The purpose of the different colored inks was to enable the ex- perimenters to .... The history of this doctrine is summarized beautifully in a paper by ... thorough reviews) support the view that tactile perception in man is equally represented in ...
BRAIN
AND
LANGUAGE
Dichhaptic
MARLENE
6, 323-333 (1978)
Hand-Order Effects with Verbal and Nonverbal Tactile Stimulation OSCAR-BERMAN, LUCIO REHBEIN, AND HAROLD GOODGLASS
ALAN
PORFERT,
Boston University School of Medicine und Boston Veteruns Administrution Hospitul The palms of normal right-handed subjects were stimulated dichhaptically, i.e., with competing, bimanually presented tactile stimuli consisting of pairs of letters, pairs of digits, or pairs of line orientations. The subjects were required to identify both stimuli in a particular order, and order of report was compared between hands and across stimulus materials. Results indicated right-hand superiority for letters and left-hand superiority for lines; no hand differences occurred for digits. However, observed differences between hands appeared with second reports only, suggesting that measures of tactile storage are more sensitive to laterality differences than measures closer in time to actual stimulation.
From clinical and experimental evidence, it is well established that each of the cerebral hemispheres in humans is better equipped to handle some kinds of information than others (see Blakemore, Iversen, & Zangwill, 1972; Broadbent, 1974; Dimond & Beaumont, 1974; Kinsbourne, 1975;and Liberman, 1974, for thorough reviews). The asymmetries typically are reflected in differential performance levels by the two ears or the two visual fields (with stronger contralateral than ipsilateral functional connections to the cerebral hemispheres), the superior side depending upon the nature of the stimuli presented. That is, verbal stimuli are processed better by the left half of the brain than by the right, and nonverbal stimuli are better processed by the right hemisphere than by the left. Recently, work in this area has turned toward an analysis of task requirements which can bring out or exaggerate hemispheric asymmetries (e.g., Goodglass & Peck, 1972; Moscovitch, Scullion, & Christie, 1976; Oscar-Berman, Goodglass, & Cherlow, 1973; Oscar-Berman, Goodglass, & Donnenfeld, 1974; Witelson, 1974). For example, until the late 1950s hemispheric dominance in dichotic listening tasks was assumed to reflect superior perception rather than This work was supported by USPHS Research Grants NS 10577, NS 06209, and NS 0761.5, by USPHS Research Career Development Award K04 NS 00161, and by funds from the Medical Research Service of the Veterans Administration. The help of Drs. Marylou Ried, Roger Graves, and Mary Hyde is gratefully acknowledged. Address reprint requests to Dr. M. Oscar-Berman, Aphasia Research Center, BVAH, 150 South Huntington Avenue, Boston, MA 02 130. 323
0093-934x178/0063-0323$02.00/O Copyright 0 1978 by Academic Press, Inc. All rights of reproduction in any form reserved.
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storage of the stimulus material. Based upon Broadbent’s (1957) observation that subjects listening to dichotically presented digits, tended to report first from the right ear before reporting from the left, a series of experiments had been reported with the specific aim of clarifying the “ear-order” contributions to perceptual laterality effects. All of these studies (e.g., Inglis & Sykes, 1967; Satz, Aschenbach, Pattishall, & Fennell, 1965;Goodglass & Peck, 1972) accepted Broadbent’s original assumptions (a) that a perceptual system governs first-ear reports (by passing along information to the next stage of processing as soon as it is received), and (b) that a storage system governs second-ear reports (by holding its information in reserve while the other system is occupied). By and large, studies of the ear-order effect have supported the idea that second-ear reports result in greater laterality effects than first-ear reports (Broadbent, 1958; Bryden, 1963; Goodglass & Peck, 1972; Oscar-Berman et al., 1974). For example, Satz et al. (1965) and Goodglass and Peck (1972) have found that the right ear is far superior to the left under second order of report of dichotically presented verbal stimuli, and Oscar-Berman et al. (1974) found a reversed second-ear asymmetry (i.e., left-ear superiority for nonverbal tonal contours). Because there is also considerable evidence that hemispheric differences in processing a variety of visual materials may be exaggerated when storage is required (Dee & Fontenot, 1973; Hines, Satz & Clementino, 1973; Moscovitch et al., 1976; Oscar-Berman et al., 1973), the present study was designed to examine the phenomenon in a third modality, touch. Several studies of somesthetic function in unilaterally brain-damaged patients already have provided evidence for greater right than left hemisphere involvement in the analysis of nonlinguistic cues, e.g., shapes, orientations and whole-part relationships (Corkin, 1978; Nebes, 1974). The findings obtained on unilaterally damaged subjects call into question the generality of the long held “doctrine of contralateral innervation” (see Benton, 1972, for a historical review); this doctrine states that somesthetic functions of each ‘half of the body are controlled in the contralateral cerebral hemisphere. Examination of normal hemispheric contributions to the analysis of verbal and nonverbal tactual materials has been neglected, however, primarily because the methods used to date have been unsuccessful in demonstrating clear-cut left-right dissociations (Corkin, 1978; Witelson, 1974, 1976).The present study employs a variant of a “dichhaptic” (competing simultaneous bimanual tactile) stimulation technique’ to explore the possibility of superior temporary storage of verbal vs nonverbal materials presented dichhaptically to the two hemispheres. Specifically, we wished to determine in normal subjects whether or not (a) the right and left hands, with predominantly contralateral connections to the brain, are 1 For a description (1949), and Witelson
of this general procedure, (1976).
see Bender (1943, Corkin (1978), Critchley
DICHHAI’TIC
STORAGE
325
differentially sensitive to haptically presented letters, digits and line orientations, and (b) storage of these materials, as measured by secondhand reports, is a more sensitive measure of laterality than a measure closer in time to actual perception (first-hand reports). METHOD Subjects The subjects were 15right-handed students or hospital staff members ranging in age from 19 to 41 years, with an average age of 25. There were eight women and seven men. All had formal educational backgrounds which were above average (from 13 to 24 years, with a mean of 17 years).
Apparatus
and Procedure
The procedure involved identification of two stimuli presented simultaneously, one to each hand, by two experimenters seated next to each other, opposite the subject and out of his/her view (see Fig. 1). There were three separate sets of stimuli: letters, digits, and lines differing in orientation. Each set of stimuli contained four different items which could be drawn by a single continuous stroke onto a 1.5-in. square within the center of the subjects’ palms. The four letters were capital consonants C, N, S, and V; the four digits were 2, 3, 6, and 8; the four lines were /, \, -, and ) . For each set of stimuli, the palms were clearly marked in ink with overlapping but differently colored tracings made through a set of standard cardboard stencils. The purpose of the different colored inks was to enable the experimenters to differentiate the stimuli when they were to be traced. Order of presentation of stimulus series was counterbalanced across subjects. Each stimulus was presented six times on each palm, twice with each other stimulus in its class of material (for a total of 24 trials per class), and no item was ever paired with itself. From the subject’s view, the stimuli were drawn in the direction in which the subjects would write them on their own palms, i.e., from the top of the palm to the bottom, in most instances. The use of a “passive” procedure (experimenters controlling delivery of the stimuli) with brief exposure durations (less than two seconds) prevented differential palpation times by the two hands, and feedback from ipsilateral motor pathways involved in gross movement (Sperry, Gazzaniga, & Bogen, 1969). Response mode was verbal for letters and digits, and nonverbal for lines (pointing to the correct stimuli on an available choice card containing the four stimuli) in order to maximize involvement of the hemisphere most involved in processing the information (after Witelson, 1974). The subject and the experimenters were separated by a table containing a curtain that could be raised and lowered (see Fig. 1). The curtain prevented visual contact between subject and experimenters, and kept subjects from seeing their own hands during testing. The subjects rested their forearms, palms up, on the table under the curtain. The table was cushioned by a sheet of 0.5-in foam rubber. The table also contained leather straps which served as thumb holders to keep subjects’ hand positions constant during the sessions. The distance between palms was approximately 15 in. Stimuli were drawn within a l.S-in. square in the middle of the palms, using the back ends of Delta No. 2 paint brushes. Midway through each set of stimuli, the experimenters changed positions to control for possible differences in pressure and speed of stimulus delivery by the two experimenters. Subjects were instructed that they would feel a different stimulus on each palm, and they were encouraged to identify both if possible. Further, they were instructed to identify the stimuli in a particular order, indicated only after each instance of dichhaptic presentation (to avoid an attentional bias to one side or the other during stimulus delivery). Thus, at the instant
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FIG. 1. Testing apparatus with the subject’s palms prepared for presentation of letter stimuli (C, N, S, V). Signal lights, mounted laterally to, and slightly above each forearm, were used to indicate side of first report. of stimulus completion, one experimenter depressed a foot-operated microswitch which illuminated a light attached to the frame of the apparatus and located to the left or right of the subjects’ palms (Fig. 1). According to a modified random schedule (Gellermann, 1933),on any given trial, the side on which the light became illuminated indicated the side of firsthand report; this allowed both hands to be represented equally in each order-of-report condition. On trials in which nonverbal reports were required, the curtain was raised between trials to allow for the pointing response. Intertrial intervals varied in length from approximately 5 to 35 sec. Each trial began on the silent count of three by one experimenter nodding his head; this was the signal for the start of simultaneous drawing of the stimuli by both experimenters.
DICHHAPTIC
327
STORAGE
Prior to testing, subjects were screened for gross sensory loss in the palms. A clinical aesthesiometer (Rowan Products) was used on the lowest value of its scale (2 g) to deliver single-stroke applications to each of five different points within the center area of the palms. As expected, no subject evidenced tactile impairment by this measure. Also, preceding each of the actual test sessions was a practice session to ensure that subjects could identify individual stimuli correctly. In addition, there were four dichhaptic practice trials to familiarize subjects with the double-stimulus, double-report procedure.
RESULTS
Results are summarized in Tables 1 and 2 and Fig. 2. As can be seen in Table 1, a four-way analysis of variance (sex x stimulus materials x hands x order of report) indicated a significant effect of stimulus materials, and a near significant interaction among materials x hands x order of report. Consequently, subsequentt tests were performed within stimulus-material TABLE I SUMMARY (SEX,
OF THE ANALYSIS OF VARIANCE STIMULUS MATERIALS, HANDS,
Source of variance
WITH FOUR MAIN EFFECTS AND ORDER OF REPORT)
Significance levels
df
Mean square
F ratio
Sex Error term
I 13
0.05 0.04
1.40
0.258
Material Sex X material Error term
2 2 26
0.10 0.00 0.02
4.99 0.23
0.015 Over 0.500
Hand Sex x hand Error term
I I 13
0.01 0.01 0.03
0.22 0.23
Over 0.500 Over 0.500
Order of report Sex x order Error term
1 I 13
0.05 0.01 0.03
1.65 0.38
0.222 Over 0.500
Material x hand Sex x material x hand Error term
2 2 26
0.03 0.01 0.01
2.21 0.80
0.130 0.462
Material x order Sex x material x order Error term
2 2 26
0.01 0.00 0.01
1.02 0.40
0.374 Over 0.500
Hand x order Sex x hand x order Error term
1 1 13
0.02 0.00 0.01
1.23 0.08
0.287 Over 0.500
Material x hand x order Sex x material x hand x order Error term
2 2 26
0.03 0.00 0.01
2.66 0.28
0.089 Over 0.500
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TABLE 2 PERCENTAGE CORRECT IDENTIFICATIONS (MEANS AND STANDARD DEVIATIONS) BY THE RIGHT AND LEFT HANDS ACCORDING TO ORDER OF REPORT AND TYPE OF STIMULUS MATERIAL
Stimulus materials
Letters
Left hand first
Right hand second
81 14
79 14
r 83 12
SD
T 77 12
c.09 * 71 14
\ 69 12
X SD
I 80 16
