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The extrastriate generators of the EP to checkerboard onset. A source localization approach
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Electroencephalography and clinical Neurophysiology, 1991, 80:181-193 © 1991 Elsevier Scientific Publishers Ireland, Ltd. 0168-5597/91/$03.50 ADONIS 016855979100075W
EVOPOT 89651
The extrastriate generators of the E P to checkerboard onset. A source localization approach P. Ossenblok and H. Spekreijse The Netherlands Ophthalmic Research Institute, 1100 A C Amsterdam (The Netherlands) (Accepted for publication: 23 July 1990)
Summary The cortical origin of the pattern onset EP has been investigated over a time window which covers the entire positive-negative-positive complex of the pattern onset EP. On the basis of a dipole source localization approach, the position, orientation and strength of the underlying sources of the pattern onset EP were estimated. For large check stimuli, chosen to have a weak edge specific component in the response, still two components are needed to account for the variance of the responses. Each component corresponds to a single dipole source, and both originate in the extrastriate cortex. These components dominate, respectively, the initial and the late positive peaks of the pattern onset EP. The equivalent dipole sources of the two components show different behaviors with respect to the position of the stimulus in the visual field. The topography and behavior of the equivalent dipole source underlying the early positive component suggest an origin in area 18. The invariance with stimulus location of the dipole source underlying the late positive component suggests an origin beyond area 18. The different topographies of the components also account for the differences in surface distribution of the pattern onset EP to large check stimulation of the upper and lower sectors of the visual field. Key words: VEP; Brain mapping; Extrastriate activity; Dipole sources; Pattern stimulation
Visual evoked potentials (VEPs) can be used to determine the location of brain activity. We studied the origin of the positive-negative-positive complex of the pattern onset EP evoked by the onset of a checkerboard stimulus. According to Maier et al. (1987) and Van Dijk and Spekreijse (1989), the sources underlying the first positive (CI) and negative peaks (CII) of the pattern onset EP have respectively an extrastriate and striate origin. Jeffreys (1977) suggested an extrastriate origin of the late positive peak (CIII); however, no further information is available about the cortical generators of this peak. The extrastriate origin of the initial part of the pattern onset EP is not accepted generally; others have reported a striate origin (Jeffreys and Axford 1972a; Kriss and Halliday 1980; Butler et al. 1987). Jeffreys and Axford (1972a,b) proposed that CI originates from the primary visual cortex located mainly on the medial surfaces in and around the calcarine fissure. Their main argument is based upon the classical model of the representation of the peripheral visual fields in striate cortex (Holmes 1945; Spalding 1952). This model implies that the vertical halves of the visual fields are
Correspondence to." Dr. P. Ossenblok, The Netherlands Ophthalmic Research Institute, P.O. Box 12141, 1100 AC Amsterdam (The Netherlands).
projected in the contralateral hemisphere with the upper and lower half-fields represented on the lower and upper leaves of the calcarine fissure, respectively. If the assumption is made that the surface potential field produced by cortical activity can be described by an equivalent dipole source perpendicular to the cortical surface, then the symmetrical arrangement of the paramacular visual field projection to the striate cortex will result in potentials of opposite polarity for right and left half-fields and for upper and lower field stimulation (Fig. la). In accordance with this dipole-sheet model of striate cortex a reversal in polarity of the initial peak of the pattern onset EP for left and right half-field stimulation (Jeffreys and Axford 1972a; Jeffreys 1977) and for upper and lower field stimulation (Jeffreys and Smith 1979; Sencaj and Aunon 1982; Butler et al. 1987) has been reported. Note, however, that the short duration of the onset period used by these authors may lead to ambiguities in the results, since the components of the pattern onset EP contain contributions related to the offset of the pattern (Spekreijse and Estrvez 1972). Les~vre and Joseph (1979) and Les+vre (1982) also found a reversal in polarity of the initial part of the pattern onset EP. However, they attributed this polarity inversion to a lower field generator lying on the upper convexity of the hemisphere in extrastriate cortical regions and an upper field generator lying on the undersurface of the lobe. The equivalent dipole
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P. OSSENBLOK, H. SPEKREIJSE a
sources for the extrastriate cortical regions representing the upper and lower visual fields will then be roughly perpendicular. The analysis of the results and hence the conclusions will depend on the way the EP components are defined. A weak point in the reasoning of most of the authors is the a priori assumption that the activity of each peak may be attributed to a single dipole source. Les6vre and Joseph (1979) were the first to propose that the peaks in the pattern onset EP may reflect simultaneous activity of several visual areas. Evidence against the assumption that the potential distribution of the underlying cortical activity at each time instant can be ascribed to a single equivalent dipole source was given by Stok (1986) and Maier et al. (1987). Stok (1986), whose analysis was based on the model of a moving dipole, found that the equipotential distribution of the pattern onset EP is stable from 80 msec to about 120 msec, whereas at 125 msec the maximum started to move towards the midline while slowly rotating. These observations were confirmed by Maier et al. (1987) who found that the activity underlying the first positive peak of the pattern onset EP with a peak latency of about 100 msec was dominated by an extrastriate component, whereas the negative peak with a peak latency of about 130 msec was dominated by a striate component. Since these components overlap, one should be cautious in defining EP components on the basis of the appearance of positive or negative peaks. Because of the substantial overlap of the responses originating in different visual areas (Les6vre and Joseph 1979; Maier et al. 1987), it is necessary to separate the constituent components of the pattern onset EP before the causes of eventual polarity reversals can be established. Maier et al. (1987) described a method that determines the dimensionality of the response space and provides the constituent components of the composite responses. It is assumed that the responses originate from a fixed number of dipole sources, each with a fixed position and orientation, but varying in strength. The components then reflect the variation of the strength of the dipole sources as a function of time. Maier et al. (1987) and Van Dijk and Spekreijse (1989) used localized stimulation of the visual field ~o determine from which visual areas the responses originate. The size of the stimulus field was as small as possible in order to activate a small part of the cortex and hence to mimic a single current dipole. Their analysis over a time window covering CI and CII yielded 2 components. The first component, consisting mainly of CI, was ascribed to an equivalent dipole source situated in the extrastriate visual cortex and the second component, in which CII dominates, to the primary visual cortex. Using the same dipole source localization approach, we tried to account for the variance of the responses over a window which covered the entire C I - C I I - C I I I
interval. Since in source localization a large number of parameters has to be estimated, the stimulus parameters were chosen such that a minimum number of sources was activated. So, a single hemisphere was always activated. Furthermore, to weaken the striate component in the response, large checks were used since the striate component is selectively sensitive to the edges of a pattern stimulus. Moreover, large checks favor CI whereas CIII can be enhanced by binocular presentation (Spekreijse et al. 1973). Therefore, we stimulated with checkerboards with large check sizes viewed binocularly. The analysis of the pattern onset EP over its total response period further requires a good signal-tonoise ratio. The signal-to-noise ratio is increased by using large visual fields for stimulation and high modulation depths (the significant power of the responses then exceeds 90%). One may use large visual fields because the effect of source extension on the parameters of the equivalent dipole is very small (De Munck et al. 1988a,b; De Munck and Spekreijse 1989). Furthermore, we studied the retino-cortical organization to gain information about the anatomical location of the generators underlying the CI and CIII peak of the pattern onset EP. We shall discuss also the results of stimulation of distinct sectors of the upper and lower visual fields to clarify the controversy about the differences in polarity and latency of the upper and lower field EPs. These results provide further evidence for the cortical origin of the first positive peak of the pattern onset EP.
Methods
Stimuli Stimuli were generated on a digital display system (Genisco 3000). The subject viewed a TV monitor with a mean illumination of 65 c d / m 2, subtending 8 × 8 °. All stimuli were presented in the left or right hemifield surrounded by a steady homogeneous field of the same mean luminance. VEPs were also recorded with stimuli restricted to quadrants or octants in the upper or lower visual field. A checkerboard pattern with checks of 24' or 48' and a contrast of 80% was used, presented for 300 msec once in 800 msec, without net variation in overall luminance level. In this way the responses evoked by the onset and offset of the checkerboard could be registered separately.
Recording VEPs were recorded from 24 scalp electrodes. To minimize the effect of systematic errors on the location of the equivalent dipole we used a wide "electrode-grid" with a vertical and horizontal spacing of the electrodes of 4 cm. The lowest row of electrodes was positioned 2 cm below the inion. A reference electrode was placed on the frontal midline; the ground was located near the
EXTRASTRIATE GENERATORS OF PATTERN ONSET EP
183
vertex. Signals were amplified (Medelec 5000) and bandpass filtered between 0.5 and 70 Hz. T h e high cut-off frequency (70 Hz) was set b y a low-pass fourthorder Butterworth filter, which introduced a phase shift increasing the response latencies b y 7 msec. Thus, peak latencies estimated f r o m the records in this p a p e r should be corrected for this latency increase. T h e E E G s were A / D converted with a 240 H z sample frequency and on-line averaged with an H P 2100 minicomputer. Since only 16 E E G amplifiers were available, all records were made in 2 sessions. The 8 double records were used to estimate the signal-to-noise ratio and to obtain an impression of the reproducibility of the responses. The length of the time w i n d o w for analysis was optimized in accordance with the best possible signal-to-noise ratio,
thus accounting for the variance over the total response period. The maximal time w i n d o w chosen was 58.4204.3 msec after stimulus onset, thus containing the p r o m i n e n t peaks (CI, C I I and C I I I ) of the pattern onset EP. In this p a p e r the EP results of 5 healthy subjects are presented. The results of stimulation with the distinct sectors of a hemifield, as depicted in Fig. la, are illustrated b y the results obtained in a single subject who is representative for the group.
Source localization T h e analysis is based on the assumption that the sources of the V E P m a y be modeled by equivalent dipole sources, which m a y be activated simultaneously u p o n visual stimulation. The role of the principal com-
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Fig. 1. a: the distinct sectors of the first 4 ° of the right half visual field are depicted schematically. A fixation-cross Is situated in the middle of the visual field. The lower part of the figure gives the upper and lower leaves of the calcarine fissure lying near the medial plane together with the area activated (solid lines) and the predicted orientation of the equivalent dipole sources, b: in the orthogonal xyz-frame a point p is defined in the azimuth-elevation co-ordinate system. Phi is the angle with the x-axis of the projection of p in the x-y plane and theta is the angle between the vector to p and the x-y plane. The orientation of the equivalent dipoles is determined in the local x'y'z'-frame. The projection along the z'-axis yields the radial part of the dipole and the projection in the x'y'-plane yields the tangential part. The orientation of the dipole projection in the tangential plane is denoted as the angle psi. In the x'y'-plane the y '-axis points towards the z-axis of the xyz-frame.
184
ponent analysis (PCA) in the analysis is that it provides a good estimate of the dimensionality of the data space (see Kavanagh et al. 1976). PCA reveals how many principal components (PCs) are needed to account for the significant variance within the time window chosen for a set of EP records. This estimate provides the number of underlying sources of the VEP that are necessary to account for the data (see Kavanagh et al. 1976; Donchin and Heffley 1978). However, the statistically independent PCs do not necessarily have any physiological significance (Van Rotterdam 1970). Therefore, the mathematically obtained PCs are transformed into physiologically meaningful components. The criterion for this transformation is that the weights of the corresponding potential distributions satisfy a dipole-like potential distribution (Maier et al. 1987). If linear combinations of the factor loading distributions of the PCs yield potential distributions that are in accordance with dipole sources, then we shall have explained the significant power in the responses in terms of dipole sources. The position and orientation of the equivalent dipole sources are obtained by fitting dipole fields to these best possible dipole-like potential distributions. Thus, the whole procedure, which is described in detail in Maier et al. (1987), yields the constituent components of the pattern onset EP, each corresponding to a single dipole source with a time profile reflecting the variation of strength of that dipole source. To determine the location of the dipole sources the head is described as a volume conductor with 3 concentric spheres of different conductivity. Using a homogeneous sphere model an initial set of source parameters is calculated. With a correction table the eccentricity found is converted to the 3-shell model consisting of a homogeneous sphere of neural tissue, surrounded by 2 concentric spherical shells representing, respectively, the skull and the skin (Ary et al. 1981). The converted parameters of the source are used as a starting point for the computation in the inhomogeneous 3-sphere model.
Presentation of data The electrode positions are defined in an orthogonal (xyz) co-ordinate system with the x- and y-axes in a horizontal plane when the subject looks straight ahead (Fig. lb). The x-axis points towards the inion. The position of the equivalent dipole is given by the coordinates f, phi and theta. The factor f, which can vary between 0 (mid of the best fitting sphere) and 1, represents the eccentricity of the dipole source and phi and theta give the azimuth and elevation co-ordinates, respectively. Note that f should have values smaller than 0.87 for a dipole situated in the cortex (Rush and Driscoll 1968). The orientation of the equivalent dipole is represented in a local co-ordinate frame x ' y ' z ' . The projection along the z'-axis yields the radial part of the dipole and the projection in the x'y'-plane yields the
P. OSSENBLOK, H. SPEKREIJSE
tangential part. The z'-axis points radially outward, whereas the orientation of the dipole projection in the tangential plane is denoted as the angle psi. This angle can b e visualized by placing a tangential plane at the radial projection of the dipole position on the surface of the sphere; it is measured counterclockwise with respect to the local x'-axis. Both the radial and tangential parts of the dipole are expressed in percentages of the strength of the equivalent dipole. The potential distributions are plotted in a topographic map in the phi-theta co-ordinate frame (see Figs. 3-6). Since the occipital part of the head is assumed to be spherical, the zero-point of the co-ordinate frame (i.e., phi = 0 and theta = 0) coincides with the inion. The asterisks in the topographic maps indicate the electrode positions. The equipotential lines are drawn according to a linear interpolation between the potentials of 3 neighboring recording positions. A maximum in the equipotential maps always coincides with the recording site of the maximal response of that component. In the topographic maps the positions of the dipole sources are depicted by large dots, their orientations in the tangential plane by arrows. The inset gives the radial part of the dipole strength (% rad) and the depth by the factor f. The quality of dipole fit is indicated by the % error based on the sum of the squared potential differences.
Results
Hemifield stimulation Fig. 2 shows VEPs recorded at the 24 scalp electrodes to a pattern onset stimulus presented in the right hemifield. In the positive-negative-positive complex of the response the early positive peak (CI) dominates at the contralateral recording site. From the figure it is evident, however, that there are large differences in the wave forms of the responses. For instance, the early positive peak of the onset EP seems to reverse in polarity for the left and right hemisphere. However, the initial peaks in the pattern onset EP may differ by as much as 15 msec on opposite sides of the head. It should be noted also that the pattern onset responses recorded at the electrode sites 14 and 16 show a single prominent positive peak coinciding with, respectively, CI (at 80.5 msec) or with CIII (at 138.8 msec). Thus, it appears that the pattern onset EP, evoked by large visual fields and large high contrast checks, is due to a dual contribution of an early and a late component. Calculation of the equivalent dipoles and their corresponding time functions indeed shows that the pattern onset EP consists of 2 components. The strength of one of the dipole sources changes in a triphasic way in time (insert Fig. 3a), that of the other in a biphasic way (insertFig. 3b). Although the components overlap in
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Fig. 2. The visual evoked responses to the appearance of a 48', 80% modulated checkerboard in the central 4° of the right visual field. The responses are plotted according to a 2-dimensional projection of a spherical surface with horizontal and vertical distances of 4 cm. The asterisk indicates the position of the inion, which is located at the midline of the head 2 cm above the bottom row of electrodes.The responses recorded at the electrodesites 14 and 16 are marked with.the respectivenumbers.
time, the first dominates the initial positive peak (CI) and the second component the late positive peak (CIII). Therefore, these components are referred to as the early or the late component and are, respectively, designated as C1 and C3. From the amplitude distributions of the components equivalent dipoles were computed, giving the best approximation to the recorded potential distributions. Fig. 3 depicts the recorded potential distributions of both components, as well as the best fitting dipole distributions. The difference between adjacent contours is given in microvolts and reflects the strength of the dipole source. In accordance with the results of Maier et al. (1987) the spatial distribution of the early component shows a contralateral maximum, whereas the orientation of the underlying dipole source is mainly radial (Fig. 3a). The radial part of the equivalent dipole source (% rad) amounts to 77.5% of the strength of the source. The dipole source underlying the late component has about the same position. However, its orientation is predominantly tangential (Fig. 3b). The positive part of the potential distribution is ipsilateral, whereas contralaterally a negative distribution emerges. The shift from a maximum in the potential distribution to a minimum corresponds to a polarity reversal of the component. The depth (f) indicates that both dipole sources are situated in the cortex. Upper versus lower field EPs In Fig. 4 the results of quadrant-field stimulation are depicted. Lower quadrant-field stimulation generates an
equivalent dipole source underlying the early component (C1) which has shifted outward with respect to half-field stimulation, but maintains a predominant radial orientation (Fig. 4a). For this component the projection of the lower quadrant-field lies at the upper part of the outer occipital lobes, whereas the upper quadrant-field is projected at the bottom of the hemisphere and will contribute less to the surface potentials evoked by half-field stimulation (Fig. 4b). In accordance with the dipole-sheet model of extrastriate cortex, the orientation of the equivalent dipole source of the early component becomes predominantly tangential for upper quadrant-field stimulation. The position and orientation of the equivalent dipole source underlying the activity of the late component (C3) remains the same for upper and lower quadrant-field stimulation. The time functions of both equivalent dipole sources have the same polarity for upper and lower quadrantfield stimulation (inserts Fig. 4). Apart from the less pronounced initial peak of the early component for upper quadrant-field stimulation the shapes of the components are rather similar, suggesting the same cortical origin. Furthermore, the peak latencies of the components generated by upper quadrant-field stimulation are the same as those to lower quadrant-field stimulation. If the responses evoked by upper and lower quadrant-field stimulation originate in adjacent cortical areas, then the responses evoked by half-field stimulation should approximate the sum of the quadrant responses. Fig. 4c shows that the algebraic summated responses have indeed the same topographic distribu-
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Fig. 3. Equipotential maps derived from the responses depicted in Fig. 2. The upper map holds for the early component (C1) and the lower map for the late component (C3). Heavy lines represent the positive values of the recorded potentials, heavy dotted lines the zero values, and heavy dashed lines the negative values. Also shown are the best fitting dipole distributions, represented by thin continuous, thin dotted and thin dashed lines for positive, zero and negative potentials respectively. The asterisks give the positions of the electrodes in the topographic map. In each plot the location of the equivalent dipole source is depicted by a dot. The arrow gives the orientation of the dipole projection in the tangential plane. AV gives the difference between adjacent contours in microvolts. The inserts in the upper left corner give the depth f, the radial part of the dipole strength (% rad) and the error in percentages of the sum of the squared potentials. The shape of the components is shown in the inserts in the upper right corner. The components covered a time window that started 58.3 msec after pattern onset and ended at 204.3 msec.
tions for C1 as well as for C3, similar w a v e shapes, a n d c o m p a r a b l e l o c a t i o n s for the e q u i v a l e n t d i p o l e sources to those g e n e r a t e d b y half-field s t i m u l a t i o n (see Fig. 3). N o t e , however, t h a t the d i p o l e source u n d e r l y i n g C1 is s i t u a t e d m o r e eccentrically t h a n the e q u i v a l e n t d i p o l e source g e n e r a t e d b y half-field stimulation. I n accord a n c e with the a n a t o m i c a l o r g a n i z a t i o n of the visual c o r t e x the shape a n d t o p o g r a p h i c d i s t r i b u t i o n o f the
early c o m p o n e n t (C1) e x t r a c t e d f r o m the s u m m a t e d p o t e n t i a l s are d o m i n a t e d b y the r e s p o n s e to lower q u a d r a n t - f i e l d s t i m u l a t i o n . Also, the o r i e n t a t i o n of the e q u i v a l e n t d i p o l e source is m a i n l y radial. N o t e t h a t the e q u i v a l e n t d i p o l e source of the early c o m p o n e n t is l o c a t e d a p p r o x i m a t e l y b e t w e e n the d i p o l e sources gene r a t e d b y u p p e r a n d lower q u a d r a n t - f i e l d stimulation. Since the r e t i n o t o p i c p r o j e c t i o n of the late c o m p o n e n t
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(C3) seems to be invariant for upper and lower quadrant-field stimulation, this component remains in place.
contrary to the dipole-sheet model of striate cortex the equivalent dipole sources generated by stimuli lying near the horizontal meridian do not reverse in polarity, whereas the equivalent dipole sources activated by stimuli lying near the vertical meridian have a different orientation. Moreover, in accordance with the results of quadrant-field stimulation both lower octant-field generators of C1 have a predominant radial orientation, whereas the orientation of the upper field generators becomes tangential. The dipole sources of the early
Stimulation with sectors of the lateral hemifields Figs. 5 and 6 depict the potential distributions of the equivalent dipole sources generated by octant-field stimulation. The equipotential maps are arranged according to the retinotopic projections of the octantfields on the striate cortex (see Fig. la). However,
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Fig. 4. Equipotential maps derived from the responses evoked by the onset of a 24', 80% modulated checkerboard in the central 4 ° of the lower (upper part of the figure) and upper (middle part of the figure) right quadrant of the visual field. In the lower part of the figure the potential distributions of the components extracted from the algebraic sum of the recorded potentials evoked by the upper and lower right quadrants of the visual field are plotted. The maps in the left column hold for the early component (C1) and in the right column for the late component (C3). The inserts in the lower right corner give the depth f, the radial part of the dipole strength (% rad) and the error in percentages of the sum of the squared potentials. The shape of the components is shown in the inserts in the upper right corner. The components covered a time window that started 58.3 msec after pattern onset and ended at 204.3 msec. For further information on equipotential maps see the legend of Fig. 3.
188 component also lie deeper in the head for upper than for lower octant-field stimulation and the strength of these dipole sources has diminished. The equivalent dipole sources underlying the late component have about the same position for all 4 octants (Fig. 6). The corresponding potential fields are mainly tangential, whereas the position of the dipole sources is clearly contralateral. Compared to the lower field the orientation of the upper field generators tends to be more radial. However, these differences may partly be due to the differences in the quality of the fits for both conditions of stimulation. Also the time functions of the equivalent dipole sources activated by octant-field stimulation show a high degree of similarity. For instance, the peak latency of the positive peak (84.7 msec) of the early component is remarkably constant (inserts Fig. 5). The variability becomes apparent in the later parts of the components. Although the late component always has a biphasic shape the variability of this component is large, especially for the negative peak (inserts Fig. 6). Note the difference of about 20 msec in the peak latency of the early peak of C3 for octant-fields lying near the vertical and horizontal meridians.
Discussion The positive-negative-positive complex of the pattern onset EP, evoked by large visual fields with large checks, consists of at least two components, each generated by one equivalent dipole source. It was found, in accordance with Maier et al. (1987), that the initial positive peak of the early component coincides with CI. Other correspondences between the study of Maier et al. (1987) and the present study are the shape and spatial distribution of C1 and the position and orientation of the underlying sources of this component. This paper shows further that the positive peak of the late component coincides with CIII. The accumulated power of the two components, which amounts, for the 5 subjects studied, to 90% or more of the mean power of their responses, can account almost completely for the significant variance of the sets of responses. The responses to patterns with large checks can, therefore, be described adequately by a linear combination of these two components.
P. OSSENBLOK,H. SPEKREIJSE The equivalent dipole sources underlying the early and late components both have large phi values, indicating a position away from the midline. On the basis of the symmetrical arrangement of the striate cortex (see Fig. 1) it can be expected that sources of striate origin lying near the midline generate potential fields of opposite polarity for left and right and upper and lower field stimulation. However, the potential fields corresponding to the early and late components do not reverse in polarity for stimulation with any of these parts of the visual fields. Moreover, both the position and orientation of the equivalent dipole sources generated by left and right and upper and lower visual field stimulation suggest an extrastriate origin of these components.
The topographic representation of the components In Fig. 7 the position of the equivalent dipole sources activated by left and right half-field stimulation are depicted for the 5 subjects studied. The sources for both hemispheres are situated far apart and clearly contralateral with respect to the midline, demonstrating the extrastriate origins of both components. There is large intersubject variability in the position and orientation of the equivalent dipole sources underlying both extrastriate components, which is probably due to the variability in the geometry of the various visual areas of extrastriate cortex between subjects (see also Table I). The intrasubject variability of the equivalent dipole sources may reflect the variability of the geometry of the hemispheres with respect to e a c h other, as was shown, for instance, by Stensaas e~ al: (1974). However, in spite of marked differences between the hemispheres of a subject the potential distributions of the equivalent dipole sources and the corresponding components of the pattern onset EP are quite symmetrical for left and right half-field stimulation. The early component (C1) reflects also a representation of the upper and lower quadrants of the contralateral hemifields. Since at the lower border of the extrastriate cortex the cortical surface bends strongly inward, stimulation of the lower field results in a more anteriorly positioned and stronger maximum than that to upper field stimulation. Moreover, the downward shift of the maximum in the potential distribution for upper field stimulation corresponds to an equivalent dipole which lies deeper in the head and is oriented
Fig. 5. Equipotentialmaps derived from the responses evoked by the onset of a 24', 80% modulated checkerboardrestricted to the central 4 ° of the individual upper and lower right octant-fields.All maps in this figure hold for the early component (C1). In each part of the figure the potential distribution and the octant-fieldused for stimulation are depicted. The inserts in the lower right corner give the depth f, the radial part of the dipole strength (% rad) and the error in percentagesof the sum of the squared potentials. Also the shape of the components is shown for each potential distribution. The componentscovereda time window that started at 79.2 msec after pattern onset and ended at 183.5 msec. For further information on the equipotential maps see the legend of Fig. 3.
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more tangentially. These changes are in agreement with an extrastriate cortical origin of C1. The results of octant-field stimulation also show that the potential distributions corresponding to the early component closely follow the spherical surface of the extrastriate cortex (see Fig. 5). This topographic representation of the early component supports the view that this component is generated in area 18 (Van Dijk and Spekreijse 1989), an area which is rather unfolded and runs parallel to the spherical surface of the cortex. We did not find a reversal in polarity for upper and lower octantfield stimulation, as was reported by Butler et al. (1987). In accordance with the dipole-sheet model of extrastriate cortex the equivalent dipole sources change from a radial to an almost completely tangential orientation for upper octant-field stimulation. The less consistent changes in the orientation of the dipole projection in the tangential plane with the position of the stimulus in the visual field are probably due to local curvatures of the visual cortex. The picture of C3 is less clear. The parameters of the equivalent dipole sources underlying the late component (C3) seem to be invariant for upper and lower visual field stimulation. It is known from recent work on monkey that, contrary to area 18, area 19 is far from representing a functional unity. It is more likely to be made up of multiple independent areas, each of which represents the whole hemiretinal projections (Van Essen and Zeki 1978; Van Essen 1985; Burkhalter 1986). It was found that these areas occupy much of the lateral portion of the anterior bank of the lunate sulcus or are submerged in the superior temporal sulcus (Van Essen 1985; Gattass et al. 1988). If it is assumed that the extrastriate cortex in m a n corresponds to the prestriate cortex of monkey, then sources originating in area 19 should be located in the contralateral hemisphere, but the topography of these components should not change when turning from lower to upper field stimulation. The topographical characteristics of the late component (C3) and the invariance of the parameters of the corresponding dipole sources for upper and lower field stimulation, therefore, suggest an origin in area 19. However, according to Gattass et al. (1988), it is possible that the retinotopic organization is not discerned for recordings within 5 - 1 0 ° of the visual field, because of the large size of the receptive fields and the somewhat disorderly representation of the visual field in this area.
P. OSSENBLOK, H. SPEKREIJSE
The equivalent dipole description In general the correspondence between the measured and dipole-like distributions is quite good, suggesting that a single equivalent dipole also provides an adequate description of the activity evoked by extended visual fields. However, for large stimulus fields the error of the depth and strength parameters will be larger (De Munck et al. 1988b). Equivalent dipole sources with a radial orientation will be somewhat deeper than the midpoint of the extended source and tangential equivalent dipole sources will be more eccentric. Another possible source of error is the sensitivity of the localization methods to the geometry chosen for the head model and the sensitivity to the parameters that describe the conductivity properties of the head. For instance, scalp and skull thickness are known to vary from one person to another and from location to location over the head. For these kinds of model errors the depth of the equivalent dipole sources tends to be more superficial than the depth of the cortical sources (Ary et al. 1981). Similarly, the conductivity ratio and the degree of anisotropy in the skull can only be approximated (De Munck 1988). The largest deviations from the spherical model are present at the lower border of the prestriate cortex, where the cortical surface bends strongly inward. It is probably due to these deviations that the quality of fit is generally worse for upper field stimulation. Especially the sources found for vertical upper octant-field stimulation are rather poorly matched to a dipole (see Figs. 5 and 6). This is in agreement with the results of Butler et al. (1987), who also had difficulty in resolving the upper vertical octant generators as a single dipole. Thus, the inconsistency of the depth and strength parameters will be larger for upper field stimulation than for the projection of the lower visual field. The errors for the strength of the activity tend to be much larger; therefore only a rough indication of source strength will be justified in most cases. However, in spite of the possible sources of error, the effect of the position of the stimulus in the visual field on the location and orientation of the dipole sources underlying both components is rather consistent and agrees with the known structure of extrastriate cortex. Also, the components elicited by different sectors of the visual field are rather invariant in shape. However, due to changes in the topography of the components the pattern onset EP changes for the distinct sectors of the
Fig. 6. Equipotential maps derived from the responses evoked by the onset of a 24', 80% modulated checkerboard restricted to the central 4° of the individual upper and lower right octant-fields. All maps in this figure hold for the late component (C3). In each part of the figure the potential distribution and the octant-field used for stimulation are depicted. The inserts in the lower right corner give the depth f, the radial part of the dipole strength (% rad) and the error in percentages of the sum of the squared potentials. Also the shape of the components is shown for each potential distribution. The components covered a time window that started 79.2 msec after pattern onset and ended at 183.5 msec. For further information on the equipotential maps see the legend of Fig. 3.
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v i s u a l field. T h u s , t h e s e s t i m u l u s - b o u n d d i f f e r e n c e s in the p a t t e r n o n s e t E P a r e n o t d u e to c h a n g e s in the t i m e course of the components. The intersubject variability in t h e p a t t e r n o n s e t E P is also s u b s t a n t i a l , b u t less so for the c o m p o n e n t s . H o w e v e r , d u e to t h e v a r i a b i l i t y o f the g e o m e t r y o f t h e v a r i o u s v i s u a l areas, the t o p o g r a p h y o f t h e c o m p o n e n t s v a r i e s b e t w e e n subjects. T h e r e f o r e , the i n t e r s u b j e c t v a r i a b i l i t y o f t h e p a t t e r n o n s e t E P will b e d u e m a i n l y to t h e v a r i a b i l i t y in c o r t i c a l t o p o g r a p h y .
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Conclusions
W e h a v e s h o w n in this p a p e r t h a t t h e i n t e r p r e t a t i o n o f t o p o g r a p h i c d a t a o f the p a t t e r n o n s e t E P o n t h e basis o f t h e p e a k s in t h e r e s p o n s e s m a y l e a d to e r r o n e o u s conclusions, since the peaks reflect activity from different parts of the visual cortex. For instance, we found t h a t t h e d i f f e r e n c e s in the s u r f a c e d i s t r i b u t i o n of c o r r e s p o n d i n g c o m p o n e n t s f o r left a n d r i g h t h a l f - f i e l d s t i m u l a t i o n a n d f o r u p p e r a n d l o w e r field s t i m u l a t i o n are n o t due to a r e v e r s a l in p o l a r i t y of t h e initial p e a k of the p a t t e r n o n s e t E P , as o b s e r v e d b y J e f f r e y s a n d cow o r k e r s . T h e r e is a l s o n o e v i d e n c e f o r a d e l a y o f the u p p e r field E P s as s u g g e s t e d b y L e h m a n n et al. (1977). Thus, the results of the present study do not support the i d e a o f a s y m m e t r i e s in signal p r o c e s s i n g in the u p p e r a n d l o w e r h e m i r e t i n a . It was s h o w n t h a t t h e p a t t e r n o n s e t E P , e v o k e d b y l a r g e v i s u a l fields a n d l a r g e elem e n t sizes o f h i g h c o n t r a s t , c o n s i s t s o f at least t w o c o m p o n e n t s , e a c h c o r r e s p o n d i n g to o n e single d i p o l e
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Fig. 7. The positions of the equivalent dipole sources generated by left and right half-field stimulation are depicted in the phi-theta coordinate frame for the 5 subjects studied. The positions of the sources of the 5 subjects are numbered 1-5. The positions of the equivalent dipole sources underlying the early component (C1) of the pattern onset EP are depicted in the upper part of the figure and the positions of the equivalent dipole sources underlying the late component (C3) in the lower part. It can be seen that for all subjects the theta values of the late component are significantly smaller than those of the early component.
TABLE I Positions and orientations of the equivalent dipole sources underlying C1 and C3 generated by left (lhf) and right (rhf) half-field stimulation. The last column gives the error of the fit. For explanation of the position and orientation parameters see Fig. lb. Subject
Field
f
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% error
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0.69 0.79 0.57 0.73 0.46 0.54 0.71 0.55 0.73 0.57
- 22.8 26.6 - 25.7 33.5 - 37.7 32.9 - 15.0 8.9 - 34.5 18.3
38.4 50.5 42.9 38.1 26.7 36.9 22.9 28.0 13.4 24.3
77.5 12.6 82.2 90.8 20.2 16.3 73.3 60.2 59.0 59.9
143.1 - 166.4 110.6 - 155.9 140.1 - 150.3 124.4 - 126.2 177.3 - 137.9
0.7 1.3 1.1 0.6 1.8 0.8 1.3 1.0 6.1 0.6
rhf lhf rhf lhf rhf lhf rhf lhf rhf lhf
0.86 0.87 0.78 0.79 0.71 0.76 0.74 0.78 0.86 0.86
- 23.3 23.8 - 29.3 32.3 - 12.7 28.1 - 21.2 15.8 - 20.1 23.9
32.8 23.6 25.0 14.5 7.7 10.4 11.4 2l .5 7.6 7.0
9.1 8.6 13.4 7.2 0.3 11.7 4.3 3.2 2.5 3.2
- 91.3 68.1 - 101.9 90.9 - 48.0 73.3 23.6 133.5 103.9 - 105.5
8.0 25.2 8.1 5.3 2.6 1.l 2.8 20.7 19.0 7.8
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EXTRASTRIATE GENERATORS OF PATTERN ONSET EP source and overlapping in time. These components
show
different behaviors with respect to the position of the s t i m u l u s i n t h e v i s u a l field. O n t h e b a s i s o f t h e r e s u l t s p r e s e n t e d i n t h i s p a p e r it s e e m s l i k e l y t h a t t h e d i f ferences in surface distribution of the pattern onset EP for upper and lower field stimulation are due to changes of topography of the early component, whereas the topography of the late component does not change for upper and lower field stimulation. The authors acknowledge the suggestions of Dr. J.C. De Munck, Dr. D. Reits and Dr. B.W. Van Dijk to improve the manuscript. They further wish to express their appreciation to the subjects who took part in this research project.
References Ary, J.P., Klein, S.A. and Fender, D.H. Location of sources of evoked scalp potentials: correction for skull and scalp thickness. IEEE Trans. Biomed. Eng., 1981, 28: 447-452. Burkhalter, A., Felleman, D.J., Newsome, W.T. and Van Essen, D.C. Anatomical and physiological asymmetries related to visual areas V3 and VP in macaque extrastriate cortex. Vision Res., 1986, 26: 63-80. Butler, S.R., Georgiou, G.A., Glass, A., Hancox, R.J., Hopper, J.M. and Smith, K.R.H. Cortical generators of the CI component of the pattern onset visual evoked potential. Electroenceph. clin. Neurophysiol., 1987, 68: 256-267. De Munck, J.C. The potential distributions in a layered anisotropic spheroidal volume conductor. J. Appl. Phys., 1988, 64: 464-470. De Munck, J.C. and Spekreijse, H. The effect of source extension on the location and components of the equivalent dipole. In: K. Maurer (Ed.), Topographic Brain Mapping of EEG and Evoked Potentials. Springer, Berlin, 1989: 141-146. De Munck, J.C., Van Dijk, B.W. and Spekreijse, H. Mathemathical dipoles are adequate to describe realistic generators of human brain activity. IEEE Trans. Biomed. Eng., 1988a, 35: 960-966. De Munck, J.C., Van Dijk, B.W. and Spekreijse, H. An analytic method to determine the effect of source modelling errors on the apparent location and direction of biological sources. J. Appl. Phys., 1988b, 63: 944-956. Donchin, E. and Heffley, E.F. Multivariate analysis of event related potential data: a tutorial review. In: D.A. Otto (Ed.), Multidisciplinary Perspectives in Event-Related Brain Potential Research. Environmental Protection Agency, Washington, DC, 1978: 555572. Gattass, R., Sousa, A.P.B. and Gross, C.G. Visuotopic organization and extent of V3 and V4 of the macaque. J. Neurosci., 1988, 8: 1831-1845. Holmes, G. The organization of the visual cortex in man. Proc. Roy. Soc. B, 1945, 132: 348-361. Jeffreys, D.A. The physiological significance of pattern visual evoked potentials. In: J.E. Desmedt (Ed.), Visual Evoked Potentials in Man: New Developments. Clarendon Press, Oxford, 1977: 134167.
193 Jeffreys, D.A. and Axford, J.G. Source locations of pattern specific components of human visual evoked potentials. I. Component of striate cortical origin. Exp. Brain Res., 1972a, 16: 1-21. Jeffreys, D.A. and Axford, J.G. Source locations of pattern specific components of human visual evoked potentials. II. Component of extrastriate cortical origin. Exp. Brain Res., 1972b, 16: 22-40. Jeffreys, D.A. and Smith, A.T. The polarity inversion of scalp potentials evoked by upper and lower half-field stimulus patterns: latency or surface distribution differences?. Electroenceph. clin. Neurophysiol., 1979, 46: 409-415. Kavanagh, R.N., Darcey, T.M. and Fender, D.H. The dimensionality of the h u m a n visual evoked scalp potential. Electroenceph. clin. Neurophysiol., 1976, 40: 633-644. Kriss, A. and Halliday, A.M. A comparison of occipital potentials evoked by pattern onset, offset, and reversal by movement. In: C. Barber (Ed.), Evoked Potentials. MTP Press, Lancaster, 1980: 213-218. Lehmann, D., Meles, H.P. and Mir, Z. Average multichannel EEG potential fields evoked from upper and lower hemi-retina: latency differences. Electroenceph. clin. Neurophysiol., 1977, 43: 725-731. Lesrvre, N. Chronotopographical analysis of the human evoked potential in relation to the visual field (data from normal individuals and hemianopic patients). In: I. Bodis-Wollner (Ed.), Evoked Potentials. Ann. NY Acad. Sci., 1982, 388: 156-182. Lesrvre, N. and Joseph, J.P. Modifications of the pattern evoked potential (PEP) in relation to the stimulated part of the visual field. Electroenceph. clin. Neurophysiol., 1979, 47: 183-203. Maier, J., Dagnelie, G., Spekreijse, H. and Van Dijk, B.W. Principal components analysis for source localization of VEPs in man. Vision Res., 1987, 27: 165-177. Rush, S. and Driscoll, D.A. Current distribution in the brain from surface electrodes. Anesth. Analg. Curr. Res., 1968, 47: 727-733. Sencaj, R.W. and Aunon, J.I. Dipole localization of average and single visual evoked potentials. IEEE Trans. Biomed. Eng., 1982, 29: 26-33. Spalding, J.M.K. Wounds of the visual pathway. Part II. The striate cortex. J. Neurol. Neurosurg. Psychiat., 1952, 15: 169-176. Spekreijse, H. and Estevez, O. The pattern appearance-disappearance response. Trace, 1972, 6: 13. Spekreijse, H., Van der Tweel, C.H. and Zuidema, T. Contrast evoked responses in man. Vision Res., 1973, 13: 1577-1601. Stensaas, S.S., Edington, D.K. and Dobelle, W.H. The topography and variability of the primary visual cortex in man. J. Neurosurg., 1974, 40: 747-755. Stok, C.J. The Inverse Problem in EEG and MEG with Application to Visual Evoked Responses. Thesis. Krips Repro, Meppel, 1986. Van Dijk, B.W. and Spekreijse, H. Localization of the visually evoked response: the pattern appearance response. In: K. Maurer (Ed.), Topographic Brain Mapping of EEG and Evoked Potentials. Springer, Berlin, 1989: 360-365. Van Essen, D.C. Functional organization of primate visual cortex. In: Jones and Peters (Eds.), Cerebral Cortex, Vol. 3. Plenum Press, New York, 1985: 259-329. Van Essen, D.C. and Zeki, S.M. The topographic organization of rhesus monkey prestriate cortex. J. Physiol. (Lond.), 1978, 277: 193-227. Van Rotterdam, A. Limitations and difficulties in signal processing by means of the principal component analysis. IEEE Trans. Biomed. Eng., 1970, 17: 268-269.
Electroencephalography and clinical Neurophysiology, 1991, 80:181-193 © 1991 Elsevier Scientific Publishers Ireland, Ltd. 0168-5597/91/$03.50 ADONIS 016855979100075W
EVOPOT 89651
The extrastriate generators of the E P to checkerboard onset. A source localization approach P. Ossenblok and H. Spekreijse The Netherlands Ophthalmic Research Institute, 1100 A C Amsterdam (The Netherlands) (Accepted for publication: 23 July 1990)
Summary The cortical origin of the pattern onset EP has been investigated over a time window which covers the entire positive-negative-positive complex of the pattern onset EP. On the basis of a dipole source localization approach, the position, orientation and strength of the underlying sources of the pattern onset EP were estimated. For large check stimuli, chosen to have a weak edge specific component in the response, still two components are needed to account for the variance of the responses. Each component corresponds to a single dipole source, and both originate in the extrastriate cortex. These components dominate, respectively, the initial and the late positive peaks of the pattern onset EP. The equivalent dipole sources of the two components show different behaviors with respect to the position of the stimulus in the visual field. The topography and behavior of the equivalent dipole source underlying the early positive component suggest an origin in area 18. The invariance with stimulus location of the dipole source underlying the late positive component suggests an origin beyond area 18. The different topographies of the components also account for the differences in surface distribution of the pattern onset EP to large check stimulation of the upper and lower sectors of the visual field. Key words: VEP; Brain mapping; Extrastriate activity; Dipole sources; Pattern stimulation
Visual evoked potentials (VEPs) can be used to determine the location of brain activity. We studied the origin of the positive-negative-positive complex of the pattern onset EP evoked by the onset of a checkerboard stimulus. According to Maier et al. (1987) and Van Dijk and Spekreijse (1989), the sources underlying the first positive (CI) and negative peaks (CII) of the pattern onset EP have respectively an extrastriate and striate origin. Jeffreys (1977) suggested an extrastriate origin of the late positive peak (CIII); however, no further information is available about the cortical generators of this peak. The extrastriate origin of the initial part of the pattern onset EP is not accepted generally; others have reported a striate origin (Jeffreys and Axford 1972a; Kriss and Halliday 1980; Butler et al. 1987). Jeffreys and Axford (1972a,b) proposed that CI originates from the primary visual cortex located mainly on the medial surfaces in and around the calcarine fissure. Their main argument is based upon the classical model of the representation of the peripheral visual fields in striate cortex (Holmes 1945; Spalding 1952). This model implies that the vertical halves of the visual fields are
Correspondence to." Dr. P. Ossenblok, The Netherlands Ophthalmic Research Institute, P.O. Box 12141, 1100 AC Amsterdam (The Netherlands).
projected in the contralateral hemisphere with the upper and lower half-fields represented on the lower and upper leaves of the calcarine fissure, respectively. If the assumption is made that the surface potential field produced by cortical activity can be described by an equivalent dipole source perpendicular to the cortical surface, then the symmetrical arrangement of the paramacular visual field projection to the striate cortex will result in potentials of opposite polarity for right and left half-fields and for upper and lower field stimulation (Fig. la). In accordance with this dipole-sheet model of striate cortex a reversal in polarity of the initial peak of the pattern onset EP for left and right half-field stimulation (Jeffreys and Axford 1972a; Jeffreys 1977) and for upper and lower field stimulation (Jeffreys and Smith 1979; Sencaj and Aunon 1982; Butler et al. 1987) has been reported. Note, however, that the short duration of the onset period used by these authors may lead to ambiguities in the results, since the components of the pattern onset EP contain contributions related to the offset of the pattern (Spekreijse and Estrvez 1972). Les~vre and Joseph (1979) and Les+vre (1982) also found a reversal in polarity of the initial part of the pattern onset EP. However, they attributed this polarity inversion to a lower field generator lying on the upper convexity of the hemisphere in extrastriate cortical regions and an upper field generator lying on the undersurface of the lobe. The equivalent dipole
182
P. OSSENBLOK, H. SPEKREIJSE a
sources for the extrastriate cortical regions representing the upper and lower visual fields will then be roughly perpendicular. The analysis of the results and hence the conclusions will depend on the way the EP components are defined. A weak point in the reasoning of most of the authors is the a priori assumption that the activity of each peak may be attributed to a single dipole source. Les6vre and Joseph (1979) were the first to propose that the peaks in the pattern onset EP may reflect simultaneous activity of several visual areas. Evidence against the assumption that the potential distribution of the underlying cortical activity at each time instant can be ascribed to a single equivalent dipole source was given by Stok (1986) and Maier et al. (1987). Stok (1986), whose analysis was based on the model of a moving dipole, found that the equipotential distribution of the pattern onset EP is stable from 80 msec to about 120 msec, whereas at 125 msec the maximum started to move towards the midline while slowly rotating. These observations were confirmed by Maier et al. (1987) who found that the activity underlying the first positive peak of the pattern onset EP with a peak latency of about 100 msec was dominated by an extrastriate component, whereas the negative peak with a peak latency of about 130 msec was dominated by a striate component. Since these components overlap, one should be cautious in defining EP components on the basis of the appearance of positive or negative peaks. Because of the substantial overlap of the responses originating in different visual areas (Les6vre and Joseph 1979; Maier et al. 1987), it is necessary to separate the constituent components of the pattern onset EP before the causes of eventual polarity reversals can be established. Maier et al. (1987) described a method that determines the dimensionality of the response space and provides the constituent components of the composite responses. It is assumed that the responses originate from a fixed number of dipole sources, each with a fixed position and orientation, but varying in strength. The components then reflect the variation of the strength of the dipole sources as a function of time. Maier et al. (1987) and Van Dijk and Spekreijse (1989) used localized stimulation of the visual field ~o determine from which visual areas the responses originate. The size of the stimulus field was as small as possible in order to activate a small part of the cortex and hence to mimic a single current dipole. Their analysis over a time window covering CI and CII yielded 2 components. The first component, consisting mainly of CI, was ascribed to an equivalent dipole source situated in the extrastriate visual cortex and the second component, in which CII dominates, to the primary visual cortex. Using the same dipole source localization approach, we tried to account for the variance of the responses over a window which covered the entire C I - C I I - C I I I
interval. Since in source localization a large number of parameters has to be estimated, the stimulus parameters were chosen such that a minimum number of sources was activated. So, a single hemisphere was always activated. Furthermore, to weaken the striate component in the response, large checks were used since the striate component is selectively sensitive to the edges of a pattern stimulus. Moreover, large checks favor CI whereas CIII can be enhanced by binocular presentation (Spekreijse et al. 1973). Therefore, we stimulated with checkerboards with large check sizes viewed binocularly. The analysis of the pattern onset EP over its total response period further requires a good signal-tonoise ratio. The signal-to-noise ratio is increased by using large visual fields for stimulation and high modulation depths (the significant power of the responses then exceeds 90%). One may use large visual fields because the effect of source extension on the parameters of the equivalent dipole is very small (De Munck et al. 1988a,b; De Munck and Spekreijse 1989). Furthermore, we studied the retino-cortical organization to gain information about the anatomical location of the generators underlying the CI and CIII peak of the pattern onset EP. We shall discuss also the results of stimulation of distinct sectors of the upper and lower visual fields to clarify the controversy about the differences in polarity and latency of the upper and lower field EPs. These results provide further evidence for the cortical origin of the first positive peak of the pattern onset EP.
Methods
Stimuli Stimuli were generated on a digital display system (Genisco 3000). The subject viewed a TV monitor with a mean illumination of 65 c d / m 2, subtending 8 × 8 °. All stimuli were presented in the left or right hemifield surrounded by a steady homogeneous field of the same mean luminance. VEPs were also recorded with stimuli restricted to quadrants or octants in the upper or lower visual field. A checkerboard pattern with checks of 24' or 48' and a contrast of 80% was used, presented for 300 msec once in 800 msec, without net variation in overall luminance level. In this way the responses evoked by the onset and offset of the checkerboard could be registered separately.
Recording VEPs were recorded from 24 scalp electrodes. To minimize the effect of systematic errors on the location of the equivalent dipole we used a wide "electrode-grid" with a vertical and horizontal spacing of the electrodes of 4 cm. The lowest row of electrodes was positioned 2 cm below the inion. A reference electrode was placed on the frontal midline; the ground was located near the
EXTRASTRIATE GENERATORS OF PATTERN ONSET EP
183
vertex. Signals were amplified (Medelec 5000) and bandpass filtered between 0.5 and 70 Hz. T h e high cut-off frequency (70 Hz) was set b y a low-pass fourthorder Butterworth filter, which introduced a phase shift increasing the response latencies b y 7 msec. Thus, peak latencies estimated f r o m the records in this p a p e r should be corrected for this latency increase. T h e E E G s were A / D converted with a 240 H z sample frequency and on-line averaged with an H P 2100 minicomputer. Since only 16 E E G amplifiers were available, all records were made in 2 sessions. The 8 double records were used to estimate the signal-to-noise ratio and to obtain an impression of the reproducibility of the responses. The length of the time w i n d o w for analysis was optimized in accordance with the best possible signal-to-noise ratio,
thus accounting for the variance over the total response period. The maximal time w i n d o w chosen was 58.4204.3 msec after stimulus onset, thus containing the p r o m i n e n t peaks (CI, C I I and C I I I ) of the pattern onset EP. In this p a p e r the EP results of 5 healthy subjects are presented. The results of stimulation with the distinct sectors of a hemifield, as depicted in Fig. la, are illustrated b y the results obtained in a single subject who is representative for the group.
Source localization T h e analysis is based on the assumption that the sources of the V E P m a y be modeled by equivalent dipole sources, which m a y be activated simultaneously u p o n visual stimulation. The role of the principal com-
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Fig. 1. a: the distinct sectors of the first 4 ° of the right half visual field are depicted schematically. A fixation-cross Is situated in the middle of the visual field. The lower part of the figure gives the upper and lower leaves of the calcarine fissure lying near the medial plane together with the area activated (solid lines) and the predicted orientation of the equivalent dipole sources, b: in the orthogonal xyz-frame a point p is defined in the azimuth-elevation co-ordinate system. Phi is the angle with the x-axis of the projection of p in the x-y plane and theta is the angle between the vector to p and the x-y plane. The orientation of the equivalent dipoles is determined in the local x'y'z'-frame. The projection along the z'-axis yields the radial part of the dipole and the projection in the x'y'-plane yields the tangential part. The orientation of the dipole projection in the tangential plane is denoted as the angle psi. In the x'y'-plane the y '-axis points towards the z-axis of the xyz-frame.
184
ponent analysis (PCA) in the analysis is that it provides a good estimate of the dimensionality of the data space (see Kavanagh et al. 1976). PCA reveals how many principal components (PCs) are needed to account for the significant variance within the time window chosen for a set of EP records. This estimate provides the number of underlying sources of the VEP that are necessary to account for the data (see Kavanagh et al. 1976; Donchin and Heffley 1978). However, the statistically independent PCs do not necessarily have any physiological significance (Van Rotterdam 1970). Therefore, the mathematically obtained PCs are transformed into physiologically meaningful components. The criterion for this transformation is that the weights of the corresponding potential distributions satisfy a dipole-like potential distribution (Maier et al. 1987). If linear combinations of the factor loading distributions of the PCs yield potential distributions that are in accordance with dipole sources, then we shall have explained the significant power in the responses in terms of dipole sources. The position and orientation of the equivalent dipole sources are obtained by fitting dipole fields to these best possible dipole-like potential distributions. Thus, the whole procedure, which is described in detail in Maier et al. (1987), yields the constituent components of the pattern onset EP, each corresponding to a single dipole source with a time profile reflecting the variation of strength of that dipole source. To determine the location of the dipole sources the head is described as a volume conductor with 3 concentric spheres of different conductivity. Using a homogeneous sphere model an initial set of source parameters is calculated. With a correction table the eccentricity found is converted to the 3-shell model consisting of a homogeneous sphere of neural tissue, surrounded by 2 concentric spherical shells representing, respectively, the skull and the skin (Ary et al. 1981). The converted parameters of the source are used as a starting point for the computation in the inhomogeneous 3-sphere model.
Presentation of data The electrode positions are defined in an orthogonal (xyz) co-ordinate system with the x- and y-axes in a horizontal plane when the subject looks straight ahead (Fig. lb). The x-axis points towards the inion. The position of the equivalent dipole is given by the coordinates f, phi and theta. The factor f, which can vary between 0 (mid of the best fitting sphere) and 1, represents the eccentricity of the dipole source and phi and theta give the azimuth and elevation co-ordinates, respectively. Note that f should have values smaller than 0.87 for a dipole situated in the cortex (Rush and Driscoll 1968). The orientation of the equivalent dipole is represented in a local co-ordinate frame x ' y ' z ' . The projection along the z'-axis yields the radial part of the dipole and the projection in the x'y'-plane yields the
P. OSSENBLOK, H. SPEKREIJSE
tangential part. The z'-axis points radially outward, whereas the orientation of the dipole projection in the tangential plane is denoted as the angle psi. This angle can b e visualized by placing a tangential plane at the radial projection of the dipole position on the surface of the sphere; it is measured counterclockwise with respect to the local x'-axis. Both the radial and tangential parts of the dipole are expressed in percentages of the strength of the equivalent dipole. The potential distributions are plotted in a topographic map in the phi-theta co-ordinate frame (see Figs. 3-6). Since the occipital part of the head is assumed to be spherical, the zero-point of the co-ordinate frame (i.e., phi = 0 and theta = 0) coincides with the inion. The asterisks in the topographic maps indicate the electrode positions. The equipotential lines are drawn according to a linear interpolation between the potentials of 3 neighboring recording positions. A maximum in the equipotential maps always coincides with the recording site of the maximal response of that component. In the topographic maps the positions of the dipole sources are depicted by large dots, their orientations in the tangential plane by arrows. The inset gives the radial part of the dipole strength (% rad) and the depth by the factor f. The quality of dipole fit is indicated by the % error based on the sum of the squared potential differences.
Results
Hemifield stimulation Fig. 2 shows VEPs recorded at the 24 scalp electrodes to a pattern onset stimulus presented in the right hemifield. In the positive-negative-positive complex of the response the early positive peak (CI) dominates at the contralateral recording site. From the figure it is evident, however, that there are large differences in the wave forms of the responses. For instance, the early positive peak of the onset EP seems to reverse in polarity for the left and right hemisphere. However, the initial peaks in the pattern onset EP may differ by as much as 15 msec on opposite sides of the head. It should be noted also that the pattern onset responses recorded at the electrode sites 14 and 16 show a single prominent positive peak coinciding with, respectively, CI (at 80.5 msec) or with CIII (at 138.8 msec). Thus, it appears that the pattern onset EP, evoked by large visual fields and large high contrast checks, is due to a dual contribution of an early and a late component. Calculation of the equivalent dipoles and their corresponding time functions indeed shows that the pattern onset EP consists of 2 components. The strength of one of the dipole sources changes in a triphasic way in time (insert Fig. 3a), that of the other in a biphasic way (insertFig. 3b). Although the components overlap in
EXTRASTRIATE GENERATORS OF PATTERN ONSET EP
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Fig. 2. The visual evoked responses to the appearance of a 48', 80% modulated checkerboard in the central 4° of the right visual field. The responses are plotted according to a 2-dimensional projection of a spherical surface with horizontal and vertical distances of 4 cm. The asterisk indicates the position of the inion, which is located at the midline of the head 2 cm above the bottom row of electrodes.The responses recorded at the electrodesites 14 and 16 are marked with.the respectivenumbers.
time, the first dominates the initial positive peak (CI) and the second component the late positive peak (CIII). Therefore, these components are referred to as the early or the late component and are, respectively, designated as C1 and C3. From the amplitude distributions of the components equivalent dipoles were computed, giving the best approximation to the recorded potential distributions. Fig. 3 depicts the recorded potential distributions of both components, as well as the best fitting dipole distributions. The difference between adjacent contours is given in microvolts and reflects the strength of the dipole source. In accordance with the results of Maier et al. (1987) the spatial distribution of the early component shows a contralateral maximum, whereas the orientation of the underlying dipole source is mainly radial (Fig. 3a). The radial part of the equivalent dipole source (% rad) amounts to 77.5% of the strength of the source. The dipole source underlying the late component has about the same position. However, its orientation is predominantly tangential (Fig. 3b). The positive part of the potential distribution is ipsilateral, whereas contralaterally a negative distribution emerges. The shift from a maximum in the potential distribution to a minimum corresponds to a polarity reversal of the component. The depth (f) indicates that both dipole sources are situated in the cortex. Upper versus lower field EPs In Fig. 4 the results of quadrant-field stimulation are depicted. Lower quadrant-field stimulation generates an
equivalent dipole source underlying the early component (C1) which has shifted outward with respect to half-field stimulation, but maintains a predominant radial orientation (Fig. 4a). For this component the projection of the lower quadrant-field lies at the upper part of the outer occipital lobes, whereas the upper quadrant-field is projected at the bottom of the hemisphere and will contribute less to the surface potentials evoked by half-field stimulation (Fig. 4b). In accordance with the dipole-sheet model of extrastriate cortex, the orientation of the equivalent dipole source of the early component becomes predominantly tangential for upper quadrant-field stimulation. The position and orientation of the equivalent dipole source underlying the activity of the late component (C3) remains the same for upper and lower quadrant-field stimulation. The time functions of both equivalent dipole sources have the same polarity for upper and lower quadrantfield stimulation (inserts Fig. 4). Apart from the less pronounced initial peak of the early component for upper quadrant-field stimulation the shapes of the components are rather similar, suggesting the same cortical origin. Furthermore, the peak latencies of the components generated by upper quadrant-field stimulation are the same as those to lower quadrant-field stimulation. If the responses evoked by upper and lower quadrant-field stimulation originate in adjacent cortical areas, then the responses evoked by half-field stimulation should approximate the sum of the quadrant responses. Fig. 4c shows that the algebraic summated responses have indeed the same topographic distribu-
186
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Fig. 3. Equipotential maps derived from the responses depicted in Fig. 2. The upper map holds for the early component (C1) and the lower map for the late component (C3). Heavy lines represent the positive values of the recorded potentials, heavy dotted lines the zero values, and heavy dashed lines the negative values. Also shown are the best fitting dipole distributions, represented by thin continuous, thin dotted and thin dashed lines for positive, zero and negative potentials respectively. The asterisks give the positions of the electrodes in the topographic map. In each plot the location of the equivalent dipole source is depicted by a dot. The arrow gives the orientation of the dipole projection in the tangential plane. AV gives the difference between adjacent contours in microvolts. The inserts in the upper left corner give the depth f, the radial part of the dipole strength (% rad) and the error in percentages of the sum of the squared potentials. The shape of the components is shown in the inserts in the upper right corner. The components covered a time window that started 58.3 msec after pattern onset and ended at 204.3 msec.
tions for C1 as well as for C3, similar w a v e shapes, a n d c o m p a r a b l e l o c a t i o n s for the e q u i v a l e n t d i p o l e sources to those g e n e r a t e d b y half-field s t i m u l a t i o n (see Fig. 3). N o t e , however, t h a t the d i p o l e source u n d e r l y i n g C1 is s i t u a t e d m o r e eccentrically t h a n the e q u i v a l e n t d i p o l e source g e n e r a t e d b y half-field stimulation. I n accord a n c e with the a n a t o m i c a l o r g a n i z a t i o n of the visual c o r t e x the shape a n d t o p o g r a p h i c d i s t r i b u t i o n o f the
early c o m p o n e n t (C1) e x t r a c t e d f r o m the s u m m a t e d p o t e n t i a l s are d o m i n a t e d b y the r e s p o n s e to lower q u a d r a n t - f i e l d s t i m u l a t i o n . Also, the o r i e n t a t i o n of the e q u i v a l e n t d i p o l e source is m a i n l y radial. N o t e t h a t the e q u i v a l e n t d i p o l e source of the early c o m p o n e n t is l o c a t e d a p p r o x i m a t e l y b e t w e e n the d i p o l e sources gene r a t e d b y u p p e r a n d lower q u a d r a n t - f i e l d stimulation. Since the r e t i n o t o p i c p r o j e c t i o n of the late c o m p o n e n t
EXTRASTRIATE GENERATORS OF PATTERN ONSET EP
187
(C3) seems to be invariant for upper and lower quadrant-field stimulation, this component remains in place.
contrary to the dipole-sheet model of striate cortex the equivalent dipole sources generated by stimuli lying near the horizontal meridian do not reverse in polarity, whereas the equivalent dipole sources activated by stimuli lying near the vertical meridian have a different orientation. Moreover, in accordance with the results of quadrant-field stimulation both lower octant-field generators of C1 have a predominant radial orientation, whereas the orientation of the upper field generators becomes tangential. The dipole sources of the early
Stimulation with sectors of the lateral hemifields Figs. 5 and 6 depict the potential distributions of the equivalent dipole sources generated by octant-field stimulation. The equipotential maps are arranged according to the retinotopic projections of the octantfields on the striate cortex (see Fig. la). However,
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Fig. 4. Equipotential maps derived from the responses evoked by the onset of a 24', 80% modulated checkerboard in the central 4 ° of the lower (upper part of the figure) and upper (middle part of the figure) right quadrant of the visual field. In the lower part of the figure the potential distributions of the components extracted from the algebraic sum of the recorded potentials evoked by the upper and lower right quadrants of the visual field are plotted. The maps in the left column hold for the early component (C1) and in the right column for the late component (C3). The inserts in the lower right corner give the depth f, the radial part of the dipole strength (% rad) and the error in percentages of the sum of the squared potentials. The shape of the components is shown in the inserts in the upper right corner. The components covered a time window that started 58.3 msec after pattern onset and ended at 204.3 msec. For further information on equipotential maps see the legend of Fig. 3.
188 component also lie deeper in the head for upper than for lower octant-field stimulation and the strength of these dipole sources has diminished. The equivalent dipole sources underlying the late component have about the same position for all 4 octants (Fig. 6). The corresponding potential fields are mainly tangential, whereas the position of the dipole sources is clearly contralateral. Compared to the lower field the orientation of the upper field generators tends to be more radial. However, these differences may partly be due to the differences in the quality of the fits for both conditions of stimulation. Also the time functions of the equivalent dipole sources activated by octant-field stimulation show a high degree of similarity. For instance, the peak latency of the positive peak (84.7 msec) of the early component is remarkably constant (inserts Fig. 5). The variability becomes apparent in the later parts of the components. Although the late component always has a biphasic shape the variability of this component is large, especially for the negative peak (inserts Fig. 6). Note the difference of about 20 msec in the peak latency of the early peak of C3 for octant-fields lying near the vertical and horizontal meridians.
Discussion The positive-negative-positive complex of the pattern onset EP, evoked by large visual fields with large checks, consists of at least two components, each generated by one equivalent dipole source. It was found, in accordance with Maier et al. (1987), that the initial positive peak of the early component coincides with CI. Other correspondences between the study of Maier et al. (1987) and the present study are the shape and spatial distribution of C1 and the position and orientation of the underlying sources of this component. This paper shows further that the positive peak of the late component coincides with CIII. The accumulated power of the two components, which amounts, for the 5 subjects studied, to 90% or more of the mean power of their responses, can account almost completely for the significant variance of the sets of responses. The responses to patterns with large checks can, therefore, be described adequately by a linear combination of these two components.
P. OSSENBLOK,H. SPEKREIJSE The equivalent dipole sources underlying the early and late components both have large phi values, indicating a position away from the midline. On the basis of the symmetrical arrangement of the striate cortex (see Fig. 1) it can be expected that sources of striate origin lying near the midline generate potential fields of opposite polarity for left and right and upper and lower field stimulation. However, the potential fields corresponding to the early and late components do not reverse in polarity for stimulation with any of these parts of the visual fields. Moreover, both the position and orientation of the equivalent dipole sources generated by left and right and upper and lower visual field stimulation suggest an extrastriate origin of these components.
The topographic representation of the components In Fig. 7 the position of the equivalent dipole sources activated by left and right half-field stimulation are depicted for the 5 subjects studied. The sources for both hemispheres are situated far apart and clearly contralateral with respect to the midline, demonstrating the extrastriate origins of both components. There is large intersubject variability in the position and orientation of the equivalent dipole sources underlying both extrastriate components, which is probably due to the variability in the geometry of the various visual areas of extrastriate cortex between subjects (see also Table I). The intrasubject variability of the equivalent dipole sources may reflect the variability of the geometry of the hemispheres with respect to e a c h other, as was shown, for instance, by Stensaas e~ al: (1974). However, in spite of marked differences between the hemispheres of a subject the potential distributions of the equivalent dipole sources and the corresponding components of the pattern onset EP are quite symmetrical for left and right half-field stimulation. The early component (C1) reflects also a representation of the upper and lower quadrants of the contralateral hemifields. Since at the lower border of the extrastriate cortex the cortical surface bends strongly inward, stimulation of the lower field results in a more anteriorly positioned and stronger maximum than that to upper field stimulation. Moreover, the downward shift of the maximum in the potential distribution for upper field stimulation corresponds to an equivalent dipole which lies deeper in the head and is oriented
Fig. 5. Equipotentialmaps derived from the responses evoked by the onset of a 24', 80% modulated checkerboardrestricted to the central 4 ° of the individual upper and lower right octant-fields.All maps in this figure hold for the early component (C1). In each part of the figure the potential distribution and the octant-fieldused for stimulation are depicted. The inserts in the lower right corner give the depth f, the radial part of the dipole strength (% rad) and the error in percentagesof the sum of the squared potentials. Also the shape of the components is shown for each potential distribution. The componentscovereda time window that started at 79.2 msec after pattern onset and ended at 183.5 msec. For further information on the equipotential maps see the legend of Fig. 3.
EXTRASTRIATE GENERATORS OF PATTERN ONSET EP
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more tangentially. These changes are in agreement with an extrastriate cortical origin of C1. The results of octant-field stimulation also show that the potential distributions corresponding to the early component closely follow the spherical surface of the extrastriate cortex (see Fig. 5). This topographic representation of the early component supports the view that this component is generated in area 18 (Van Dijk and Spekreijse 1989), an area which is rather unfolded and runs parallel to the spherical surface of the cortex. We did not find a reversal in polarity for upper and lower octantfield stimulation, as was reported by Butler et al. (1987). In accordance with the dipole-sheet model of extrastriate cortex the equivalent dipole sources change from a radial to an almost completely tangential orientation for upper octant-field stimulation. The less consistent changes in the orientation of the dipole projection in the tangential plane with the position of the stimulus in the visual field are probably due to local curvatures of the visual cortex. The picture of C3 is less clear. The parameters of the equivalent dipole sources underlying the late component (C3) seem to be invariant for upper and lower visual field stimulation. It is known from recent work on monkey that, contrary to area 18, area 19 is far from representing a functional unity. It is more likely to be made up of multiple independent areas, each of which represents the whole hemiretinal projections (Van Essen and Zeki 1978; Van Essen 1985; Burkhalter 1986). It was found that these areas occupy much of the lateral portion of the anterior bank of the lunate sulcus or are submerged in the superior temporal sulcus (Van Essen 1985; Gattass et al. 1988). If it is assumed that the extrastriate cortex in m a n corresponds to the prestriate cortex of monkey, then sources originating in area 19 should be located in the contralateral hemisphere, but the topography of these components should not change when turning from lower to upper field stimulation. The topographical characteristics of the late component (C3) and the invariance of the parameters of the corresponding dipole sources for upper and lower field stimulation, therefore, suggest an origin in area 19. However, according to Gattass et al. (1988), it is possible that the retinotopic organization is not discerned for recordings within 5 - 1 0 ° of the visual field, because of the large size of the receptive fields and the somewhat disorderly representation of the visual field in this area.
P. OSSENBLOK, H. SPEKREIJSE
The equivalent dipole description In general the correspondence between the measured and dipole-like distributions is quite good, suggesting that a single equivalent dipole also provides an adequate description of the activity evoked by extended visual fields. However, for large stimulus fields the error of the depth and strength parameters will be larger (De Munck et al. 1988b). Equivalent dipole sources with a radial orientation will be somewhat deeper than the midpoint of the extended source and tangential equivalent dipole sources will be more eccentric. Another possible source of error is the sensitivity of the localization methods to the geometry chosen for the head model and the sensitivity to the parameters that describe the conductivity properties of the head. For instance, scalp and skull thickness are known to vary from one person to another and from location to location over the head. For these kinds of model errors the depth of the equivalent dipole sources tends to be more superficial than the depth of the cortical sources (Ary et al. 1981). Similarly, the conductivity ratio and the degree of anisotropy in the skull can only be approximated (De Munck 1988). The largest deviations from the spherical model are present at the lower border of the prestriate cortex, where the cortical surface bends strongly inward. It is probably due to these deviations that the quality of fit is generally worse for upper field stimulation. Especially the sources found for vertical upper octant-field stimulation are rather poorly matched to a dipole (see Figs. 5 and 6). This is in agreement with the results of Butler et al. (1987), who also had difficulty in resolving the upper vertical octant generators as a single dipole. Thus, the inconsistency of the depth and strength parameters will be larger for upper field stimulation than for the projection of the lower visual field. The errors for the strength of the activity tend to be much larger; therefore only a rough indication of source strength will be justified in most cases. However, in spite of the possible sources of error, the effect of the position of the stimulus in the visual field on the location and orientation of the dipole sources underlying both components is rather consistent and agrees with the known structure of extrastriate cortex. Also, the components elicited by different sectors of the visual field are rather invariant in shape. However, due to changes in the topography of the components the pattern onset EP changes for the distinct sectors of the
Fig. 6. Equipotential maps derived from the responses evoked by the onset of a 24', 80% modulated checkerboard restricted to the central 4° of the individual upper and lower right octant-fields. All maps in this figure hold for the late component (C3). In each part of the figure the potential distribution and the octant-field used for stimulation are depicted. The inserts in the lower right corner give the depth f, the radial part of the dipole strength (% rad) and the error in percentages of the sum of the squared potentials. Also the shape of the components is shown for each potential distribution. The components covered a time window that started 79.2 msec after pattern onset and ended at 183.5 msec. For further information on the equipotential maps see the legend of Fig. 3.
EXTRASTRIATE G E N E R A T O R S OF PATTERN ONSET EP
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P. OSSENBLOK, H. SPEKREIJSE
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v i s u a l field. T h u s , t h e s e s t i m u l u s - b o u n d d i f f e r e n c e s in the p a t t e r n o n s e t E P a r e n o t d u e to c h a n g e s in the t i m e course of the components. The intersubject variability in t h e p a t t e r n o n s e t E P is also s u b s t a n t i a l , b u t less so for the c o m p o n e n t s . H o w e v e r , d u e to t h e v a r i a b i l i t y o f the g e o m e t r y o f t h e v a r i o u s v i s u a l areas, the t o p o g r a p h y o f t h e c o m p o n e n t s v a r i e s b e t w e e n subjects. T h e r e f o r e , the i n t e r s u b j e c t v a r i a b i l i t y o f t h e p a t t e r n o n s e t E P will b e d u e m a i n l y to t h e v a r i a b i l i t y in c o r t i c a l t o p o g r a p h y .
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Conclusions
W e h a v e s h o w n in this p a p e r t h a t t h e i n t e r p r e t a t i o n o f t o p o g r a p h i c d a t a o f the p a t t e r n o n s e t E P o n t h e basis o f t h e p e a k s in t h e r e s p o n s e s m a y l e a d to e r r o n e o u s conclusions, since the peaks reflect activity from different parts of the visual cortex. For instance, we found t h a t t h e d i f f e r e n c e s in the s u r f a c e d i s t r i b u t i o n of c o r r e s p o n d i n g c o m p o n e n t s f o r left a n d r i g h t h a l f - f i e l d s t i m u l a t i o n a n d f o r u p p e r a n d l o w e r field s t i m u l a t i o n are n o t due to a r e v e r s a l in p o l a r i t y of t h e initial p e a k of the p a t t e r n o n s e t E P , as o b s e r v e d b y J e f f r e y s a n d cow o r k e r s . T h e r e is a l s o n o e v i d e n c e f o r a d e l a y o f the u p p e r field E P s as s u g g e s t e d b y L e h m a n n et al. (1977). Thus, the results of the present study do not support the i d e a o f a s y m m e t r i e s in signal p r o c e s s i n g in the u p p e r a n d l o w e r h e m i r e t i n a . It was s h o w n t h a t t h e p a t t e r n o n s e t E P , e v o k e d b y l a r g e v i s u a l fields a n d l a r g e elem e n t sizes o f h i g h c o n t r a s t , c o n s i s t s o f at least t w o c o m p o n e n t s , e a c h c o r r e s p o n d i n g to o n e single d i p o l e
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20
40
Fig. 7. The positions of the equivalent dipole sources generated by left and right half-field stimulation are depicted in the phi-theta coordinate frame for the 5 subjects studied. The positions of the sources of the 5 subjects are numbered 1-5. The positions of the equivalent dipole sources underlying the early component (C1) of the pattern onset EP are depicted in the upper part of the figure and the positions of the equivalent dipole sources underlying the late component (C3) in the lower part. It can be seen that for all subjects the theta values of the late component are significantly smaller than those of the early component.
TABLE I Positions and orientations of the equivalent dipole sources underlying C1 and C3 generated by left (lhf) and right (rhf) half-field stimulation. The last column gives the error of the fit. For explanation of the position and orientation parameters see Fig. lb. Subject
Field
f
Phi
Theta
% rad
Psi
% error
rhf lhf rhf lhf rhf lhf rhf lhf rhf lhf
0.69 0.79 0.57 0.73 0.46 0.54 0.71 0.55 0.73 0.57
- 22.8 26.6 - 25.7 33.5 - 37.7 32.9 - 15.0 8.9 - 34.5 18.3
38.4 50.5 42.9 38.1 26.7 36.9 22.9 28.0 13.4 24.3
77.5 12.6 82.2 90.8 20.2 16.3 73.3 60.2 59.0 59.9
143.1 - 166.4 110.6 - 155.9 140.1 - 150.3 124.4 - 126.2 177.3 - 137.9
0.7 1.3 1.1 0.6 1.8 0.8 1.3 1.0 6.1 0.6
rhf lhf rhf lhf rhf lhf rhf lhf rhf lhf
0.86 0.87 0.78 0.79 0.71 0.76 0.74 0.78 0.86 0.86
- 23.3 23.8 - 29.3 32.3 - 12.7 28.1 - 21.2 15.8 - 20.1 23.9
32.8 23.6 25.0 14.5 7.7 10.4 11.4 2l .5 7.6 7.0
9.1 8.6 13.4 7.2 0.3 11.7 4.3 3.2 2.5 3.2
- 91.3 68.1 - 101.9 90.9 - 48.0 73.3 23.6 133.5 103.9 - 105.5
8.0 25.2 8.1 5.3 2.6 1.l 2.8 20.7 19.0 7.8
C1
FR AB HS DR VL C3
FR AB HS DR VL
EXTRASTRIATE GENERATORS OF PATTERN ONSET EP source and overlapping in time. These components
show
different behaviors with respect to the position of the s t i m u l u s i n t h e v i s u a l field. O n t h e b a s i s o f t h e r e s u l t s p r e s e n t e d i n t h i s p a p e r it s e e m s l i k e l y t h a t t h e d i f ferences in surface distribution of the pattern onset EP for upper and lower field stimulation are due to changes of topography of the early component, whereas the topography of the late component does not change for upper and lower field stimulation. The authors acknowledge the suggestions of Dr. J.C. De Munck, Dr. D. Reits and Dr. B.W. Van Dijk to improve the manuscript. They further wish to express their appreciation to the subjects who took part in this research project.
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