Perceptrral andMofor Skills, 2000,91, 123-130. O Perceptual and Motor Skills 2000
STUTTERERS' VOCAL REACTION TIMES' TO UNILATERALLY
PRESENTED HIGH AND LOW FREQUENCY VERBS '
MANISH K. RAM1
University of Norfh Dakota
RICHARD
E. SHINE AND MICHAEL P. RASTATTER
Tachistoscopic Viewing Laboratory
Department of Cornmunica~iotzSciences and Disorders
East Carolina Utzjversity
Szrmmary.-The current study investigated vocal reaction times of 11 stuttering
participants to verbs with high and low frequencies of occurrence when these verbs
were unilaterally visually presented. N o significant main effects or interactions between stimuli and visual-half field were found. Reaction times were 16 msec. faster
after right visual-field presentations and the Pearson correlation coefficient between
visual fields .71 (p=.02). These results were interpreted as suggesting h a t the Iefc
hemisphere was dominant for processing the current lexical items, findings which parallel those for normal speakers.
The use of tachistoscopic viewing procedures with people who stutter
has yielded data from which we may infer that both the right and left hemispheres are involved in responses to visually presented information. Specifically, while it has been shown that the left hemisphere5 of stuttering subjects
are involved in response to language, evidence suggcscs that the right hemispheres contain the dominant neuronal structures underlying many lquistic
events, includmg object and letter naming, performing verbal processing
strategies, and evokmg lexical decisions (Hand & Haynes, 1983; Johannsen
& Victor, 1986; Rastatter & Dell, 1987a, 1987b, 1988; Rastatter, Loren, &
Colcord, 1987; Rastatter & Loren, 1988; Rastatter, McGuire, & Loren, 1988;
Rastatter & Stuart, 1995).
While studes indicate that the right hemisphere is dominant primarily
for visual language processing in those who stutter, issues pertaining to the
nature of hemispheric organization for a larger set of hguistic elements in
stuttering subjects remain equivocal. Specifically, it is not known whether
the left and right hemispheres of stutterers are involved in processing other
forms of language as they are for normal speakers. Research has shown that
the intact, normal right hemisphere shows limited linguistic processing char-
'Please direct correspondence to hfichael P. Rastatter, Ph.D., Chair, Department of Communication Sciences and Disorders, East Carolina University, Greenville, NC 27858-4353 or e-mail
(rastatterm@mail.ecu.edu).
124
M.K. RAMI, ET AL.
acteristics. Various studies have indexed the semantic capacity of the right
hemisphere using various tachistoscopic viewing techniques and stimuli (Day,
1977, 1979; Rastatter, Dell, McGuire, & Loren, 1987; Rastatter & McGuire,
1990). Rastatter and Loren (1988) provided information pertaining to the involvement of the left and right hemispheres in processing verbs by normal
subjects. In that investigation vocal reaction times of normal subjects were
measured to unilaterally presented verbs of high and low frequency. There
was a significant interaction between the stimulus items a n d visual fields.
Post hoc tests showed that vocal reaction times to verbs of high frequency
were significantly faster following presentations to the right visual field. No
significant differences in reaction times were found between the two visual
fields for the verbs of low frequency. Also, significant differences were obtained benveen the two types of verbs following presentations to the left visual field but not the right. These results were interpreted as suggesting that
right hemispheric analysis was restricted to the verbs of high frequency in the
presence of a dominant left hemisphere.
Under experimental conditions in which both the left and right hemispheres of normal subjects are involved in processing a certain lexicon, the
verbal competence of the minor (right) hemisphere is said to be inhibited by
the superior (left) hemisphere via an interhemispheric process referred to as
reciprocal inhibition ( b s b o u r n e , 1975; Moscovitch, 1976; Rastatter, et al.,
1987). This phenomenon, based on general biological principles that govern
the relationships among many laterally symmetrical structures, is related to
the magnitude of cortical substrate subserving a given function. Because the
neurological structures mediating language processing are larger in the left
hemisphere in normal, right-handed speakers, the corresponding interhemispheric inhibitory effect reflects this asymmetry and is expressed as a suppression of right-hemisphere linguistic abhties. From their study, Rastatter
and Loren (1988) inferred that, compared with unfamhar, action-oriented
hguistic processes, left hemispheric-processing mechanisms responsible for
analyzing familiar verbs impose an interhemispheric inhibition on parallel
right hemispheric hguistic functions. Unhke the high frequency items, the
latency findmgs suggested that the right hemisphere was more restricted in
its processing functions, serving possibly as an afferent channel shuttling the
verbs of low frequency to the left hemisphere for signal analysis. The curve
obtained for the verbs of low frequency was nearly horizontal, with the
intercept slightly higher (14 msec.) for the left visual-field. Based on the principles of interhemispheric inhibition, active right hemispheric processing of
the verbs of low frequency should have produced a more robust intervisualfield response time due to the reciprocal inhibiting effects of the left hemisphere. Since this was not the case, right hemisphere signal analysis was not
predicted for the low frequency items.
VOCAL REACTION TIMES FOR VERBS IN STUTTERERS
125
While information pertaining to neurolinguistic organization needs further exploration in stuttering individuals, the inferred nature of interhemispheric interaction governing the events associated with processing verbs of
high and low frequency remains uncertain. This is important since functional
differences in reciprocal, interhemispheric inhibition for other language processing tasks have been observed for the stuttering population (Rastatter &
Dell, 1987b). The purpose of the current experiment was to measure vocal
reaction times of people who stutter to unilateral, tachistoscopically presented verbs of high and low frequency to study the response configuration and
suggest a processing model. Zaidel (1983) argued that callosal relay mode of
processing is manifested as significant positive correlation while the direct access mode of processing is evidenced by a low positive correlation or a high
negative correlation. So a correlation analysis between the two lateralized
conditions should top the mode of processing for action-oriented lexical information. The concern here was primarily with examination of response
configurations and applicable models.
Eleven people who stuttered (8 males, 3 females; age range=14-56
years) and were native English-only speakers served as participants. It should
be noted that although there might be some change in reaction time over
the age range, the configuration of the responses, which is of primary inrerest, should remain constant. No effects for sex have been reported in the literature for this type of task with stutterers so none were examined. AU participants had normal or corrected-to-normal visual acuity. Case history information was used to validate the inclusion criteria of no other speech or language disorder than stuttering. Severity of stuttering ranged from mild to severe. AU participants indcated a right-hand preference (Annett, 1970).
Ten high frequency and 10 low frequency four-letter verbs from Rastatter and Loren (1988), and 20 four-letter, pronounceable nonwords were
used as stimuli. The frequency of occurrence of the high-frequency and lowfrequency verbs was at least 100 and 9.5 per mdhon, respectively, in general
reading material (Thorndlke & Lorge, 1944). The nonword stimuli were randomly arranged letters constructed from the experimental items. All nonword stimuh were developed in accordance with the rules of English phonetic sequencing. Each stimulus word was presented vertically in capital letters (Font: Geneva; Size: 14; Style: Bold) using a PICT f i e format created
with a commercially available software (Claris Corporation Model MacDraw
I1 Ver. 1.1). All stimuli were presented unilaterally to both the left and the
126
M.K. RAMI, ET AL
right visual fields at a subtended angle of approximately 3.5" (83 pixels)
from the central fixation to the lateral periphery.
Apparatus
The visual stimuh were presented to both visual fields (right and left)
through the use of SuperLab Pro (Cedrus Corp., 1990), an experimental software program that presents visual stimuli on a 12-in. monitor (Applevision
Model 1710 AV Display) with the screen resolution set at 640. 480 pixels,
which served as a tachistoscope. Each stimulus item was presented for a duration of 100 msec. Simultaneously with the onset of illumination, SuperLab
Pro's presentation timer activated a second dtgital timer, accurate to 1 msec.
A signal from the voice-operated relay stopped the timer, which provided a
vocal reaction time (rnsec.) for each stimulus item. The timer was stopped
by a signal from a voice-operated relay (SuperLab Pro) activated by a microphone (Apple Model Plaintalk) placed approximately 65 cm from the lips
with an orientation of 0" azimuth and 20" altitude. This timed activation
yielded a lexical decision response time for each stimulus item.
Procedure
A participant was seated in front of a computer screen a t a distance of
64 cm so that a constant visual angle was maintained throughout the experiment. The head position was secured on a chin rest, and each person was
asked to focus on a central fixation point present on the computer screen.
The experimenter explained that a series of vertically typed words would appear to- the left or the right of the fixation point. The participant's task was
to phonate the vowel /a/ into the microphone when the stimulus was an actual word. Participants were asked not to phonate when a nonword was presented. All stimulus words were presented for an exposure duration of a 100
msec. An interstimulus interval of 2000 msec. was employed. The order of
stimulus presentation was randomized, with the restriction that an identical
four-letter stimulus would not appear in direct sequence. The order of presentation was identical for all participants.
RESULTS
AND DISCUSSION
The means and standard deviations summarized in Table 1 were subjected to a repeated two-factor analysis of variance to test the significance of
main effects and interactions. Main effects for visual field (F1,,,=0.12, ns,
o2= 0), verb frequency ( F , , , =3.36,
,
ns, oZ= 0), and the interaction of visual
field by verb fluency (F,,8=1.62, ns, w2=0) were not significant.
The Pearson correlation between the reaction times for the right and
the left visual fields, collapsed across verb frequency, was statistically sign&cant (r = .71, p = .02).
These results showed that the main effects for stimuli and visual-half
VOCAL REACTION TIMES FOR VERBS IN STUTTERERS
127
TABLE 1
A N D STANDARD
DEVIATIONS
O F REACTION
TIMES(MSEC.)FOR
GRANDMEANS
~ G H T A N DLEFTVISUAL
FIELDS
FORALLVERBSCOMBINED
(n= 11)
Right Visual Field
Verbs
Left Visual Field
M
SD
M
SD
873
172
889
205
field were nonsignificant, with vocal reaction times favoring the right sensory
inputs by 16 msec. Such a finding best fits an interhemispheric transfer interpretation. This indicates that, although each hemisphere may be capable
of performing a component of a given processing task, the stages of processing required to complete the operation are functionally localized to one
hemisphere (Moscovitch, 1976). When stimuli are delivered to the hemisphere capable of partial processing only, the information eventually must be
sent to the specialized hemisphere via a transcallosal route.
Left
Right
Visual Field
FIG. 1. Mean reaction times (in msec.) for verbs of high ( 0 ) and low ( A ) frequency by
visual field
The correlations between the two visual-half fields support an inference
of interhemispheric transfer as well. Correlational analyses between lateralized conhtions have been used to prehct the presence of left and right
hemispheric processing capacities. Zaidel (1983) argued that the presence of
a high positive correlation predicts an interhenlispheric transfer interpretation since the processing strategy of a single hemisphere is responsible for
completing the task demands. Under these conditions the dominant hemi-
128
M. K. M I . ET AL.
sphere must be accessed for the task to be completed, regardless of the
hemisphere stimulated. The correlation between visual fields was high and
significant in this experiment, hence the inference of an interhemispheric
transfer interpretation is supported for these stuttering subjects' data.
Such findings parallel the normal subjects of Rastatter and Loren (1988)
in certain respects but diverge in others. Specifically, the finding that reaction times were faster following- right visual-field stimulation suggests that
the left hemispheres of the stuttering participants were dominant for the
current lexical decision tasks, which findings are consistent with the data derived from the normal speakers. Zaidel (1983) suggested that a consistent
laterality effect of modest magnitude would be predicted in the intact brain
based on conditions of exclusive specialization of hemispheric processing.
Under these processing condtions, wherein perceptual laterality effects are
based on "callosal connectivity," information delivered initially to the hemisphere not containing the resources required for stimulus analysis must be
shuttled via the corpus callosum to the hemisphere specialized for performing the task demands required for analysis.
In addition to the finding that the left hemisphere was responsible for
analyzing the current verb s t i m d , the main effect for verb type was nonsignificant. Such a fiidmg is consistent with the normal speaker's data, suggesting that there is no processing hierarchy for analyzing verbs of high versus
low frequency. Ln other words, the time involved in generating a lexical decision for the two types of verbs was not related to frequency of occurrence
of the hguistic set. Such a finding is intriguing and suggests that neurolinguistic organization for certain lexical classes are parallel for stuttering and
nonstuttering speakers. These findmgs are not consistent with hemispheric
organization for other lexical classes in the stuttering population. While it
has been inferred from empirical work that the left hemispheres of stuttering
subjects can process visual language, evidence suggests that the right hemisphere is dominant for performing many hguistic events, including object
and letter naming, performing verbal processing strategies, and evoking lexical decisions (Hand & Haynes, 1983; Johannsen & Victor, 1986; Rastatter &
Dell, 1987a, 1987b, 1988; Rastatter, Loren, & Colcord, 1987; Rastatter &
Loren, 1988; Rastatter, McGuire, & Loren, 1988; Rastatter & Stuart, 1995).
While the results obtained for the stuttering subjects seem parallel to
those observed for the nonstuttering participants of Rastatter and Loren
(1988) with respect to laterality characteristics, the vocal reaction times for
the stuttering participants suggest that interhemispheric inhibiting patterns
underlying the processing operations for completing the current task demands are likely not the same as those of normal speakers. As noted above,
the stuttering participants' latencies were considered consistent with an inference of interhemispheric transfer wherein the left hemisphere is responsi-
VOCAL REACTION TIMES FOR VERBS IN STUTTERERS
129
ble for verb recognition. Such a finding may indicate that inhibitory interactions between the two hemispheres governing verb assimilation among stutterers may not be related to frequency of occurrence. As noted earlier, Rastatter and Loren (1988) suggested the right hemispheres of the normal participants were involved in completing lexical decisions for the verbs of high frequency. On such findings were based the inference that the right-hernispheric, linguistic substrates are capable of processing such information in nonstuttering samples.
The vocal-reaction findings obtained for the stuttering participants, however, suggest that the right hemisphere was not involved in the performance
of lexical decisions on the verb stirnd. Given that the inference the right
hemisphere is organized and based on associations, emotions, and structural
correspondence, and responsible for various hguistic decoding and encoding events in stutterers, limitations in neural resources may necessitate bilateral linguistic processing related to neurohguistics by stuttering people.
Finally, the current results, taken in concert with past neurolinguistic
evidence, suggest that a diversity of sensorilinguistic models may be operational for stuttering samples. While the relation between hemispheric-hguistic processing and stuttering behavior is not clear, the present findmgs increase our understanding of the psychophysical dimensions subserving language-related events in stuttering subjects. Such avenues of experimentation
may broaden our empirical base and foster theoretical efforts pertaining to
the complex neuronal dynamics that underlie or accompany stuttering behavior.
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Accepted ]tine 13, 2000.