81 33.28 28.59 OT 8.44 6.25 10.31 4.69 13.44 7.19 25.94 22.19

II

L R

39.68 43.44 55.47 62.03 39.60 32.81 21.56 13.13

2F 1.09 3.28 5.63 5.16 2.19 1.25 13.59 14.38 24.06 26.72 1F 0.31 0.70 1.02 0.47 0.86 0.70 2.81 1.95 7.89 6.17


that the target is present can overcome weak evidence that it is correctly localized and also lead to a positive response.

A Model

In order to develop a model of dichotic performance, let us consider how evidence for target presence and evidence for target location may differ for the six stimulus complexes of Table 5. In terms of evidence for the presence of the target, the strongest evidence occurs in conditions Tl and IT, since one of the target features is enhanced by feature sharing with the competing item. Somewhat less evidence for target presence is available in TO and OT, since there is no feature sharing. 2F also produces reasonable evidence that the target is present, since both features are present, but because they are on opposite ears, this evidence may be less than that in TO and OT trials. Finally, there is relatively little evidence that the target is present on 1F trials.

In terms of localization, the target is most clearly localized to the attended ear on TO trials, and least localized to the attended ear (best localized to the unattended ear) on OT trials. The feature sharing on Tl and IT trials not only enhances the strength of the evidence for the presence of the target, but because it makes the location of one of the relevant features ambiguous, it also diminishes the evidence for localization to the correct ear. Finally, 2F and IF trials are localized neither to the correct nor to the incorrect ear. If subjects based their decisions solely on the evidence for localization to the attended ear, one would not expect the high false alarm rate on OT and 1T trials, and TO trials should be more accurate than OT trials. On the other hand, if evidence for the presence of the target were the sole criterion, positive responses should be most common to Tl and 1T trials, and attentional instructions should accomplish nothing. Instead, the data pattern suggests that subjects use both sources of evidence in reaching their decision.

Within this framework, how can one account for lateral asymmetries? To do so, one must either follow Kimura (1967) and postulate that right- ear information is more strongly represented than left-ear information, or follow Kinsbourne (1975) and argue that different criteria are used for deciding that a target is on the left ear and that a target is on the right ear. However, if subjects adopted a more lenient criterion for deciding that an item is present when attending to right ear, thus detecting more targets, they should also show an increased intrusion rate from the left ear. The effect of attention can therefore be examined by looking at intrusions in a rather different way than we did earlier. A specific input to the right ear can be attended to (attend right) or not (attend left), and, similarly, a specific input to the left ear can be attended to or not. If one adopts a more lenient criterion for deciding that an input is on the right ear than that it is on the left ear, then there should be a greater


discrepancy between attended and unattended inputs on the right ear than on the left. This is not the case. Rather, the effect of attention on left-ear items (detection rate when attending versus that when not attending) is about the same as that on right-ear items (see Table 6). This pattern indicates that attention to one ear does not serve to alter the selection criterion, but rather is a later process, dictating whether or not a positive response is to be made. Such a result would seem to provide support for the contention that right-ear items are more strongly represented either because of better access, as envisioned by Kimura (1967) or because there is an initial attentional primary bias, as suggesed by Kins- bourne (1970). Figure 1 provides a general representation of this model, in which trials of particular types are represented as having a reasonable degree of uncertainty with respect to both strength of evidence and localization. The strength of evidence centroids for right-ear items have been drawn slightly above those for left-ear items to indicate the input difference between left- and right-ear items, and the two criterion lines have been drawn symmetrically. Since the figure is only a first approx- imation, the criterion lines do not divide the trial types into the proper segments, although the general argument should be clear.

What about the fricatives, where a significant overall REA was not observed? While it is possible that a right-ear input asymmetry occurs only for certain classes of sounds (cf. Dwyer et al., 1982), this is not particularly parsimonious and alternative explanations are possible. Recall that fricatives show an REA that is not significantly different from the ear advantages obtained for stops and nasals although it is not of a sufficient magnitude to reach significance. This observation suggests that there really is an input difference between left- and right-ear fricatives, but that an overall REA is not observed with fricatives because most of the pairings are so poorly localized. Perhaps fricative pairings, containing as they do fairly long bursts of broadband noise, show subcortical acoustic fusion of the sort suggested by Cutting (1976), and this fusion makes localization more difficult. Given this view, fricative targets are difficult to discriminate and unlikely to show an ear advantage. In terms of the model, subjects continue to use evidence for presence and evidence for localization jointly in reaching a decision as to whether a target is present

TABLE 6 PERCENTAGE “YES” RESPONSES ON EACH EAR

AS A FUNCTION OF ATTENTION

Attended Unattended Difference

Left ear Right ear

64.2s 75.63 17.04 25.00 47.21 50.63


Localization

FIG. 1. Diagrammatic representation of the model of dichotic performance. Vertical dimension represents strength of evidence for target, while horizontai dimension represents target localization. Each stimulus complex is conceived as falling in a fuzzy area represented by a circle with a dot at its center. TO (OT) indicates complexes in which target is on L(R) ear and the competing item shares no features with it; Tl(lT) indicate complexes in which target is on L(R) ear and the competing item shares one feature with it. 1 and 2 indicate complexes in which the target is not present, but either one or both features are available. The solid and dotted lines represent criterion lines above which one judges the target as present on the left or right ear. Further explanation is given in the text.

or not, but now the evidence along the localization dimension is much weaker.

The basic idea of Fig. 1 can be seen if we plot the joint probability of left- and right-ear responses for each combination, as is done for each set in Fig. 2. By doing this, we have simply rotated Fig. 1 by 45”, with the “strength” dimension now corresponding to the main diagonal and the “localization” dimension being orthogonal to it.

As localization evidence becomes weaker, the relative weight given to strength increases, and Tl and IT become more difficult to discriminate, as do TO and OT. At the same time, the difference between Tl and TO, and between 1T and OT, increases. The pattern of results obtained with Sets I, IV, and V suggests a roughly equal weighting of strength and


100 Set I

75 0

25 50 75 100

Right Ear

100 Set V

75 .

50

liLc!L

25 .

A 0

25 50 75 100

25 50 75 100

A OT, TO . lT, Tl -1 l 2

BYes

FIG. 2. Joint probability of left- and right-ear responses for each stimulus complex in each set. The main diagonal represents the strength of evidence dimension, with the localization dimension orthogonal to it.

localization evidence. For Set III, the nasal-fricative combination, relatively more weight seems to be placed on strength, and the false positive rates to 1T and OT increase. For the fricative set (Set II), the relative weight on strength is even greater, and the false positive rate increases even further.

Repp (1978) has emphasized the fact that, in some dichotic pairings, one syllable is dominant over the other, in the sense that it is heard regardless of the ear to which it is presented. To see whether dominance effects existed in our data, and how they related to other facets of the data, we examined the data for each of the 30 different stimulus pairings employed. Each stimulus pair (e.g., /bo/-/do/) contributed eight scores to this analysis: the number of hits and intrusions for each target while attending to each ear. Dominance was measured by summing the “yes” responses to each of the two targets separately, and computing the ratio. The overall ear effect was calculated by determining the ratio of “yes” responses to right-ear items to those to left-ear items. An attention effect was measured by the ratio of attended responses to intrusions. All ratios were then subjected to a natural log transformation. Although dominance


effects do exist (there were six times as many “yes” responses to /fo/ as to /zTo/ in the /fo/-/ao/ pairing, for example), they are almost entirely confined to the fricative pairings, and they are uncorrelated with the REA [r (28) = - .13, n.s.1. Dominance effects do, however, correlate with the attentional effects [Y (28) = - .48, p < .Ol]: when a stimulus pair shows a strong dominance effect, it is more difficult to attend suc- cessfully. In terms of the model developed in Fig. 1, when dominance effects occur, stimuli are not clearly differentiated along the localization axis.

Individual Differences

One of the advantages of the Bryden-Sprott statistic (A) is that it permits us to test the data of each individual subject for statistical significance, The results of such tests have been shown in Table 1: On any given set, fewer than half of the subjects show significant ear advantages, but almost all the significant effects favor the right ear. If we combine the data across all five sets, 9 subjects show significant REAs and 1 shows a significant LEA, and 12 show positive average h’s and 3 show negative averages. While this is not as strong an effect as might be expected from the clinical literature on the incidence of left-hemispheric speech representation in right-handers (Segalowitz & Bryden, 1983), it is in general agreement with most other studies of dichotic listening performance. While few researchers report significance tests on individual subjects, Wexler, Halwes, and Heninger (1981) found 14 REAs and 1 LEA in 31 subjects, and Speakes, Niccum, and Carney (1982) reported 12 REAs and 3 LEAS among 24 subjects. Thus, even with extensive testing, only about half the population shows a significant REA. Of those who show significant ear advantages in these studies, though, about 87% exhibit REAs rather than LEAS; this is not a substantially higher figure than the 75% observed in the present study.

The present study, and that of Speakes et al. (1982) employed many trials, and thus it is unlikely that the failure to observe an REA results from an insufficient number of observations. It seems to be a real phenomenon that some subjects show a strong REA while others do not. There are several possible explanations for this. One might argue, for example, that subcortical asymmetries in the ascending auditory pathways introduce enough variance to obscure right-ear effects in some subjects (cf. Sidtis, 1982). Alternatively, it may be that the left hemisphere is not as completely dominant for the reception of linguistic input as it is for the production of language, and that the present figures are a fair estimate of the distribution of receptive processes.

Even with only 16 subjects, the intercorrelations between the A values for the various sets are fairly high (Table 7). It is clear that the two stop sets are the most closely related, but the generally high correlations


TABLE 7 INTERCORRELATIONS BETWEEN A VALUES FOR DIFFERENT SETS

SET

I II III IV

* p < .05.

II III IV V

,387 .549* .571* .754* ,428 .574* .454*

,424 .407 ,480

suggest that the dichotic task measures some common property in all subjects, regardless of the type of material employed. These correlations might be taken as evidence for an effect of subcortical asymmetries. For example, the three subjects with the largest A values on the fricative set (Set II), all have even larger A values on the other four sets: These people might be considered to have a cortical right-ear processing effect su- perimposed upon a subcortical right-ear advantage that was material independent. However, the two subjects with the largest negative mean A have very small effects with the fricatives: They might be taken to be examples of right-hemispheric speech processing.

Finally, one should mention the lack of sex differences in the present study. Although it is often reported that men are more lateralized than women (Bryden, 1979; McGlone, 1980) the only significant sex differences in this study appeared with Set I, and they favored women rather than men. Overall, A values tended to be larger for the women, with six showing significant REAs, one a nonsignificant REA, and one a nonsignificant LEA. Of the men, four showed significant REAs and two LEAS, with the remainder showing nonsignificant REAs. Bryden (1980) has previously noted that sex differences in dichotic listening seem to disappear under conditions of controlled attention.

DISCUSSION

Experimentally, this study has demonstrated that a REA can be obtained in dichotic listening not only with stop consonants, but also with nasals. Under conditions in which the target and competing stimuli differ in both place of articulation and voicing, a significant REA can also be observed with fricatives. These findings are consistent with the observations of Hayden et al. (1979), who also found fairly strong REAs for nasals. The observation that REAs can be obtained with other than stop consonants would suggest that the very rapid onset of stop consonants is not the critical determinant of the REA (Dwyer et al., 1982).

A model of dichotic listening performance has been proposed in which both the ease of localizing the item and the strength of the evidence for the presence of the item are seen as relevant factors. This model is able


to encompass both the data of the present experiment and the results of studies in which subjects are permitted to divide their attention between the two ears. Furthermore, the model suggests further experiments. It predicts, for example, that manipulations that increase the ease with which stimuli are localized will increase the REA. It also predicts that REAs should be observable even with vowels, if our measuring instruments are sensitive enough. For example, presenting /ae/ to the right ear and /i/ to the left ear should lead to a resulting fused percept that is closer to /ae/ than to /i/, while reversing the presentation should lead to a percept closer to /i/. Since the opposite effect occurs with tonal signals in that the fused percept is judged more similar to the left-ear input (Gregory et al., 1983; Yund 8z Efron, 1975, 1976, 1977), such a result would provide evidence for a special speech mode of perception.

In these experiments, we have been unable to rule out the possibility of major subcortical asymmetries. Although Levine, Isenberg, and McGaffigan (1983) found no correlation between dichotic REAs and brainstem-evoked potential asymmetries, this does not necessarily mean that the REA is not affected by any subcortical asymmetry. Conversely, the fact that some subjects seem to have shifted along the left-right continuum to a point where they show strong REAs for all types of material does not require that one assume subcortical asymmetry. To examine the possible effect of subcortical asymmetries, we should test left-handed subjects on both speech and nonspeech tasks varying in the magnitude of the ear effect found in right-handers. If we found that the ear effects consistently followed the same rank order or its reverse, then we might have some reason to accept the notion of subcortical asymmetry and use some neutral task as an anchor or zero point.

Finally, the present data provide evidence that attentional biases are not the sole determinant of dichotic laterality effects. Directed attention serves to select the stimuli that will be reported, but at a late stage and only after an initial right-ear perceptual advantage has had its effect.

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Toward a model of dichotic listening performance