Visual information processing in the left and right hemispheres during unilateral tachistoscopic stimulation of stutterers

J. FLUENCY DISORD. I1 (1986), 285-291

VISUAL INFORMATION PROCESSING IN THE LEFT AND RIGHT HEMISPHERES DURING UNILATERAL TACHISTOSCOPIC STIMULATION OF STUTTERERS HELGE S. JOHANNSEN Phoniutrir

Oufputient

DepurtmcJnt.

and CLAUDIA Uni\,ersity

VICTOR

of U/m, Fcdercrl Rcprrhlic, c$Getmtrn~

Tachistoscopy was used to investigate 42 stutterer\ and a corresponding control group for their speech dominance. A bilateral cortical responsibility for speech was equally frequent in the two groups. However. 2.5 times as many stutterers showed a superiority in the information processing in the right hemisphere than did nonstuttererb.

INTRODUCTION Various methods have been used to investigate the functional specialization of the two brain hemispheres in stutterers and control subjects who speak fluently. These include temporary anesthetization of the cerebral hemispheres (Jones, 1966; Andrews et al., 1972; Luessenhop et al., 1973), dichotic listening (Curry and Gregory, 1969; Quinn, 1972; Brady and Berson, 1975; Rosenfield and Gdodglass. 19801, averaged evoked responses (Zimmermann and Knott, 1974; Ponsford et al., 1975), hemispheric alpha asymmetries (Moore and Lang, 1977; Pinsky and McAdam, 1980). and cortical blood flow (Wood et al., 1980; Boberg et al., 1983). The method of tachistoscopic visual stimulation has been applied by Moore (1976) and Hand and Haynes (1983), but only by Moore (1976) in a way comparable with that in our investigation since Hand and Haynes (1983) investigated the manual and voice reaction time responses to tachistoscopically presented meaningful and nonmeaningful stimuli in a different paradigm. A restriction of the visual information transmission into only one hemisphere can be attained with tachistoscopic presentation of visual stimuli. The very short tachistoscopic presentation time should prevent possible eye movements and, thus, bilateral cortical projection of the stimuli. Speech stimuli presented in the right visual field and, thus, transmitted Address University

correspondence

to Dr. Helge S. Johannsen,

of Ulm,

Ulm.

D-7900

Federal

0 1986 by Elsevier Science Publishing Co.. Inc. Ave.. New York. NY 10017

52Vanderbilt

Republic

Phoniatric of Germany.

Outpatient

Department.

285

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H. S. JOHANNSEN

and C. VICTOR

primarily to the left hemisphere are recognized and named more rapidly and certainly than stimuli presented in the left visual field (McKeever and Huling, 1970; Hines, 1972). The dominance of the left hemisphere is shown more distinctly in recognition of abstract rather than concrete nouns (Ellis and Shepard, 1974, Hines, 1978) and also of words that only elicit a visual imagination with difficulty (Day, 1979). We used tachistoscopy to determine if there are differences in brain dominance for language between stutterers and nonstutterers, especially because the findings of Moore (1976) are questionable as a result of quantitative and methodologic limitations.

METHODS

Subjects Forty-two stutterers were investigated (36 men and six women, aged between 1 I and 40 years. with an average age of 18.8 years). Younger stutterers were not included in the study because of possible uncertainties in reading. The control subjects with the same distribution were fluently speaking medical students and conscripts who had never stuttered as far as they could remember. Thirty-eight of the stutterers and 39 of the control subjects were right-handed; four of the stutterers and three of the fluently speaking subjects were left-handed. None of the persons participating in the study had indications of brain disease or cerebra1 trauma in the case history.

Stimuli One hundred and twenty slides consisted of 60 different combinations of one number (from one to nine) and a nonsense syllable. The number and syllable were randomly combined. Letters and numbers were black. and the background was white. The nonsense syllables were combined from the consonants D, F, G, K, P. S, and T and the vowels A, E, and 0 in the arrangement of consonantvowel-consonant. The height of the individual letters or figures was 0.4 cm; total height of the vertically arranged syllable was 1.45 cm. The distance between the number arranged in the middle and the syllable to the left or right of it was 1.6 cm.

Apparatus A two-channel projection tachistoscope Carousel S-AV 1000 projectors (Getra, used.

(GA 1198 A-2) with two Kodak Albrecht Instruments Ltd.) was

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Procedure A slide with a black fixation point on a white field was shown before each of the 120 slides. The test subjects sat so far from the projection screen that they saw the vertically arranged nonsense syllable 2” to the right or left of the number that had replaced the fixation point. The presentation time for the slides with number/syllable varied between 90 and 200 msec; a graduation into time steps of 10 msec was possible. It was normed individually for each subject so that about 50% of the 120 slides presented were correctly recognized. The fixation point was shown until the subject specified the number and syllable; this was an average of 4 seconds in the control subjects and was occasionally longer in the stutterers (between 4 and 12 seconds). The test subjects first reproduced the number, and then the syllable. One slide was rated as correctly recognized only when the number and syllable were correctly specified. The results were evaluated as follows: the test subjects who recognized more syllables correctly when they were arranged to the right of the number were regarded as left-lateralized. Those who correctly specified more syllables to the left of the number were designated as right-lateralized. Subjects were regarded as not unequivocally lateralized when they correctly recognized the same number of syllables on both sides or when the difference between the two sides was less than or equal to 2. RESULTS The average tachistoscopic presentation time differed in the suttering and nonstuttering test subjects. In the stutterers, it was 140 msec, and in the control subjects, it was 120 msec. Twenty-nine (69%) of the 42 control subjects but only 17 (41%) of the stutterers required up to 120 msec to recognize approximately 50% of the slides correctly. Twenty-one stutterers (50%) recognized more syllables in the right visual field and were considered to be left-lateralized. Eleven stutterers (26%) recognized more syllables in the left visual field and were considered to be right-lateralized. Ten stutterers (24%) had no unequivocal hemispheric dominance for the stimulus material used. The latter observation also applied to the control subjects (24% without unequivocal lateralization), whereas 28 (66%) of them were left-lateraiized and 4 (10%) were right-lateralized (Fig. 1). Analogous to the mode of evaluation applied by Levy and Reid (1976), the performance for each visual field was measured as a ratio of the correctly identified syllables to the presentation time. In application of the Wilcoxon test for independent random samples, a significant difference was shown between the stutterers and the control subjects for alpha = 0.05.

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No.

of subjects

40

n=4 *left field advantage n=ll

30

20

10

right field *advantage

-

n=21

n=28

equal *scores

field -

n=lO

n=lO

stutterers

Figure 1. Distribution of right-field field scores in a group of stutterers

nonstuttering controls

advantages, and a control

left-field advantages.

and equal-

group.

If one considers the ratio of the syllables correctly recognized in the left visual field to the total number of correctly reproduced stimuli (that is, the sum of the syllables correctly specified on the right and left) for each test subject, the distributions shown in Figure 2 result. In the stutterers, the maximum was in the region of 50% with a second peak at around 55%. In the distribution curve of the control subjects, two maxima were substantially below the 50% mark. The 50% mark was exceeded by 35.7% of the stutterers and 20.45% of the nonstutterers, which corresponds to a dominance of the right brain for the stimulus material used in this subgroup of stutterers and nonstutterers.*

* The difference in the two types of calculation for lateralization resulted from the application of more stringent criteria in the first group. In this group. only those subjects who correctly recognized two stimuli more on one side than on the other side were rated as contralateral having an advantage in the right or left field and. thus. corresponding dominance.

TACHISTOSCOPIC

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and control subjects (m1. m)on the basis of stutterers (-) of the ratio of tachistoscopically presented stimuli correctly recognized in the left half-field and the total number of correct recognitions.

Figure 2. Distribution

DISCUSSION In our study,

the group of stutterers required a longer average time of tachistoscopic presentation to recognize 50% of the numbers/nonsense syllables on the stimulus slides. This finding may support the results of Hand and Haynes (1983), who found longer vocal and manual reaction times for the stutterers compared with a control group of fluently speaking subjects. However, whether this is caused by a lack of efficiency of the central nervous system as discussed by Wall and Myers (1984) must remain an open question for now. We decided on the stimulus material used here because it is known from neurophsychologic investigations in normal subjects (as mentioned) that meaningful words are processed differently in the two hemispheres, depending on their degree of abstraction and the readiness with which optical images are formed. It appeared to us to be practically impossible to find 60 words that were regarded as equivalent in terms of these criteria. Under the investigation conditions described, 66% of the control subjects showed dominance of the left brain as compared with 50% of the stutterers, but 2.5 times as many stutterers (26%) than control subjects (10%) had a dominance of the right brain. Furthermore, the results summarized in Figure 2 show that a total of seven stutterers (17%) deviated from the normal distributation of the control group (i.e., a subgroup of stutterers, but not all stutterers, displayed different dominance conditions for speech than did normal subjects). This

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support the thesis that stuttering is not an etiologically homogeneous disorder. a theory that is gaining ever greater acceptance. Using the same method as shown in Figure 2 (i.e., ratio of the syllables correctly recognized in the left visual half-field to the total of correctly reproduced stimuli). Moore (1976) found that 53.3%’ of the stutterers and only 20%~ of the control subjects had rates above 50%. As shown before, the rate was 35.7% in the stutterers in our study and 20.45% in the nonstutterers. The result of Moore (1976) were based on a small sample of 15 stutterers and IS control subjects. He used only four pairs of words during bilateral tachistoscopic presentation (i.e., eight meaningful words such as “pears. tears.” etc., so that each combination turned up six times in his set of 24 slides). The fact that three of the IS stutterers were not able to identify any item in the right visual half-field leads to the conclusion that they concentrated only on the left half-field, which is an unlikely result when using our stimulus material. In spite of these differences, the statement that more of our nonstuttering subjects had a dominance in the right visual half-field, whereas more of the stutterers had a dominance in the left visual half-field is identical.

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