electrodermal orienting responses to verbal and geometrical visual stimuli projected to the left or...

Acta Psychologica 53 (1983) 141-154

North-Holland

141

ELECTRODERMAL ORIENTING RESPONSES TO VERBAL AND GEOMETRICAL VISUAL STIMULI PROJECTED TO THE LEFT OR RIGHT RETINAL HALF-FIELDS: SEX DIFFERENCES *

Kenneth HUGDAHL, Mikael FRANZON and Eva FRISTORP-WASTEBY University of Uppsala, Sweden

Accepted October 1982

Using the visual half-field technique, verbal and geometrical slides were repeatedly projected to

either the left or right visual field in an electrodermal orienting paradigm. Bilateral skin conduc-

tance response-magnitudes were recorded continuously over trials. Half the subjects (8 males and 8

females) had a verbal and a geometrical slide repeatedly presented to the right of a central

fixation-point, (i.e. initial left hemisphere input) and the other half had the same kinds of stimuli

presented to the left (i.e. initial right hemisphere input). There were 32 presentations of each

stimulus, i.e. a total of 64 trials. The intertrial interval (ITI) varied between 25 to 40 sec. Results

showed significantly larger response-magnitudes in the female group having the stimuli presented

in the left visual field and especially to the geometrical slide. The same trend in data was also

found for the male subgroup having the stimuli presented in the left visual field. No significant

differences between the bilateral left and right hand recordings were found in the main analysis.

However, a closer inspection of the data indicated the left hand recording to be more sensitive than

the right hand recording.

Usually the term functional cerebral, or hemispheric, asymmetry is used in experimental psychology to denote the fact that the two halves of the brain are differentially specialized to process different aspects of information from the surrounding environment (Dimond and Beaumont 1974; Springer and Deutsch 1981). In general, there is evidence that the left hemisphere is dominant in analysing and processing verbal material,

* The present investigation was supported by a grant to the first author from the Swedish Council

for Research in the Humanities and Social Sciences (F33/81). The help and assistance of

photographer Eje Nord in preparing the stimulus material and to Tomas Palm in running a

pilot-study is greatly acknowledged.

Requests for reprints should be sent to Kenneth Hugdahl, Department of Psychology,

University of Uppsala, Box 227, 75 1 04 Uppsala, Sweden.

OOOl-6918/83/$3.00 0 1983, Elsevier Science Publishers B.V. (North-Holland)

142 K. Hugdahl et al. / Orienting responses and lateralization of input

while the right hemisphere is dominant in the analysis of non-verbal tasks and especially in processing spatial aspects of the environment (Harnad et al. 1977, for a review). j,

The suggestion of differential hemisphere function has received support not only from clinical data on brain-lesions (Goldstein 1974), or commisurotomy (Gazzaniga and Sperry 1967), but also from studies on unilateral asymmetries in the normal individual using both auditory (Kimura 1961, 1964) and visual (Kimura and Durnford 1974) stimulation.

Within the visual sphere, the use of the visual half-field technique (Springer 1977) has been successful in supplying the experimenter with a method for initial unilateral stimulation of only one half of the brain. In essence, the visual half-field technique takes advantage of the left-right separation of information from each hemiretina, by present- ing stimuli very briefly in the right or left visual fields, before saccadic eye-movements may re-center stimulation. Thus, information in the left visual field has initial input to only the right hemisphere and vice versa (Springer 1977).

Using the half-field technique, recent research in our laboratory on the relation between electrodermal orienting activity and hemispheric asymmetry (Hugdahl et al. 1982b) showed a slowing of habituation when geometrical stimuli briefly were projected to the left visual field (i.e initial right hemisphere input). In interpreting these findings the authors suggested that stimulus significance may be retained, delaying habituation, when there is both an initial advantage in time for one hemisphere, and an optimal overlap between the processing capacity of that hemisphere and the salient features of the stimulus.

A first test of this suggestion would be to include verbal stimuli presented to the right visual field (i.e. with initial left hemisphere input). A reversal of the effect would be predicted compared to when geometrical stimuli are used.

Second, since evidence from both brain lesioned and intact subjects (McGlone 1980) suggest that males tend to be more lateralized for both verbal and spatial-geometrical abilities and women show greater bilateral representation for both types of functions (see also I ake and Bryden 1976), gender was included as a separate factor with an equal number of males and females in all cells.

Furthermore, a within-subjects design was used, i.e. having each subject view both verbal and geometrical stimuli repeatedly presented

K. Hugdahl et al. / Orienting responses and Iateralization of input 143

to only one visual field. If it could be shown that the same subject is able to differentially retain response-magnitudes over time to the verbal and geometrical stimuli, then previous data from our laboratory showing lateralization of orienting attention in a between-subjects design (Hugdahl et al. 1982b) would get a firmer empirical support.

Finally, electrodermal recordings were taken from both the left and right hands. Recent research suggests that the electrodermal system is contralaterally controlled by the cerebral hemispheres, with responses from the contralateral hand to the more active hemisphere to any given task being inhibited (Lacroix and Comper 1979; Ketterer and Smith 1981). The logic behind this is that although both excitatory and inhibitory centers controlling EDA may be found at the cortical level, the effects are for most parts inhibitory lateralized to the hemisphere stimulated. In his review of neurophysiological control of EDA, Dar- row ( 1937) cites evidence showing an increase in sweating from the hand contralateral to unilateral lesions of the premotor area (see also Wilcott and Bradley (1970)). However, a reverse pattern of responding to about the same types of verbal and geometrical tasks has been found by Myslobodsky and Rattok (1977) indicating contralateral excitation, while both Erwin et al. (1980) and Gross and Stern (1980) have failed to report any differences across the two hands.

The experiment

Method

Subjects Thirty-two right-handed undergraduate students at the University of Uppsala served

as Ss. Handedness was determined first by the S’s own assertion and second by questioning Ss about preferred hand for various manual items like writing, drawing, throwing a ball, use of hammer, toothbrush etc. The age range was between 18-28 years. There were 16 males and 16 females. All were paid for participation (25 SEK,

approximately $4.5).

Apparatus Skin conductance (SC) was recorded directly in micromhos by two constant voltage

circuits with two 1.35 v mercury cells supplying each circuit (Venables and Christie 1973), one connected to the left and the other to the right hand. The two bridges were systematically shifted between Ss in order to control for any differences in recording accuracy between the circuits. Beckman Ag/AgCl, 8 mm diameter, cup-electrodes were


used. A 0.3 g NaCl-solution/l00 ml aqua purificata was used as the electrolyte (Venables and Christie 1980). In order to detect and control for unwanted saccadic eye-movements during stimulus presentations, electro-oculographic (EOG) recordings were taken during the entire experiment. This was done with two Beckman miniature 1.5 mm diameter cup-electrodes connected to a DC-amplifier with Grass EC2 paste as the electrolyte. Both SC and the EOG were recorded on paper on a Hewlett-Packard 7754A four-channel polygraph. In addition, SC was also read digitally from a display connected to the polygraph.

Colour slides were projected from two Sawyer projectors with a Vincent Associates 225L high-speed shutter mounted in front of the lens. The two projectors, one on top of the other, were mounted in a rack which could be moved on a rail either to the left or to the right of the center fixation-point. The shutters were adjusted to attain maximum speed of slide onset and offset on each trial. Opening and closing time for the shutters were approximately 2 msec. The slides were projected onto a 50 X 40 cm milkglass screen inside a 2,5 X 2,0 x 1,8 m sound-attenuated chamber in which the S was seated. The S was facing a plywood screen with a small opening for the eyes exactly 100 cm in front of the milk-glass screen. The distance between the plywood and milk-glass screens was covered by black satin cloth. A 2 mm diameter light-emitting diode (LED), as fixation point, was-placed in the center of the milk-glass projection screen. Duration of the slides as well as onset and offset of the LED was controlled by electronic timers which were triggered from relay-detectors activated by signals prerecorded on a

Tandberg tape-recorder.

Two different sets of slides were used, one verbal category and one geometrical category. In the verbal category five different slides were prepared containing the capital letters A, E, H, R and S in white against a blue background. The geometrical category consisted of five simple geometric figures (square, rectangle, triangle, circle, and pentagon), also in white lines against a blue background. In order to randomize out irrelevant aspects of the slides, different slides within each category were used. The picture on the milk-glass screen subtended 6.28’ of visual angle (11 x 16 cm) with the edge closest to the fixation point 5.4” of visual angle removed from it. The visual angle between the LED and center of stimulus was approximately 8”. The projection screen was dimly illuminated during the entire experiment in order to avoid effects of glare

during stimulus presentations.

Design The basic design was a 2 X 2 X 2 factorial split-plot design (Kirk 1968) with Field

(left vs right visual half-field); Stimulus (verbal vs. geometrical), and Sex (male vs. female) as separate factors. The first and third variables were randomized, and the second variable involved repeated measurement. In the statistical analyses both the trial factor (i.e. repeated presentations of the slides) and electrode-site (left vs. right hand recording) were included in some comparisons, both with repeated measurement.

Procedure

Using the visual-half field technique (Springer 1977) adapted to a two-stimulus habituation paradigm, slides were binocularly projected either to the left or to the right

K. Hugdahl et al. / Orienting responses and lateralization of input 145

of the center LED-fixation point. Half of the Ss had the two sets of slides projected in the left field and the other half had the same stimuli projected in the right field, i.e. initial right and left hemisphere input, respectively. A total of 64 trials were presented to each S (32 verbal and 32 geometrical trials). The two stimulus categories were randomly mixed across trials with the restriction that not more than three consecutive appearances of the same category was allowed. The intertrial interval (ITI) was varied in steps of 5 sets between 25 and 40 sec.

Ss were randomly assigned either to the left or right field group depending on their order of appearance in the laboratory. After arrival they were seated in a comfortable armchair with a specially designed neck-support to “fixate” the head, and asked to lean their forehead against a rubber eye-mask fastened to the plywood screen, looking through the small opening onto the milk-glass projection screen. They were instructed to fixate the LED whenever it was lit but relax between trials. The LED was lit 5 sets prior to slide onset on every trial, going off when the slide disappeared. The duration of the slide was 200 msec. Such a short duration was necessary in order to avoid saccadic eye-movements during stimulus onset, which could otherwise confound the initial unilateral input.

Trials where eye-movements occurred, registered by the EOG, were replaced by the mean of the trials occurring just prior and after the contaminated trial. (Only eight trials were actually distorted.)

All Ss were told that the purpose of the experiment was to monitor bodily responses when having slides presented to them, and that they would be shown several presentations of two different slides, a letter and a common geometric figure. They were finally told to not deliberately move their eyes during the fixation-intervals. They were, however, free to move their eyes during the ITI, although not to remove their forehead from the rubber mask in the viewing opening of the plywood screen.

The general purpose of the experiment was explained to the S upon entry to the laboratory and the SC electrodes fastened with the help of adhesive collars to the palmar medial phalanx of the second and third fingers of each hand. Electrode sites were first washed with distilled water. The two EOG-electrodes were placed, also with the help of adhesive collars, at the temporal corners of the left and right eyeball in the medial plane across the eyes, with a reference-electrode placed on the back of the right shoulder. EOG-sites were smoothly rubbed before fastening of the electrodes in order to bring skin-resistance below 10 Kohms. The experiment proper started after a 5- 10 min adaptation period during which time the instruments were calibrated. Communica- tion, if necessary, during the experiment was done through an intercom system.

Definition of phasic SC-responses Phasic skin conductance responses were scored in the interval l-4 set (Prokasy and

Kumpfer 1973) after onset of both the LED and the slide. If more than one response occurred in an interval, the largest was scored. The scoring of responses in the LED-interval was done in order to control for any differences between stimulus-categories, or groups, to the LED-stimulus in itself prior to slide onset.

146 K. Hugdahl et al. / Orienting responses and lateraliration of input

Results

All responses were recorded and scored directly in micromho ( y mho) and subjected to analyses of variance (ANOVA).

Responses to the slides

Mean response-magnitudes, averaged across trials and separated for the left and right hand recording, are seen in fig. 1. The results of the analysis of variance are shown in table 1 with the F-ratios for each source of comparison. As can be seen in table 1 there were significant main effects of sex and trials for both the left and right hand recordings. An inspection of fig. 1 reveals overall larger mean magnitudes among the females as compared to the males. The significant trials factor relate to a general decrease in responding over trials from the first one. Furthermore, as also seen in table 1, there was a significant effect of the interaction of set by projection-field (i.e. left vs right half-field), also observed in both hand recordings. It is easily seen in fig. 1

Table 1

F-ratios from the ANOVAs, separated for responses to the slides and to the LED, and for the right and left hand recordings. Designation of sources of variance same as in text.

source df Slide LED

Left hand Right hand Left hand Right hand

Sex (Se) Field (F)

SexF

Subj. w. groups

Trials (T)

TxF

TxSe TXFxSe

T x Subj. w. groups

Stimulus (St)

StxF

StXSe

StXFxSe

St x Subj. w. groups

TxSt

TxStxF

TxStxSe TxStxFxSe

TX St X Subj. w. groups

1 1

1 28

31 31 31 31

868

1 1 1 1

28

31 31 31 31

868

13.71 b 8.20 b 5.12 = 6.00 b 3.64 2.80 Cl -Cl

6.28 = S.80a <I Cl

(0.145) (0.154) (0.912) ( 1.670)

2.78 b 2.38 b 3.18 b 4.36 b

1.16 1.42 <l (1

1.13 1.32 I .03 iI

1.42 1.12 1.03 1.18

(0.031) (0.020) (0.043) (0.034)

3.56 2.57 1.83 il

4.78 a 3.65 1.50 il

1.33 (1 <1 -Cl

4.73 a 1.39 Cl -=I

(0.019) (0.07 I) (0.055) (0.073)

1.42 1.99 a 1.80 a 1.31 1.25 1.38 <l Cl

<l Cl -Cl Cl

I .29 1.33 1.27 1.08 (0.024) (0.02 1) (0.030) (0.027)

Note: Error terms within parenthesis. B p < 0.05; b p -c 0.01

K. Hugdahl et al. / Orienting responses and lateraliration of input 147

LEFT HAND

20

r

V G V G V G V G Rloht field Left ‘kid

RIGHT HAND

.20 r

F.rn.1.

Right flald Left field

Fig. 1. Mean skin conductance response-magnitudes to the slide stimuli for the left and right hand recording. V = verbal stimuli, G = geometrical stimuli. Right and left field = hemiretina stimu-

lated.

that this pertains to larger responses in the female left-field groups in comparison to the other groups and fields. Looking only at the left-hand recording both the two- and three-way interactions of type of stimulus (verbal vs. geometrical) by projection-field, and sex by type of stimulus by projection-field were significant (table 1). A look at fig. 1 reveals that the same trend was also seen in the right hand recording.

In order to evaluate any differences in response-magnitudes between the two hands recordings, a second ANOVA was run including recording-site as a separate factor. No differences were found between the hands in any comparison. In fact, most F-ratios -C 1.

To follow up the significant three-way interaction of sex by stimulus by field for the left hand recording, Tukey’s HSD a posteriori test (Kirk 1968) was applied. These data are shown in table 2. For the sake of convenience, data from the HSD-test for the right-hand recording has also been included in the table. This is justified partly because of ease of comparison with the left-hand recording and partly because the trend in the right-hand recordings is in the same direction as for the left-hand recordings. As seen in table 2 the left hand female/left-field/geometrical stimulus/group showed higher scores compared to all other groups except the female/left-field/verbal stimulus/group, i.e. the within-subject comparison. Thus, this group showed higher scores in six of the seven comparisons it entered. The corresponding data for the right hand recording showed superior magnitudes for this group in only two of the seven comparisons. Finally, the left hand recording for the female/left-field/verbal stimulus group was significantly above both left-field male groups.

Because of the very conservative nature of the HSD-test for within-subjects comparisons, yielding a low power in such evaluations (Kirk 1968), the critical within-subjects discriminations between the geometrical and verbal stimulus for the female left-field groups (see figs. 1 and 2) were in addition also evaluated by a priori t-tests. This statistic showed significant discriminations among the females on both hand

148 K. Hugdahl et al. / O&wring responses and lateraliration of inpul

Table 2

Matrix of differences between means (columns minus rows) to the slide stimulus for the various

cells in the experiment for comparison with Tukey’s HSD-test, separately for the left and right

hand recording.

(a) Lejt - hand

MRVe FLVe MLVe FRGe MRGe FLGe MLGe

FRVe 0.026 - 0.048 0.036 0.001 0.014 -0.102 a 0.027 MRVe - 0.074 0.010 - 0.025 -0.012 -0.125 a -0.001

FLVe 0.084 a 0.049 0.062 0.05 1 0.086 a

MLVe - 0.035 - 0.022 -0.135 a -0.011

FRGe 0.013 -0.100 a 0.024

MRGe -0.113= 0.011

FLGe 0.124a

HSD (0.05; 28) = 0.084.

a p < 0.05

(b) Right-hand

MRVe

FRVe 0.038

MRVe

FLVe

MLVe

FRGe

MRGe

FLGe

FLVe MLVe FRGe MRGe FLGe MLGe

- 0.022 0.033 - 0.002 0.032 - 0.063 0.006

- 0.060 -0.005 - 0.040 a - 0.006 -0.101 - 0.032

0.055 0.020 0.054 -0.041 0.028

0.035 -0.001 0.099 = - - - 0.027

0.034 -0.061 0.008

- 0.095 - 0.026

0.069

HSD (0.05; 28) = 0.098. M = Males; F = Females; R = Right visual half-field; L = Left visual

half-field; Ve = Verbal stimuli; Ge = Geometrical stimuli.

a p < 0.05

recordings with larger responses to the geometrical stimulus compared with the responses to the verbal stimulus, r(28) = 2.25, p < 0.01, and 1.78, p < 0.05 for the left and right hand recordings, respectively. The corresponding comparison in all other groups yielded t-values less than one except for the right-hand recording for the male/left-field group where t(28) = 1.17.

To include a direct measure of habituation, i.e. response-decrement within each stimulus category as a function of repeated stimulation, an individual criterion of habituation was scored for each S as the number of trials with responses (> 0.05 ELmho) prior to three consecutive trials with “zero-responses”. In order to get an estimate of any differences in frequency of responses between the two stimulus categories number of trials prior to the criterion of three “zero-responses” were counted separately for the verbal and geometrical stimulus category. The mean number of trials to criterion for each group averaged across both hand recordings is shown in table 3. As is obvious

K. Hugdahl et al. / Orienting responses and lateralizarion of input 149

Table 3

Mean number of trials to criterion ( = 3 “zero-responses”) as a measure of habituation for the

various cells in the experiment separated for slide and LED responses and averaged across the left

and right hand recordings.

Right field

Left field

Slide

Females

Ve Ge

4.31 3.56

7.56 12.50

LED

Males Females Males

Ve Ge Ve Ge Ve Ge

4.88 4.75 9.38 11.13 10.00 7.50

2.94 4.68 14.50 12.63 9.50 6.88

Nore: Ve = Verbal stimuli; Ge = Geometrical stimuli.

from an inspection of the table, the results are very similar to the results for the magnitude-data, with the female left-field cells having higher scores than all other cells in the matrix. In order to elucidate the within-subject comparisons, separate t-tests were performed within each group comparing the verbal mean against the geometrical mean (see table 3). The mean sum of squares for the within-subjects source of variation in the overall ANOVA was used as error-terms. The corresponding t(28)-values for the comparison of the female left-field geometrical stimulus vs verbal stimulus; male left-field; female right-field; and male right-field, were 3.18, p < 0.01; 1.31, p < 0.10;

-=C 1, n.s.; < 1, n.s. Individual data from the female left-field group show that seven of the eight subjects

.20 F.nn.6.

r

LEFT HAND RIGHT NAND

0,

0.

” G V G V G V G

Fig. 2. Mean skin conductance response-magnitudes to the LED-stimulus for the left and right hand recording. V = verbal stimuli, G = geometrical stimuli. Right and left field = hemiretina

stimulated.


had larger response-magnitudes to the geometrical stimulus as compared to the verbal stimulus. The probability of obtaining such proportions by chance alone is p = 0.035 by the binomial distribution.

Responses to the LED

Mean response-magnitudes are seen in fig. 2 separated for the left and right hand recording. The F-ratios are shown in the right columns of table 1. The only statistically reliable effects seen in response to the LED-stimulus prior to slide-onset was a significant difference between sexes, with responses in the female groups to exceed the male groups (see fig. 2; and table l), and an overall difference across trials. This latter effect pertained to a general decrease in responding over trials. When uncorrected inflated degrees of freedom were used in evaluating the interaction of trials by type of stimulus this source also turned out to be significant. However, the effect disappeared when the Geisser-Greenhouse technique was applied. No differences between recording sites were detected. The mean number of trials to reach the criterion of three consecutive “zero-responses” are seen in the right half of table 3. t-tests among cell-means for the verbal-geometrical comparison in each group showed no significant effects; all t’s < 1.

Discussion

The basic findings reported in the Results section were first of all; larger response-magnitudes, averaged across the entire session, for the female left-field group, and especially to the geometrical slides. Second, this group was the only to show a statistically reliable within-subjects discrimination between the verbal and geometrical cues, although the trend in data was the same for the male left-field group. Third, the female left-field group displayed increased response-magnitudes also to the verbal stimuli compared to the other groups, although not large enough to prevent discrimination between the two stimuli to occur. Thus, the female left-field group showed evidence of asymmetric processing, with increased right-hemisphere activation to both types of stimuli but especially to the geometrically more relevant slides.

A possible explanation for the absence of an increase in response- magnitudes to the verbal stimuli in the right-field groups could be that these stimuli were too verbally degraded, leaving little verbally relevant features to be processed. Such an explanation may be supported by recent analysis of hemispheric asymmetry during information-processing to be more easily observed at later higher-order processing stages involving a shift from automatic to controlled processing (Moscovitch

K. Hugdahl et al. / Orienting responses and lateralization of input 151

1979), whereas asymmetry for sensory features like contour, or texture, to be smaller and often inconsistent (Corkin et al. 1973). Furthermore, Maltzman (1979) has recently suggested that orienting information processed in the Pavlovian second signal system (Kimble 196 l), i.e. verbal and language related tasks, should be primarily regulated by the left hemisphere. On the other hand, orienting behavior to the presenta- tion of novel physical stimuli should be processed in the first signal system regulated by the right hemisphere.

Applying these models of hemispheric functioning and orienting behavior to the present data, it may be argued that reducing the verbal content to a single letter is a too small demand for centrally integrated processing (cf. Friedman and Polson 1981). The expected left hemisphere dominance for these stimuli may therefore have failed to emerge because the geometrically relevant features like contour, shape and texture may have been more salient.

The reported differences between the sexes in the present study seem at first hand to be contradictory to existing evidence showing males to be more lateralized than females on both geometrical and verbal tasks (McGlone 1980; Lake and Bryden 1976). However, recent data on asymmetry of autonomic and electrophysiological measures have shown females to be more sensitive than males to show right hemisphere asymmetry to spatial and geometrical tasks (Stewart et al. 1981; Andreassi and Juszczak 198 1). Using change in pulse-amplitude (PA) from the left and right side of the central forehead in response to Gestalt-closure problems and verbal analogies test Stewart et al. (1981) showed females to respond differently as a function of type of task. In a similar vein Andreassi and Juszczak (198 1) reported only females to show larger right hemisphere event related potentials (ERPs) to repeated presentations of a moving spatial pattern. Moreover, Smith et al. (198 1) also reported larger response magnitudes among right-handed females after processing of spatially relevant stimuli.

Finally, no significant differences were found between the two hands recordings in the analysis of variance including recording site as a separate factor. Figs. 1 and 2 support this with small differences to be seen between the left- and right-hand data. However, several other sources of comparisons point to a more complicated picture with the effects seen in the left hand to be more homogeneous, although not necessarily implying larger means. First of all, most significant comparisons in the left hand recordings yielded larger F-ratios than the


corresponding right hand recordings. Second, the three-way interaction of sex by projection field by type of stimulus were significant beyond the 0.05 level of significance only in the left recording. Third, the Tukey HSD-analyses for the left-hand recordings showed that the female left-field spatial group differed significantly in six out of seven comparisons, whereas the corresponding right-hand recordings showed only two out of seven comparisons to be significant (see table 2). However, the t-tests comparing responses to the verbal stimuli vs. responses to the geometrical stimuli showed reliable discrimination for the female left- field group in both hands recordings, with the same trend in the data for the corresponding comparisons in the male left-field group. Thus, although the overall parametric statistics do not support a view of lateralization in the electrodermal system (cf. Obrist 1963), several other sources of comparisons show the left hand recording to have higher amplitudes, (cf. Myslobodsky and Rattok 1977). This is partly in contrast with previous results from our own laboratory (Hugdahl et al. 1982a, b) where we have failed to detect any differences at all between the two hands to lateralized sensory input.

In conclusion then, the present results imply a functional relation- ship between orienting responses and hemispheric asymmetry, at least for females, by showing larger response-magnitudes on repeated presentations to geometrical stimuli initially fed only to the right hemisphere. As such the present data are in congruence with previous findings from our laboratory showing a delay in response-diminution when geometrical visual stimuli are repeatedly presented to the right hemisphere (Hugdahl et al. 1982b). Seen in this perspective the use of EDA in research on hemispheric asymmetry points to a new direction for studies of brain lateralization, i.e. attention and orienting behavior. For this purpose EDA may be a valuable instrument since it is one of the most sensitive autonomic indicators of these behaviors.

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