Scandinavian Journal of Psychology, 1987, 7 8 . 2 6 3 4

Dichotic listening performance to CVs between versus within groups

KENNETH HUGDAHL’ and MlKAEL FRANZON’ ‘University of Bergen und 2Uppsala University

Hugdahl, K. & Franzon, M.: Dichotic listening performance to CVs between versus within groups. Scandinavian Jourtial of Psychology, 1987,ZS, 26-34.

The present study is concerned with comparing group mean data between groups with individual performance within groups in dichotic listening (DL). It is argued that only comparing mean ear performance in dichotic listening between groups may obscure important individual differences within groups in DL performance. In the present study we compared between group vs. within group performance in four groups of subjects (male and female, right- and left-handers). The stimuli were 66 trials of consonant-vowel combi- nations with the six stop-consonants and the vowels a and u . The results showed a group mean right ear advantage for consonants in the two right-handed groups, and a left-ear advantage in the two let-handed groups. The analysis of the individual distributions within each group, however, showed clear differences between the male and female groups. Generally, though, only the male right-handed group revealed enough homogeneity within the group to warrant the conclusion that the group mean REA reflects a left hemisphere specialization for speech perception. K . Hirgdahl. Deportment of Somatic Psychology, Universitj of Bergen, Arstadveien 21, N-5000, Bergen. Norway.

The dichotic lestening (DL) procedure with simultaneous competitive auditory inputs to each ear, is today perhaps the most frequently used technique for investigation of hemi- spheric asymmetry in the intact brain (see Geffen & Quinn, 1984: Springer, 1977; Bryden. 1982; Bradshaw & Nettleton, 1983 for reviews).

The most important feature of the DL method is that under competitive conditions of two speech signals, most right-handed subjects are better at recalling the right ear input compared to the left ear input (Kirnura, 1961 a; McKeever et al., 1984). This outcome is usually called the Right-Ear Advantage (REA). and is taken as an indication of left hemisphere specialization for speech perception. Theoretically, the rationale for DL is that when two different auditory stimuli are simultaneously presented to each ear, the projec- tions from the cochlea in the contralateral ear to the auditory cortex will let these stimuli predominate. This notion is empirically supported both by recordings of evoked potentials (e.g. Rosenzweig, 1951 ; Connolly. 1985), and through monitoring of regional cerebral bloodflow (Maximilian 1982). The predominance of the contralateral cortical representa- tion of each ear is possibly a consequence of the ampler projections of second-order neurons to the inferior colliculus on the contralateral rather than to the ipsilateral side (Brodal, 1981). As a rule the projection ascending to the inferior colliculus is larger from the contralateral than from the ipsilateral ear, whereas the pathway ascending from the collicle is larger on the ipsilateral side. Thus, the net effect is a favor of the ultimate representation of the contralateral ear in the auditory cortex.

Although most frequently used as an instrument to assess language lateralization in the intact brain (e.g., Kimura, 1961 a; Studdert-Kennedy & Shankweiler, 1970; Lake & Bryden, 1976), the DL technique has also been used both in evaluations of psychiatric patients (e.g. Wexler & Heninger, 1979; Hatta et al. 1984; Bruder, 1983), and in investiga- tions of brain lesions (e.g. Kimura, 19616; Inglis, 1962; Berlin, 1977). Since the DL

Scand J Psycho1 98 (1987) fridividiiiil performance in dichoiic l isieriirip 27

method has been demonstrated to have sufficient validity and reliability on the group level (Geffen & Caudrey, 1981). and since it is demonstrated that the method yields quite remarkable similarities with results of invasive techniques like the Sodium-Amytal test (cf. Rasmussen & Pulilner, 1977). it can be argued that the DL method also should yield homogenous distributions of subjects within groups. Thus, one would expect that a significant REA on the group mean level means that a clear majority of the subjects within the group, individually shows a REA. When the D L technique is used in a clinical or applied setting, it is perhaps not so much the overall group mean differences between ear input recalls that is the critical information as to whether D L is a useful method or not, but rather how many of the individual subjects within the group that actually reveal a REA. This information is not conveyed by the traditional analysis of variance. A significant REA on the 0.05 level tells us only that the null-hypothesis of the absence of a difference between ear inputs has been rejected. It does not mean that if one compare any one subject on right ear performance with left ear performance, the probability would be 0.95 that this subject also demonstrates a REA. Thus, the sole use of group mean comparisons in DL research may obscure other clinically relevant information.

In the present paper group mean data are supplemented by also displaying the individual distributions of ear performance. Figs. 3-6 in the results section demonstrate a possible standardization of how this information may be displayed. Although this proposal also allows for an immediate visual inspection of differences in degree of REA between subjects, the issue of levels of scaling in DL will not be further discussed. As argued by Berlin (l977), the best way to treat dichotic data are as nominal data. or possibly as ordinal data (see also Porter & Hughes, 1983). Other authors (e.g. Bruder, 1983; Repp, 1977; Shankweiler & Studdert-Kennedy, 1975) have instead maintained that given sufficient methodological considerations, differences in the magnitude of ear asymmetry are a t least in part related to the degree of cerebral lateralization. It is our basic view that differences in ear magnitudes reflet differences in brain asymmetry between individuals, but further research is necessary to make any final conclusions possible.

We have compared mean DL performance to CV syllables in four different groups of subjects (male and female right- and left-handers) with the individual distributions within each group. The hypothesis is that two groups showing an almost identical right ear advantage on the group mean level may not necessarily be similar in individual perfor- mance of subjects within each group.

METHOD

Subjects A total of 60 subjects, aged between 17 and 42, participated in the study. The subjects were divided into four groups (n= 15) with the following characteristics: right-handed males, right-handed females, left-handed males, and left-handed females. Familial sinistrality was excluded since none of the subjects had left-handed close relatives (cf. Hecaen & Sauget, 1971). Handedness was tested with a Swedish translation of a modified version of the handedness questionnaire developed by Raczkowski et al. (1974). The questionnaire contains 15 items, and in order to be included in the study, 13 out of the 15 items had to be consistently performed with the preferred hand (either right or left).

Mean right-hand score was 14.60 for the right-handed males and 14.50 for the right-handed females. The corresponding scores for the left-handers were 15.0 and 14.0 respectively. Each subject was tested for hearing loss by a standard audiometer screening test, ranging from 500 Hz to 6000 Hz with a 20 db test signal. Each subject received SEK 35 (approximately four US dollars) as remuneration.

Dichotic stimuli The dichotic stimuli consisted of the six stop-consonants b, d, g, p . t , k . which were paired with the vowels a and u to form 12 consonant-vowel (CV) syllables. The syllables were paired with each other

28 K . Hiigdahl and M. Franzon Scand J Psycho1 28 (1987)

SECTOR NUMBER= 1, STARTING FRhME=15036, NUMBER FRAMES= 329, CTX= 10 DURATION IN MSEC 320.90

SECTOR NUMBER= i r FRAMES- 32.n.. CTX= 10 fliJRATiON I N MSEC =

Fig. 1. An example of the visual display of syllables (oscillograms) in the CADDIC software for exact temporal alignment between channels. Note "starting frame" coordinate as the point of onset- alignment for each syllable. Synchronization made at the onset of the consonant-segment.

to form 66 dichotic pairs, excluding the homonymic pairs. The 66 trials were divided into 3 sequences with 22 trials each, with a brief pause after each sequence. The intertrial interval between each stimulus presentation was 5-+1 SCC. The subjects gave their oral reply immediately after each trial.

The dichotic tape was prepared on a PDP 11/45 computer with A/D and D/A converters and with a D/A multiplexer. The dichotic tape was prepared with the help of the CADDIC-software adapted to a RT-I1 operative system. See Hugdahl et at. (1986) for a technical description of the CADDIC- selfware. Each CV-syllable had a duration of 320 msec. Temporal alignment between the channels w a s set at the first visually and auditorily identifiable energy-release in the consonant-segment of the syllable (see Fig. I).

Maximum onset difference for the CV syllables between the left and right channel of the tape was 0.5 msec due to the D/A multiplexer resolution.

The syllables were originally recorded from the computer output onto a NAGRA IV tape-recorder. However, in order to easily test the subjects in a more comfortable environment than in the research laboratory, the tape was copied onto a chrome dioxide cassette, and played from a Sony WM DD minicasette player equipped with an equalizer, through matched TDH-39 earphones housed in a Maico headset. The intensity of the output from the earphones was 75+3 dB when tested with a Bruel and Kjaer 2203 sound level meter and a 4152 artificial ear.

Design The basic design was a 2 ~ 2 x 2 split-plot factorial (Kirk. 1968). i.e. ear input (right vs. left) x handedness X gender. The first variable involved repeated measurement on each subject while the latter two variables were treated as between factors (Kirk, 1968). Separate analyses of variance were performed for percent correct consonant recall, and for vowel recall.

Procedure Each subject was tested individually in a separate room at the Psychology Department in Uppsala. The subject was seated in a comfortable chair beside,a desk on which the minicassette player was placed. The experimenter was seated of the other side of the desk. The experimenter had an extra set

Scund 1 Psycho1 28 (1987) Individiral perforniunce in dichotic listening 29

DEXTRALS SNISTRALS

V O W E L S

DEXTRALS i SlNlSTRALS Y F : Y ’ F

~ LE RE n Fig. 2. Mean percent correct recall from the right ear ( R E ) and left ear (LE) for the four groups in the experiment, separated for consonants and vowels. M = males, F = females. Vertical bars = standard error (Se).

of headphones, connected to the cassette player, in order to be able to monitor each dichotic trial. The experimenter further marked the subject’s oral reply on each trial on a scoring-sheet.

The subject was instructed to relax and listen to the two syllables on each trial, and to try to recall aloud both syllables as accurately as possible immediately after each trial. If he/she could not recall two different syllables on a trial he/she was instructed to recall the one that was identified. I f no syllable could be identified on a trial, the subject should simply keep silent.

Scoring of the data The data were scored as frequency of correctly recalled consonants and vowels, separately for the right and left ear inputs.

Mean correctly recalled consonants and vowels for the right and left ear inputs were then calculated for each subject. and subjected to analyses of variance and follow-up comparisons with pairwise r- tests.

RE S U LTS

Group means Percent correct recall from the right and left ear, separated for consonants and vowels, are shown in Fig. 2.

Consonants The only statistically reliable effect for the consonants was the two-way interaction of ear input by handedness (see Fig. 2), F (1, 56) = 7.12, MSe = 1032.18, p<O.OS. The left-side panel of Fig. 2 reveals that this effect is due to superior recall from the right ear (REA) for the right-handed subjects, and superior recall from the left ear (LEA) for the left-handed subjects. Follow-up tests with pairwise t-tests showed a significant difference between ear input for the right-handed males, t (14) = 1.73, p<O.O5, and for the left-handed males and females, t (15) = - 1.76,p<O.O5, and -1.82,p<O.O5, respectively. There was furthermore a trend towards a REA significance for the right-handed females, p<O. 10. No other source reached significance in the analysis of variance.

Vowels A look at the right-side panel of Fig. 2 reveals an almost perfect (near 100%) recall for the vowels in all four groups, with no apparent differences between the groups. This impres- sion was given statistical support since all comparisons turned out insignificant in the analysis of variance.

Individiral performance The number of subjects within each group showing a REA, or a LEA, separately for the consonants and vowels, are shown in Figs. 3-6.

30 K . Hugdulzl arid M . Frarizon Scand J Psycho1 2X (IYX7)

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Fig. 3. Number of subjects (along theordinate) with a REA, LEA or no ear-advantage (on midline) in the male right- handed group (DexM). The abscissa shows the relative magnitude in percent of the ear-advantage for each subject. Filled circles (C) = consonants. Unfilled circles (V) = vowels.

Fig. 4 . Number of subjects with a REA, LEA, or no ear-advantage in the female right-handed group (DexF). Abbreviations and explanations as for Fig. 3.

Fig. 5 . Number of subjects with a REA, LEA, or no ear-advantage in the male left-handed group (SinM). Abbreviations and explanations as for Fig. 3.

Fig. 6 . Number of subjects with a REA, LEA, or no ear-advantage in the female left-handed group (SinF). Abbreviations and explanations as for Fig. 3.

Scmd J P\ychol 2 X (1'9x7) Irtdirirliid prrforr?itince in dicliotic listening 3 I

Consonunts

Filled circles show the results for the consonants. Looking first at the right-handed males. Fig. 3 reveals a 12 to 2 split of the subjects in favor of a right ear advantage (REA) with 1 subject falling on the midline, i.e. showing an equal recall from the right and left ears. Using the binomial distribution (Mendenhall e t al:, 1977) with p=0.5, reveals that the probability of obtaining a split with 12 subjects showing a REA is 0.018. Fig. 3 further reveals that the magnitude of the REA varies considerably between subjects.

Fig. 4 shows the distribution of right-handed females along the REA-LEA dimension. As is evident from Fig. 4, in comparison with the group means in Fig. 2 . the individual performance is quite different between groups, although the mean REA is about equal in both groups.

First of all, there is a 7 to 2 split in favor of a REA for the females. Secondly. and perhaps more important, there are 6 females (vs. 2 males) along the midline, i.e. with neither a REA nor a LEA. The significance of this will be further addressed in the discussion.

Fig. 5 reveals the corresponding distributions for the left-handed males. The left-handed males show a 8 to 3 split in favor of a LEA, with 4 subjects along the midline.

Fig. 6, finally, shows the distribution for the left-handed females. with a 9 to 3 split in favor of a LEA, and with 3 subjects along the midline.

Vowels The unfilled circles in Figs. 3-6 show the corresponding number of subjects with either a REA. LEA, or no advantage to the vowels, separately for each group.

The overall trend in the data for the vowels is an absence of both a REA and B LEA. Thus. I 1 out of 15 subjects are on the midline for the right-handed males. with 3 subjects showing a LEA, and one a REA. The corresponding distributions for the female right- handers were 12 on the midline, 2 with a LEA, and one with a REA. The left-handed males showed a similar distribution with 14 subjects on the midline (when subject 15 in Fig. 5 is considered neither REA nor LEA), and one subject with a LEA.

The left-handed females (Fig. 6 ) , finally, revealed an individual distribution of 9 subjects with neither a REA, nor a LEA, with 4 subjects showing a LEA, and one subject showing a REA.

Thus, the overall results for the vowels is neither a right ear, nor a left ear advantage when the distributions of individual subjects in the four groups are considered.

DISCUSSION

Summarizing the main findings, the analysis of group means between groups have basical- ly revealed three findings. First of all, a significant REA is revealed in right-handed males, with a tendency towards REA in the right-handed females. Secondly. significant LEAS are revealed in both the left-handed male and female groups. Thirdly. all significant effects are exclusively related to the consonants, with no significant effects for the vowels.

Turning first to the significant REA for the right-handed males, this is an expected outcome often reported in the literature (see Bryden, 1982; Bradshaw & Nettleton. 1983: McKeever et al., 1984 for reviews). The right ear recall is between 6040% with the left ear recall about 10--15% lower. Thus, the REA in right-handed males when group means are analyzed is an expected finding. An inspection of Fig. 3 support the group mean results. Thus, 12 out of the 15 subjects reveal a REA for consonants. Furthermore, 8 out of the 12 REA subjects reveal a REA magnitude in the region of IO-IS%. Thus, there is a

32 K. Hirgdalil and M. Franzon Scand J PIychol28 (1987)

reasonable homogeneity of the individual results for the dextral males, although it is not complete since three subjects did not show a REA.

Furthermore, the overall consistency in results between subjects for the male right- handed group thus yields a conclusion (although carefully stated), that the DL technique may have clinical significance in a male right-handed population. The trend towards a REA for the dextral females are also what might be expected from the literature. That is, if it is acknowledged that females are less lateralized than males (McGlone, 1978; Inglis & Lawson, 1981), the REA in right-handed females should be less pronounced than for males. The important question, however, is whether the reduced mean REA for the female groicp is also reflected in the analysis of the individual performance.

Statistically, a reduced mean REA in the female group could either be due to an increased number of subjects with a LEA, or to an increased number of subjects with neither a REA, nor a LEA.

Looking at Fig. 4, the results for the individual distribution reveal that there is an increased number of subjects with neither a REA, nor a LEA in the female right-handed group compared to the male right-handers. An interesting aspect of the results for the female right-handers is that the mean REA found on the group level (although only approaching significance), with the same 10-15% magnitude as that found for the male right-handers, obviously, is owing to few females showing a large magnitude REA (>IS%, see Fig. 4). This should be compared with the individual distribution for the males (Fig. 3) which reveals that the group mean REA is caused by many subjects with a smaller (10-15%) REA magnitude. Thus, while the analysis of the individual data for the right- handed males are consistent with the group data, the individual data for the right-handed females are not so consistent. This difference between the sexes will not be obvious when group means are compared.

Turning to the two left-handed groups, Fig. 2 shows that both the males and the females reveal a significant LEA for the consonants on the group mean level.

This is an unexpected outcome, since invasive techniques, like the Sodium-Amytal test (Rasmussen & Milner, 1977), have revealed a clear majority (about 70%) of all left- handers to be left hemisphere speech dominant. However, a look at Figs. 5 and 6, showing the individual distributions for the left-handed groups. may perhaps explain some of the unexpected results. Fig. 6 shows that five of the 15 females reveal quite extreme LEAS (up to 70%), while six subjects show either a REA, or neither a REA, nor a LEA. Thus, the overall picture for the female left-handers is of great individual variation. It may perhaps be of interest to relate the five most extreme LEA subjects (subjects 8,27, 36,44, and 48 in Fig. 6 ) , (=33 % of the sample), to the 3040% of left-handers found with right hemisphere language in Sodium-Amytal tests (e.g. Rasmussen & Milner, 1977). Possibly, these five subjects are right hemisphere dominant.

However, the overall picture for the left-handed females is that of great individual variation despite an impressive significant LEA on the group mean level. Thus, a simple analysis of individual performance within the group has revealed a quite different “homo- geneity-pattern” behind the significant group LEA, as compared to the pattern behind the significant REA in the right-handed males. Furthermore, it should be kept in mind that the LEA is based on only 9 subjects, which in a sample of 15 is equal to a chance-probability of as much as 0.15 1.

The impression of individual heterogeneity is also obvious in the individual performance of the left-handed males, although to a lesser degree (see Fig. 5). Thus, the significant LEA for consonants found for the group mean is based on only 8 subjects (out of 15). The binomial sampling distribution reveals an “8 to 7” split to be insignificantly different from chance. Thus, the significant group LEA for the male left-handed group is based on no

Scand J Psycho1 28 (1987) Individiial performance in dichotic listening 33

more than 50.33% of the subjects actually revealing better recall from the left ear, compared to the number of subjects with better recall from the right ear or with no ear preference.

The final aspect of the data to be discussed concerns the individual performance for the vowels. Both the group mean analysis and the analysis of the individual distributions reveals an overall pattern of almost perfect recall for the vowels with no significant ear differences in any analysis. This finding is consistent with the findings of Studdert- Kennedy & Shankweiler (1970), and has been interpreted by some authors as an effect of the stop-consonants requiring more of phonetic processing, perhaps involving a lateralized phonological processor (Bradshaw & Nettleton, 1983). Furthermore, it could be that the brief duration and the rapid formant transients found in the release of the consonants, but not in the vowels, is the critical issue in the DL method (cf. Tallal, 1976).

In conclusion, then, the present findings of dichotic listening performance to CVs in a free recall paradigm have basically revealed consistency between group mean data and individual data for male right-handed subjects. For right-handed females, as well as for left-handed males and females, there is a marked inconsistency between the group mean data and the individual distributions. This may either indicate that it would be premature to apply the DL method in more applied settings, when other individuals than male right- handers are considered, or that right-handed females and all left-handers are more hetero- geneous with respect to brain asymmetry. Since arguments for both stands may be found in the literature (cf. Repp, 1977; Bruder, 1983; Porter 8t Hughes, 1983; Berlin, 1977; Bryden, 1982), this issue is not further stressed in the present paper.

What should be stressed, however, is that only reporting group mean differences between ear inputs in DL may be a misleading way of presenting DL data, where valuable information concerning individual performance within the group is lost.

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Received 20 May 1986