Asymmetry in visual evoked
potentials to gratings registered
in the two hemispheres of the
human brain
Anna Grabowska, Anna Nowicka and Iwona Szatkowska
Department of Neurophysiology, Nencki Institute of Experimental
Biology, 3 Pasteur St., 02-093 Warsaw, Poland
Abstract. The study aimed at testing, by a visual evoked potential
method, the hypothesis of the hemispheric specialization in processing
l
Twenty four right-handed subjects
of high and lo\\ ~ p a t i a frequencies.
(12 males and 12 females) were presented with square-wave vertical
gratings of various spatial frequencies (0.67,0.86, 1.20,2.00, 2.40,
3.00, 3.30,6.00 and 7.50 cldeg). Gratings were presented in nine
separate blocks each containing 64 exposures. Time of exposure was
30 ms and the interstimulus interval varied from 2 to 3.5 s. VEPs were
recorded with electrodes located at 0 I and 0 2 and referred to Cz
according to the 10120 system. Amplitudes and latencies of two VEPs
components (N 130-150 and P200-240) were analyzed. The results
showed larger amplitudes of VEPs registered in the right hemisphere of
both males and females. This difference, however, was apparent in the
earlier component of VEPs in females and in the later component in
males. The observed hemispheric asymmetry did not depend on the
spatial frequency of grating. Females demonstrated longer latencies
than males for both N and P components. Our data suggest that the
right hemisphere predominates in processing grating stimuli, but the
dynamics of this process differ in the two sexes. The results do no
support Sergent's hypothesis which postulate the right hemisphere
specialization for low spatial frequencies and the left hemisphere
specialization for high spatial frequencies.
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Key words: hemispheric asymmetry, evoked potentials, gratings,
spatial frequency
240
A. Grabowska et al.
INTRODUCTION
For the time of Sperry's pioneer investigations
on comissurotomized patients a considerable
amount of data has been collected indicating that
the two hemispheres of the human brain specialize
in various functions. That growing interest in the
lateralization of brain functions resulted also in formulation of several theoretical concepts which intended to explain the variety of experimental
findings (Zaidel 1978, Bradshaw and Nettleton
1981, Hellige 1983). According to one of the most
popular theory the left hemisphere is superior in
dealing with various kinds of verbal material and
the right hemisphere dominates in processing of
nonverbal stimuli. Another hypothesis, which
become a basis for interpretation of many researches, proposed that the left hemisphere processes the incoming information in an analytical
and sequential manner, whereas the right hemisphere does it in a global, holistic way. Unfortunately, none of these theories was able to account for the
existing data.
In early eighties there appeared a quite new theoretical approach to the problem of hemispheric
specialization. J. Sergent postulated that the two hemispheres differ in their ability to process high and
low spatial frequency information (Sergent 1982,
1983, 1985). Basing on the analysis of several earlier papers Sergent assumed that the left hemisphere
is biased toward efficient use of high spatial frequencies, whereas the right hemisphere shows an
advantage in handling low spatial frequencies.
That new hypothesis attracted much attention of
the researchers investigating the problem of brain
lateralization. Many experiments have been carried
out to verify it. Most of them used the traditional tachistoscopic, lateral presentation method, using
both simple stimuli of the type of grating controlled
with respect of their spatial frequency content (Rose
1983, Fiorentini and Berardi 1984, Kitterle and
Kaye 1985, Peterzell et al. 1989, Kitterle et al. 1990,
Nevskaya et al. 1990, Kitterle and Selig 1991), and
more complex pictures of natural objects which
were filtered or blurred to remove high frequency
components (Sergent 1985, Johnson and Hellige
1986, Michimata and Hellige 1987, Nevskaja and
Leushina 1987, Petrzell et al. 1989, Christman
1990). Those researches were accompanied by few
clinical studies on patients with unilateral brain
damage (Grabowska et al. 1989, Fendrich and
Gazzaniga 1990, Spinelli et al. 1990).
The results of these researches appeared to be
very inconsistent. Although, some of the data directly corroborated the Sergent's predictions, i. e.
they suggested the right hemisphere specialization
for low and the left hemisphere specialization for
high spatial frequencies (Sergent 1985, Kitterle et
al. 1990 exp. 4, Kitterle and Selig 1991), there was
also quite a number of studies which showed the
right hemisphere superiority in processing a wide
spectrum of spatial frequencies (Rovamo and Virsu
1979, Beaton and Blakemore 198 1, Grabowska et
al. 1989, Kitterle et al. 1990 exp. 2, Nevskaya et al.
1990). Moreover, some researchers did not find any
hemispheric differences (Peterzell et al. 1989,
Spinelli et al. 1990).
As the behavioural studies did not resolve the
question of the possible asymmetrical involvement
of the two hemispheres in the spatial frequency processing, during the last few years there have appeared
a few electrophysiological studies which have addressed the same issue.
Meccacci and Spinelli (1987) recorded, in the
left and right temporal lobes, visual evoked potentials (VEPs) to checkerboard patterns which were
phase-reversed (i.e. white checks were reversed
with black ones) at a rate of either 1 or 8 Hz. The
size of check which might be considered to be equivalent to spatial frequency, was varied from 2.8 min
of arc to 16 min. The results of this study showed
that time frequency of the reversals was a critical
factor influencing VEPs: 8 Hz frequency produced
larger visual evoked potentials amplitudes in the
left hemisphere, whereas I Hz frequency condition
yielded the right hemisphere predominance or no
clear effect. The size of check did not interact with
the hemispheres; although, the smaller the check,
the smaller the amplitude of evoked potentials was
Hemispheric asymmetry in VEPs to gratings 241
observed. The slope of the curve representing that
function seemed to be more pronounced for the left
hemisphere, though the authors did not mention
this. VEPs to black and white reversing checks were
also recorded by Cohn et al. (1985). In this study,
the temporal frequency and check size were kept
constant (21s and 70 min of arc respectively). Under
such a condition, amplitudes registered in the RH
were larger.
Another VEPs study by Rebai et al. (1989) is
even more relevant to the problem of hemispheric
specialization in spatial frequency processing because it utilized gratings of various spatial frequency (0.5- 16 cldeg). They were phase reversed at
a rate of 4 and 12 Hz. As in the previous investigations, the temporal frequency appeared to be the
major factor influencing the pattern of lateralization: the evoked activities were greater in the RH at
4 Hz and in the LH at 12 Hz. This effect was present
for the spatial frequencies ranging from 3 to 12
cldeg. Neither lower nor higher spatial frequencies
yielded pronounced differences.
The electrophysiological data presented above
suggest that the temporal rather than the spatial frequency determines the pattern of hemispheric
asymmetry. The results show that high temporal
frequencies activate the left hemisphere more,
whereas low temporal frequencies, the right hemisphere. If spatial frequency exerts an effect on hemispheric asymmetry, it is masked by a much more
pronounced influence of the temporal frequency
factor.
The Experiment we present in the this study is
another attempt to test spatial frequency theory by
an electrophysiological method. We tried to solve
the problem of temporal frequency effect by using
different method of VEPs measurement than that of
pattern reversal. We used "pattern onset" type of
stimulation, i.e. we registered VEPs to grating
stimuli separated by a few seconds of interstimulus
interval. Such a method not only avoids the temporal frequency problem, but it also makes it possible
to study the dynamics of VEP, because its earlier
and later components can be registered. This is not
possible with the other method where patterns
change several times per second. The onset stimulation method has proven to be useful in studies on
hemispheric specialization. For example such
VEPs studies have shown hemispheric asymmetry
for words, faces or emotional stimuli (Rugg 1983,
Small 1983, Sob6tka et al. 1984, Sob6tka and
Grodzicka 1989, Sob6tka et al. 1992) similar to the
RT and accuracy studies.
In the present study we registered VEPs to gratings of various spatial frequency in a condition
where no task was given to subject except to look
at the stimuli and be attentive. We chose to do this
for two reasons: first, we wished to relate our results
to electrophysiological studies with pattern reversal, and second, we wanted to see whether sensory
stimulation per se (with no task which would require higher cognitive function involvement), could
produce asymmetrical response.
METHODS
Subjects
Twelve males and 12 females ranging from 20 to
40 years of age were tested. Only right-handed subjects (according to the Edinburgh Inventory - Oldfield,
197 l), without familial left-handedness were selected, and all had normal or corrected-to-normal
vision.
Material and procedure
The stimuli consisted of square wave gratings
each subtending a circular area with a diameter of
5 deg. Nine different (0.67, 0.86, 1.20, 2.00, 2.40,
3.00,3.30,6.00 and 7.50 cldeg) spatial frequencies
were presented in separate sessions, each containing 64 exposures. Time of exposure was 30 ms and
interstimulus interval was varied randomly from 2
to 3.5 s. Stimuli were presented with a Kodak Carousel 140 slide projector equipped with a Lafayette
shutter. The order of experimental sessions was randomly counterbalanced over subjects.
Subjects were comfortably seated in an electrically and acoustically shielded chamber. They were
242
A. Grabowska et al.
SUBJECT M. D
SUBJECT D. H.
SUBJECT W. P
SUBJECT A. E.
Fig. 1 . Examples of VEPs registered in the left and right hemispheres of four subjects. An arrow indicates the stimulus onset.
requested to fixate a dot in the centre of a screen
placed 180 cm from their eyes. At the beginning of
the experiment subjects were presented with several
grating stimuli to make them familiar with the experimental situation and to reject orientation response. The subjects' behaviour was observed on a
TV monitor.
Evoked potentials recording
The evoked potentials were recorded with silver
disc electrodes located over the left and right occipital regions of the scalp ( 0 1 and 0 2 according to the
10/20 system) and referred to Cz. The grounded
electrode was placed on central forehead. Electrode
impedances were below 5,000 Q and particular care
was taken to equalize impedances of the left and
right side links. The electrical signals were amplified by differential amplifiers of bandwidth 0.23-30
Hz and gain 25x10~.Signals from the EEG amplifiers were digitized on IBM AT compatible computer. Sampling rate was 3 ms. The registration
began 250 m prior to and finished 800 ms after the
stimulus onset. In each session 64 potentials were
summed, averaged, and stored on a computer disk.
RESULTS
Figure 1 shows examples of VEPs registered in
the left and right hemispheres of four subjects. They
Hemispheric asymmetry in VEPs to gratings
243
Amplitudes of N 130
males
amplitude (uV)
Fig. 2. Mean amplitudes of N130 registered in the left and
right hemispheres of
male subjects. The amplitudes are plotted as
a function of spatial
frequency of the grating stimuli.
0.67
0.86
1.20
2.00
2.40
3.00
3.30
6.00
7.50
frequency (c/deg)
indicate that although, in the majority of subjects
two regularly appearing components could be distinguished (an earlier negative N and later positive
P waves), the individual recordings differed to some
extent in overall shape, amplitude and latency. That
created some problems for identification of particular VEPs components in different subjects. In identifying them we considered both the regularities
appearing in recordings of individual subjects
(there were 18 recordings for each subject) and
those which were evident in different subjects. That
problem can be illustrated by analyzing VEPs
presented in the upper right of Fig. 1 (subject D.H.).
The N and P components in that recording have
shorter latencies than those in other subjects. One
may doubt, therefore, whether they represent similar brain processes. However, just at those latencies
the VEPs registered in subjects D.H. contained
regular positive and negative waves (other deflections were much smaller and appeared only occasionally). Moreover, they were similar in their
shapes and latencies to some other recordings not
shown in Fig. 1 . The VEPs registered in our 24 subjects formed a continuum and thus we could not
limit the criteria of identification of particular
VEPs components to very narrow bands of latencies.
VEPs latency
A repeated measures analysis of variance with
sex (male, female), hemisphere (left, right) and spatial frequency, as factors, performed separately on
N and P latency data, revealed a significant main effect of sex (Fi,22=6.18, P<0.02 and F1,22= 12.83,
P<0.002 for N and P components respectively).
Both for earlier and for later components VEPs of
males had shorter latencies than those of females.
The mean latencies of N component were 130 and
150 ms for male and female subjects respectively,
and the mean latencies of P component were 200
and 240 ms. Neither the hemisphere nor the spatial
frequency influenced the VEPs latency. Interactions were also statistically nonsignificant.
244
A. Grabowska et al.
Amplitudes of N 150
females
amplitude ( y V )
-1
Fig. 3. Mean amplitudes of N150 registered in the left and
right hemispheres of
female subjects. The
amplitudes are plotted
as a function of spatial
frequency of the grating stimuli.
0.67
0.86
1.20
2.00
2.40
3.00
3.30
6.00
7.50
frequency (c/deg)
VEPs amplitude
Since the latencies of the two VEPs components
significantly differed in male and female subjects,
an analysis was performed separately for those two
groups of subjects. The amplitudes of the two VEPs
components were estimated in relation to zero line
determined on the basis of EEG recording during
the 250 msec period preceding the stimulus onset.
THE EARLY N130-150 COMPONENT
istical significance. Although Fig. 2 might suggest
larger amplitudes in the LH, that effect was incidental and resulted from the fact that in five subjects the
difference in favour of the LH recordings was very
large.
Figure 3 shows amplitudes of the early N150
component averaged over all female subjects. An
analysis by a two-factor repeated measures
ANOVA revealed significant effects of both hemisphere (F1,11=6.39, Pc0.03) and frequency
(F8,88=3.64, Pc0.03) factors, whereas the interaction was not statistically significant (F8,88=0.23,
P<0.98). Potentials registered in the RH were larger
than those registered in the LH. This effect was
present in all spatial frequency registratidns.
The amplitudes of N130 in males and N 1 50 in females were analyzed with two-factor repeated
measures ANOVAs with hemisphere and frequency as factors. Figure 2 shows mean amplitudes
THE LATER P200-240 COMPONENT
of N 130 for male subjects plotted as a function of
spatial frequency. The only significant main effect
Similar to the earlier N component, two analyses
was that of spatial frequency (F8,88=2.54,
P<0.016). Neither the main factor of hemisphere of variance were performed for the later compo(Fi,i i=0.38, P<0.55) nor the interaction between nents of P200 in males and P240 in females.
the two factors (F8,88=0.59, P<0.78) reached stat- ANOVA conducted on P amplitude values in males
Hemispheric asymmetry in VEPs to gratings 245
Amplitudes of P 200
males
amplitude (pV)
4 -1
Fig. 4. Mean amplitudes of P200 registered in the left and
right hemispheres of
male subjects. The amplitudes are plotted as
a function of spatial
frequency of the grating stimuli.
0.67
0.86
1.20
2.00
2.40
3.00
3.30
6.00
7.50
frequency (c/deg)
Amplitudes of P 240
females
amplitude (vV)
I
Fig. 5. Mean amplitudes of P240 registered in the left and
right hemispheres of
female subjects. The
amplitudes are plotted
as a function of spatial
frequency of the grating stimuli.
0.67
0.86
1.20
2.00
2.40
3.00
frequency (c/deg)
3.30
6.00
7.50
246
A. Grabowska et a1
revealed the statistical significance of hemisphere
(Fi,11=5.16, P<0.04), the right hemisphere amplitudes being larger than those of the left hemisphere.
Neither the frequency factor (F8,88=1.79, P<0.09)
nor the interaction (F8,88=1.09, P<0.37) were significant.
ANOVA performed on P240 amplitude values
in females (Fig. 5) did not show statistically significant main effects (Fl,li=0.24, Pc0.63 and
Fs,sg=1.12, Pc0.36 for hemisphere and frequency,
respectively). The interaction was also insignificant
(Fg,gg=0.37, P<0.94).
DISCUSSION
The results of the present study show that the
electrical activity of the two hemispheres, elicited
by grating stimuli, differ: the right hemisphere was
activated more than the left one. Interestingly these
differences were apparent either in an earlier (N) or
in a later (P) component of VEPs, depending on the
subjects' gender. In female subjects whose response
latencies were longer, the hemispheric asymmetry
emerged readily in the earlier N150 component,
whereas in male subjects who demonstrated shorter
latencies, the difference was apparent in the later
P200 component.
The most important finding, for the purpose this
study, was that we did not observe any interaction
between hemisphere and frequency. The differences, indicating stronger RH involvement, existed
essentially within the whole range of the tested spatial frequencies, and they were evident despite the
lack of any task given to subjects. Our data, therefore, do not corroborate the Sergent's model of hemispheric asymmetry either in respect to the
specific pattern of the brain lateralization or in respect to the level of processing at which the asymmetry arises. According to Sergent's view, the two
hemispheres are equipotential in the efficiency of
spatial frequency information processing at a lower,
sensory level; whereas they are biased toward efficient use of higher or lower spatial frequency at
higher level of processing which involves cognitive
functions. One problem in resolving the issue of the
level of processing at which asymmetries arise is
that the meanings of such terms as "level of processing" or " sensory or cognitive functions", are not precisely defined. If one restricts testing of the so called
"early processing" to contrast sensitivity measurement then, indeed, hemispheric asymmetry could
be rather attributed to processes that occur at a
"higher cognitive level". Even the contrast sensitivity studies, however, provide sometimes evidence
on hemispheric asymmetry. It is important to mention that if such an asymmetry exists, it always
points to the right hemisphere prevalence (Rovamo
and Virsu 1979, Beaton and Blakemore 1981, Rao
at al. 1981, Nevskaya et al. 1990).
Some data indicate that even detection tasks can
yield asymmetrical results under a condition of
suprathreshold stimulation (Fiorentini and Berardi
1984, Kitterle 1990).Moreover, several electrophysiological studies provide additional evidence of the
asymmetrical response of the brain to gratings or
check-type stimuli, controlled as to their spatial frequency content (Vella et al. 1972, Dustman and
Snyder 1981, Cohn et al. 1985). Those differences
were observed despite lack of any "cognitive" engagement of subject. Interestingly, much of these
data suggested the right hemisphere predominance
for those stimuli, independent of the spatial frequency components they contained. On the other
hand there are researches which essentially do parallel Sergent's predictions; when tasks involve relatively complicated decision making processes then
the interaction between the hemisphere and spatial
frequency becomes more evident (Kitterle et al.
1990, Kitterle and Selig 1991).
Considering these apparently discrepant findings, it is reasonable to assume that hemispheric
asymmetry emerge both at the "earlier" and "later"
level of processing and that the specific pattern of
that asymmetry differ in the two stages. The early
stage of processing may result in the right hemisphere superiority for the whole range of spatial frequencies whereas the later stage of processing in the
left hemisphere advantage for high spatial frequencies and the right hemisphere advantage for low.
There is much data which show the right hemis-
Hemispheric asymmetry in VEPs to gratings 247
phere predominance in the perception of several
basic features of visual stimuli. It has been proved
that the right hemisphere is superior in depth detection (Grabowska 1983), in colour and brightness
sensitivity (Davidoff 1975, Davidoff 1976), in dot
detection (Davidoff 1977) and localization (Kimura
1969), in orientation matching (Longden et al.
1976), or in adaptation to orientation (Grabowska
1987, Hegarty et al. 1991). The right-hemisphere
superiority for such elementary visuospatial functions may change into a different pattern of hemispheric asymmetry when later cognitive processes
are involved. According to the logic of such approach, the brain lateralization should not be considered as a stable feature which determines the left
or right hemisphere predominance in a given function, but rather as a dynamic process that depends
both on stimuli characteristics and task demands,
and that may change in the course of information
processing.
Further electrophysiological studies with various task demands would be very useful for learning
more about the information processing locus of
cerebral lateralization.
Another finding of our study was that the evoked
potentials to grating stimuli varied as a function of
spatial frequency. That was true only for the earlier
cmponents (i.e. N130 in males and N150 in females). The highest amplitudes of VEPs were observed within the region of 2 cldeg, which suggests
the highest sensitivity of the visual system for that
range of frequencies. This function resemble those
of psychophysical threshold contrast sensitivity
(Blakemore and Campbell 1969). A direct relationship between thresholds of grating visibility determined by psychophysical and visual evoked
potential methods was earlier demonstrated by
Campbell and Maffei (1970) and Cannon (1983).
Finally, we wish to note the sex related differences in evoked potentials. Unfortunately, there are
very few studies which concern that problem and
they do not provide clear data (e.g. Allison et al.
1983, Buchsbaum et al. 1974, Cohn et al. 1985).
Our results suggest that the dynamics of the hemispheric asymmetry in male and female subjects dif-
fer. In female subjects, who show longer VEPs
latencies, hemispheric asymmetry emerges already
in the earlier N150 component, whereas in male
subjects whose VEPs latencies are shorter, it emerges in the later P200 component. Further studies of
electrophysiological hemispheric asymmetries
with respect to sex differences might contribute to
our understanding of hemispheric asymmetry that
seems to be a dynamic process rather than a stable
feature.
ACKNOWLEDGMENT
The research was supported by Nencki Institute
of Experimental Biology.
REFERENCES
Allison T., Wood C.C., Goff W.R. (1983) Brain stem auditory, pattern reversal visual, and short-latency somatosensory evoked potentials: latencies in relation to age,
sex, and brain and body size. Electroencephalogr. Clin.
Neurophysiol. 55: 61 9-636.
Beaton A., Blakemore C. (198 1) Orientation selectivity of
the human visual system as a function of retinal eccentricity and visual hemifield. Perception 10: 273-282.
Blakemore C., Campbell F. W. (1969) On the existence in
the human visual system of neurones selectively sensitive to the orientation and size of retinal images. J.
Physiol. 203: 237-260.
Bradshaw J.L., Nettleton N. C. (1981) The nature of hemispheric specialization in man. Behav. Brain Sci. 4: 5 1-9 1.
Buchsbaum M.S., Henkin R.J., Christiansen, R.L. (1974)
Age and sex differences in averaged evoked responses in
a normal population with observations on patients with
gonadal dysgenesis. Electroencephalogr. Clin. Neurophysiol. 37: 137-144.
Campbell F. W., Maffei L. (1970) Electrophysiological evidence for existence of orientation and size detectors in
the human visual system. J. Physiol. 207: 635-652.
Cannon M. W. Jr. (1983) Contrast sensitivity: psychophysical and evoked potential methods compared. Vis. Res.
23: 87-95.
Christman S. (1990) Effects of luminance and blur on hemispheric asymmetries in temporal integration. Neuropsychologia 28: 361-374.
Cohn N. B., Kircher J., Emmerson R. K., Dustman R. E.
(1985) Pattern reversal evoked potentials: age, sex, and
hemispheric asymmetry. Electroencephalogr. Clin. Neurophysiol. 62: 399-405.
248
A. Grabowska et al.
Davidoff J. B. (1975) Hemispheric differences in the perception of lightness. Neuropsychologia 13: 121-124.
Davidoff J. B. (1976) Hemispheric sensitivity differences in
the perception of colour. Q. J. Exp. Psychol. 28: 387394.
Davidoff J. B. (1977) Hemispheric differences in dot detection. Cortex 13: 434-444.
Dustman R. E., Snyder E. W. (1981) Life-span changes in
visually evoked potentials at central scalp. Neurobiol.
Aging 2: 303-308.
Fendrich R., Gazzaniga, M. (1990) Hemispheric processing
of spatial frequencies in two comissurotomy patients.
Neuropsychologia 28: 657-663.
Fiorentini A., Berardi N. (1984) Right-hemisphere superiority in the discrimination of spatial phase. Perception
13: 695-708.
Grabowska A. (1983). Lateral differences in the detection of
stereoscopic depth. Neuropsychologia 21: 249-257.
Grabowska A. (1987) Visual field differences in the magnitude of the tilt after-effect. Neuropsychologia 25: 957-963.
Grabowska A., Semenza C., Denes G., Testa S. (1989) Impaired grating discrimination following right hemisphere
damage. Neuropsychologia 27: 259-263.
Hegarty S., Calvert J. E., Snelgar R. S. (1991) The tilt aftereffect in right and left visual fields. Perception 20: 124 (A62).
Hellige J. B. (Ed.) (1983) Cerebral hemisphere asymmetry:
method, theory and application. Praeger, New York.
Johnson J. E., Hellige J. B. (1986) Lateralized effects of
blurring: a test of the visual spatial frequency model of
cerebral asymmetry. Neuropsychologia 24: 351-362.
Kimura D. (1969) Spatial localization in left and right visual
fields. Can. J. Psychol. 23: 445-458.
Kitterle F. L., Christman S., Hellige J. B. (1990) Hemispheric differences are found in the identification, but not
the detection of low versus high spatial frequencies. Percept. Psychophys. 48: 297-396.
Kitterle F. L., Kaye, R. S. (1985) Hemispheric symmetry in
contrast and orientation sensitivity. Percept. Psychophys.
37: 391-396.
Kitterle F. L., Selig L. N. (1991) Visual field effects in the
discrimination of sine-wave gratings. Percept. Psychophys. 50: 15-18.
Longdon K., Ellis C., Iverson S.D. (1976) Hemispheric differences in the discrimination of curvature. Neuropsychologia 14: 195-202.
Meccaci L., Spinelli, D. (1987) Hemispheric asymmetry of
pattern reversal visual evoked potentials in healthy subjects. Internat. J. Psychophysiol. 4: 325-328.
Michimata C., Hellige J. B. (1987) Effects of blurring and
stimulus size on the lateralized processing of nonverbal
stimuli. Neuropsychologia 25: 397-402.
Nevskaya A. A,, Leushina L. I. (1987) Hemispheric differences in the perception of spatial-frequency-filtered
objects. Perception 16: 256 (A32).
Nevskaya A. A., Leushina L., Vassilev A., Manahilov V.,
Mitov D. (1990) Hemisphere asymmetry in the detection
of gratings of varying spatial frequency. Acta Physiol.
Phamacol. Bulg. 16: 3-9.
Oldfield R. C. (1971) The assessment and analysis of handedness: the Edinburgh Inventory. Neuropsychologia 9:
97-113.
Peterzell D. H., Harvey L. 0 . Jr., Hardyck C. D. (1989) Spatial frequencies and the cerebral hemispheres: Contrast
sensitivity, visible persistence, and letter classification.
Percept. Psychophys. 46: 443-455.
Rao S. M., Rourke D., Whitman R. D. (1981) Spatio-temporal discrimination of frequency in the right and left visual fields: a preliminary report. Percept. Motor Skills 53:
31 1-316.
Rebai M., Mecacci L., Bagot J., Bonnet C. (1989) Influence
of spatial frequency and handedness on hemispheric
asymmetry in visually steady-state evoked potentials.
Neuropsychologia 27: 3 15-324.
Rose D. (1983) An investigation into hemisphere differences in adaptation to contrast. Percept. Psychophys. 34:
89-95.
Rovamo J., Virsu V. (1979) An estimation and application
of the human cortical magnification factor. Exp. Brain
Res. 37: 495-5 10.
Rugg M. D. (1983) The relationship between evoked potentials and lateral asymmetries of processing. In: Tutorials
in ERP research: Endogenous components.(Eds. A. W.
K. Gaillard and W. Ritter). North Holland Publishing
Co., The Netherlands, p. 369-383
Sergent J. (1982) The cerebral balance of power: confrontation or cooperation. J. Exp. Psychol.: Hum. Percept. Perform. 8: 253-272.
Sergent J. (1983) The effect of sensory limitation on hemisphere processing. Can. J. Psychol. 37: 345-366.
Sergent J. (1985) Influence of task and input factors on hemisphere involvement in face processing. J. Exp. Psychol.: Hum. Percept. Perform. 11: 846-861.
Sergent J. (1987) Failure to confirm the spatial-frequency
hypothesis: Fatal blow or healthy complication? Can. J.
Psychol. 41 : 412-428.
Small M. (1983) Asymmetrical evoked potentials in response to face stimuli. Cortex 19: 44 1-450.
Sob6tka S., Grodzicka J. (1989) Hemispheric differences in
evoked potentials to faces and words. Acta Neurobiol.
Exp. 49: 265-280.
Sob6tka S., Grabowska A., Grodzicka J., Wasilewski R.,
Budohoska W. (1992) Hemispheric asymmetry in event
related potentials associated with positive and negative
emotions. Acta Neurobiol. Exp. 52: 25 1-260
Sobotka S., Pizto Z., Budohoska W. (1984) Hemispheric
differences in evoked potentials to pictures of faces in
the left and right visual fields. Electroencephalogr. Clin.
Neurophysiol. 58: 441-453.
Hemispheric asymmetry in VEPs to gratings
Spinelli D., Guargilia C., Massironi M., Pizzamiglio L.,
Zoccolotti P. (1990) Contrast sensitivity and low spatial frequency discrimination in hemi-neglect patients.
Neuropsychologia 28: 727-732.
Vella E. J., Butler S. R., Glass A. (1972) Electrical correlate
of right hemisphere function. Nature New Biol. 236:
125-126.
Zaidel E. (1978) Concepts of cerebral dominance in the split
brain. In: Cerebral correlates of conscious experience.
249
(Eds. P. Buser and A. Rongeul-Buser). Elsevier, Amsterdam, p. 263-284.
Received 15 September 1992, accepted 21 October 1992