Asymmetry in visual evoked potentials to gratings registered in the by linzhengnd

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									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
of high and lo\\ ~ p a t i a frequencies. Twenty four right-handed subjects
(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.
                  -   -                      --- - --




Key words: hemispheric asymmetry, evoked potentials, gratings,
spatial frequency
240     A. Grabowska et al.

   INTRODUCTION                                           were filtered or blurred to remove high frequency
                                                          components (Sergent 1985, Johnson and Hellige
                                                           1986, Michimata and Hellige 1987, Nevskaja and
    For the time of Sperry's pioneer investigations       Leushina 1987, Petrzell et al. 1989, Christman
on comissurotomized patients a considerable               1990). Those researches were accompanied by few
amount of data has been collected indicating that         clinical studies on patients with unilateral brain
the two hemispheres of the human brain specialize         damage (Grabowska et al. 1989, Fendrich and
in various functions. That growing interest in the        Gazzaniga 1990, Spinelli et al. 1990).
lateralization of brain functions resulted also in for-       The results of these researches appeared to be
mulation of several theoretical concepts which in-        very inconsistent. Although, some of the data di-
tended to explain the variety of experimental             rectly corroborated the Sergent's predictions, i. e.
findings (Zaidel 1978, Bradshaw and Nettleton             they suggested the right hemisphere specialization
 1981, Hellige 1983). According to one of the most        for low and the left hemisphere specialization for
popular theory the left hemisphere is superior in         high spatial frequencies (Sergent 1985, Kitterle et
dealing with various kinds of verbal material and         al. 1990 exp. 4, Kitterle and Selig 1991), there was
the right hemisphere dominates in processing of           also quite a number of studies which showed the
nonverbal stimuli. Another hypothesis, which              right hemisphere superiority in processing a wide
become a basis for interpretation of many re-             spectrum of spatial frequencies (Rovamo and Virsu
searches, proposed that the left hemisphere pro-           1979, Beaton and Blakemore 198 1, Grabowska et
cesses the incoming information in an analytical          al. 1989, Kitterle et al. 1990 exp. 2, Nevskaya et al.
and sequential manner, whereas the right hemis-           1990). Moreover, some researchers did not find any
phere does it in a global, holistic way. Unfortunate-     hemispheric differences (Peterzell et al. 1989,
ly, none of these theories was able to account for the    Spinelli et al. 1990).
existing data.                                                As the behavioural studies did not resolve the
    In early eighties there appeared a quite new the-     question of the possible asymmetrical involvement
oretical approach to the problem of hemispheric           of the two hemispheres in the spatial frequency pro-
specialization. J. Sergent postulated that the two he-    cessing, during the last few years there have appeared
mispheres differ in their ability to process high and     a few electrophysiological studies which have ad-
low spatial frequency information (Sergent 1982,          dressed the same issue.
 1983, 1985). Basing on the analysis of several ear-          Meccacci and Spinelli (1987) recorded, in the
lier papers Sergent assumed that the left hemisphere      left and right temporal lobes, visual evoked poten-
is biased toward efficient use of high spatial fre-       tials (VEPs) to checkerboard patterns which were
quencies, whereas the right hemisphere shows an           phase-reversed (i.e. white checks were reversed
advantage in handling low spatial frequencies.            with black ones) at a rate of either 1 or 8 Hz. The
    That new hypothesis attracted much attention of       size of check which might be considered to be equi-
the researchers investigating the problem of brain        valent to spatial frequency, was varied from 2.8 min
lateralization. Many experiments have been carried        of arc to 16 min. The results of this study showed
out to verify it. Most of them used the traditional ta-   that time frequency of the reversals was a critical
chistoscopic, lateral presentation method, using          factor influencing VEPs: 8 Hz frequency produced
both simple stimuli of the type of grating controlled     larger visual evoked potentials amplitudes in the
with respect of their spatial frequency content (Rose     left hemisphere, whereas I Hz frequency condition
 1983, Fiorentini and Berardi 1984, Kitterle and          yielded the right hemisphere predominance or no
Kaye 1985, Peterzell et al. 1989, Kitterle et al. 1990,   clear effect. The size of check did not interact with
Nevskaya et al. 1990, Kitterle and Selig 1991), and       the hemispheres; although, the smaller the check,
more complex pictures of natural objects which            the smaller the amplitude of evoked potentials was
                                                             Hemispheric asymmetry in VEPs to gratings 241

observed. The slope of the curve representing that       change several times per second. The onset stimu-
function seemed to be more pronounced for the left       lation method has proven to be useful in studies on
hemisphere, though the authors did not mention           hemispheric specialization. For example such
this. VEPs to black and white reversing checks were      VEPs studies have shown hemispheric asymmetry
also recorded by Cohn et al. (1985). In this study,      for words, faces or emotional stimuli (Rugg 1983,
the temporal frequency and check size were kept          Small 1983, Sob6tka et al. 1984, Sob6tka and
constant (21s and 70 min of arc respectively). Under     Grodzicka 1989, Sob6tka et al. 1992) similar to the
such a condition, amplitudes registered in the RH        RT and accuracy studies.
were larger.                                                 In the present study we registered VEPs to grat-
    Another VEPs study by Rebai et al. (1989) is         ings of various spatial frequency in a condition
even more relevant to the problem of hemispheric         where no task was given to subject except to look
specialization in spatial frequency processing be-       at the stimuli and be attentive. We chose to do this
cause it utilized gratings of various spatial fre-       for two reasons: first, we wished to relate our results
quency (0.5- 16 cldeg). They were phase reversed at      to electrophysiological studies with pattern rever-
a rate of 4 and 12 Hz. As in the previous investiga-     sal, and second, we wanted to see whether sensory
tions, the temporal frequency appeared to be the         stimulation per se (with no task which would re-
major factor influencing the pattern of lateraliza-      quire higher cognitive function involvement), could
tion: the evoked activities were greater in the RH at    produce asymmetrical response.
4 Hz and in the LH at 12 Hz. This effect was present
for the spatial frequencies ranging from 3 to 12            METHODS
cldeg. Neither lower nor higher spatial frequencies
yielded pronounced differences.                             Subjects
    The electrophysiological data presented above
suggest that the temporal rather than the spatial fre-       Twelve males and 12 females ranging from 20 to
quency determines the pattern of hemispheric             40 years of age were tested. Only right-handed sub-
asymmetry. The results show that high temporal           jects (according to the Edinburgh Inventory - Oldfield,
frequencies activate the left hemisphere more,            197 l), without familial left-handedness were se-
whereas low temporal frequencies, the right hemis-       lected, and all had normal or corrected-to-normal
phere. If spatial frequency exerts an effect on he-      vision.
mispheric asymmetry, it is masked by a much more
pronounced influence of the temporal frequency              Material and procedure
factor.
    The Experiment we present in the this study is          The stimuli consisted of square wave gratings
another attempt to test spatial frequency theory by      each subtending a circular area with a diameter of
an electrophysiological method. We tried to solve        5 deg. Nine different (0.67, 0.86, 1.20, 2.00, 2.40,
the problem of temporal frequency effect by using        3.00,3.30,6.00 and 7.50 cldeg) spatial frequencies
different method of VEPs measurement than that of        were presented in separate sessions, each contain-
pattern reversal. We used "pattern onset" type of        ing 64 exposures. Time of exposure was 30 ms and
stimulation, i.e. we registered VEPs to grating          interstimulus interval was varied randomly from 2
stimuli separated by a few seconds of interstimulus      to 3.5 s. Stimuli were presented with a Kodak Car-
interval. Such a method not only avoids the tempo-       ousel 140 slide projector equipped with a Lafayette
ral frequency problem, but it also makes it possible     shutter. The order of experimental sessions was ran-
to study the dynamics of VEP, because its earlier        domly counterbalanced over subjects.
and later components can be registered. This is not         Subjects were comfortably seated in an electri-
possible with the other method where patterns            cally 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              impedances were below 5,000 Q and particular care
placed 180 cm from their eyes. At the beginning of               was taken to equalize impedances of the left and
the experiment subjects were presented with several              right side links. The electrical signals were ampli-
grating stimuli to make them familiar with the ex-               fied by differential amplifiers of bandwidth 0.23-30
perimental situation and to reject orientation re-               Hz and gain 25x10~.    Signals from the EEG ampli-
sponse. The subjects' behaviour was observed on a                fiers were digitized on IBM AT compatible com-
TV monitor.                                                      puter. Sampling rate was 3 ms. The registration
                                                                 began 250 m prior to and finished 800 ms after the
   Evoked potentials recording                                   stimulus onset. In each session 64 potentials were
                                                                 summed, averaged, and stored on a computer disk.
    The evoked potentials were recorded with silver
disc electrodes located over the left and right occipi-             RESULTS
tal regions of the scalp ( 0 1 and 0 2 according to the
10/20 system) and referred to Cz. The grounded                      Figure 1 shows examples of VEPs registered in
electrode was placed on central forehead. Electrode              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 ampli-
                                                                                             tudes of N130 regis-
                                                                                             tered in the left and
                                                                                             right hemispheres of
                                                                                             male subjects. The am-
                                                                                             plitudes are plotted as
                                                                                             a function of spatial
                                                                                             frequency of the grat-
                                                                                             ing 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       shown in Fig. 1 . The VEPs registered in our 24 sub-
two regularly appearing components could be dis-          jects formed a continuum and thus we could not
tinguished (an earlier negative N and later positive      limit the criteria of identification of particular
P waves), the individual recordings differed to some      VEPs components to very narrow bands of laten-
extent in overall shape, amplitude and latency. That      cies.
created some problems for identification of particu-
lar VEPs components in different subjects. In ident-            VEPs latency
ifying them we considered both the regularities
appearing in recordings of individual subjects                A repeated measures analysis of variance with
(there were 18 recordings for each subject) and           sex (male, female), hemisphere (left, right) and spa-
those which were evident in different subjects. That      tial frequency, as factors, performed separately on
problem can be illustrated by analyzing VEPs              N and P latency data, revealed a significant main ef-
presented in the upper right of Fig. 1 (subject D.H.).    fect of sex (Fi,22=6.18, P<0.02 and F1,22= 12.83,
The N and P components in that recording have             P<0.002 for N and P components respectively).
shorter latencies than those in other subjects. One       Both for earlier and for later components VEPs of
may doubt, therefore, whether they represent simi-        males had shorter latencies than those of females.
lar brain processes. However, just at those latencies     The mean latencies of N component were 130 and
the VEPs registered in subjects D.H. contained             150 ms for male and female subjects respectively,
regular positive and negative waves (other deflec-        and the mean latencies of P component were 200
tions were much smaller and appeared only occa-           and 240 ms. Neither the hemisphere nor the spatial
sionally). Moreover, they were similar in their           frequency influenced the VEPs latency. Interac-
shapes and latencies to some other recordings not         tions were also statistically nonsignificant.
244        A. Grabowska et al.

                                 Amplitudes of N 150
                                          females




        amplitude ( y V )
   -1


                                                                                         Fig. 3. Mean ampli-
                                                                                         tudes of N150 regis-
                                                                                         tered in the left and
                                                                                         right hemispheres of
                                                                                         female subjects. The
                                                                                         amplitudes are plotted
                                                                                         as a function of spatial
                                                                                         frequency of the grat-
                                                                                         ing stimuli.
          0.67    0.86     1.20    2.00     2.40    3.00    3.30     6.00      7.50
                                  frequency (c/deg)

   VEPs amplitude                                     istical significance. Although Fig. 2 might suggest
                                                      larger amplitudes in the LH, that effect was inciden-
   Since the latencies of the two VEPs components     tal and resulted from the fact that in five subjects the
significantly differed in male and female subjects,   difference in favour of the LH recordings was very
an analysis was performed separately for those two    large.
groups of subjects. The amplitudes of the two VEPs        Figure 3 shows amplitudes of the early N150
components were estimated in relation to zero line    component averaged over all female subjects. An
determined on the basis of EEG recording during       analysis by a two-factor repeated measures
the 250 msec period preceding the stimulus onset.     ANOVA revealed significant effects of both hemis-
                                                      phere (F1,11=6.39, Pc0.03) and frequency
   THE EARLY N130-150 COMPONENT                       (F8,88=3.64, Pc0.03) factors, whereas the interac-
                                                      tion was not statistically significant (F8,88=0.23,
   The amplitudes of N130 in males and N 1 50 in fe-  P<0.98). Potentials registered in the RH were larger
males were analyzed with two-factor repeated          than those registered in the LH. This effect was
measures ANOVAs with hemisphere and fre-              present in all spatial frequency registratidns.
quency as factors. Figure 2 shows mean amplitudes
of N 130 for male subjects plotted as a function of     THE LATER P200-240 COMPONENT
spatial frequency. The only significant main effect
was that of spatial frequency (F8,88=2.54,              Similar to the earlier N component, two analyses
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 ampli-
                                                                                    tudes of P200 regis-
                                                                                    tered in the left and
                                                                                    right hemispheres of
                                                                                    male subjects. The am-
                                                                                    plitudes are plotted as
                                                                                    a function of spatial
                                                                                    frequency of the grat-
                                                                                    ing 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 ampli-
                                                                                    tudes of P240 regis-
                                                                                    tered in the left and
                                                                                    right hemispheres of
                                                                                    female subjects. The
                                                                                    amplitudes are plotted
                                                                                    as a function of spatial
                                                                                    frequency of the grat-
                                                                                    ing stimuli.
           0.67   0.86   1.20     2.00    2.40     3.00   3.30     6.00    7.50
                                 frequency (c/deg)
246     A. Grabowska et a1

revealed the statistical significance of hemisphere      level of processing at which asymmetries arise is
(Fi,11=5.16, P<0.04), the right hemisphere ampli-        that the meanings of such terms as "level of process-
tudes being larger than those of the left hemisphere.    ing" or " sensory or cognitive functions", are not pre-
Neither the frequency factor (F8,88=1.79, P<0.09)         cisely defined. If one restricts testing of the so called
nor the interaction (F8,88=1.09, P<0.37) were sig-        "early processing" to contrast sensitivity measure-
nificant.                                                 ment then, indeed, hemispheric asymmetry could
   ANOVA performed on P240 amplitude values               be rather attributed to processes that occur at a
in females (Fig. 5) did not show statistically signi-     "higher cognitive level". Even the contrast sensitiv-
ficant main effects (Fl,li=0.24, Pc0.63 and              ity studies, however, provide sometimes evidence
Fs,sg=1.12, Pc0.36 for hemisphere and frequency,         on hemispheric asymmetry. It is important to men-
respectively). The interaction was also insignificant    tion that if such an asymmetry exists, it always
(Fg,gg=0.37, P<0.94).                                    points to the right hemisphere prevalence (Rovamo
                                                         and Virsu 1979, Beaton and Blakemore 1981, Rao
   DISCUSSION                                            at al. 1981, Nevskaya et al. 1990).
                                                             Some data indicate that even detection tasks can
    The results of the present study show that the       yield asymmetrical results under a condition of
 electrical activity of the two hemispheres, elicited    suprathreshold stimulation (Fiorentini and Berardi
 by grating stimuli, differ: the right hemisphere was     1984, Kitterle 1990).Moreover, several electrophy-
 activated more than the left one. Interestingly these   siological studies provide additional evidence of the
differences were apparent either in an earlier (N) or    asymmetrical response of the brain to gratings or
 in a later (P) component of VEPs, depending on the      check-type stimuli, controlled as to their spatial fre-
 subjects' gender. In female subjects whose response     quency content (Vella et al. 1972, Dustman and
 latencies were longer, the hemispheric asymmetry        Snyder 1981, Cohn et al. 1985). Those differences
emerged readily in the earlier N150 component,           were observed despite lack of any "cognitive" en-
whereas in male subjects who demonstrated shorter        gagement of subject. Interestingly, much of these
latencies, the difference was apparent in the later      data suggested the right hemisphere predominance
P200 component.                                          for those stimuli, independent of the spatial fre-
    The most important finding, for the purpose this     quency components they contained. On the other
study, was that we did not observe any interaction       hand there are researches which essentially do par-
between hemisphere and frequency. The differen-          allel Sergent's predictions; when tasks involve rela-
ces, indicating stronger RH involvement, existed         tively complicated decision making processes then
essentially within the whole range of the tested spa-    the interaction between the hemisphere and spatial
tial frequencies, and they were evident despite the      frequency becomes more evident (Kitterle et al.
lack of any task given to subjects. Our data, there-      1990, Kitterle and Selig 1991).
fore, do not corroborate the Sergent's model of he-          Considering these apparently discrepant find-
mispheric asymmetry either in respect to the             ings, it is reasonable to assume that hemispheric
specific pattern of the brain lateralization or in re-   asymmetry emerge both at the "earlier" and "later"
spect to the level of processing at which the asym-      level of processing and that the specific pattern of
metry arises. According to Sergent's view, the two       that asymmetry differ in the two stages. The early
hemispheres are equipotential in the efficiency of       stage of processing may result in the right hemis-
spatial frequency information processing at a lower,     phere superiority for the whole range of spatial fre-
sensory level; whereas they are biased toward effi-      quencies whereas the later stage of processing in the
cient use of higher or lower spatial frequency at        left hemisphere advantage for high spatial frequen-
higher level of processing which involves cognitive      cies and the right hemisphere advantage for low.
functions. One problem in resolving the issue of the     There is much data which show the right hemis-
                                                              Hemispheric asymmetry in VEPs to gratings 247

 phere predominance in the perception of several         fer. In female subjects, who show longer VEPs
 basic features of visual stimuli. It has been proved    latencies, hemispheric asymmetry emerges already
 that the right hemisphere is superior in depth detec-   in the earlier N150 component, whereas in male
 tion (Grabowska 1983), in colour and brightness         subjects whose VEPs latencies are shorter, it emer-
 sensitivity (Davidoff 1975, Davidoff 1976), in dot      ges in the later P200 component. Further studies of
 detection (Davidoff 1977) and localization (Kimura      electrophysiological hemispheric asymmetries
 1969), in orientation matching (Longden et al.          with respect to sex differences might contribute to
 1976), or in adaptation to orientation (Grabowska       our understanding of hemispheric asymmetry that
 1987, Hegarty et al. 1991). The right-hemisphere        seems to be a dynamic process rather than a stable
 superiority for such elementary visuospatial func-      feature.
tions may change into a different pattern of hemis-
pheric asymmetry when later cognitive processes             ACKNOWLEDGMENT
are involved. According to the logic of such ap-
proach, the brain lateralization should not be con-         The research was supported by Nencki Institute
sidered as a stable feature which determines the left    of Experimental Biology.
or right hemisphere predominance in a given func-
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