002X 3932’92 S5.00+0 00 0 19Y2 Prrpmnn Press I.td

DICHOTIC LISTENING: WHAT DOES IT MEASURE?

LUTZ J~NCKE,* HELMUTH STEINMETZ~ and JENS V~LKMANN?

*Department of Psychobiology and Psychocybernetics, Heinrich-Heine-University, Universitatsstral3e 1, D-4000 Dusseldorf 1, F.R.G. and tDepartment of Neurology, Heinrich-Heine-University. Moorenstrage 5.

D-4000 Diisseldorf 1, F.R.G.

(Receiced 2 Septemhw 1991; accepted 2 1 July 1992)

Abstract-Auditory lateralization was investigated in 26 right-handed and 26 left-handed, normal subjects using seven different dichotic listening tests in each proband (free recall of digit lists, free recall ofconsonant vowel (0’) svllables, four different CV syllable monitoring paradigms, and free recall of Morse codes). Reliabilttjes calculated with the formula of Spearman Brown were low for digit recall (0.29, corrected for test length: 0.50) but good for CV recall (0.83), CV monitoring (0.75~ 0.88). and Morse code recall (0.50, corrected for test length: 0.88). Nevertheless, intertest correlations were low, both for right- and left-handers (negative correlations ranging from -0.44 to -0.05, positive correlations ranging from 0.01 to 0.51). Only 38 77% of the right-handed and left- handed subjects retained one direction of ear advantage across any combination of two tests. The data suggest that different dichotic tests reveal different results. This may be due to psychometrtc. procedural, or phonetic properties. We conclude that individual predictions of language dominance are not justified using the dichotic tests evaluated in the present study.

INTRODUCTION

SINCE KIMURA’S original digit list paradigm [18, 191, several paradigms using other verbal stimuli have been proposed to increase the predictive validity of the dichotic method with respect to language lateralization (for overviews see Refs [2], [S], [7--91 and [36]). Among these tasks, the recall or monitoring of dichotic consonant-vowel (CV) syllables [30,35,38] has been consistently found to produce the most robust right-ear advantages (REAs) in right-handed individuals, an effect thought to be due to the superior temporal discrimination of acoustic signals by the dominant hemisphere [4, 71. In accordance with this assumption. several studies demonstrated REAs also for nonverbal acoustic stimuli with rapidly changing temporal cues [23, 391. Only a few investigations, however, directly assessed the predictive validity of dichotic paradigms with respect to language lateralization as determined by invasive methods [ll, 19, 37, 441. In contrast to the encouraging results of these studies, other authors were much more cautious on the validity issue, mainly due to poor dichotic retest or intertest comparabilities [3, 5,401. The present study addresses this problem. We report intertest correlations for seven dichotic paradigms, all known to produce strong REAs and thought to reflect language lateralization, but differing with respect to stimulus material (digits, CV syllables, or nonverbal stimuli), stimulus presentation (list or single-pair) or stimulus reproduction (free recall or target detection). The investigation thus attempts to further elucidate the question whether auditory lateralization is likely to be related to one

*Address for correspondence: Dr Lutz Jancke, Department of Psychobiology and Psychocybernetics. Heinrich-Heine-University Dusseldorf, Universitatsstrabe I. D-4000 Dusseldorf I, F.R.G.

941

942 L. J;(N~KE rl trl

specialized mechanism of the dominant hemisphere [4,38] which, if this were the case, could indeed be expected to also underlie language dominance.

METHODS (a) Suhjrcrs

We examined 52 healthy, paid volunteers without hearmg deficits in the speech frequency range (less than IO dB hearing loss on either ear tested by standard audiometry). Twenty-six of the 52 subjects were self-described sinistrals (mean age: 26.4 years; age range: 21 35 years) and 26 were self-described dcxtrals (mean age: 24.9 years age; age range: 2l--34 years). Most ofthem were university students. There were 13 males and I3 females in each handedness group.

Hand preference was assosed using the five-item questionnaire OIBKYIIFY [6,7]. Subjects were classified as either left-handed (Bryden scores ~0) or right-handed (Bryden scores >O) L6.71. This classilicatlon conformed with the subject’s self-description in all casts (XC a).

All subjects were examined with the following dichotic tests: (i) digit list recall. (ii) CV recall, (iii) CV monitoring with target syllables;ta; or/da: (hit rate and reaction time). and (iv) Morse code recall. Tests (i) and (iv) were applied in one session, tests (ii) anJ (iii) In another session. Half of the subjects started with session I, the other with session 2. Interbals between sessions ranged from I to 5 days. Within each session. half of the subjects started with test (i) or (iv) and the other with test (ii) or (iii).

(i) Dig/if list vt~c~/l. The test material consisted of 96 pairs of German monosyllabic digits (/ems.. ;rwei:. idrei,‘, ,‘vier.. ‘fiinf,. . ,/sechs,,,, ,‘acht,,‘. ,‘neun,.‘. , rehn . :elf,‘. ,‘zwolf:) spoken by a male person. These stimuli were band-pass filtered (upper corner frequency 10 kHz, 24 dB,,octave and lower corner frequency 80 Hz. 24 dB,‘octave), digitized (AjD conversion. 16 bit). and stored in computer memory (PDPI I computer). Each dlgit wa, edited and synchromred for intensity and onset with a speech editor program. The criterion for temporal alignment of digits was onset of articulatory rcleasc. After randomizatmn for digit order and exclusion of pairs of identical digits. the stimuli were D A converted (20 kHr, resolution I6 hit). Digits forming a pair were rccordcd on separate tracks of a magnetic recording tape usmg different channels of the D:A conbertcr. With this technique. competing digits uerc aligned with an accuracy of 50 /Isec. Thirty-three trials. each trial consisting of three digit pairs, were recorded on tape. Within each trial, &git pairs were presented with I-XX intcr%als (interval from onset ofpreceding digit to onset of following digit). Each trial was followed hq a 6-set pause. Dichotic pairs were presented using a stereo tape recorder (AIWA AD-F360). stereo headphones (Scnnhelser HD222). and at an intensity of about 70 dB (SPL). Suhjccts were instructed to pay attenllon to both ears. They were asked to write down the percaved stimuli immediately after each trial. lrrcspective of order or ear of presentation. After 16 trials, headphone orientation was reversed. Half of the subjects started with headphone orientation A (right-car channel right. left-car channel left). The other half started with rckerscd headphone orientation B. In both groups. the numbers of right-banders, left- banders. men and women were counterbalanced. Prior to each experiment. 10 practice trials were carried out.

(ii) C‘I’wc 011. This test comprised paira of natural speech CV syllables (:ba:, :ga, , da., :p:~,‘, /ka,‘, and II ta; ) spoken by a trained phonetician (microphone .AKG 16OEl) which had heen band-pass liltercd (Rockland 432, upper corner frequency IO kHz 24 dB octave and lower corner frequency X0 Hz 24 dB:octave). Each syllabic WBS digitized and stored in computer memory (l2-hit A;D converter. digitizing rate 20.229 Hz. ATARI MEGA ST4). The onset, duration. intensity and fundamental frequency of the stimuli were then edited and synchronized by means of a speech editor. The criterion for temporal alignment of the competing syllables was the onset of articulatory release. Each syllable was preceded by IO0 msec ofzero intenTit> before articulation started and followed by 50 msec ofrero intensity after vowel end. This manipulation served to remove clicks at the beginning and end upon D:A convcraion. The duration of syllables ranged from 310 to 360 msec, depending on voice onset times, with a +owel duration of 300 msec. Voice onset times (in mscc) for the stops were approximately b- IO. g=20. d = 15. p=40, k =60 and t = 50. The duration of the entire signals (syllables. leading and end interval with Lero Intensity) ranged from 450 to 500 msec. These edited Ggnals were stored on hard disk (MEGAFILE 30) for further use. For dichotic presentation, the stored sign& were D,‘A converted (I 2.bit. four-channel D’A converter, digitizing rate 20.229 Hz) by alternately converting a value of the first syllable and then the second syllable. Each syllahlc was conberted using a diflerent channel of the D.A con\crtcr. With this technique. competmg syllables were aligned with an accuracy of49.4 /(sec. After con>eraion the output of each D A channel &as amplified (70 dB SPL as measured by a Bruel & Kjaer sound level meter) and presented via stereo headphones (Sennheiscr HD450). During the cupcriments. the subjects were seated in a sound-attenuated chamber. received instructions through a two-way intercom set and were instructed to pay attention to both cars. The presentation of dichotic stimuli and the recall procedure were controlled by the

DlCHOTIC LlSTENlNG 943

computer. For presentation, the CV pair was loaded from hard disk into computer memory and D/A converted. After stimulus application, a diagram showing six squares containing the six syllables appeared on a monitor in front of the subjects for 5 sec. The subjects were instructed to “click” the syllable or syllable pair they had heard. Prior to each experiment, 30 practice trials were carried out. Test stimuli were randomized and arranged in eight blocks of 30 CV pairs in which each ofthe six syllables was paired with the other five (no syllable was paired with itself). Thus, the whole experiment consisted of 240 test trials, each trial consisting of one pair of syllables followed by the 5-set response interval. Headphone orientation was reversed after each block. Halfof the subjects started with headphone orientation A (right-ear channel right, left-ear channel left), whereas the other half of the subjects started with reversed headphone orientation B. In both groups, the numbers of right-handers, left-handers, men and women were counterbalanced. Previous experiments in our laboratory had shown a Pearson test-retest correlation of 0.81 for the laterality index (“e”) (see below) (15 healthy male right-handers: test-intervals: 3 4 weeks).

(iii) CVmonitoring. The same stimuli and technical equipment as in the CV recall test were used. The stimuli were randomized and arranged in eight blocks, each containing 30 syllable pairs. resulting in a total of 240 trials. Each of the 30 possible pair-wise permutations of the six CV syllables occurred equally often. Because all possible CV pairs were equiprobable, one-third of the trials consisted of pairs containing the target. Fifty per cent of the targets were delivered to the right and 50% to the left ear. Thus, the a priori probability of target occurrence for each ear was 0.167. The interval between syllable pairs was 1.9 sec. Subjects were instructed to pay attention to both ears and press a button with his or her forefinger when the target syllable was heard. Reactions within 100~1000 msec after stimulus onset were accepted. Subjects underwent this procedure twice, once with the target :ta/ and once with the target ,‘da/. Headphone orientation and responding hand were counterbalanced for the whole experiment. Thus, 26 subjects started with headphone orientation A. I3 of whom were asked to respond to with the right hand (R) and another 13 of whom were to respond with the left hand (L). The other 26 subjects started with reversed headphone orientation (B) using the same order of responding hands. Respondmg hand and headphone orientation were changed after each block (ABABABAB with RLRLRLRL or LRLRLRLR: BABABABAB with RLRLRLRL or LRLRLRLR). In both groups, the numbers of right-handers. left-handers, men and women were counterbalanced. The frequency of target detection (hit rate), false alarms and the reaction times (RT) for each ear were stored. Previousexperiments in our laboratory had shown Pearson test-retest correlations of0.78 and 0.72 for the laterality indices (“e”) for ,/(a/ and ,‘da,‘hit rates. and correlations of 0.73 and 0.64 for the differences of right- and left-ear ,‘ta,’ and ,idai reaction times (15 healthy right-handed, malt subjects; test intervals: 34 weeks). All subjects were given 30 practice trials for each target syllable prior to the experiment.

(iv) Morse code recr~I1. Seven “letters” represented by either dots or dashes were chosen from the international Morsecodealphabet (e. ,i . . ,s.. . ,h . . . . , t -, m --, o ---). A single stimulus ofa dichotic pair consisted of one or two Morse letters. A personal computer was used to control two electronic oscillators which produced the signals. The oscillators were set at frequencies of 1000 and 2000 Hz to allow distinction between the two channels. Stimuli were presented via stereo headphones (Sennheiser HD222) at an intensity level of 65 dB (SPL). All dichotic Morse pairs were matched in length. A dot was counted as one unit long, a dash as three units. a pause in a Morse letter as one unit. and a pause between subsequent Morse letters as three units. Unit length was 75 msec. During the whole test, 30 Morse pairs were presented dichotically. In 10 ofthese trials, dots were presented to the left ear, while dashes were presented to the right ear. Ten trials were reversed. and 10 trials employed mixed dot-dash sequences on both ears. The order of these three blocks of 10 trials was randomized across subjects. Each ear received identical numbers ofdots and dashes. None of the subjects had previous knowledge of Morse code. The probands proceeded at their own pace by pressing a button to summon a stimulus pair. They were instructed to direct attention to both ears and to report the perceived Morse sequences immediately after stimulus presentation in self-chosen order. (The “Morse” sequences were vocalized using two different lengths of the syllable /ta,‘.) The test was scored such that partial credit was given if one letter in a Morse “syllable” was correctly reported. For a “completely correct” response, one point was given. After live trials, headphone orientation was reversed. Half of the subjects started with headphone orientation A (right-ear channel right, left-ear channel left). The other subjects started with reversed headphone orientation B. In both groups, the numbers of right-handers. left-handers, men and women were counterbalanced. All subjects were given 10 practice trials prior to the experiment.

Design of this paradigm was based on the assumption that ear advantages produced by lOOO- or 2000-Hz Morse code signals are likely to reflect hemispheric left-right differences in the processing of rapidly changing temporal cues. It is unlikely from the data of Gt?FFEN and REYNOLDS [13] that lOOO- or 2000-Hz tones are subject to a bias due to tone height.

(d) Daru anul~.sis

(i) Accurac): score and hteraliry index “e”. Accuracy scores were determined for each test as percentages of correct responses. To assess ear asymmetry, the laterality index “e” according to HALWES 173 was used because this coefficient is independent of the subjects’ overall accuracy and has been shown to produce sensitive isolaterality curves 171. The following formulas applied: e= R-L/(R+ L) if overall accuracy was less than .50X, or e = R ~ L,‘(R, - L,) if overall accuracy was greater than 50% [R, L: total numbers of correct responses for targets presented to the right (R) or left (L) ear, R,, L,: total numbers oferrors for targets presented to the right (R,) or left (L,) ear]. It should be noted at this point that other indicators ofear asymmetry were also determined in the present

944 L. J~CKF et ul.

study without revealing different results (e.g. simple left -right difference. left right difference relative to total score). In addition, differences between reaction times for correct right- or left-ear target detection were calculated in the monitoring tests. A negative reaction time difference indicated a REA, a positive difference a left-ear advantage (LEA). Laterality indices and reaction time differences were used for further analysis.

(ii) Splil-half‘ correlations. To estimate test reliability, we calculated the “consistency” using the formula 01 Spearman-Brown (R,, = (2 * R, *)/( I + R, J. R,,: consistency coefficient, R, z: Pearson correlation between halves of each test 12271). Variables of the Spearman Brown calculations were determined as follows, taking counterbalancing of headphone orientation into account. For digit list recall, the data of the first eight trials of the first half of the test and of the first eight trials of the second half of the test were pooled. From these, a latcrality index was computed. A second laterality coeficient was computed from the pooled data of the other trials. For CV recall and CV monitoring. laterality indices were determined from the first and last four blocks. For Morse code recall. the data from the first five trials of each block were pooled. From these a laterality index was computed. A second laterahty coeficient was determined from the second five trials of each block.

Because reliability depends heavily on test length (high reliability with long tests), the Spearman- Brown reliabilities had to be corrected for this effect. Thus, we calculated an estimate of reliability which corrected for overall test length by using the Spearman Brown relation (LR;, = (n’,‘n * R,,)/( 1 + (n’,‘n - I ) * R,,)], R;, : corrected reliability, R,,: original reliability, n: number of test stimuli. n’: number oftest stimuli for the longer test version). For digit list recall and Morse code recall we estimated the reliability for a test length of 240 stimuli pairs. that is, for the same number of stimuli as in CV recall and CV monitoring.

(iii) Sturistical ana/~.sis. A two-way multivariate analysis of variance (MANOVA) with “handedness” and “sex” as factors was computed for the laterality coefficients. The accuracy scores were analysed with a three-way univariate analysis of variance (ANOVA), with “sex”, ” handedness” and “dichotic tebts” (repeated measurements factor) as factors. In addition. frequency data for all dichotic tests concerning the Incidence of ear advantage in connection with handedness and sex w-ere subjected to seven Hierarchical Log- Linear analyses [I 51. Furthermore, 21 pair-by- pair cross-tabulation analyses were computed and analysed with the 2’ test for each handedness group. Finally. 21 Spearman- Rank-intertest correlations were performed for each handedness group. Because several tests were computed for one sample, type-l-error was adjusted according to HOLM [16]. A significance level of 10% and a two- sided test problem was chosen. All statistical analysts were performed with the statistical software package SPSSPC-V2.I.

RESULTS

We obtained the following reliabilities based on the Spearman-Brown formula (see Methods): digit list recall, 0.29 (RI,: 0.50); CV recall, 0.83; /ta/ hit rate, 0.85; /a/ reaction time, 0.88; /da/ hit rate, 0.75; /da/ reaction time, 0.78; Morse code recall, 0.50 (R;,: 0.88). These reliabilities were similar for right- and left-handers.

As shown in Table 1, the REA incidences in the different dichotic tests (Table 1) corresponded roughly to those reported in other studies on normal individuals [7], ranging from 58 to 85% in the 26 right-handers and from 54 to 70% in the 26 left-handers of the present study. Due to sample size, there was no statistically significant effect of handedness on REA or LEA incidence in any test. Furthermore, there was no evidence for an influence of sex or sex x hand interaction. Hit rate and reaction time for detection of the syllable /da/ in the CV monitoring task were surprisingly low in right-handers (Table 1).

Table 2 shows the group means and standard deviations of laterality scores obtained for the different tests. There was considerable variation of laterality scores in both hand groups. The coefficient of variation (S.D. * loo/mean) ranged from 133 to 780”/0 in right-handers. In left-handers. these coefficients were higher, ranging from 224 to 1542%. Table 2 demonstrates that right-handers showed a significant REA only for digit list recall. Four tests yielded a trend towards a REA in this group (CV recall, /ta/ hit rate, ita/ reaction time, Morse code recall). In left-handers, there was no significant REA in any of the dichotic tests. Two tests revealed a trend towards a REA in this hand group (ita/ and /da/ hit rates).

The MANOVA performed for the laterality coefficients revealed no significant result [handedness: F(7,42)=1.51, P=O.191; sex: F(7,42)=2.39, P=O.O37; handxsex: F (7,42)= 1.10. P=O.379]. However, this MANOVA and subsequent ANOVAs revealed

DICHOTIC LISTENIMi 945

Table 1. Percentages of right-handers (RH, n = 26) and left-handers (LH, n=26) showing a right-ear advantage (REA), a left-ear advantage

(LEA) or no ear advantage (NEA) in seven different dichotic tests

REA (%) LEA or NEA (%) RH LH RH LH

Digit list recall 85 61 15 39 CV recall 77 61 23 39 CVmon /ta/ hit rate 73 61 27 39 CVmon /ta/ RT r 1 73 61 27 39 CVmon /da,’ hit rate 58 54 42 46 CVmon /da/ RT r-l 58 70 42 30 Morse code recall 70 54 30 46

CVmon: CV monitoring, RT r I: difference between reaction times for targets presented to the right (r) or left (I) ear.

Table 2. Group means and standard deviations (S.D.) of laterality scores for right-handers (RH, n =26) and left-handers (LH, n = 26)

RH LH Mean S.D. t P Mean S.D. r P

Digit list recall 0.21 (0.28) 3.75 <0.001* 0.04 (0.38) 0.50 0.620 CV recall 0.04 (0.10) 2.23 0.034 0.04 (0.21) 0.96 0.346 CVmon ita,’ hit rate 0.09 (0.13) 3.40 0.003 0.09 (0.20) 2.22 0.036 CVmon ;ta/ RT r -1 -36.95 (65.10) 2.84 0.008 -24.95 (73.38) 1.70 0.114 CVmon /da,/ hit rate 0.01 (0.14) 0.46 0.649 0.12 (0.31) 1.96 0.061 CVmon ,/da/ RT r- 1 9.19 (71.73) 0.63 0.534 -8.62 (133.81) 0.32 0.750 Morse code recall 0.06 (0.15) 2.06 0.050 -0.01 (0.21) 0.24 0.812

*P<O.lO after alpha-adjusting, t: r-values for t-tests of paired data comparing laterality scores for deviation from zero. P: two-sided

probability of t-value. RT r-l: difference between reaction times (msec) for targets presented to right (r) or left (1) ear.

trends towards a sex difference for digit list recall [F (1, 48) = 7.43, P=O.O09] and /da/ hit rate [F(l. 48)=6.57, P=O.O14] with higher laterality scores in men than women.

The three-way ANOVA for the accuracy scores (Table 3) revealed a significant result [dichotic tests, F (4, 192)=253.77, P<O.OOl], as did all subsequent r-tests for paired data (Table 4). There was no significant sex or hand effect and no sex x hand interaction. However, a trend towards a sex difference emerged with women responding more accurately than men [sex: F(1,48)=7.49, P=O.Ol; hand: F(1, 48)= 1.53, P=O.222, sex x hand: F (1, 48)=2.32, P=O.179; sex x dichotic tests: F (4, 192)=2.21, P=O.O69; hand x dichotic tests: F (4, 192)= 1.43, P=O.226; hand x sex x dichotic tests: F (4, 192)=0.80, P=O.527].

Pair-by-pair cross-tabulation analysis (Table 5) revealed percentages of right-handed subjects with a consistent ear advantage in any combination of two of the seven tests ranging from 38% (Ita/ hit rate paired with /da/ reaction time and /da/ hit rate paired with /da/ reaction time) to 77% (J’ta/ hit rate paired with Morse code recall). Pair-by-pair cross- tabulation revealed percentages of left-handed subjects with a consistent ear advantage in any combination of two tests ranging from 38% (digit list recall with /ta/ reaction time and /ta/ reaction time paired with /da/ reaction time) to 77% (digit list recall paired with Ita/ hit rate, digit list recall paired with ida,’ hit rate, and /ta/ hit rate paired with /da/ hit rate).

946 L. JANCKE et al.

Table 3. Mean percentages and standard deviations (in brackets) of correct responses observed in five different

dichotic tests for right-handed (RH: n=26) and left-handed subjects (LH: n=26)

RH LH

Digit list recall 78 (7.5) 72 (13.7) CV recall 48 (6.6) 49 (8.9) CV monitoring: /ta/ hit rate 36 (9.2) 35 (5.9) CV monitoring: /da/ hit rate 24 (6.6) 25 (8.2) Morse code recall 56 (12.62) 53 (10.5)

Table 4. Absolute t-values (d.f.: 51) for comparisons of accuracy scores

2 3 4 5

(1) Digit list recall 14.43* 21.66* 26.70* 10.88* (2) CV recall 9.70* 17.23* 3.39* (3) ,‘ta/ hit rate 7.63* 10.81* (4) /da/ hit rate - 14.60* (5) Morse code recall - -

*P-c =O.lO after alpha-adjusting.

Among a total of 21 intertest correlations for each handedness group, none was significant (Table 6).

Only one subject (right-hander) retained one direction of ear advantage (REA) across all seven tests. Only 15 subjects (29%) retained one direction of ear advantage across those four tests showing a > 65% REA incidence in right-handers (digit list, CV recall, /ta/ reaction time and Morse code recall). Of these 15 individuals, 13 exhibited a REA (with 10 right- handers and three left-handers) and two a LEA (all left-handers).

DISCUSSION

The present investigation confirms and extends earlier studies of TENC; [40] and WEXLER

and KING [43] who compared different dichotic recall tests in right-handed subjects. TENC

[40] evaluated four paradigms (free recall of word lists and single-pair CV syllables, free and balanced-order recall of digit lists). Intertest correlations ranged from 0.01 to 0.77 in her study 1401. WEXLER and KING [43] compared three tests (free recall of words and non-words differing only in the final consonant, free recall of vowel-consonant-vowel (VCV) syllables). In their study, intertest correlations were low for dissimilar stimuli (words/VCV: 0.20-0.31; non-words/VCV: 0.16-0.24) but high for similar stimuli (words/non-words: 0.84) [43].

The main findings of the present investigation were low or non-existent intertest correlations between a variety of tests of auditory lateralization (Table 6), most of them thought to be related to language dominance [7]. Concordance rates within any test pair were only slightly above chance level (38-77X) for each handedness group. Nevertheless, most of the tests produced a clear average REA in the same group of 26 healthy right- handers, with REA incidences ranging between 58% (/da/ hit rate and reaction time) and

DICHOTIC LISTENING 947

Table 5. Percentages of right-handed (RH, n:26) and left-handed (LH, n:26) subjects with a consistent ear advantage in any

combination of two dichotic tests

Dichotic tests

(1) Digit

(2) CVrec

(3) /ta/ hit rate

(4) /ta/ RT r-1

(5) /da/ hit rate

(6) /da/ RT r-l

(7) Morse

(2) (3) (4) (5) (6) (7)

RH: 61 65 65 50 50 69 LH: 69 71 38 77 54 62

RH: ~ 58 58 58 50 69 LH: ~ 61 54 62 54 54

RH: ~ 61 54 38 77 LH: - 61 71 46 54

RH: ~ 54 53 65 LH: - 54 38 54

RH: - 38 58 LH: ~ 46 54

RH: ~ 46 LH: - 46

RH: LH:

Digit: digit list recall, CVrec: CV recall, /ta/ hit rate: hit rate on CV monitoring using target syllable /ta/, /ta/ RT r-1: reaction time difference on CV monitoring using target syllable /ta/, /da/ hit rate: hit rate on CV monitoring using target syllable /da/, /da/ RT r-1: reaction time difference on CV monitoring using target syllable /da/, Morse: Morse code recall.

Table 6. SpearmanRank intertest correlations between auditory laterality indices (“e”) in 26 right-handers (RH) and 26 left-handers (LH)

Dichotic tests

(I) Digit

(2) CVrec

(3) /ta/ hit rate

(4) /ta/ RT r-1

(5) /da; hit rate

(6) ,/da/ RT r-1

(7) Morse

(2)

RH: 0.11 LH: 0.40

RH: - LH: -~

RH: LH:

RH: LH:

RH: LH:

RH: LH:

RH: LH:

(3) (4) (5)

-0.15 -0.22 -0.05 0.32 0.16 0.25

0.51 -0.30 0.11 0.38 -0.06 0.07

- -0.34 0.29 - -0.44 0.26

-0.14 0.11

(6) (7)

~0.08 0.42 0.21 0.39

0.28 0.22 0.40 0.31

0.19 -0.13 0.48 0.35

~0.03 -0.20 -0.34 0.01

0.24 -0.15 0.17 0.18

0.31 - -0.08

Digit: digit list recall, CVrec: CV recall, /ta,I hit rate: hit rate on CV monitoring using target syllable /ta/, /ta/ RT r-l: reaction time difference on CV monitoring using target syllable /ta/, /da/ hit rate: hit rate on CV monitoring using target syllable /da/, /da/ RT r-1: reaction time difference on CV monitoring using target syllable /da,‘, Morse: Morse code recall.

948 L. J~NCKE et a/

85% (digit list recall) (see Table 1). For the most part, this surprisingly high intrasubject variability could not be explained by low dichotic test reliabilities. Previous, unpublished experiments in our laboratory had demonstrated favourable retest reliabilities for CV recall (0.X1), and CV monitoring (/a/: 0.72 to 0.78 /da/: 0.64 to 0.73, see Methods). These reliabilities were similar to those obtained by other auhors [3, 29, 31, 33, 42, 431. Furthermore, the consistency of these tests as determined from the present data was in the same range (CV recall: 0.83, CV monitoring: 0.78-0.88, Morse code recall: 0.50 CR,‘,: O.SS] see Results). Only digit list recall revealed a low reliability of 0.29 [RI,: 0.501 (see Ref. [40] for similar data). Poor intertest correlations thus could be attributed to poor test reliability only for those test pairs of Table 6 which included digit list recall. The other dichotic tests must have measured diverging functions. What are these functions, and can any of them be regarded as an index of language dominance’?

A multitude of factors are known to influence performance on dichotic listening tasks [2,5, 7, 8, 24, 361. Among them are stimulus material, stimulus presentation and stimulus reproduction, all of which varied between the dichotic tests of the present investigation. Thus, digit list recall used stimuli which were meaningful, acoustically complex, easy to detect, and which consisted of a number of phonemes which may not lateralize to the same ear-one possible explanation for the low reliability (R,,: 0.29, R:, : 0.50) of this procedure. In contrast, the CV stimuli were meaningless, acoustically simple, difficult to detect, and differed mainly in their initial consonants, which may account for the much better reliability of our CV paradigms (0.75-0.8X). Factors related to the tasks themselves may also partly explain the considerable heterogeneity in our results (Tables 24). Memory was a factor in the recall tasks, but presumably not in CV monitoring. The recall tests required written, visuomotor, or oral responses, the monitoring task only pressing a button. However, not all of the low intertest comparabilities (see Table 6) can be attributed to such relatively coarse methodological differences. It was particularly surprising that even the correlation between hit rates of the two CV monitoring tasks using /ta/ or /da/ as targets did not exceed 0.29 (Table 6), with only a low concordance rate (54% for RH and 77% for LH) between these very “similar” tests. This could not be due to procedural factors, which were identical in these tasks. Instead, we must assume that different processing pathways of different lateralization are accessible in one individual not only by lexico-semantic, phonetic. or Morse code stimuli, but even by two phonemes (/ta,’ or /da/) differing only in their voice-onset time 1211. Perceptual studies have also suggested that the accuracy oftarget detection within a set of CV syllables presented dichotically are not invariant [25, 28, 341. In these studies, accuracy scores were higher for voiceless stops than for voiced stops. Our data accord with these results, showing higher hit rates for the detection of /taj than for the detection of /da/ (see Tables 3 and 4). This difference in test dificulty may well have led to a differential selection of prephonetic or phonetic processing pathways in our CV monitoring tasks.

Considering such an intraindividual diversity in auditory lateralization, the present data do not support a positive answer to the question whether any of the dichotic tests employed here are likely to detect a single “linguistic device” [38] of the left or right hemisphere. Using word list recall techniques similar to ours, KIMIJRA [ 191 and STRAIX et al. [37] reported 80% correct predictions of the language-dominant hemisphere in patients examined with intracarotid sodium amobarbital injections. GEFFEN, TRAUB and STIERMAN [ 1 l] reported an even better performance of their word-monitoring technique, which allowed a 97% correct prediction of language lateralization in patients treated with unilateral electroconvulsive therapy. Marc recently, ZATORRE 1441 reported similar validity for the dichotic fused-words

DlCHOTIC LISTENING 949

test as determined in 61 epileptic patients who were also examined with intracarotid sodium amobarbital injections. Ninety-four per cent of his patients with left-hemispheric dominance revealed a REA, and all patients with right-hemispheric dominance (n = 4) revealed a LEA. Unfortunately, comparable data for CV or Morse code stimuli are lacking. Our digit recall test showed an extremely low reliability of only 0.29 (R;,: OSO), thus arguing against even a moderate validity of this technique. This assessment accords with other authors [3,40]. Also conforming with previous studies [43], our reliabilities were much better for CV stimuli. However, the CV paradigms produced relatively low, 58--77X REA incidences in our right- handers (Table l), thus almost excluding a higher predictive validity with regard to left hemisphere language dominance.

It has been argued by others that dichotic listening and invasive tests of language dominance are probably not evaluating the same phenomenon [37]. Whereas invasive methods, such as intracarotid amobarbital injection [19, 37, 41, 441 or unilateral electroconvulsive treatment [l 1,12,27], primarily assess disturbances of speech production, dichotic tests are generally thought to emphasize perceptive mechanisms. There is a wealth of data from studies on aphasic, hemispherectomized or postcommissurotomy patients suggesting that perceptive language functions are less lateralized than language production Cl, 10: 14, 17, 20, 26, 321. In the absence of quantitative information about language lateralization in our subjects, it cannot be excluded that any one particular of the dichotic tests employed in the present study indeed measured something like “receptive language dominance”. However, the intra- and interindividual diversity in the results does not favour a simple dichotomous left--right conception. Our data suggest an array of separate processing mechanisms in dichotic listening, and for each of these the significance regarding language lateralization remains to be elucidated.

A~knoMlI~dyrmPnrs~This study was supported by grants from the Deutsche Forschungsgemeinschaft to H.S. (SFB 194iA7). We also wish to thank the colleagues from the Institute of Phonetics at the University of Cologne for providing us with the stimulus material for the digit list recall test, and Dr Rolf Diehl for carrying out a number of digit list experiments.

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