Visual coding dominance in stuttering: Some evidence from central tachistoscopic stimulation (tachistoscopic viewing and stuttering)

J. FLUENCY DISORD. 13 (1988), 89-95

VISUAL CODING DOMINANCE IN STUTTERING: SOME EVIDENCE FROM CENTRAL TACHISTOSCOPIC STIMULATION (TACHISTOSCOPIC VIEWING AND STUTTERING) MICHAEL

P. RASTATTER

Department of Communication Disorders Bowling Green State University Bowling Green, Ohio

CATHERINE

A. LOREN

Program in Communication University of Houston Houston, Texas

Disorders

The current study examined central visual information processing style in a group of stuttering and nonstuttering subjects. Subjects were required to phonate the vowel /al when tachistoscopically presented a flash of light. Following this procedure, central, tachistoscopic letter-naming reaction times were obtained for the subjects at a rate of presentation producing approximately 90% errors. Adjusted letter-naming reaction times were derived by subtracting the former from the latter value. Results of a r-test procedure showed that significant differences in letter naming reaction time existed between the two groups with the stutterers being slower than the nonstutterers. Analysis of errors showed that the nonstuttering subjects’ confusions were primarily visual while the stutterers’ recognition errors were auditorily based. These findings were interpreted to suggest that the right hemisphere maintains the greatest interhemispheric inhibiting effect in stutterers and will govern responses regardless of whether the processing task can be completed more efficiently (in time) by the left hemisphere.

INTRODUCTION Recently, we investigated hemispheric information processing (or coding strategies) and its relationship to linguistic dominance for written material in stutterers (Rastatter et al., 1987b). In that study, unilateral tachistoscopic letter-naming reaction times were obtained for stuttering and non-

Address correspondence to Dr. Michael Rastatter, Associate Dean, College of Health and Human Services, Bowling Green State University, Bowling Green, Ohio 43403-0280. 0 1988 by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Ave., New York, NY 10017

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stuttering subjects at presentation windows producing an average group error rate of approximately 90%. Visual processing errors were classified as either visual confusions (errors of appearance) or auditory confusions (errors of sound). Results showed that the left hemisphere of stutterers processed the letter stimuli within the parameters delivered to the linguistic cortex and generated vocal reaction times commensurate with those of the normal controls. The right hemisphere, however, was found to perform a transcoding operation where the visual information was processed in an auditory mode. Additionally, vocal reaction times following left visual field stimulation proved to be significantly slower as compared to the nonstuttering subjects. Such findings indicated that when the two hemispheres of stutterers are isolated through unilateral stimulation, and are required to respond optimally to the information delivered, they are capable of decoding the stimuli in a style mandated possibly by the underlying organizational structure of each hemisphere. What was not resolved from that work, however, is whether the left or right hemisphere dominates in its processing style since the existence of bimodel hemispheric coding is now evident in the stuttering population. Although the left hemisphere was capable of performing its operation with greater speed in our stuttering subjects, it remains possible that the style of linguistic processing subserving the superior hemisphere will govern the processing characteristics of the intact brain of stutterers when the two hemispheres are simultaneously presented a series of identical lexical items. Such a theory would call for the existence of a linguistic processing style/speed of operation trade-off in stutterers, a phenomenon that may well account for the current picture of right hemispheric dominance for visually-presented material in stutterers (Hand and Haynes, 1983; Johannsen and Victor, 1986; Moore, 1976; Rastatter and Dell, 1987b). The purpose of the present study was to assess central information processing style in a stuttering population. A tachistoscopic viewing procedure employing a letter confusion and vocal reaction time task was used to address this issue. In doing so we wished to measure interhemispheric coding competition under a condition where each hemisphere is simultaneously activated via central tachistoscopic stimulation.

METHOD Subjects Stuttering Subjects. The experimental procedures were administered to 16 subjects (ten male and six female, aged 19 to 34 years) with a selfreported history of stuttering. Seven of the stutterers were rated as being very severe while six were rated as moderate and three as mild during

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reading and conversation by a speech-language pathologist. All subjects had normal hearing bilaterally; pure-tone, air-conduction levels were 10 dB HL for the octave frequencies 250-1000 Hz. They were native speakers of English and had no history of neurological damage. All subjects were right-handed as determined by the Classification of Hand Preference by Association Analysis (Annett, 1970). Additionally, each subject passed the visual screening procedure for tachistoscopic viewing experimentation proposed by McKeever and Huling (1971). Normal Subjects. Sixteen normal subjects (ten male and six female) matched with the experimental subjects for age and educational level were administered the experimental procedures. None of the subjects had a history of speech, hearing, or neurological anomalies. All were righthanded and evidenced normal hearing and visual acuity at the time of testing as determined by the procedures described above.

stimuli

Stimuli were eleven capital letters: A, H, I, 0, S, U, X, Y, Z, T, and V. These letters, the same as those used in our earlier study (Rastatter et al., 1987c), were chosen for their symmetry around the vertical axis. Each letter was typed (IBM Selectric II electric typewriter) on an index card cut to fit the card tray of a constant-illumination tachistoscope (Lafayette Model U-l). The letters were positioned in the center of each index card, corresponding to the central fixation point of the constant illumination mode of the tachistoscope. The order of the letters was random with the criteria that an identical letter did not appear in direct sequence. All subjects received the same order of presentation. Instrumentation The letters were presented centrally by the tachistoscope. Simultaneously with onset of illumination, the tachistoscope’s presentation timer activated a second digital timer (CMC, Model 7078) accurate to 1 ms. The timer was stopped by a signal from a voice-operated relay (Grason-Stadlet-, Model E7300-1) activated by a microphone (Grason-Stadler, Model E7300A-2) after a subject phonated, rendering a vocal reaction time value for that particular stimulus item. Procedure Simple vocal reaction time condition. Each subject focused on a circular fixation point that was presented under the constant illumination mode (of the tachistoscope). It was read to them that a light would flash

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Table 1. Letter Recognition Reaction Times and Visual the Stuttering and Nonstuttering Subjects Letter recognition reaction timesa Rule Stutterers Nonstutterers

Confusion

A. LOREN

Errors

for

Visuai confusiot&

T7

SD

Auditory

Visual

386 321

31 20

74 8

26 92

L?Data presented in ms. Means significantly different at p < 0.01. b Data presented in %. All pair-wise comparisons significantly different at p cc 0.01.

at a random interval (ranging from 1 to 3 sec.) following the word “focus” spoken by the examiner. The subject’s task was to phonate the vowel /a/ as rapidly as possible into the microphone (located approximately 10 cm from the subject’s mouth) when the flash of light occurred. Each subject made 30 responses. This procedure was done in an attempt to eliminate individual differences in laryngeal psychomotor latency (Rastatter et al., 1987c).

Stimulus Detection Condition Each subject focused again on the circular fixation point that was presented under the constant illumination mode (of the tachistoscope). It was read to them that a series of letters would appear in the fixation circle. The subject’s task was to name the letter as fast and accurately as possible into the microphone. In order to determine stimulus detection thresholds for each subject, a descending procedure was employed. Stimuli were presented initially at 100 msec followed by successive 10 msec decreases. At each interval the 11 letters were presented. This procedure continued until approximately a 90% error rate was produced by the subjects. It should be noted that, although in error, subjects reported that they were able to detect the form of the letters, thereby enabling them to provide a response to each stimulus input. Once stimulus detection thresholds were determined, the series of 11 letters were presented five times consecutively, generating 55 responses per subject.

RESULTS Adjusted letter-naming reaction times were obtained for the stimulus detection condition. Mean simple vocal reaction times were calculated and subtracted from the mean letter naming response time for each subject (Table l).’ These results indicate that the stutterers showed increased i It should be noted that the group mean letter naming reaction time value is based on approximately 90% of the total number of responses since only those items producing an error in recognition were submitted to statistical analysis.

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latencies when responding to the letters. Appropriate pair-wise t-tests showed the average difference in reaction time between the two groups of subjects was significant [t(31) = 3.66, p < 0.011.

Error Data Each subject produced approximately a 90% error rate across the experiment. From these data, errors of recognition were classified as either visual or auditory confusions. The six pairs of letters, V-U, V-Y, V-X, O-U, X-Y, and X-Z were considered confusable along the visual axis while the six pairs of letters, A-H, I-Y, X-S, H-S, T-Z, and H-X were deemed confusable along auditory lines. A percentage of visual and auditory confusion errors were then derived for the normal and stuttering groups and are shown in Table 1.’ Results of an analysis of variance procedure showed that a significant interaction existed between the group and error variables [F(1,30) = 16.82, p < 0.011. Post-hoc Boneferroni pair-wise tests were employed to determine the sources of the significant interaction. Results showed that all pair-wise comparisons were significantly different (p < 0.01) (Nonstutterers: Visual Errors = 8%; Auditory Errors = 92%; Stutterers: Visual Errors = 74%; Auditory Errors = 26%).

DISCUSSION The current results showed that a significant interaction existed between the group and visual-confusion variables. Post-hoc tests revealed that the stuttering subjects evidenced significantly more errors that were characterized by auditory confusions while the nonstuttering subjects produced errors that were visually based. Additionally, letter naming reaction times generated by the stuttering subjects were significantly slower than corresponding reaction times obtained for the fluent controls. Collectively, such findings suggest that when the left and right hemispheres of stutterers were simultaneously stimulated by the tachistoscope, the processing strategy employed by the right hemisphere proved dominant in completing the perceptual task. If the reverse of this process were found to occur, vocal reaction times of the stutterers would have been similar to those generated by the nonstutterers. Our earlier work involving hemispheric coding strategies showed that unilateral, left hemispheric stimulations produced comparable psychomotor latencies between stuttering and nonstuttering subjects (Rastatter et al., 1987~). We suggested that such a finding was most likely a concomitant of the information processing style of the stutterers’ left hemi’ Approximately 31% and 35% of the total number of errors for the nonstuttering and stuttering subjects, respectively, were eliminated from the confusion analysis since neither an auditory nor visual type error occurred.

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sphere. As noted earlier, letter-naming processes of the stuttering subjects following right visual field stimulations proved to be founded primarily on visual information. Because of this, the current results should have yielded a larger percentage of visual errors for the stutterers if the speed of the coding operation in the left hemisphere was capable of surmounting the less efficient stimulus transformational functions of the right hemisphere. Obviously, such was not the case when hemispheric processing styles were rivaled in the current group of stutterers. Since previous tachistoscopic viewing research has shown that the interhemispheric inhibiting effect of the right hemisphere in stutterers is greater than the corresponding effect of the left hemisphere (Rastatter and Dell, 1987b), it follows that under central tachistoscopic conditions the hemisphere maintaining the greatest interhemispheric inhibiting effect should govern the response, regardless of whether the task can be completed more efficiently (in time) by the other hemisphere. More specifically, the principles of reciprocal interhemispheric inhibition (Kinsbourne, 1975; Rastatter and Dell, 1987b; Rastatter et al., 1987~) appear to be the prevailing force accounting for the current results. This phenomenon, which is related to the extent and direction of inhibition imposed by many lateral symmetrical neurological structures, suggests that the hemisphere maintaining the greatest representation of neurological substrate will govern or suppress the functions of the homologous area of the other hemisphere in the intact brain. It appears that an interhemispheric “coding” dominance characteristic may be functioning that serves to overrule or inhibit certain more efficient linguistic functions in the intact brain of stutterers. Because of these interhemispheric interactions, processing efficiency, as defined by the speed of stimulus analysis, does not appear to be the critical variable in determining perceptual processing characteristics of written material in the stuttering population. Such a theory may well account for our earlier findings showing that visual lexical decision processing times of a group of stutterers were significantly more latent as compared with their normal controls (Rastatter and Dell, 1987a). Based on our findings further research is warranted studying the effects of other language processing events in stutterers by contrasting decoding strategies and processing latency in order to test the findings presented in the current paper. REFERENCES Annett, M. (1970). A classification British Journal

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