Can expectancy explain reaction time ear asymmetries?

Neuropsychologia, 1978, Vol. 16, pp. 225 to 228. PergamonPress.Printed in England.

NOTE CAN

EXPECTANCY

EXPLA1N

REACTION

TIME

EAR

ASYMMETRIES?

HOWARD J. KALLMAN* McGill University, Montreal, Quebec, Canada H3A 1B1

(Received 23 September 1977) Abstract---Subjects were presented with dichotic pairs of musical and speech sounds randomly intermixed and the task was to press quickly a response button whenever a pre-specified target sound was detected at either ear. Reaction times were faster to right- than to left-ear speech targets; there was also a trend favoring the left ear in detection of musical targets. Since subjects did not know in advance of each trial whether the dichotic pair would consist of music or speech, the significant interaction between stimulus types and ears suggests that an attentional explanation based on expectancy cannot fully explain ear asymmetries. THERE is a large body of evidence demonstrating that when two different sounds are presented dichotically lo a human subject, the sound arriving at one ear may be more efficiently processed than that arriving at the other (see RICHARDSONand KNIGHTS [1] for a bibliography). Which ear is favored depends, at least in part, on the nature of the stimuli: speech sounds are usually processed more efficiently when presented to the right ear, while certain nonverbal sounds, including musical sounds, are better processed when presented to the left ear. The first to provide an explanation for laterality effects in dichotic listening was KIMURA [2, 3]. She suggested that with dichotic presentation, activity in the contralateral pathways between ear and cortex may occlude activity in the ipsilateral pathways (cf. ROSENZWEIG [4]). Information presented to the right ear would therefore be relayed directly to the left hemisphere, which in most people is the more specialized for the processing of language, while information presented to the left ear would be relayed to the right hemisphere, which is specialized for processing certain kinds of nonverbal information. This would explain why, with dichotic presentation, the right ear is favored for verbal input and the left ear for nonverbal input. Recent studies have, however, demonstrated ear asymmetries with monaurally-presented stimuli and thus have suggested that Kimura's explanation may need revision [5-10]. KINSBOURNE [11, 12] has proposed an alternative explanation of laterality effects. He argues that since the left hemisphere is specialized for processing language, repeated exposure to verbal messages should result in an increase in the level of left-hemisphere activity relative to right-hemisphere activity. This in turn should direct attention to that side of the body controlled by the left hemisphere, namely, the right side. Because of this, messages delivered to the right ear would be more efficiently processed. Similarly, exposure to musical or other "right-hemisphere" stimuli would direct attention to the left, and messages d~elivered to the left ear would be favored. In the typical dichotic-listening experiment the subject is repeatedly exposed to stimuli of the same class. The resulting attentional bias, according to Kinsbourne, raay be sufficient to explain ear asymmetries in such experiments. KINSBOURNE [11] himself has suggested that one way to test his theory would be to present, in random c,rder, two different classes of stimuli, one which normally yields a right-ear advantage (REA) and the other a left-ear advantage (LEA). If both asymmetries were in fact present, this would argue against the theory, at least as the sole explanation for differences between the ears, since one could not explain both the REA and the LEA in terms of attention to one side of the body. On the other hand, a failure to find differentia' asymmetries would be consistent with Kinsbourne's theory. In lhe present study, subjects were presented with dichotic pairs of consonant-vowel (CV) syllables or musical sounds, randomly intermixed from trial to trial, and pressed a response key whenever they heard a pre-specified target sound. SPRINGER [13, 14] found that reaction times (RTs) to target CV syllables were faster when the target was presented to the right ear rather than to the left ear. KALLMANand CORBALLIS[15] found that RTs to a target musical sound *Requests for reprints may be sent to the author who is now at the Department of Psychology, University of Wisconsin, Madison, Wisconsin 53706, U.S.A. 225

226

NOTE

were faster when that sound was presented to the left ear, although this effect was evident only during the first of four consecutive experimental blocks of trials. By combining speech and musical targets in the same experimental task and by limiting the experimental session to one experimental block, it was hoped that it could be determined whether these RT asymmetries are primarily attentional as Kinsbourne's theory suggests. METHOD Twenty-nine right-handed university students with no known history of hearing disorder were paid to participate in one 20-min session. The speech sounds used in the experiment were the CV syllables/ba/, /pa/, /ga/, and /ka/, spoken by an adult male. The nonverbal sounds were 500-msec samples of the musical note " A " (440 Hz) produced on different instruments--a bassoon, viola, piano, and cello (the cello was plucked so as to sound the harmonic with frequency 440 Hz). The stimulus tape was produced with the aid of a PDP-I1 computer. Forty-eight dichotic pairs with interstimulus interval of 5.5 sec were recorded; half were musical and half were speech. In no case were musical sounds paired with CV syllables nor was any sound paired with itself. There were 12 possible pairwise permutations of the four musical sounds and likewise 12 possible permutations of the four CV syllables. The 24 possible pairs of sounds occurred randomly without replacement with the restriction that no more than five speech or five musical pairs could appear consecutively. Subjects were familiarized with the stimuli prior to the experiment. In the experiment proper, the task was to depress quickly a response button whenever a musical or speech target was heard in either ear. The CV syllable /ga/ and the sound produced on the cello acted as speech and musical targets, respectively. The probability of a target occurring on a trial in the experiment was 0.5. Response hand and headphone orientation were counterbalanced between subjects. RTs were read from a Hunter Klockcounter which was triggered by a voice key. The dichotic stimulus pairs were delivered to subjects through Koss Pro 4AA headphones powered by a Melos stereo headphone amplifier. A General Radio type 1551-C sound level meter was used to adjust sound intensity in each headphone channel to an average intensity of 76 dB (SPL). Data from subjects who did not correctly identify more than 5 0 ~ of the target speech sounds or 50~o of the target musical sounds were excluded from analysis and new subjects were run in their place. Also, any subject who failed to identify at least two right- or two left-ear targets under either the speech or musical condition was excluded. While these criteria were rather liberal in that a subject need have performed only slightly better than chance in order to be included (assuming no bias to respond positive or negative), these criteria should, if anything, bias the results against obtaining consistent RT trends. Five subjects had to be excluded due to failure to meet the performance criteria. Thus data from 24 subjects were analysed. RESULTS Harmonic means RTs for left and right ears with musical and speech sounds appear in Table 1.* The interaction between stimulus types and ears was significant, F(1,22) = 5'04, P < 0.05. Tests of simple effects (WINER [16]) demonstrated a significant REA with speech sounds, F(I,43) = 4.37, P < 0.05, but no significant ear advantage with music, F < 1. Responses made with the right hand were faster than were those made with the left, F(1,22) = 6.54, P < 0.05, but hand assignment did not interact with any other factor. Mean accuracy scores for left and right ears also appear in Table 1. A one-tailed test showed a significant speech REA (Wilcoxon matched-pairs signed-ranks test, P < 0.05). With music, the accuracy difference between ears was not significant. False alarm rates were 11 ~o with speech and 29 ~ with music. Table 1. Harmonic mean reaction times (in msec) and percentage of correct detections for speech and musical sounds presented to the left and right ears

Speech Music

(RT) 671 677

Left ear ( ~ correct) 71 85

Right ear (RT) (~o correct) 627 79 698 81

DISCUSSION The purpose of the present experiment was to test a limiting case for Kinsbourne's theory that lateral asymmetries in the processing of verbal and nonverbal sounds are due to an attentional bias to one or *Harmonic means were used to correct for positive skewing in the distributions of RT scores. An analysis of variance performed on median RT scores yielded essentially the same results.

NOTE

227

other side of the body. The subjects could not have known in advance of each trial whether they would hear speech or musical sounds, so there was no way they could have systematically directed their attention to one or other ear in anticipation of a particular category. To this limited extent, the asymmetries cannot have depended on attentional factors operating prior to the presentation of each stimulus pair. However, it is still possible to argue that the RT differences could have been due to rapid switches in attention occurring after stimulus presentation, that is, after it had been determined that a given stimulus pair was verbal or nonverbal, attention might have switched to the appropriate hemisphere for more detailed processing (cf. KINSnOURNE [12, 17]). In fact, the results of a recent study (DoNNENFELD et al. [18])conducted to test Kinsbourne's attentional model could be interpreted in this light. In that study, subjects were presented with dichotic CV syllables and dichotic pitch contours randomly interspersed. The task was to identify the sounds constituting each dichotic pair. Ear of first report was controlled so that on half of the trials subjects first reported the message presented to the right ear whereas on the remaining trials the message presented to the left ear was reported first. Ear advantages in the appropriate directions occurred only with second-ear reports. In control conditions, subjects listened to homogeneous blocks of either dichotic CV syllables or pitch contours and these yielded, respectively, right- and left-ear advantages in identification performance. However, in the control conditions, order of report was not a significant factor nor did it interact with other factors. Based on these results, Donnenfeld et al. proposed that two independent mechanisms might produce auditory asymmetries. Following BROADBENT [19] they assume that first-ear reports reflect perceptual processing whereas second-ear reports are more dependent on memory. Thus Donnenfeld et al. argue that asymmetries based in the perceptual system occur only when an expectancy (or attentional bias) develops, whereas asymmetries rising out of the memory system are relatively independent of attentional biases. However, it might be supposed that in order to find asymmetries with the ,ear of the first report, attention must be directed appropriately prior to stimulus presentation, but that by the time the subject reports the message at the second ear, an attentional bias induced by the to-be-reported dichotic pair is sufficient to produce ear asymmetries. The results of the present experiment show that the presence of an attentional bias prior to the presentation of a dichotic stimulus pair is not a necessary condition for demonstrating RT ear asymmetries. However, future research should concentrate on determining the proportion of variance that can be accounted for by either pre- or post-stimulus attentional biases under various experimental conditions.

Acknowledgement This research was supported by a grant from the National Research Council of Canada to MtCHAEL C. CORBALLIS. Thanks are expressed for his support and encouragement.

REFERENCES 1. RICHARDSON, D. H. and KNIGHTS,R. M. A bibliography on dichotic listening. Cortex 6,236-240, 1970. 2. KIMURA,D. Cerebral dominance and the perception of verbal stimuli. Can. J. Psyehol. 15, 166-171, 196l. 3. KIMURA,D. Functional asymmetry of the brain in dichotic listening. Cortex 3, 163-178, 1967. 4. ROSENZWEIG,M. R. Representations of the two ears at the auditory cortex. Am. J. Physiol. 167, 147158, 1951. 5. BAKKER, D. J. Ear-asymmetry with monaural stimulation: task influences. Cortex 5, 36-42, 1969. 6. CATLIN, J., VANDERVEER, N. J. and TriCKER, R. D. Monaural right-ear advantage in a target-identification task. Brain & Lang. 3, 470-481, 1976. 7. DOEnRtN% D. Ear asymmetry in the discrimination of monaural tonal sequences. Can. J. Psyehol. 26, 106-110, 1972. 8. FRY, D. B. Right ear advantage for speech presented monauraUy. Lang. & Speech 17, 142-151, 1974. 9. KALLMAY, H. J. Ear asymmetries with monanrally-presented sounds. Neuropsychologia. In press. ] 0. MORAIS,J. Monaural ear differences for reaction times to speech with a many-to-one mapping paradigm. Percept. & Psychophys. 19, 144-148, 1976. 1I. KINSBOURNE, M. The cerebral basis of lateral asymmetries in attention. Aeta Psychol. 33, 193-201, 1970. ;I2. KINSBOURNE,M. The control of attention by interaction between the cerebral hemispheres. In Attention and Performance. S. KORNnLUM (Editor), Vol. 4, pp. 239-256. Academic Press, New York, 1973. 13. SPRINGER, S. P. Ear asymmetry in a dichotic detection task. Percept. & Psyehophys. 10, 239-241, 1971. 14. SPmYGER, S. P. Hemispheric specialization for speech opposed by contralateral noise. Percept. & Psychophys. 13, 391-393, 1973. 15. KALLMAN, H. J. and CORBALLIS, M. C. Ear asymmetry in reaction time to musical sounds. Percept. & Psyehophys. 17, 368-370, 1975. 16. WINER, B. J. Statistical Principles in Experimental Design, p., 545. McGraw-Hill, New York, 1971. 17. KINSBOURNE, M. Eye and head turning indicates cerebral lateralization. Science 176, 539-541, 1972.

228

NOTE

18. DONNENFELD,H., ROSEN, J. J., MACKAVEY,W. and CURC10, ~7. Effects of expectancy and order of report on auditory asymmetries. Brain & Lang. 3, 350-358, 1976. 19. BROADBENT, D. E. Immediate memory and simultaneous stimuli. Quart. J. exp. Psychol. 4, 1-11, 1957. R~ sum@

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