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Mapped distribution of pattern reversal VEPs to central field and lateral half-field stimuli of different spatial frequencies
Electroencephalography and clinical Neurophysiology , 1991, 8 0 : 1 6 7 - 1 8 0 © 1991 Elsevier Scientific Publishers Ireland, Ltd. 0168-5597/91/$03.50 ADONIS 016855979100074J
167
EVOPOT 89628
Mapped distribution of pattern reversal VEPs to central field and lateral half-field stimuli of different spatial frequencies M. Onofrj, S. Bazzano, G. Malatesta and T. Fulgente Department of Neurology, Istituto di Clinica Neurologica e Scienze del Comportamento, State Unioersity of Chieti, Chieti (Italy) (Accepted for publication: 13 July 1990)
Summary Visual evoked potentials (VEPs) to pattern reversal vertical bar stimuli of 3 different sizes (1, 2, 4 c / d e g ) were recorded from 19 scalp derivations in 50 controls. The stimuli were presented on a full-field (FF) screen of 24 ° visual angle, and on left and fight half-fields (HF) of 12 ° radius. In 15 controls partial H F stimuli were presented on the central 3 and 6 o and as hemiannular stimuli of 12 ° with occlusion of the central 3 and 6 o. An antero-posterior polarity reversal of the N1-P1-N2 sequence was observed for F F VEPs. A tangential polarity reversal was observed for H F VEPs. Also with central or hemiannular stimuli polarity reversals of all VEP components were observed within the scalp. Van~ants of VEP distribution, absence or prominence of some of the ipsi- or contralateral VEP components were observed in 8-40% of controls. The FF and H F VEP distribution, and the variant VEP asymmetries were partly dependent on the pattern spatial frequency.
Key words: Visual evoked responses; Visual cortex; Visual pathways; Topographic mapping
The paradoxical distribution of pattern reversal VEPs to half-field (HF) stimuli was reported in several studies (Barrett et al. 1976; Shagass et al. 1976; Arruga et al. 1980; Kuroiwa and Celesia 1981; Streletz et al. 1981; Rowe 1982; Jones and Blume 1985; H a m m o n d et al. 1987; Paulus et al. 1988). These studies showed that the negative positive negative VEP sequence, corresponding to components recorded at the midline with full-field (FF) stimuli, was recorded ipsilaterally to H F stimuli, while contralaterally polarity reversed counterparts (positive, negative, positive) were recorded. Since the contralateral components become prominent in posterior derivations when the central part of H F patterns are occluded (Blumhardt et al. 1978, 1989), a theory of VEP generators was proposed, suggesting that two generators should be found in the visual areas: one for projections from the central retina (generating ipsilateral components) and one for projections from the parafoveal-paracentral retina (contralateral components). 'The relationship between eccentricity and visual evoked potential' was found to be 'not straightforward' (Blumhardt et al. 1989) since ipsilateral and contralateral components were not found to have precise boundaries in the visual field: ipsilateral N75, P100 and
Correspondence to: Marco Onofrj, Clinica Neurologica, Ospedale ex Pediatrico, 66100 Chieti (Italy).
N140 were recorded with stimuli as eccentric as 5-10 ° from the fovea and contralateral P80-N105-P135 were recorded with stimuli covering only the central 2-5 ° radius (Blumhardt et al. 1978; 1989). Moreover, the characteristic pattern of VEP distribution is not observed in all controls, and variants with asymmetries of VEPs, preponderance at the midline of ipsi- or contralateral components and bifid or 'w'shaped P100 are described in varying (6-25%) percentages of controls (Blumhardt et al. 1982; Chiappa 1983; Jones and Blume 1985; H a m m o n d et al. 1987). Blumhardt et al. (1978, 1982) suggest that preponderance of ipsi- or contralateral components and 'normal wave shape asymmetry' depend on variations of the representation of area 17, at the tip of the occipital pole or on its medial surface. All the studies quoted above were conducted with a small number of derivations (5-13) placed on the postcentral area of the scalp, and with coarse pattern element stimuli (more than 30 min visual angle per element). In only 4 other studies, pattern elements of 15 min visual angle or less were used but records were obtained only from posterior leads (Harding et al. 1980; Onofrj et al. 1982; Skrandies 1984; Novak et al. 1988). In two studies, references other than the midfrontal were used (Arrnga et al. 1980; Mauguirre et al. 1984) and in one records were obtained also from anterior leads (Maugui&e et al. 1984) but in both studies the patterns consisted of coarse elements.
168 Recent reports suggest that the detection of visual pathway anomalies is dependent on the size of pattern elements used for stimulation (Plant 1983; Oishi et al. 1985; Onofrj et al. 1986; Novak et al. 1988) and that human VEPs have an amplitude tuning approximating the spatial frequency tuning of psychophysical contrast sensitivity (Campbell and Maffei 1970; Bodis-Wollner et al. 1972; Plant 1983; Skrandies 1984). The peaks of the contrast sensitivity curve vary in proportion to the eccentricity from the fovea: the peak is at 4-6 cycles/degree (cpd) for the fovea, at 2 cpd for 10 ° eccentricity, at 1 cpd for 30 ° eccentricity (BodisWollner et al. 1987). The relationship of contrast sensitivity to eccentricity from the fovea is thought to be related to the receptive field size of retinal ganglion cells, that is, the smallest with the highest density of ganglion cells per unit area, at the foveal level (Hubel and Wiesel 1968). The different receptive field sizes at the foveal and paracentral levels might suggest that the chances of identifying a paracentral complex are greater when coarse rather than small patterned element stimuli are used. The present paper therefore describes in controls the distribution over the entire scalp of VEPs obtained with pattern reversal F F and HF, and central and annular H F stimuli of various spatial frequencies (i.e., various sizes of the pattern elements). Whereas prior studies on pattern VEP distribution were mostly focused on the distribution of the major positive component, the present paper describes the distribution of all VEP components. Furthermore, since the 'normal' asymmetric VEP variants are thought to be dependent on anatomical variants of area 17 (Blumhardt et al. 1978, 1982, 1989) a relevant point of our study, because of implicit applications in the clinical use of VEPs, was to try to understand whether VEP variants were dependent or not on the spatial frequency of patterned stimuli. We had to record VEPs with a reference placed outside the mapped scalp, rather than in the commonly used Queen Square position (Barrett et al. 1976). Since in preliminary studies (Onofrj 1990) the single earlobe reference was found to unbalance the VEP distribution, as already reported by Halliday (1982), we used a calibrated linked earlobe reference, that yielded the same results as the average reference or the extracephalic reference, with the lowest presence of artifacts. A fast scanning oscilloscope had also to be used for pattern presentation instead of the commonly used TV monitors, since TV monitors generate the pattern on the screen from the right border to the left, and completion of the pattern takes 25 msec, the right half of the pattern being therefore completed earlier than the left half (Van Lith et al. 1979), and this left-right delay might be misleading.
M. ONOFRJ ET AL. Materials and methods
Pattern VEP stimuli were vertical bars with a square wave luminance profile presented on the screen of a LACA monitor placed at 1 m from the observer's eye. The completion of the pattern on the screen required 6 msec. The FF screen was circular, with a diameter of 24 ° of visual angle at the observer's eye. The mean luminance of the monitor was 75 c d / m 2. The contrast, defined as the ratio of the difference of maximum and minimum luminance of adjacent bars over their summed luminances was 50%. Minimum and maximum luminances were calibrated as in previous reports (Onofrj et al. 1982, 1986). Monocular fixation was provided by a dark spot subtending 5' of visual angle centered on the screen and was constantly monitored by two examiners. During H F stimulation, patterns were presented on the 12 ° radius half (left, L or right, R) of the screen and controls fixated the same central spot, displaced by 30' of arc from the H F border. FF, L-R H F VEPs were recorded in 50 controls (22 males, age range 20-51 years with normal ocular media and normal visual acuity). In 15 of the same controls VEPs were also recorded with H F stimuli subtending 3 ° and 6 °, and hemiannular stimuli in which the central 3 ° or the central 6 ° were occluded (i.e., were substituted by an homogenous blank central field). In the unstimulated parts of the screen the overall luminance was the same as the average luminance of the pattern. Three different pattern element sizes were tested in all recording conditions: coarse bars of 30' of arc, corresponding to a dominant spatial frequency of 1 cpd, intermediate bars of 15' of arc (2 cpd) and bars of 7.5' of arc (4 cpd). The contrast of the grating was reversed (i.e., dark bars became bright and vice versa) every 510 msec. The signals effecting the reversal of patterns triggered the Brain Surveyor (Basis) averager. The dwell time of the averager was 0.400 msec. The averager was set for 100, 200 or occasionally 400 sums. Two or three sets of traces were recorded for each stimulating pattern, stored on floppy disk and eventually reaveraged on a grand average (GA) program. VEPs were recorded with Ag/AgC1 disk electrodes placed in the 19 positions of the international 10-20 system, except that P3, Pz and P4 were posteriorly displaced and O1-O2 laterally displaced by 5%. A calibrated linked earlobe reference was used. Following our modified system the head volume was represented on a single plane octagonal projection with the center at Cz. Color images of electrical distributions were obtained by the classic 4 nearest neighbors (4-NN) interpolation method (Shepard 1968) and were drawn on paper by a microprocessor-based Fujitsu Dx 2100 cartridge copier which bit-mapped at 100 dots/inch. The maps of Figs. 4 - 6 were directly photographed from the computer monitors, maps of Fig. 7 were photo-
VEP MAPS TO 1, 2, 4 C / D E G P A T T E R N E D S T I M U L I
169
graphed from paper drawings. After computing 3860 pixels at any given latency the map program assigned them to appropriate voltage and color steps. The selected gradient of colors was red for negative and blue for positive (Desmedt et al. 1987). VEP measurements were automatically performed by the computer as the amplitude of components from the baselines and matched by measurements from onset to peak of each deflection performed by visual inspection of traces. Data recorded from single derivations in normals in the different recording conditions were statistically compared by means of 2-tailed Student's t test and 1-way ANOVA.
Results
Figs. 1-3 show grand average (GA) VEP distributions in 50 controls to FF and HF (LF, RF) stimuli of 1, 2, 4 cpd. Fig. 4 shows the corresponding maps. FF VEPs (Fig. 1) consist of negative-positive-negative (N1-P1-N2) components distributed over derivations T5 to 02. Mean latencies and amplitudes are
(
F7 ~
lepd
lepd
~
F8 ~
~
T3 ~
~
reported in Table I. The latencies, amplitudes and (in part) distribution of these components change depending on the pattern element size, as shown in Fig. 1 and Table I. N1 latency increases with the spatial frequency. P1 latency is longer for 4 cpd stimuli. N2 looks 'broad' compared to the other components, the peak is not defined at 1 cpd stimuli, and a 'notch' on the slope is recorded with both 2 and 4 cpd stimuli. N1 is of largest amplitude at O1-O2 when 2 and 4 cpd stimuli are used. P1 amplitude is largest to 1 cpd and smallest to 2 cpd stimuli. The amplitude of N2 is also largest for 1 cpd stimuli. In anterior derivations (F1 to F4) and in the central coronal row positive-negativepositive components are recorded with 2 and 4 cpd FF patterns. These anterior potentials have latencies partly corresponding to, although not always matching, the latencies of N1-P1-N2 posterior components. On GA traces an early positivity (Po) could be identified preceding the N1 component to 2 and 4 cpd FF stimuli (Figs. 2-4). The VEP distribution to HF stimuli changes according to a characteristic pattern: N1, P1 and N2 components, similar to N1, P1 and N2 components recorded with FF stimuli, are recorded over derivations ipsi-
lepd
Slim
I +RF
FF+.qum
~
"
-~
~ ~
c4
Fig. 1. GA VEPs to 1 cpd stimuli, full-field (FF), right (RF) and left (LF) hemifields. Algebraic s u m m a t i o n of LF and R F VEPs in the fourth column. The last column on the right shows superimposition of FF VEPs and of the s u m of R F and LF VEPs. Vertical arrows identify N1, P1 and N2 at 83, 118 and 167 msec. Black horizontal arrows identify ipsilateral components (white horizontal arrows, contralateral activities). Time scale 250 msec.
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M. ONOFRJ ET AL.
LF
T
3
RF
LF+RF
FF+sum
~
t Fig. 2. GA VEPs to 2 cpd stimuli, with the same presentation as in Fig. 1. Vertical arrows point to FF N1 (87 msec), P1 (115 msec), to the positive notch on the slope of N2 (P143), and to N2 (167 msec). Notice that FF N1 is more evident than N1 to 1 cpd stimuli. Contralateral cP1, cN1, cP2 (white arrows) can be clearly identified in temporal derivations to RF and LF stimuli and either match the ipsilateral component or the ipsilateral and contralateral peaks are shifted. Notice that the algebraic sum of LF and RF looks like, but is not the same as the FF response (mostly in anterior leads).
lateral to the stimulating HF; while FF VEPs are symmetrically distributed only in derivations T5 to 0 2 , ipsilateral VEP components to HF stimuli are recorded
also at C3-C4, T3-T4 and F3-F4 (Figs. 1-3). The early positivity antecedent to N1 recorded with 2 and 4 cpd FF stimuli is not recorded with HF stimuli.
TABLE I Latencies (msec) and amplitude (/~V) of VEPs recorded on posterior leads to FF stimuli. 1 cpd
2 cpd
4 cpd
N1
P1
N2
N1
P1
N2
86+4.7 83+4.2 865:3.8 825:3.8 835:4.8 835:3.6 815:3.7
112+6.8 111+5.4 1105:5.8 1105:5.9 1095:6.2 111+5.9 1105:6.1
156+7.6 155+7.8 1575:7.7 1545:7.9 1565:7.8 1545:7.4 1545:7.3
86+4.6 88+4.3 895:4.2 885:4.4 845:4.7 87+3.6 885:3.8
112+7.2 112+6.2 110+6.4 110+6.3 1095:7.4 1125:5.3 1135:5.4
162+ 9.9 167+10.3 1635:9.1 1665:8.4 1635:8.2 1655:9.8 166+ 8.7
2.0+1.4 3.8+2.1 5.45:2.6 3.95:2.1 1.9+1.4 6.25:2.8 6.35:2.9
2.25:1.2 5.35:1.9 7.45:2.2 5.4+1.8 2.25:1.2 8.15:2.8 8.25:2.8
1.25:0.8 2.8+1.7 3.15:1.5 2.8+1.7 1.15:0.7 3.75:1.6 3.85:1.6
1.65:1.3 2.8+1.6 3.9+1.5 2.8+1.6 1.75:1.2 4.65:1.7 4.5-t-1.8
1.9+0.7 3.7+1.3 6.15:3.2 3.75:1.3 1.95:0.7 5.4+1.6 5.6+1.5
N1
P1
N2
91+4.4 915:4.2 935:4.3 905:4.2 855:4.7 91+3.2 905:3.1
113+5.8 1165:4.3 119+4.1 1185:4.2 1165:5.1 118+3.8 1185:3.9
166+ 8.2 1645:9.8 1695:9.8 1675:9.7 1645:9.9 1665:9.9 166+10.1
1.3+0.9 3.95:1.8 4.05:1.8 3.95:1.7 1.4+0.9 4.55:1.8 4.5+1.8
1.4_+1.2 1.85:1.2 2.6+1.2 1.95:1.1 1.5+1.3 3.4+1.6 3.45:1.6
2.15:1.3 2.4-t-1.3 3.7+1.3 2.4+1.2 2.25:1.4 3.95:1.8 4.15:1.7
Latency T5 P3 Pz P4 T6 O1 02
Amplitude T5 P3 Pz P4 T6 O1 02
0.5_+0.6 1.35:1.1 1.8+1.6 1.3+1.1 0.65:0.7 2.85:2.6 2.7+2.5
VEP MAPS TO 1, 2, 4 C / D E G P A T T E R N E D S T I M U L I
171
Contralaterally to the stimulating HF, 3 deflections (polarity reversed as compared to N1-P1-N2) are recorded which will be marked as C (contralateral) cP1, cN1, cP2, and have largest amplitudes over T5-6 and O1-2 leads and are recorded also in contralateral parietal and central derivations. Table II reports latency and amplitude measurements of N1, P1, N2 and cP1, cN1 and cP2 recorded with H F stimuli of the 3 tested spatial frequencies. Since Student's t comparisons of latencies and amplitudes of matched ipsi- or contralateral components to L and R H F stimuli were not significant the data obtained with H F stimuli are presented after pooling. On color maps of FF GA VEPs (Fig. 4) a polarity inversion of N1, P1, N2 is observed anterior to the central coronal row, for stimuli of 1, 2 and 4 cpd. Therefore the inspection of traces to F F 1 cpd stimuli cannot identify anterior components like those observed with 2 and 4 cpd VEPs, whereas antero-posterior polarity gradients can be observed in maps also with 1 cpd stimuli. Maps of H F VEPs (left or right) show a transverse inversion of polarity with the same polarities on each hemisphere (Fig. 4). Independently of the element size, N1 to H F stimuli has larger amplitudes in ipsilateral temporo-parietal
leads than N1 recorded with F F stimuli. The ipsilateral distribution of N1 is clearest with 2 and 4 cpd H F stimuli. The contralateral (cP1) polarity reversed potential could look like a separate component, since the sequence of maps and traces of Figs. 1 - 4 shows that the peaks of N1 and cP1 potentials do not correspond, to the point that in the maps of Fig. 4 the contralateral posterior positivity (cP1) is recorded when the ipsilateral N1 has already disappeared from posterior leads. The map of the whole scalp shows, however, that cP1 is now faced by a negativity recorded at the coronal midline level. N1 and cP1 potentials are also recorded with 1 cpd H F stimuli, and since the N1 to 1 cpd F F stimuli is small the hemifield stimuli apparently facilitate the appearance of N1 to 1 cpd patterns. The peak latencies of N1 recorded with H F stimuli are 2-8 msec shorter than N1 peaks to FF stimuli, at all the tested spatial frequencies (Figs. 1-4). P1 is recorded mostly ipsilateral to the hemifield but, more than N1, spreads over contralateral leads. With 1 cpd stimuli an amplitude attenuation of P1 rather than a polarity reversal is recorded over contralateral parieto-occipital derivations. When 2 and 4 cpd stimuli are used a polarity reversal, identified at the peak as cN1 component, of P1 is instead recorded, though not
4cpd RF F7 ~
sum
LF+RF
FF+. um
~
Fz F8
Cz
1'4 T6
Fig. 3. G A VEPs to 4 cpd stimuli. Same presentation as in Figs. 1 and 2. Notice that polarity reversed peaks matching the N1-P1-N2 peaks (vertical arrows) are recorded in anterior derivations with F F stimuli. Notice also that the L F - R F s u m does not reproduce the wave shape recorded in anterior leads with FF stimuli.
172
M. O N O F R J ET AL.
lcpd
lVF N83
Pl18
N167
LF N78
cPl92
P1112
P-cN136
N154
RF N181
Pill7
P-cN128
N146
N157
2cpd
FF P67
N87
PlI5
P143
N168
LF -0
N, 85
cP197 P l - c N l l l 4 Ne 138
cPa 146
N172
Rlr ÷
N~ 84
cPl94 Pl-cNll15cl~139
N154
N168
4cpd P68
NI 93
P1 120
P152
Na 174
LF N, 87
P, 117
P136
N148
N170
RF N191 P I - c N I l l 8
P136
Ne148
N168
VEP MAPS T O 1, 2, 4 C / D E G P A T T E R N E D S T I M U L I
173
T A B L E II Latency in msec. Amplitude in #V. Measurements of N1, P1, N2 and cP1, cN1, cP2 recorded with hemifield stimuli of the 3 different spatial frequencies, from the posterior derivations. Ipsi indicates derivations ipsilateral to the hemifield; Contra indicates derivations contralateral to the stimulated hemifield. 1 cpd
2 cpd
4 cpd
T
P
O
Pz
T
P
O
Pz
T
P
O
Pz
80±3,9 109±5.6 136±6.8 87±5.1 118±4.6 138±5.3
79±3.8 108±5.1 136±6.9 88±4.9 114±4.4 136±6.1
79±3.8 110±5.9 137±6.1 88±4.2 116±4.3 132±5.7
80±4.0 110±5.2 138±5.7 85±3.9 109±4.2 1~±5.2
84±3.7 111±5.1 138±3.6 88±4.3 112±4.6 132±4.8
84±3.8 110±6.1 134±4.2 90±4.8 113±4.9 134±4.7
83±3.9 109±6.2 138±4.9 89±4.7 110±5.0 131±4.6
84±3.9 110±6.0 139±4.4 85±3.7 112±4.6 133±4.1
87±3.8 112±4.8 1~±4.4 86±3.8 114±4.3 139±4.3
87±3.9 111±4.2 1~±4.7 88±4.1 111±4.7 1~±4.6
87±3.9 112±4.1 139±4.8 86±3.6 113±4.6 137±4.5
86±3.8 113±4.4 1~±4.6 87±4.1 112±4.4 139±4.2
1.2±0.7 1.3±0.9 1.8±0.9 0.9±0.3 0.6±0.3 0.8±0.3
1.5±0.9 3.3±1.8 4.2±1.8 0.8±0.4 0.7±0.3 0.6±0.3
2.6±1.7 4.8±2.1 5.9±2.2 0.9±0.5 0.5±0.4 0.7±0.3
1.2~0.8 3.6±2.3 5.7!1.9 0.3~0.2 0.5±0.3 0.5±0.4
1.8±0.9 1.2±0.8 2.3±0.7 2.6±0.6 1.4±0.4 2.4±0.4
2.9±1.9 2.3±1.6 3.0±1.4 2.1±0.4 0.6±0.3 0.7±0.2
3.3±1.5 3.6±1.6 4.1±1.7 2.9±1.0 0.7±0.3 0.9±0.4
2.9±1.4 3.4±1.3 4.3±1.9 0.9±0.4 0.6±0.2 0.6±0.3
2.1±1.1 1.2±1.0 1,6±0.9 1.9±1.2 0.9±0.4 1.6±0.7
3.8±1.7 1.3±1.0 2.1±0.9 1.2±0.8 1.1±0.4 1.3±0.4
4.2±1.6 2.8±0.8 3.1±1.2 1.8±1.3 1.6±0.5 1.5±0.3
3.1±1.6 1.8±0.9 2.1±0.9 0.9±0.6 1.1±0.4 1.1±0.4
Latency lpsi
Contra
N1 P1 N2 cP1 cN1 cP2
Amplitude Ipsi
Contra
NI P1 N2 cP1 cN1 cP2
T A B L E III The table reports, in percent values, in how m a n y control subjects the different VEP components could be identified. From the table it is evident that in some controls the ipsilaterat or contralateral components could spread to the opposite derivations. In some derivations the responses to full-field stimuli could consist of components with the same polarity as the contralateral components recorded with H F stimuli. The discrepant percentage of ipsilateral or contralateral component identification depends on the fact that in some controls none of the components could be identified inside their latency ranges, reported in Tables II and III. F r o m the table it should also be evident that in some controls VEPs could be asymetrically distributed, i.e., presence of 'contralateral' components to F F stimuli (and mostly when the stimulated spatial frequency was 1 cpd). 1 cpd
FF
H F ±psi
H F contra
HF
4 cpd
N1
P1
N2
cP1
cN1
cP2
NI
P1
N2
cP1
cN1
cP2
N1
P1
N2
cP1
cN1
cP2
T5 P3 Pz P4 T6 O1 02
3t 42 56 43 32 58 60
62 96 100 98 63 98 96
78 84 100 86 82 82 80
16 4 3 14 4 6
13 2 12 4 4
31 12
71 94 100 92 68 98 98
64 92 100 88 64 98 100
18 16 2 2
21 2 2 26 2 2
33 36 10 33 2 -
84 98 100 98 88 98 98
80 98 100 98 82 96 98
76 96 100 94 74 98 98
14 -
11 29 15 17
74 92 100 94 76 96 98
12 2 2
18 2 16 4 2
24 2 4 24 2 2
T P O
86 84 84
100 98 100
96 98 100
-
2 -
2 2 -
100 100 100
100 98 98
96 92 100
-
2 2
4 -
100 100 100
98 96 100
96 96 98
2 2 -
-
-
-
T P O
34 38
42 46 48
32 34 48
78 64 62
64 52 50
66 64 48
6 2
8 24 22
6 22 24
100 94 98
90 76 78
86 72 75
4 6
8 22 24
20 22 20
100 96 94
92 76 78
78 74 78
P2
68
94
92
28
4
6
92
96
88
6
2
14
91
94
86
4
8
8
The
peak
stimuli
has
as clear-cut
as N1.
stimuli are 2-12
1-4).
2 cpd
msec shorter
latencies than
of P1
P1 to FF
to HF
P1 (Figs.
The
N2
ipsilateral
component
to HF
shorter latency than the peak of the broad with
FF.
In VEPs
to 4 cpd
(LF)
and
a
N2 recorded
2 cpd
t h e ±psi-
Fig. 4. Color maps of polarity distributions of the different VEP components to FF, LF and R F patterns of 1, 2, 4 cpd. The calibration is 3.2 /~V, blue, positive and red, negative. The latency of measured polarity gradients is reported below each map. Notice that N1-P1-N2 components to LF and RF have shorter latencies than to F F stimuli. In FF VEPs the polarities are antero-posteriorly oriented. LF and R F maps show the predominant ipsilateral distribution of N1 and P1 components, with polarities oriented transversely (sideways) on the scalp. Notice that ±psi- and contralateral potentials can have peaks with unmatched latencies (as reported below the maps). Yet polarity reversed counterparts of each component can be observed within the scalp. Notice also that P1 to 1 cpd stimuli is widespread. The first maps in the top rows to FF stimuli of 2 and 4 cpd ~timuli show the distribution of tile early positivity (Po) preceding N1 to FF stimuli.
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M. O N O F R J ET AL.
lateral N2 is faced by the contralateral positivity cP2. The latencies of ipsilateral N2-contralateral cP2 to 2 and 4 cpd match the latency of notches evident on O1-O2 derivations on the broad slope of F F N2 (Figs. 2-4). GA traces to 1 cpd H F stimuli show that P1 is followed by a cP2 component recorded over contralateral temporal derivations, while the negativity N2 is widespread and the only identifiable peak has a latency approximating that of F F N2. A late negativity with a peak latency matching that of FF N2 is also recorded with 2 and 4 cpd H F stimuli. This latest negativity is distributed either contralaterally (2 cpd, 4 cpd RF) or ipsilaterally (4 cpd) or is widespread (1 cpd). Since FF N2 and H F N2 do not match, the F F N2 appears to be the summation of the ipsilateral N2 component, distributed like P1, and the latest negative peak. When VEPs t'0 right and left H F stimuli were summed (Figs. 1-3), the distribution over posterior leads was similar to the distribution of FF VEPs (Fig. 1). However, the peak latencies of N1 and P1 obtained by summation were shorter by 2 - 9 msec than N1-P1 to real FF stimuli ( P < 0.05-0.001). Moreover, none of the components recorded in anterior leads with FF stimuli could be observed. Table III reports the percentage of controls in whom the different components could be identified over different derivations, to F F and H F stimuli of the 3 spatial frequencies, and shows that single VEP components were not identified in all controls.
The early positivity (Po) preceding N1 to 2 and 4 cpd FF stimuli was only recorded in 23 controls (42 eyes, 42%). Polarity reversed counterparts of posterior VEP components were recorded as identifiable peaks in 65% of controls with 1 cpd F F stimuli, in 85 and 90% with 2 and 4 cpd F F stimuli. Table IV reports amplitude ratios of the different VEP components recorded with the 3 different spatial frequencies from symmetrical posterior leads. The amplitude ratios of FF VEP components recorded from corresponding lateral derivations ranged from 1 / 1 to 2 / 3 in 76% of controls to 1 cpd stimuli, in 84% to 2 cpd stimuli and in 92% to 4 cpd stimuli. The amplitude ratios of the N1-P1-N2 components and cP1, cN1 and cP2 components recorded over the two different hemispheres with H F stimuli were 1/1 to 2 / 3 in 74% of controls to 1 cpd stimuli, in 82% to 2 and 4 cpd stimuli. The comparison of H F VEPs vs. FF VEPs showed that N1 amplitude to 2 and 4 cpd H F stimuli was significantly larger over temporo-parietal derivations ( P < 0.05), and significantly smaller in occipital derivations ( P < 0.02). F F P1 was always larger than H F P1. There was no significant difference between matched VEP components to right and left H F stimuli of left and right eyes. As on GA traces, the single components recorded in individual controls to F F and H F stimuli and their polarity reversed counterparts were found to have matching or partly shifted peak latencies over the different scalp derivations.
T A B L E IV Amplitude ratios of the VEP components recorded from posterior derivations in different stimulus conditions. 1 cpd
FF (R vs. L derivations) FF vs. H F (L vs. L, R vs. R) HF (ipsilateral derivations R vs. L)
HF (contralateral derivations R vs. L)
HF (contralateral vs. ipsilateral derivations)
O P T O P T O P T
O P T
O P T
2 CPD
4 cpd
N1
P1
N2
N1
P1
N2
N1
P1
N2
97_+22 91+26 99_+21 64_+24 63-+16 91_+19 99 _+22 98 _+24 98 _ 24
99+21 90-+23 97_+21 52_+21 63+22 64_+12 97 _+23 97 _+24 98 -I-27
89-+22 100-+21 91_+24 49_+16 58_+14 62_+18 98 _+24 99 _+23 98 -+ 22
99-+22 100_+24 90_+26 82_+18 80_+21 92_+20 99 _+28 97 _+26 98 -+ 25
91-+22 101_+23 96_+22 64+21 65_+22 73_+12 98 _+24 98 _+26 96 -+ 24
100-+21 98_+22 98+21 74_+21 64_+24 67_+11 98 _+26 99 _+23 96 _+23
99_+21 101_+22 96-+24 85_+12 83_+17 92_+16 99 _+24 96 _+24 95 _+21
94_+26 97_+28 98_+28 64_+16 73_+12 84_+15 98 _+29 96 + 24 98 _+27
94_+22 99_+26 99_+24 68_+14 66_+12 72_+21 98 _+27 96 _+24 98 _+26
cP1
cN1
cP2
cP1
cN1
cP2
cP1
cNl
cP2
98_+21 98_+26 99 -+ 27
96_+22 94_+22 92 -+ 24
98_+22 99-+27 97 _+23
99_+19 99+21 98 + 22
cP1-N1
cN1-P1
cNI-P1
cP2-N2
61_+21 73 _+12 76-+14
48_+19 61 _+16 72_+15
80-+21 84 -+ 12 92-+24
72_+12 76 + 20 78_+16
96+25 99_+26 97 _+27 cP2-N2 54_+16 66 _+ 18 74+_13
97-+26 99_+24 93 _+22 cP1-N1 90_+18 92 -+ 19 98-+21
96-+21 98_+21 98 _+25 cN1-P1 84_+16 86 _+ 14 93_+21
99+23 98+28 98 +_27 cP2-N2 79_+10 81 + 14 82_+13
99_+26 98-+28 99 + 25 cP1-N1 90_+22 93 _+21 99-+25
VEP MAPS TO 1, 2, 4 C / D E G PATTERNED STIMULI
175
The mean data with S.D.s reported in Tables I - I V imply that some differences in comparison with the G A patterns of distribution were observed in a minority of controls. These differences, in individual controls, i.e., ' n o r m a l variants,' were mostly confined to the use of coarse (1 cpd) stimuli and consisted of asymmetries of FF VEP distributions absence of some of the components and bifid ('w-shaped') components. The VEP 'normal variants' were identical in tests of either eye. The N1 component was recorded only in 60% of normal controls when 1 cpd FF stimuli were used, but in all normal controls when 2 and 4 cpd FF stimuli were used.
The N1 distribution to FF patterns was asymmetrical (ratio between corresponding leads below 1 / 2 ) in 16% of 1 cpd VEPs, in 8-10% of 2 and 4 cpd VEPs. The N1 amplitude ratio over the two sides of the scalp was below 1 / 2 in 4 subjects (8%) to 1 cpd FF stimuli, in 2 subjects to 2 cpd stimuli. In 2 controls (4%) the asymmetry of FF VEPs was such that N1 components to 1, 2 and 4 cpd stimuli were recorded only on one side of the scalp and midline, and a positivity (cP1, 1 / 5 of N1 amplitude) was recorded on the other side. H F stimuli elicited lateralized N1 distributions in 86% of controls with 4 cpd patterns, in 88% with 2 cpd
B
I
cpd
FF LF RF
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~
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P3
Pz
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®
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02
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02
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rg
02
I}
RF
Fig. 5. VEP distribution variants in a normal control. Time scale 200 msec. Scale of map colors as in Fig. 4. In 1 cpd VEPs the N1 component is not recorded when FF stimuli are used, and P1 has, in several derivations, a bifid, or 'w,' wave shape. P1 has an early peak at 118 msec and a second peak, of its 'w' shape, which is predominant in temporal derivations. In 1 cpd LF VEPs the N1 component can be identified with the contralateral cPl component. 1 cpd RF VEPs are instead bifid and widespread. N1 to 4 cpd FF stimuli is symmetrical. In H F 4 cpd VEPs the N1 components are ipsilateral to the stimulating hemifield, and contralateral cP1 activities can also be identified. The P1 wave is widespread to LF stimuli and is ipsilateral to RF stimuli. The late negativity (at 164-172 msec) is contralateral to both LF and RF stimuli.
176
patterns, in 78% with 1 cpd patterns. In the other controls (i.e., 14%, 8%, 22% respectively for 4, 2 and 1 cpd stimuli) the N1 components to H F patterns spread also over contralateral derivations. Even in the 4 controls having anomalous asymmetries of N1 to 1 cpd F F stimuli, the H F stimuli evoked N1 components over the same side of the scalp. In these 4 controls ipsilateral N1 was still 1 / 3 smaller on one side than on the other. P1 was recorded in all controls with the 3 tested spatial frequencies. The P1 component recorded at the midline with FF stimuli had a bifid or 'w-shaped' wave form in 24% of controls with 1 cpd and in 12% and 4% with 2 and 4 cpd. Asymmetries of P1 distribution to F F stimuli (ratio below 1 / 2 ) were observed with 1 cpd patterns in the same controls having asymmetrical N1 distributions, and in two more. P1, more than N1, spread to the midline and contralateral parietal derivation when H F stimuli were used. Widespread P1 components were obtained in 46% of controls to 1 cpd H F stimuli, in 30% of controls to 2 and 4 cpd H F patterns. Furthermore, in 2 controls (4%) the distribution of P1 to 1 cpd H F stimuli was prominent in contralateral derivations, whereas N1 was normally ipsilaterally distributed. N2 to H F stimuli was usually widespread: an ipsilateral N2 component, with a contralateral cP2, was observed in 54% of controls when patterns were 2 and 4 cpd, in 46% to 1 cpd patterns. In the remaining controls N2 was widespread or no distinction could be made between the 'early' and 'late' N2 deflections. An asymmetry or absence of N1 and bifid P1 wave shape coexisted in 84% of ' a n o m a l o u s ' F F VEP distributions. The use of H F stimuli clarified the ' a n o m a l o u s ' distribution, since ipsilateral N1-P1-N2 components appeared or were enhanced in temporo-parietal derivations where the responses to F F stimuli were of small amplitude. Fig. 5 shows examples of coexisting anomalies in one control. V E P s to "central" or "annular' stimuli
In all controls ipsilateral N1-P1-N2 and contralateral cP1, cN1 and cP2 components were recorded to 1, 2 and 4 cpd 12 ° H F stimuli. Fig. 6 reports comparisons of G A traces and maps of VEP recordings performed with 'central' and 'annular' stimuli of 1 and 4 cpd. The comparison of maps of different components (Fig. 6) shows variations of the distribution, yet polarity reversed counterparts could
M. O N O F R J ET AL.
always be recorded either tangentially oriented over the coronal rows of electrodes, or obliquely or antero-posteriorly. Fig. 7 shows amplitude plots of ipsi- and contralateral components to 1, 2 and 4 cpd patterns, recorded from temporal and occipital leads with 3, 6 and 12 central stimuli and annular stimuli with 3 ° and 6 ° central occlusion. With 3 ° central hemifields VEPs to 2 and 4 cpd patterns consist of ipsilateral N1-P1-N2 and contralateral c P l - c N l - c P 2 components at posterior derivations. In comparison with VEPs to 12 ° H F the amplitude decrement over the different derivations is 50% for both ipsi- and contralateral activities. In 7 controls VEPs to 1 cpd 3 ° central H F stimuli consisted only of a widespread P1 component, 50% smaller than P1 to 12 ° HF. In 8 controls all ipsi- and contralateral components could be identified, the same as with 2 and 4 cp~] stimuli. The G A of Fig. 6 shows small amplitude but identifiable contralateral activities. The distributions of VEPs obtained with 6 o H F or annular H F with 3 ° occlusion are scarcely different from the distribution of VEPs obtained with 12 ° HF. Amplitudes of ipsi- and contralateral components are reduced by 10-30%; only P1 amplitude, to hemiannular 3 ° occluded stimuli, is reduced by more than 40%. When the central 6 ° portion of the 12 ° H F is occluded the response over occipital derivations consists of widespread cP1, cN1 and cP2 activities: their amplitudes are 50% smaller than contralateral activities recorded with un-occluded 12 ° H F and polarity reversals and ipsilateral components can be recorded over temporal and anterior derivations. Fig. 6 shows identifiable ipsilateral components, and the mean amplitude plots of Fig. 7 show the reduction of amplitude of all ipsilateral components to hemiannular stimuli. In 5 controls the prominence of contralateral activity was not observed, and also with 6 ° occlusion, VEPs consisted of ipsi- and contralateral components. The amplitude reduction of VEPs recorded over the different leads was significant ( P < 0.05-0.001) for VEPs to 12 ° H F stimuli vs. 3 ° central H F stimuli and vs. annular stimuli with 6 o central occlusion ( P < 0.010.0001) for all ipsi- and contralateral components. With
Fig. 6. 1 and 4 cpd VEPs to 3 °, 6 ° , 12 ° HF; hemiannular 12 ° H F with 3 ° and 6 ° central occlusion. Calibration is 5/~V for central rows, 3/tV for the left and right rows. Color scale is 1.6 gV for lateral maps and 3.2 /~V for central maps. The vertical lines mark N1-P1 ipsilateral potentials (mapped below). Notice that N1 to 4 cpd stimuli is recorded with 3 °, 6 o, 12 o and 3 o occluded H F stimuli. With 6 ° occlusion the cP1 component is predominant in contralateral derivations but negative counterparts can be observed in traces and maps and its amplitude is decreased rather than increased. P1 amplitude increases when the size of the stimulating field is increased from 3 ° to 12 ° and drops when the 3 central degrees are occluded.
VEP MAPS TO 1, 2, 4 C / D E G PATTERNED STIMULI
177
4cpd
•f•T5
P3 Ol
o2
~
0
pt T6
lcpd T5 P3 O1
L
4-
f
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~ ~
02 P4
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178
M. O N O F R J ET AL.
I cpd
2
epd
4 cpd
---
cP~
---
P1 cN2
T
0
,
4o
4°
0
4°
Fig. 7. Mean amplitude ratios of N1, cP1, P1 and cN2 components recorded with central or occluded HF. 3c, 6c, 12c indicate the visual angle subtended by the central (c) stimuli, 3 co, 6 co indicate 3 ° and 6 ° central occlusions of the 12 ° HF. The amplitude of the same components recorded with the 12 ° H F is taken as 100%. Standard deviations ranged from 7% to 16%. Measurements are reported for responses to 1, 2 and 4 Cpd stimuli, and for responses recorded from matched ipsilateral and contralateral occipital derivations.
6 o central H F stimuli only the amplitude reduction of VEPs recorded from temporal and occipital derivations was significant ( P < 0.05, 12 ° vs. 6 ° H F stimuli). With 3 ° occlusion only the amplitude of P1 at Pz and O 1 - 2 was significantly smaller than P1 amplitude recorded with 12 ° H F ( P < 0.01 for 1 cpd, P < 0.05 for 2, 4 cpd stimuli). As for the 12 ° H F stimuli, in VEPs obtained with central or annular H F patterns the peak latencies of ipsi- and contralateral components were not matched across the different leads, as can be seen in Fig. 6. The latency differences were inconsistent across controls: only N1 latency was consistently shorter when the 3 ° central part was occluded ( P < 0.02). The evaluation in single subjects of m a p distributions did not show consistent prominence of any of the contralateral VEP components in any of the different recording conditions.
Discussion
The present paper shows that F F responses consist of a sequence of N1-P1-N2 components with a symmetri-
cal distribution over posterior derivations and that polarity reversed counterparts can be identified over anterior derivations, either in maps (1, 2, 4 cpd stimuli) or, as polarity reversed peaks by inspecting VEP traces to 2 and 4 cpd stimuli (Figs. 1 and 4). When right or left H F stimuli are used the polarity gradients rotate on one side: N1-P1-N2 are distributed ipsilaterally, i.e., on the same side as the stimulated visual field, and polarity reversed activities (cPl-cN1cP2) are distributed on the side of the unstimulated visual field (Figs. 1-4). The N2 potential results from the sum of a first negativity immediately following P1 and having the same distribution as N1 and P1, and a late negativity, widespread or contralateral to the stimulating hemifield. N1 can be preceded by an early positivity (Po), but this potential was recognized only in half of the controls when FF stimuli were used, and was absent in the majority of controls when H F stimuli were used. Stok (1986) also noticed a similar potential but found it inconsistent. The present paper shows furthermore that the distribution of different components is sensitive to variations of the spatial frequency. All the components, ipsi- and
VEP MAPS TO 1, 2, 4 C/DEG PATTERNED STIMULI contralateral, are best identified when 2 or 4 cpd stimuli are used. N1 is often absent when 1 cpd F F stimuli are used (Figs. 1, 4, 6 and 7). The N1 recorded with 2 and 4 cpd H F stimuli is lateralized more often than P1 and N2 (Figs. 1-4, 6, 7 and Tables I-III). Since N1 is paralleled by a contralateral positivity, cP1, N1 could be identified as the lateralizing and reversible (because of the contralateral positivity) component of VEPs. Recently Paulus et al. (1988) have recorded with colored patterns a negativity at 80 msec (similar latency as N1 in our study) and showed that this component was ipsilaterally distributed when H F stimuli were used. They suggested that this component could allow recognition of field defects. Our study shows that a lateralizing negativity at 80-90 msec (N1) can be recorded with black and white stimuli, if the size of pattern elements is appropriate (2-4 cpd). P1, which is currently used in diagnostic VEP measurements, is, among the ipsilateral components, the one to show the greatest tendency to spread diffusely over posterior derivations, mainly when 1 cpd H F stimuli are used (Figs. 1-4, 6, 7 and Table III). The evaluation of the variability of VEP components in controls showed that P1 is the least sensitive component to VEP lateralization. All in all the use of spatial frequencies above 1 cpd is advantageous in VEP diagnosis because with 2 and 4 cpd stimuli the lateralization of hemifield VEPs is enhanced either because N1 is distributed only on one side of the scalp or because all the components show a specific lateralization. A recent report showed that in patients with retrochiasmatic visual field defects the highest diagnostic yield was obtained with stimuli of 2 and 4 cpd (Onofrj 1990). Our study conducted with 19 derivations placed over the entire scalp shows that any positivity or negativity of VEPs has a counterpart of opposite polarity which is always within the map of the scalp (Figs. 1-4, 6 and 7), and this also when small central H F or hemiannular stimuli are used. In our study occlusion of central or peripheral parts of the stimulating hemifield could enhance or reduce the amplitude ratio between ipsi- and contralateral components though never leading to complete disappearance of N1-P1-N2 or cPI-cNI-cP2 (Fig. 7). Ipsilateral N1-P1-N2 were recorded in our studies with stimuli as eccentric as 6 o from the fovea, and cPl-cN1cP2 were recorded with H F stimuli covering only the central 3 ° . When annular stimuli occluding the central part of the visual field were used the contralateral cPl-cNl-cP2 components were prominent in posterior leads, but their amplitudes were smaller than when stimuli were presented also to the central field (Fig. 7), and the polarity reversed counterparts were evident in anterior leads (Fig. 7).
179 It is not surprising that the anterior counterparts of VEPs were not described accurately by other researchers since their studies were conducted with a small number of derivations placed only around the occipital pole and referred to the midfrontal position (Blumhardt et al. 1978, 1982, 1989; Wood 1982; Skrandies 1984; Butler et al. 1987). Only one previous study (Mauguirre et al. 1984) described anterior counterparts of VEPs. A study of Darcey et al. (1980) also described maps of VEP distribution over the entire scalp, but their results cannot be compared with the present ones since these authors were using onset-offset (pulse) presentation of patterns, and the distribution of onset-offset VEPs is profoundly controversial (for a review of the literature see Onofrj 1990). The maps shown in the present study (Figs. 4, 6 and 7) suggest that it is difficult, if not impossible, to separate contralateral components (paracentral?, parafoveal?) from the activity which might correspond simply to the polarity reversal of the ipsilateral VEPs. Moreover, if one accepts the idea that parafoveal retinal receptors are more likely to be stimulated by low (1-2 cpd) spatial frequency stimuli (Hubel and Wiesel 1968; Campbell and Maffei 1970), one might expect greater chances of identifying a paramacular VEP complex (i.e., the contralateral VEP components) with low (1 cpd) than with high (4 cpd) spatial frequency stimuli. Ipsiand contralateral VEP components were instead recorded in our study with all the tested spatial frequencies (Figs. 1 - 6 and Tables I-IV). Based on the maps, a simple model of VEP generators can be suggested: VEPs elicited by pattern reversal stimuli should be organized as a sequence of N1-P1-N2 components at the surface of the visual cortex; on the side of the scalp facing the inner surface of the visual cortex the recorded potentials will be polarity reversed (cPl-cNl-cP2). The prevalence of ipsi- (N1-P1-N2) or contralateral (cPl-cNl-cP2) components over anterior, posterior or lateral leads of the scalp can be explained by the orientation of the dipole sheet, which will be mostly radial if the afference is mostly foveal, thus projecting to the tip of the occipital pole, or tangential (medial surface of the occipital pole, where foveal, parafoveal, paracentral afferences arrive) or even oblique, if the afference is to a part of the visual cortex folded inside lateral sulci (Polyak 1957) of the calcarine fissure. With F F stimuli the activities of VEP generators located inside the medial occipital sulcus will mostly annul each other because they are facing: VEP maps, comparably to our findings, will have antero-posterior reversals of polarity. The model can also explain why the peak latencies of components do not match in the various leads and why the sum of H F VEPs does not result in the same VEP distributions and latencies as FF VEPs. In fact in our
180 s t u d y , in o r d e r to r e d u c e f i x a t i o n v a g a r i e s , t h e f i x a t i o n p o i n t was at 0.5 o f r o m t h e H F , w h i l e F F s t i m u l a t e d t h e e n t i r e fovea, As a concluding remark we emphasize that variants o f the n o r m a l V E P s a r e f o u n d m o s t l y w h e n c o a r s e p a t t e r n s t i m u l i a r e used, a n d d i s a p p e a r w i t h 2 o r 4 c p d p a t t e r n s . T h i s f i n d i n g has c l i n i c a l a p p l i c a t i o n s f o r t h e interpretation of abnormal VEP distributions. A few p r e v i o u s r e p o r t s ( O i s h i et al. 1985; O n o f r j et al. 1987; N o v a k et al. 1988) h a d a l r e a d y s h o w n t h a t a b n o r m a l VEP wave shapes were observed with coarse stimuli, while with finer patterns, normal wave shapes with i d e n t i f i a b l e c o m p o n e n t s w e r e o b s e r v e d in the s a m e s u b jects.
References Arruga, I., Feldon, S.E., Hoyt, W.F. and Aminoff, M.J. Monocularly and binocularly evoked responses to patterned HF stimulation. J. Neurol. Sci., 1980, 46: 281-290. Barrett, G., Blumhardt, L.D., Halliday, A.M., Halliday, E. and Kriss, A. A paradox in the lateralization of the visual evoked response. Nature, 1976, 261: 253-255. Blumhardt, L.D., Barrett, G., Halliday, A.M. and Kriss, A. The effect of experimental 'SCOTOMATA' on the ipsilateral and contralateral responses to pattern-reversal in one HF. Electroenceph. clin. Neurophysiol., 1978, 45: 376-392. Blumhardt, L.D., Barrett, G., Kriss, A. and Halliday, A.M. The pattern evoked potential in lesions of the posterior visual pathways. Ann. NY Acad. Sci., 1982, 388: 264-289. Blumhardt, L.D., Barrett, G., Halliday, A.M. and Kriss, A. The effect of field size on the pattern reversal visual evoked response (PREVER). Clin. Vision Sci., 1989, 4: 27-40. Bodis-Wollner, I., Hendley, C.D. and Kulikowski, J.J. Electrophysiological and psychophysical responses to modulation of contrast of a grating pattern. Perception, 1972, 1: 341-349. Bodis-Wollner, I., Ghilardi, M.F. and Mylin, I.H. The importance of stimulus selection in VEP practice: the clinical relevance in visual physiology. In: R.Q Cracco and I. Bodis-Wollner (Eds.), Evoked Potentials. Liss, New York, 1987: 15-27. 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. Campbell, F.W. and Maffei, L. Electrophysiological evidence for the existence of orientation and size detectors in the human visual system. J. Physiol. (Lond.), 1970, 207: 635. Chiappa, K.H. Evoked Potentials in Clinical Medicine. Raven Press, New York, 1983:216 pp. Darcey, T.M., Ary, J.P. and Fender, D.H. Spatio-temporal visually evoked scalp potentials in response to partial-field patterned stimulation. Electroenceph. clin. Neurophysiol., 1980, 50: 348-355. Desmedt, J.E., Tran Huy Ngnyen and Bourguet, M. Bit mapped color imaging of human evoked potentials with reference to the N20, P22, P27 and N30 somatosensory responses. Electroenceph. clin. Neurophysiol., 1987, 68: 1-19. Halliday, A.M. Visual evoked potentials. In: A.M. Halliday (Ed.), Evoked Potentials in Clinical Testing. Churchill Livingstone, Edinburgh, 1982: 24. Hammond, S.R., MacCallum, S., Yiannakis, C., Walsh, J.C. and McLeod, J.G. Variability on serial testing of pattern reversal VEP latencies from f-f, hf and foveal stimulation in control subjects. Electroenceph. clin. Neurophysiol., 1987, 66: 401-408. Harding, G.F.A., Smith, G.F. and Smith, P.A. The effect of various
M. ONOFRJ ET AL. stimulus parameters on the lateralization of the VEP. In: C. Barber (Ed.), Evoked Potentials I. MTP Press, Lancaster, 1980: 212-222. Hubel, D.H. and Wiesel, T.N. Receptive fields and functional architecture of monkey striate cortex. J. Physiol. (Lond.), 1968, 195: 215-243. Jones, D.C., and Blume, W.T. Aberrant wave forms to pattern stimulation: clinical significance and electrographic 'solutions.' Electroenceph, clin. Neurophysiol., 1985, 61: 472-481. Kuroiwa, Y. and Celesia, G.G. Visual evoked potentials with hemifield pattern stimulation. Their use in diagnosis of retrochiasmal lesions. Arch. Neurol., 1981, 38: 66-90. Mauguirre, F., Giard, M.H., Ibafaez, V. and Pernier, J. Sequential spatial maps of evoked potential by checkerboard pattern response topography. Rev. EEG Neurophysiol. Clin., 1984, 15: 129-137. Novak, G.P., Wiznitzer, M., Kurtzberg, D., Giesser, B.S. and Vaughan, Jr., H.G. The utility of visual evoked potentials using hemifield stimulation and several check sizes in the evaluation of suspected multiple sclerosis. Electroenceph. clin. Neurophysiol., 1988, 71: 1-9. Oishi, M., Yamada, T., Dickins, Q.S. and Kimura, J. Visual evoked potentials by different check sizes in patients with multiple sclerosis. Neurology, 1985, 35: 1461-1465. Onofrj, M. Generators of pattern visual evoked potentials in normals and in patients with retrochiasmatic lesions. In: J.E. Desmedt (Ed.), Visual Evoked Potentials. Clinical Neurophysiology Updates, Vol. 3. Elsevier, Amsterdam, 1990: 87-113. Onofrj, M., Bodis Wollner, I. and Mylin, L. Visual evoked potential diagnosis of field defects in patients with chiasmatic and retrochiasmatic lesions. J. Neurol. Neurosurg. Psychiat., 1982, 45: 294-302. Onofrj, M., Ghilardi, M.F., Basciani, M. and Gambi, D. Visual evoked potentials in parkinsonism and dopamine blockade reveal a stimulus-dependent dopamine function in humans. J. Neurol. Neurosurg. Psychiat., 1986, 49: 1150-1159. Onofrj, M., Basciani, M., Censoni, A.G. and Gambi, D. Serial recording of VEPs to different stimuli in multiple sclerosis patients. Ital. J. Neurol. Sci., 1987, 56: 113-120. Paulus, W.M., Plendl, H. and Krafczyk, S. Spatial dissociation of early and late colour evoked components. Electroenceph. clin. Neurophysiol., 1988, 71: 81-88. Plant, G.T. Transient visually evoked potentials to sinusoidal gratings in optic neuritis. J. Neurol. Neurosurg. Psychiat., 1983, 46: 11251133. Polyak, S. The Vertebrate Visual System. University of Chicago Press, Chicago, IL, 1957. Rowe, M.J. The clinical utility of half-field pattern reversal visual evoked potential testing. Electroenceph. clin. Neurophysiol., 1982, 53: 73-77. Shagass, C., Amadeo, M. and Roemer, R.A. Spatial distribution of potentials evoked by HF pattern-reversal and pattern-onset stimuli. Electroenceph. clin. Neurophysiol., 1976, 41: 609-622. Shepard, D. A two dimensional interpolation function for irregularly spaced data. Proc. 1968 ACM National Conf., 1968: 517-524. Skrandies, W. Scalp potential fields evoked by grating stimuli: effects of spatial frequency and orientation. Electroenceph. clin. Neurophysiol., 1984, 58: 325-332. Stok, C.I. The Inverse Problem in EEG and MEG with Application to Visual Evoked Response. Dissertation Rijksuniversiteit Leiden, Leiden, 1986: 1-149. Streletz, L.J., Bae, S.H., Roeshmon, R.M., Schatz, N.J. and Savino, P.J. Visual evoked potentials in occipital lobe lesions. Arch. Neurol., 1981, 38: 80-85. Van Lith, G.H.M., Van Merle, G.W. and Vijfvinkel-Brninerga, S.M. Two disadvantages of a televisive system as pattern stimulator for evoked responses. Docum. Ophthalmol., 1979, 48: 261-263. Wood, C.C. Application of dipole localization methods to source identification of human evoked potentials. Ann. NY Acad. Sci., 1982, 388: 139-155.
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EVOPOT 89628
Mapped distribution of pattern reversal VEPs to central field and lateral half-field stimuli of different spatial frequencies M. Onofrj, S. Bazzano, G. Malatesta and T. Fulgente Department of Neurology, Istituto di Clinica Neurologica e Scienze del Comportamento, State Unioersity of Chieti, Chieti (Italy) (Accepted for publication: 13 July 1990)
Summary Visual evoked potentials (VEPs) to pattern reversal vertical bar stimuli of 3 different sizes (1, 2, 4 c / d e g ) were recorded from 19 scalp derivations in 50 controls. The stimuli were presented on a full-field (FF) screen of 24 ° visual angle, and on left and fight half-fields (HF) of 12 ° radius. In 15 controls partial H F stimuli were presented on the central 3 and 6 o and as hemiannular stimuli of 12 ° with occlusion of the central 3 and 6 o. An antero-posterior polarity reversal of the N1-P1-N2 sequence was observed for F F VEPs. A tangential polarity reversal was observed for H F VEPs. Also with central or hemiannular stimuli polarity reversals of all VEP components were observed within the scalp. Van~ants of VEP distribution, absence or prominence of some of the ipsi- or contralateral VEP components were observed in 8-40% of controls. The FF and H F VEP distribution, and the variant VEP asymmetries were partly dependent on the pattern spatial frequency.
Key words: Visual evoked responses; Visual cortex; Visual pathways; Topographic mapping
The paradoxical distribution of pattern reversal VEPs to half-field (HF) stimuli was reported in several studies (Barrett et al. 1976; Shagass et al. 1976; Arruga et al. 1980; Kuroiwa and Celesia 1981; Streletz et al. 1981; Rowe 1982; Jones and Blume 1985; H a m m o n d et al. 1987; Paulus et al. 1988). These studies showed that the negative positive negative VEP sequence, corresponding to components recorded at the midline with full-field (FF) stimuli, was recorded ipsilaterally to H F stimuli, while contralaterally polarity reversed counterparts (positive, negative, positive) were recorded. Since the contralateral components become prominent in posterior derivations when the central part of H F patterns are occluded (Blumhardt et al. 1978, 1989), a theory of VEP generators was proposed, suggesting that two generators should be found in the visual areas: one for projections from the central retina (generating ipsilateral components) and one for projections from the parafoveal-paracentral retina (contralateral components). 'The relationship between eccentricity and visual evoked potential' was found to be 'not straightforward' (Blumhardt et al. 1989) since ipsilateral and contralateral components were not found to have precise boundaries in the visual field: ipsilateral N75, P100 and
Correspondence to: Marco Onofrj, Clinica Neurologica, Ospedale ex Pediatrico, 66100 Chieti (Italy).
N140 were recorded with stimuli as eccentric as 5-10 ° from the fovea and contralateral P80-N105-P135 were recorded with stimuli covering only the central 2-5 ° radius (Blumhardt et al. 1978; 1989). Moreover, the characteristic pattern of VEP distribution is not observed in all controls, and variants with asymmetries of VEPs, preponderance at the midline of ipsi- or contralateral components and bifid or 'w'shaped P100 are described in varying (6-25%) percentages of controls (Blumhardt et al. 1982; Chiappa 1983; Jones and Blume 1985; H a m m o n d et al. 1987). Blumhardt et al. (1978, 1982) suggest that preponderance of ipsi- or contralateral components and 'normal wave shape asymmetry' depend on variations of the representation of area 17, at the tip of the occipital pole or on its medial surface. All the studies quoted above were conducted with a small number of derivations (5-13) placed on the postcentral area of the scalp, and with coarse pattern element stimuli (more than 30 min visual angle per element). In only 4 other studies, pattern elements of 15 min visual angle or less were used but records were obtained only from posterior leads (Harding et al. 1980; Onofrj et al. 1982; Skrandies 1984; Novak et al. 1988). In two studies, references other than the midfrontal were used (Arrnga et al. 1980; Mauguirre et al. 1984) and in one records were obtained also from anterior leads (Maugui&e et al. 1984) but in both studies the patterns consisted of coarse elements.
168 Recent reports suggest that the detection of visual pathway anomalies is dependent on the size of pattern elements used for stimulation (Plant 1983; Oishi et al. 1985; Onofrj et al. 1986; Novak et al. 1988) and that human VEPs have an amplitude tuning approximating the spatial frequency tuning of psychophysical contrast sensitivity (Campbell and Maffei 1970; Bodis-Wollner et al. 1972; Plant 1983; Skrandies 1984). The peaks of the contrast sensitivity curve vary in proportion to the eccentricity from the fovea: the peak is at 4-6 cycles/degree (cpd) for the fovea, at 2 cpd for 10 ° eccentricity, at 1 cpd for 30 ° eccentricity (BodisWollner et al. 1987). The relationship of contrast sensitivity to eccentricity from the fovea is thought to be related to the receptive field size of retinal ganglion cells, that is, the smallest with the highest density of ganglion cells per unit area, at the foveal level (Hubel and Wiesel 1968). The different receptive field sizes at the foveal and paracentral levels might suggest that the chances of identifying a paracentral complex are greater when coarse rather than small patterned element stimuli are used. The present paper therefore describes in controls the distribution over the entire scalp of VEPs obtained with pattern reversal F F and HF, and central and annular H F stimuli of various spatial frequencies (i.e., various sizes of the pattern elements). Whereas prior studies on pattern VEP distribution were mostly focused on the distribution of the major positive component, the present paper describes the distribution of all VEP components. Furthermore, since the 'normal' asymmetric VEP variants are thought to be dependent on anatomical variants of area 17 (Blumhardt et al. 1978, 1982, 1989) a relevant point of our study, because of implicit applications in the clinical use of VEPs, was to try to understand whether VEP variants were dependent or not on the spatial frequency of patterned stimuli. We had to record VEPs with a reference placed outside the mapped scalp, rather than in the commonly used Queen Square position (Barrett et al. 1976). Since in preliminary studies (Onofrj 1990) the single earlobe reference was found to unbalance the VEP distribution, as already reported by Halliday (1982), we used a calibrated linked earlobe reference, that yielded the same results as the average reference or the extracephalic reference, with the lowest presence of artifacts. A fast scanning oscilloscope had also to be used for pattern presentation instead of the commonly used TV monitors, since TV monitors generate the pattern on the screen from the right border to the left, and completion of the pattern takes 25 msec, the right half of the pattern being therefore completed earlier than the left half (Van Lith et al. 1979), and this left-right delay might be misleading.
M. ONOFRJ ET AL. Materials and methods
Pattern VEP stimuli were vertical bars with a square wave luminance profile presented on the screen of a LACA monitor placed at 1 m from the observer's eye. The completion of the pattern on the screen required 6 msec. The FF screen was circular, with a diameter of 24 ° of visual angle at the observer's eye. The mean luminance of the monitor was 75 c d / m 2. The contrast, defined as the ratio of the difference of maximum and minimum luminance of adjacent bars over their summed luminances was 50%. Minimum and maximum luminances were calibrated as in previous reports (Onofrj et al. 1982, 1986). Monocular fixation was provided by a dark spot subtending 5' of visual angle centered on the screen and was constantly monitored by two examiners. During H F stimulation, patterns were presented on the 12 ° radius half (left, L or right, R) of the screen and controls fixated the same central spot, displaced by 30' of arc from the H F border. FF, L-R H F VEPs were recorded in 50 controls (22 males, age range 20-51 years with normal ocular media and normal visual acuity). In 15 of the same controls VEPs were also recorded with H F stimuli subtending 3 ° and 6 °, and hemiannular stimuli in which the central 3 ° or the central 6 ° were occluded (i.e., were substituted by an homogenous blank central field). In the unstimulated parts of the screen the overall luminance was the same as the average luminance of the pattern. Three different pattern element sizes were tested in all recording conditions: coarse bars of 30' of arc, corresponding to a dominant spatial frequency of 1 cpd, intermediate bars of 15' of arc (2 cpd) and bars of 7.5' of arc (4 cpd). The contrast of the grating was reversed (i.e., dark bars became bright and vice versa) every 510 msec. The signals effecting the reversal of patterns triggered the Brain Surveyor (Basis) averager. The dwell time of the averager was 0.400 msec. The averager was set for 100, 200 or occasionally 400 sums. Two or three sets of traces were recorded for each stimulating pattern, stored on floppy disk and eventually reaveraged on a grand average (GA) program. VEPs were recorded with Ag/AgC1 disk electrodes placed in the 19 positions of the international 10-20 system, except that P3, Pz and P4 were posteriorly displaced and O1-O2 laterally displaced by 5%. A calibrated linked earlobe reference was used. Following our modified system the head volume was represented on a single plane octagonal projection with the center at Cz. Color images of electrical distributions were obtained by the classic 4 nearest neighbors (4-NN) interpolation method (Shepard 1968) and were drawn on paper by a microprocessor-based Fujitsu Dx 2100 cartridge copier which bit-mapped at 100 dots/inch. The maps of Figs. 4 - 6 were directly photographed from the computer monitors, maps of Fig. 7 were photo-
VEP MAPS TO 1, 2, 4 C / D E G P A T T E R N E D S T I M U L I
169
graphed from paper drawings. After computing 3860 pixels at any given latency the map program assigned them to appropriate voltage and color steps. The selected gradient of colors was red for negative and blue for positive (Desmedt et al. 1987). VEP measurements were automatically performed by the computer as the amplitude of components from the baselines and matched by measurements from onset to peak of each deflection performed by visual inspection of traces. Data recorded from single derivations in normals in the different recording conditions were statistically compared by means of 2-tailed Student's t test and 1-way ANOVA.
Results
Figs. 1-3 show grand average (GA) VEP distributions in 50 controls to FF and HF (LF, RF) stimuli of 1, 2, 4 cpd. Fig. 4 shows the corresponding maps. FF VEPs (Fig. 1) consist of negative-positive-negative (N1-P1-N2) components distributed over derivations T5 to 02. Mean latencies and amplitudes are
(
F7 ~
lepd
lepd
~
F8 ~
~
T3 ~
~
reported in Table I. The latencies, amplitudes and (in part) distribution of these components change depending on the pattern element size, as shown in Fig. 1 and Table I. N1 latency increases with the spatial frequency. P1 latency is longer for 4 cpd stimuli. N2 looks 'broad' compared to the other components, the peak is not defined at 1 cpd stimuli, and a 'notch' on the slope is recorded with both 2 and 4 cpd stimuli. N1 is of largest amplitude at O1-O2 when 2 and 4 cpd stimuli are used. P1 amplitude is largest to 1 cpd and smallest to 2 cpd stimuli. The amplitude of N2 is also largest for 1 cpd stimuli. In anterior derivations (F1 to F4) and in the central coronal row positive-negativepositive components are recorded with 2 and 4 cpd FF patterns. These anterior potentials have latencies partly corresponding to, although not always matching, the latencies of N1-P1-N2 posterior components. On GA traces an early positivity (Po) could be identified preceding the N1 component to 2 and 4 cpd FF stimuli (Figs. 2-4). The VEP distribution to HF stimuli changes according to a characteristic pattern: N1, P1 and N2 components, similar to N1, P1 and N2 components recorded with FF stimuli, are recorded over derivations ipsi-
lepd
Slim
I +RF
FF+.qum
~
"
-~
~ ~
c4
Fig. 1. GA VEPs to 1 cpd stimuli, full-field (FF), right (RF) and left (LF) hemifields. Algebraic s u m m a t i o n of LF and R F VEPs in the fourth column. The last column on the right shows superimposition of FF VEPs and of the s u m of R F and LF VEPs. Vertical arrows identify N1, P1 and N2 at 83, 118 and 167 msec. Black horizontal arrows identify ipsilateral components (white horizontal arrows, contralateral activities). Time scale 250 msec.
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M. ONOFRJ ET AL.
LF
T
3
RF
LF+RF
FF+sum
~
t Fig. 2. GA VEPs to 2 cpd stimuli, with the same presentation as in Fig. 1. Vertical arrows point to FF N1 (87 msec), P1 (115 msec), to the positive notch on the slope of N2 (P143), and to N2 (167 msec). Notice that FF N1 is more evident than N1 to 1 cpd stimuli. Contralateral cP1, cN1, cP2 (white arrows) can be clearly identified in temporal derivations to RF and LF stimuli and either match the ipsilateral component or the ipsilateral and contralateral peaks are shifted. Notice that the algebraic sum of LF and RF looks like, but is not the same as the FF response (mostly in anterior leads).
lateral to the stimulating HF; while FF VEPs are symmetrically distributed only in derivations T5 to 0 2 , ipsilateral VEP components to HF stimuli are recorded
also at C3-C4, T3-T4 and F3-F4 (Figs. 1-3). The early positivity antecedent to N1 recorded with 2 and 4 cpd FF stimuli is not recorded with HF stimuli.
TABLE I Latencies (msec) and amplitude (/~V) of VEPs recorded on posterior leads to FF stimuli. 1 cpd
2 cpd
4 cpd
N1
P1
N2
N1
P1
N2
86+4.7 83+4.2 865:3.8 825:3.8 835:4.8 835:3.6 815:3.7
112+6.8 111+5.4 1105:5.8 1105:5.9 1095:6.2 111+5.9 1105:6.1
156+7.6 155+7.8 1575:7.7 1545:7.9 1565:7.8 1545:7.4 1545:7.3
86+4.6 88+4.3 895:4.2 885:4.4 845:4.7 87+3.6 885:3.8
112+7.2 112+6.2 110+6.4 110+6.3 1095:7.4 1125:5.3 1135:5.4
162+ 9.9 167+10.3 1635:9.1 1665:8.4 1635:8.2 1655:9.8 166+ 8.7
2.0+1.4 3.8+2.1 5.45:2.6 3.95:2.1 1.9+1.4 6.25:2.8 6.35:2.9
2.25:1.2 5.35:1.9 7.45:2.2 5.4+1.8 2.25:1.2 8.15:2.8 8.25:2.8
1.25:0.8 2.8+1.7 3.15:1.5 2.8+1.7 1.15:0.7 3.75:1.6 3.85:1.6
1.65:1.3 2.8+1.6 3.9+1.5 2.8+1.6 1.75:1.2 4.65:1.7 4.5-t-1.8
1.9+0.7 3.7+1.3 6.15:3.2 3.75:1.3 1.95:0.7 5.4+1.6 5.6+1.5
N1
P1
N2
91+4.4 915:4.2 935:4.3 905:4.2 855:4.7 91+3.2 905:3.1
113+5.8 1165:4.3 119+4.1 1185:4.2 1165:5.1 118+3.8 1185:3.9
166+ 8.2 1645:9.8 1695:9.8 1675:9.7 1645:9.9 1665:9.9 166+10.1
1.3+0.9 3.95:1.8 4.05:1.8 3.95:1.7 1.4+0.9 4.55:1.8 4.5+1.8
1.4_+1.2 1.85:1.2 2.6+1.2 1.95:1.1 1.5+1.3 3.4+1.6 3.45:1.6
2.15:1.3 2.4-t-1.3 3.7+1.3 2.4+1.2 2.25:1.4 3.95:1.8 4.15:1.7
Latency T5 P3 Pz P4 T6 O1 02
Amplitude T5 P3 Pz P4 T6 O1 02
0.5_+0.6 1.35:1.1 1.8+1.6 1.3+1.1 0.65:0.7 2.85:2.6 2.7+2.5
VEP MAPS TO 1, 2, 4 C / D E G P A T T E R N E D S T I M U L I
171
Contralaterally to the stimulating HF, 3 deflections (polarity reversed as compared to N1-P1-N2) are recorded which will be marked as C (contralateral) cP1, cN1, cP2, and have largest amplitudes over T5-6 and O1-2 leads and are recorded also in contralateral parietal and central derivations. Table II reports latency and amplitude measurements of N1, P1, N2 and cP1, cN1 and cP2 recorded with H F stimuli of the 3 tested spatial frequencies. Since Student's t comparisons of latencies and amplitudes of matched ipsi- or contralateral components to L and R H F stimuli were not significant the data obtained with H F stimuli are presented after pooling. On color maps of FF GA VEPs (Fig. 4) a polarity inversion of N1, P1, N2 is observed anterior to the central coronal row, for stimuli of 1, 2 and 4 cpd. Therefore the inspection of traces to F F 1 cpd stimuli cannot identify anterior components like those observed with 2 and 4 cpd VEPs, whereas antero-posterior polarity gradients can be observed in maps also with 1 cpd stimuli. Maps of H F VEPs (left or right) show a transverse inversion of polarity with the same polarities on each hemisphere (Fig. 4). Independently of the element size, N1 to H F stimuli has larger amplitudes in ipsilateral temporo-parietal
leads than N1 recorded with F F stimuli. The ipsilateral distribution of N1 is clearest with 2 and 4 cpd H F stimuli. The contralateral (cP1) polarity reversed potential could look like a separate component, since the sequence of maps and traces of Figs. 1 - 4 shows that the peaks of N1 and cP1 potentials do not correspond, to the point that in the maps of Fig. 4 the contralateral posterior positivity (cP1) is recorded when the ipsilateral N1 has already disappeared from posterior leads. The map of the whole scalp shows, however, that cP1 is now faced by a negativity recorded at the coronal midline level. N1 and cP1 potentials are also recorded with 1 cpd H F stimuli, and since the N1 to 1 cpd F F stimuli is small the hemifield stimuli apparently facilitate the appearance of N1 to 1 cpd patterns. The peak latencies of N1 recorded with H F stimuli are 2-8 msec shorter than N1 peaks to FF stimuli, at all the tested spatial frequencies (Figs. 1-4). P1 is recorded mostly ipsilateral to the hemifield but, more than N1, spreads over contralateral leads. With 1 cpd stimuli an amplitude attenuation of P1 rather than a polarity reversal is recorded over contralateral parieto-occipital derivations. When 2 and 4 cpd stimuli are used a polarity reversal, identified at the peak as cN1 component, of P1 is instead recorded, though not
4cpd RF F7 ~
sum
LF+RF
FF+. um
~
Fz F8
Cz
1'4 T6
Fig. 3. G A VEPs to 4 cpd stimuli. Same presentation as in Figs. 1 and 2. Notice that polarity reversed peaks matching the N1-P1-N2 peaks (vertical arrows) are recorded in anterior derivations with F F stimuli. Notice also that the L F - R F s u m does not reproduce the wave shape recorded in anterior leads with FF stimuli.
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M. O N O F R J ET AL.
lcpd
lVF N83
Pl18
N167
LF N78
cPl92
P1112
P-cN136
N154
RF N181
Pill7
P-cN128
N146
N157
2cpd
FF P67
N87
PlI5
P143
N168
LF -0
N, 85
cP197 P l - c N l l l 4 Ne 138
cPa 146
N172
Rlr ÷
N~ 84
cPl94 Pl-cNll15cl~139
N154
N168
4cpd P68
NI 93
P1 120
P152
Na 174
LF N, 87
P, 117
P136
N148
N170
RF N191 P I - c N I l l 8
P136
Ne148
N168
VEP MAPS T O 1, 2, 4 C / D E G P A T T E R N E D S T I M U L I
173
T A B L E II Latency in msec. Amplitude in #V. Measurements of N1, P1, N2 and cP1, cN1, cP2 recorded with hemifield stimuli of the 3 different spatial frequencies, from the posterior derivations. Ipsi indicates derivations ipsilateral to the hemifield; Contra indicates derivations contralateral to the stimulated hemifield. 1 cpd
2 cpd
4 cpd
T
P
O
Pz
T
P
O
Pz
T
P
O
Pz
80±3,9 109±5.6 136±6.8 87±5.1 118±4.6 138±5.3
79±3.8 108±5.1 136±6.9 88±4.9 114±4.4 136±6.1
79±3.8 110±5.9 137±6.1 88±4.2 116±4.3 132±5.7
80±4.0 110±5.2 138±5.7 85±3.9 109±4.2 1~±5.2
84±3.7 111±5.1 138±3.6 88±4.3 112±4.6 132±4.8
84±3.8 110±6.1 134±4.2 90±4.8 113±4.9 134±4.7
83±3.9 109±6.2 138±4.9 89±4.7 110±5.0 131±4.6
84±3.9 110±6.0 139±4.4 85±3.7 112±4.6 133±4.1
87±3.8 112±4.8 1~±4.4 86±3.8 114±4.3 139±4.3
87±3.9 111±4.2 1~±4.7 88±4.1 111±4.7 1~±4.6
87±3.9 112±4.1 139±4.8 86±3.6 113±4.6 137±4.5
86±3.8 113±4.4 1~±4.6 87±4.1 112±4.4 139±4.2
1.2±0.7 1.3±0.9 1.8±0.9 0.9±0.3 0.6±0.3 0.8±0.3
1.5±0.9 3.3±1.8 4.2±1.8 0.8±0.4 0.7±0.3 0.6±0.3
2.6±1.7 4.8±2.1 5.9±2.2 0.9±0.5 0.5±0.4 0.7±0.3
1.2~0.8 3.6±2.3 5.7!1.9 0.3~0.2 0.5±0.3 0.5±0.4
1.8±0.9 1.2±0.8 2.3±0.7 2.6±0.6 1.4±0.4 2.4±0.4
2.9±1.9 2.3±1.6 3.0±1.4 2.1±0.4 0.6±0.3 0.7±0.2
3.3±1.5 3.6±1.6 4.1±1.7 2.9±1.0 0.7±0.3 0.9±0.4
2.9±1.4 3.4±1.3 4.3±1.9 0.9±0.4 0.6±0.2 0.6±0.3
2.1±1.1 1.2±1.0 1,6±0.9 1.9±1.2 0.9±0.4 1.6±0.7
3.8±1.7 1.3±1.0 2.1±0.9 1.2±0.8 1.1±0.4 1.3±0.4
4.2±1.6 2.8±0.8 3.1±1.2 1.8±1.3 1.6±0.5 1.5±0.3
3.1±1.6 1.8±0.9 2.1±0.9 0.9±0.6 1.1±0.4 1.1±0.4
Latency lpsi
Contra
N1 P1 N2 cP1 cN1 cP2
Amplitude Ipsi
Contra
NI P1 N2 cP1 cN1 cP2
T A B L E III The table reports, in percent values, in how m a n y control subjects the different VEP components could be identified. From the table it is evident that in some controls the ipsilaterat or contralateral components could spread to the opposite derivations. In some derivations the responses to full-field stimuli could consist of components with the same polarity as the contralateral components recorded with H F stimuli. The discrepant percentage of ipsilateral or contralateral component identification depends on the fact that in some controls none of the components could be identified inside their latency ranges, reported in Tables II and III. F r o m the table it should also be evident that in some controls VEPs could be asymetrically distributed, i.e., presence of 'contralateral' components to F F stimuli (and mostly when the stimulated spatial frequency was 1 cpd). 1 cpd
FF
H F ±psi
H F contra
HF
4 cpd
N1
P1
N2
cP1
cN1
cP2
NI
P1
N2
cP1
cN1
cP2
N1
P1
N2
cP1
cN1
cP2
T5 P3 Pz P4 T6 O1 02
3t 42 56 43 32 58 60
62 96 100 98 63 98 96
78 84 100 86 82 82 80
16 4 3 14 4 6
13 2 12 4 4
31 12
71 94 100 92 68 98 98
64 92 100 88 64 98 100
18 16 2 2
21 2 2 26 2 2
33 36 10 33 2 -
84 98 100 98 88 98 98
80 98 100 98 82 96 98
76 96 100 94 74 98 98
14 -
11 29 15 17
74 92 100 94 76 96 98
12 2 2
18 2 16 4 2
24 2 4 24 2 2
T P O
86 84 84
100 98 100
96 98 100
-
2 -
2 2 -
100 100 100
100 98 98
96 92 100
-
2 2
4 -
100 100 100
98 96 100
96 96 98
2 2 -
-
-
-
T P O
34 38
42 46 48
32 34 48
78 64 62
64 52 50
66 64 48
6 2
8 24 22
6 22 24
100 94 98
90 76 78
86 72 75
4 6
8 22 24
20 22 20
100 96 94
92 76 78
78 74 78
P2
68
94
92
28
4
6
92
96
88
6
2
14
91
94
86
4
8
8
The
peak
stimuli
has
as clear-cut
as N1.
stimuli are 2-12
1-4).
2 cpd
msec shorter
latencies than
of P1
P1 to FF
to HF
P1 (Figs.
The
N2
ipsilateral
component
to HF
shorter latency than the peak of the broad with
FF.
In VEPs
to 4 cpd
(LF)
and
a
N2 recorded
2 cpd
t h e ±psi-
Fig. 4. Color maps of polarity distributions of the different VEP components to FF, LF and R F patterns of 1, 2, 4 cpd. The calibration is 3.2 /~V, blue, positive and red, negative. The latency of measured polarity gradients is reported below each map. Notice that N1-P1-N2 components to LF and RF have shorter latencies than to F F stimuli. In FF VEPs the polarities are antero-posteriorly oriented. LF and R F maps show the predominant ipsilateral distribution of N1 and P1 components, with polarities oriented transversely (sideways) on the scalp. Notice that ±psi- and contralateral potentials can have peaks with unmatched latencies (as reported below the maps). Yet polarity reversed counterparts of each component can be observed within the scalp. Notice also that P1 to 1 cpd stimuli is widespread. The first maps in the top rows to FF stimuli of 2 and 4 cpd ~timuli show the distribution of tile early positivity (Po) preceding N1 to FF stimuli.
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M. O N O F R J ET AL.
lateral N2 is faced by the contralateral positivity cP2. The latencies of ipsilateral N2-contralateral cP2 to 2 and 4 cpd match the latency of notches evident on O1-O2 derivations on the broad slope of F F N2 (Figs. 2-4). GA traces to 1 cpd H F stimuli show that P1 is followed by a cP2 component recorded over contralateral temporal derivations, while the negativity N2 is widespread and the only identifiable peak has a latency approximating that of F F N2. A late negativity with a peak latency matching that of FF N2 is also recorded with 2 and 4 cpd H F stimuli. This latest negativity is distributed either contralaterally (2 cpd, 4 cpd RF) or ipsilaterally (4 cpd) or is widespread (1 cpd). Since FF N2 and H F N2 do not match, the F F N2 appears to be the summation of the ipsilateral N2 component, distributed like P1, and the latest negative peak. When VEPs t'0 right and left H F stimuli were summed (Figs. 1-3), the distribution over posterior leads was similar to the distribution of FF VEPs (Fig. 1). However, the peak latencies of N1 and P1 obtained by summation were shorter by 2 - 9 msec than N1-P1 to real FF stimuli ( P < 0.05-0.001). Moreover, none of the components recorded in anterior leads with FF stimuli could be observed. Table III reports the percentage of controls in whom the different components could be identified over different derivations, to F F and H F stimuli of the 3 spatial frequencies, and shows that single VEP components were not identified in all controls.
The early positivity (Po) preceding N1 to 2 and 4 cpd FF stimuli was only recorded in 23 controls (42 eyes, 42%). Polarity reversed counterparts of posterior VEP components were recorded as identifiable peaks in 65% of controls with 1 cpd F F stimuli, in 85 and 90% with 2 and 4 cpd F F stimuli. Table IV reports amplitude ratios of the different VEP components recorded with the 3 different spatial frequencies from symmetrical posterior leads. The amplitude ratios of FF VEP components recorded from corresponding lateral derivations ranged from 1 / 1 to 2 / 3 in 76% of controls to 1 cpd stimuli, in 84% to 2 cpd stimuli and in 92% to 4 cpd stimuli. The amplitude ratios of the N1-P1-N2 components and cP1, cN1 and cP2 components recorded over the two different hemispheres with H F stimuli were 1/1 to 2 / 3 in 74% of controls to 1 cpd stimuli, in 82% to 2 and 4 cpd stimuli. The comparison of H F VEPs vs. FF VEPs showed that N1 amplitude to 2 and 4 cpd H F stimuli was significantly larger over temporo-parietal derivations ( P < 0.05), and significantly smaller in occipital derivations ( P < 0.02). F F P1 was always larger than H F P1. There was no significant difference between matched VEP components to right and left H F stimuli of left and right eyes. As on GA traces, the single components recorded in individual controls to F F and H F stimuli and their polarity reversed counterparts were found to have matching or partly shifted peak latencies over the different scalp derivations.
T A B L E IV Amplitude ratios of the VEP components recorded from posterior derivations in different stimulus conditions. 1 cpd
FF (R vs. L derivations) FF vs. H F (L vs. L, R vs. R) HF (ipsilateral derivations R vs. L)
HF (contralateral derivations R vs. L)
HF (contralateral vs. ipsilateral derivations)
O P T O P T O P T
O P T
O P T
2 CPD
4 cpd
N1
P1
N2
N1
P1
N2
N1
P1
N2
97_+22 91+26 99_+21 64_+24 63-+16 91_+19 99 _+22 98 _+24 98 _ 24
99+21 90-+23 97_+21 52_+21 63+22 64_+12 97 _+23 97 _+24 98 -I-27
89-+22 100-+21 91_+24 49_+16 58_+14 62_+18 98 _+24 99 _+23 98 -+ 22
99-+22 100_+24 90_+26 82_+18 80_+21 92_+20 99 _+28 97 _+26 98 -+ 25
91-+22 101_+23 96_+22 64+21 65_+22 73_+12 98 _+24 98 _+26 96 -+ 24
100-+21 98_+22 98+21 74_+21 64_+24 67_+11 98 _+26 99 _+23 96 _+23
99_+21 101_+22 96-+24 85_+12 83_+17 92_+16 99 _+24 96 _+24 95 _+21
94_+26 97_+28 98_+28 64_+16 73_+12 84_+15 98 _+29 96 + 24 98 _+27
94_+22 99_+26 99_+24 68_+14 66_+12 72_+21 98 _+27 96 _+24 98 _+26
cP1
cN1
cP2
cP1
cN1
cP2
cP1
cNl
cP2
98_+21 98_+26 99 -+ 27
96_+22 94_+22 92 -+ 24
98_+22 99-+27 97 _+23
99_+19 99+21 98 + 22
cP1-N1
cN1-P1
cNI-P1
cP2-N2
61_+21 73 _+12 76-+14
48_+19 61 _+16 72_+15
80-+21 84 -+ 12 92-+24
72_+12 76 + 20 78_+16
96+25 99_+26 97 _+27 cP2-N2 54_+16 66 _+ 18 74+_13
97-+26 99_+24 93 _+22 cP1-N1 90_+18 92 -+ 19 98-+21
96-+21 98_+21 98 _+25 cN1-P1 84_+16 86 _+ 14 93_+21
99+23 98+28 98 +_27 cP2-N2 79_+10 81 + 14 82_+13
99_+26 98-+28 99 + 25 cP1-N1 90_+22 93 _+21 99-+25
VEP MAPS TO 1, 2, 4 C / D E G PATTERNED STIMULI
175
The mean data with S.D.s reported in Tables I - I V imply that some differences in comparison with the G A patterns of distribution were observed in a minority of controls. These differences, in individual controls, i.e., ' n o r m a l variants,' were mostly confined to the use of coarse (1 cpd) stimuli and consisted of asymmetries of FF VEP distributions absence of some of the components and bifid ('w-shaped') components. The VEP 'normal variants' were identical in tests of either eye. The N1 component was recorded only in 60% of normal controls when 1 cpd FF stimuli were used, but in all normal controls when 2 and 4 cpd FF stimuli were used.
The N1 distribution to FF patterns was asymmetrical (ratio between corresponding leads below 1 / 2 ) in 16% of 1 cpd VEPs, in 8-10% of 2 and 4 cpd VEPs. The N1 amplitude ratio over the two sides of the scalp was below 1 / 2 in 4 subjects (8%) to 1 cpd FF stimuli, in 2 subjects to 2 cpd stimuli. In 2 controls (4%) the asymmetry of FF VEPs was such that N1 components to 1, 2 and 4 cpd stimuli were recorded only on one side of the scalp and midline, and a positivity (cP1, 1 / 5 of N1 amplitude) was recorded on the other side. H F stimuli elicited lateralized N1 distributions in 86% of controls with 4 cpd patterns, in 88% with 2 cpd
B
I
cpd
FF LF RF
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~
P3
P3
Pz
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®
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FF
02
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~ / ~ ' ~
02
LF
rg
02
I}
RF
Fig. 5. VEP distribution variants in a normal control. Time scale 200 msec. Scale of map colors as in Fig. 4. In 1 cpd VEPs the N1 component is not recorded when FF stimuli are used, and P1 has, in several derivations, a bifid, or 'w,' wave shape. P1 has an early peak at 118 msec and a second peak, of its 'w' shape, which is predominant in temporal derivations. In 1 cpd LF VEPs the N1 component can be identified with the contralateral cPl component. 1 cpd RF VEPs are instead bifid and widespread. N1 to 4 cpd FF stimuli is symmetrical. In H F 4 cpd VEPs the N1 components are ipsilateral to the stimulating hemifield, and contralateral cP1 activities can also be identified. The P1 wave is widespread to LF stimuli and is ipsilateral to RF stimuli. The late negativity (at 164-172 msec) is contralateral to both LF and RF stimuli.
176
patterns, in 78% with 1 cpd patterns. In the other controls (i.e., 14%, 8%, 22% respectively for 4, 2 and 1 cpd stimuli) the N1 components to H F patterns spread also over contralateral derivations. Even in the 4 controls having anomalous asymmetries of N1 to 1 cpd F F stimuli, the H F stimuli evoked N1 components over the same side of the scalp. In these 4 controls ipsilateral N1 was still 1 / 3 smaller on one side than on the other. P1 was recorded in all controls with the 3 tested spatial frequencies. The P1 component recorded at the midline with FF stimuli had a bifid or 'w-shaped' wave form in 24% of controls with 1 cpd and in 12% and 4% with 2 and 4 cpd. Asymmetries of P1 distribution to F F stimuli (ratio below 1 / 2 ) were observed with 1 cpd patterns in the same controls having asymmetrical N1 distributions, and in two more. P1, more than N1, spread to the midline and contralateral parietal derivation when H F stimuli were used. Widespread P1 components were obtained in 46% of controls to 1 cpd H F stimuli, in 30% of controls to 2 and 4 cpd H F patterns. Furthermore, in 2 controls (4%) the distribution of P1 to 1 cpd H F stimuli was prominent in contralateral derivations, whereas N1 was normally ipsilaterally distributed. N2 to H F stimuli was usually widespread: an ipsilateral N2 component, with a contralateral cP2, was observed in 54% of controls when patterns were 2 and 4 cpd, in 46% to 1 cpd patterns. In the remaining controls N2 was widespread or no distinction could be made between the 'early' and 'late' N2 deflections. An asymmetry or absence of N1 and bifid P1 wave shape coexisted in 84% of ' a n o m a l o u s ' F F VEP distributions. The use of H F stimuli clarified the ' a n o m a l o u s ' distribution, since ipsilateral N1-P1-N2 components appeared or were enhanced in temporo-parietal derivations where the responses to F F stimuli were of small amplitude. Fig. 5 shows examples of coexisting anomalies in one control. V E P s to "central" or "annular' stimuli
In all controls ipsilateral N1-P1-N2 and contralateral cP1, cN1 and cP2 components were recorded to 1, 2 and 4 cpd 12 ° H F stimuli. Fig. 6 reports comparisons of G A traces and maps of VEP recordings performed with 'central' and 'annular' stimuli of 1 and 4 cpd. The comparison of maps of different components (Fig. 6) shows variations of the distribution, yet polarity reversed counterparts could
M. O N O F R J ET AL.
always be recorded either tangentially oriented over the coronal rows of electrodes, or obliquely or antero-posteriorly. Fig. 7 shows amplitude plots of ipsi- and contralateral components to 1, 2 and 4 cpd patterns, recorded from temporal and occipital leads with 3, 6 and 12 central stimuli and annular stimuli with 3 ° and 6 ° central occlusion. With 3 ° central hemifields VEPs to 2 and 4 cpd patterns consist of ipsilateral N1-P1-N2 and contralateral c P l - c N l - c P 2 components at posterior derivations. In comparison with VEPs to 12 ° H F the amplitude decrement over the different derivations is 50% for both ipsi- and contralateral activities. In 7 controls VEPs to 1 cpd 3 ° central H F stimuli consisted only of a widespread P1 component, 50% smaller than P1 to 12 ° HF. In 8 controls all ipsi- and contralateral components could be identified, the same as with 2 and 4 cp~] stimuli. The G A of Fig. 6 shows small amplitude but identifiable contralateral activities. The distributions of VEPs obtained with 6 o H F or annular H F with 3 ° occlusion are scarcely different from the distribution of VEPs obtained with 12 ° HF. Amplitudes of ipsi- and contralateral components are reduced by 10-30%; only P1 amplitude, to hemiannular 3 ° occluded stimuli, is reduced by more than 40%. When the central 6 ° portion of the 12 ° H F is occluded the response over occipital derivations consists of widespread cP1, cN1 and cP2 activities: their amplitudes are 50% smaller than contralateral activities recorded with un-occluded 12 ° H F and polarity reversals and ipsilateral components can be recorded over temporal and anterior derivations. Fig. 6 shows identifiable ipsilateral components, and the mean amplitude plots of Fig. 7 show the reduction of amplitude of all ipsilateral components to hemiannular stimuli. In 5 controls the prominence of contralateral activity was not observed, and also with 6 ° occlusion, VEPs consisted of ipsi- and contralateral components. The amplitude reduction of VEPs recorded over the different leads was significant ( P < 0.05-0.001) for VEPs to 12 ° H F stimuli vs. 3 ° central H F stimuli and vs. annular stimuli with 6 o central occlusion ( P < 0.010.0001) for all ipsi- and contralateral components. With
Fig. 6. 1 and 4 cpd VEPs to 3 °, 6 ° , 12 ° HF; hemiannular 12 ° H F with 3 ° and 6 ° central occlusion. Calibration is 5/~V for central rows, 3/tV for the left and right rows. Color scale is 1.6 gV for lateral maps and 3.2 /~V for central maps. The vertical lines mark N1-P1 ipsilateral potentials (mapped below). Notice that N1 to 4 cpd stimuli is recorded with 3 °, 6 o, 12 o and 3 o occluded H F stimuli. With 6 ° occlusion the cP1 component is predominant in contralateral derivations but negative counterparts can be observed in traces and maps and its amplitude is decreased rather than increased. P1 amplitude increases when the size of the stimulating field is increased from 3 ° to 12 ° and drops when the 3 central degrees are occluded.
VEP MAPS TO 1, 2, 4 C / D E G PATTERNED STIMULI
177
4cpd
•f•T5
P3 Ol
o2
~
0
pt T6
lcpd T5 P3 O1
L
4-
f
~
~ ~
02 P4
T6
178
M. O N O F R J ET AL.
I cpd
2
epd
4 cpd
---
cP~
---
P1 cN2
T
0
,
4o
4°
0
4°
Fig. 7. Mean amplitude ratios of N1, cP1, P1 and cN2 components recorded with central or occluded HF. 3c, 6c, 12c indicate the visual angle subtended by the central (c) stimuli, 3 co, 6 co indicate 3 ° and 6 ° central occlusions of the 12 ° HF. The amplitude of the same components recorded with the 12 ° H F is taken as 100%. Standard deviations ranged from 7% to 16%. Measurements are reported for responses to 1, 2 and 4 Cpd stimuli, and for responses recorded from matched ipsilateral and contralateral occipital derivations.
6 o central H F stimuli only the amplitude reduction of VEPs recorded from temporal and occipital derivations was significant ( P < 0.05, 12 ° vs. 6 ° H F stimuli). With 3 ° occlusion only the amplitude of P1 at Pz and O 1 - 2 was significantly smaller than P1 amplitude recorded with 12 ° H F ( P < 0.01 for 1 cpd, P < 0.05 for 2, 4 cpd stimuli). As for the 12 ° H F stimuli, in VEPs obtained with central or annular H F patterns the peak latencies of ipsi- and contralateral components were not matched across the different leads, as can be seen in Fig. 6. The latency differences were inconsistent across controls: only N1 latency was consistently shorter when the 3 ° central part was occluded ( P < 0.02). The evaluation in single subjects of m a p distributions did not show consistent prominence of any of the contralateral VEP components in any of the different recording conditions.
Discussion
The present paper shows that F F responses consist of a sequence of N1-P1-N2 components with a symmetri-
cal distribution over posterior derivations and that polarity reversed counterparts can be identified over anterior derivations, either in maps (1, 2, 4 cpd stimuli) or, as polarity reversed peaks by inspecting VEP traces to 2 and 4 cpd stimuli (Figs. 1 and 4). When right or left H F stimuli are used the polarity gradients rotate on one side: N1-P1-N2 are distributed ipsilaterally, i.e., on the same side as the stimulated visual field, and polarity reversed activities (cPl-cN1cP2) are distributed on the side of the unstimulated visual field (Figs. 1-4). The N2 potential results from the sum of a first negativity immediately following P1 and having the same distribution as N1 and P1, and a late negativity, widespread or contralateral to the stimulating hemifield. N1 can be preceded by an early positivity (Po), but this potential was recognized only in half of the controls when FF stimuli were used, and was absent in the majority of controls when H F stimuli were used. Stok (1986) also noticed a similar potential but found it inconsistent. The present paper shows furthermore that the distribution of different components is sensitive to variations of the spatial frequency. All the components, ipsi- and
VEP MAPS TO 1, 2, 4 C/DEG PATTERNED STIMULI contralateral, are best identified when 2 or 4 cpd stimuli are used. N1 is often absent when 1 cpd F F stimuli are used (Figs. 1, 4, 6 and 7). The N1 recorded with 2 and 4 cpd H F stimuli is lateralized more often than P1 and N2 (Figs. 1-4, 6, 7 and Tables I-III). Since N1 is paralleled by a contralateral positivity, cP1, N1 could be identified as the lateralizing and reversible (because of the contralateral positivity) component of VEPs. Recently Paulus et al. (1988) have recorded with colored patterns a negativity at 80 msec (similar latency as N1 in our study) and showed that this component was ipsilaterally distributed when H F stimuli were used. They suggested that this component could allow recognition of field defects. Our study shows that a lateralizing negativity at 80-90 msec (N1) can be recorded with black and white stimuli, if the size of pattern elements is appropriate (2-4 cpd). P1, which is currently used in diagnostic VEP measurements, is, among the ipsilateral components, the one to show the greatest tendency to spread diffusely over posterior derivations, mainly when 1 cpd H F stimuli are used (Figs. 1-4, 6, 7 and Table III). The evaluation of the variability of VEP components in controls showed that P1 is the least sensitive component to VEP lateralization. All in all the use of spatial frequencies above 1 cpd is advantageous in VEP diagnosis because with 2 and 4 cpd stimuli the lateralization of hemifield VEPs is enhanced either because N1 is distributed only on one side of the scalp or because all the components show a specific lateralization. A recent report showed that in patients with retrochiasmatic visual field defects the highest diagnostic yield was obtained with stimuli of 2 and 4 cpd (Onofrj 1990). Our study conducted with 19 derivations placed over the entire scalp shows that any positivity or negativity of VEPs has a counterpart of opposite polarity which is always within the map of the scalp (Figs. 1-4, 6 and 7), and this also when small central H F or hemiannular stimuli are used. In our study occlusion of central or peripheral parts of the stimulating hemifield could enhance or reduce the amplitude ratio between ipsi- and contralateral components though never leading to complete disappearance of N1-P1-N2 or cPI-cNI-cP2 (Fig. 7). Ipsilateral N1-P1-N2 were recorded in our studies with stimuli as eccentric as 6 o from the fovea, and cPl-cN1cP2 were recorded with H F stimuli covering only the central 3 ° . When annular stimuli occluding the central part of the visual field were used the contralateral cPl-cNl-cP2 components were prominent in posterior leads, but their amplitudes were smaller than when stimuli were presented also to the central field (Fig. 7), and the polarity reversed counterparts were evident in anterior leads (Fig. 7).
179 It is not surprising that the anterior counterparts of VEPs were not described accurately by other researchers since their studies were conducted with a small number of derivations placed only around the occipital pole and referred to the midfrontal position (Blumhardt et al. 1978, 1982, 1989; Wood 1982; Skrandies 1984; Butler et al. 1987). Only one previous study (Mauguirre et al. 1984) described anterior counterparts of VEPs. A study of Darcey et al. (1980) also described maps of VEP distribution over the entire scalp, but their results cannot be compared with the present ones since these authors were using onset-offset (pulse) presentation of patterns, and the distribution of onset-offset VEPs is profoundly controversial (for a review of the literature see Onofrj 1990). The maps shown in the present study (Figs. 4, 6 and 7) suggest that it is difficult, if not impossible, to separate contralateral components (paracentral?, parafoveal?) from the activity which might correspond simply to the polarity reversal of the ipsilateral VEPs. Moreover, if one accepts the idea that parafoveal retinal receptors are more likely to be stimulated by low (1-2 cpd) spatial frequency stimuli (Hubel and Wiesel 1968; Campbell and Maffei 1970), one might expect greater chances of identifying a paramacular VEP complex (i.e., the contralateral VEP components) with low (1 cpd) than with high (4 cpd) spatial frequency stimuli. Ipsiand contralateral VEP components were instead recorded in our study with all the tested spatial frequencies (Figs. 1 - 6 and Tables I-IV). Based on the maps, a simple model of VEP generators can be suggested: VEPs elicited by pattern reversal stimuli should be organized as a sequence of N1-P1-N2 components at the surface of the visual cortex; on the side of the scalp facing the inner surface of the visual cortex the recorded potentials will be polarity reversed (cPl-cNl-cP2). The prevalence of ipsi- (N1-P1-N2) or contralateral (cPl-cNl-cP2) components over anterior, posterior or lateral leads of the scalp can be explained by the orientation of the dipole sheet, which will be mostly radial if the afference is mostly foveal, thus projecting to the tip of the occipital pole, or tangential (medial surface of the occipital pole, where foveal, parafoveal, paracentral afferences arrive) or even oblique, if the afference is to a part of the visual cortex folded inside lateral sulci (Polyak 1957) of the calcarine fissure. With F F stimuli the activities of VEP generators located inside the medial occipital sulcus will mostly annul each other because they are facing: VEP maps, comparably to our findings, will have antero-posterior reversals of polarity. The model can also explain why the peak latencies of components do not match in the various leads and why the sum of H F VEPs does not result in the same VEP distributions and latencies as FF VEPs. In fact in our
180 s t u d y , in o r d e r to r e d u c e f i x a t i o n v a g a r i e s , t h e f i x a t i o n p o i n t was at 0.5 o f r o m t h e H F , w h i l e F F s t i m u l a t e d t h e e n t i r e fovea, As a concluding remark we emphasize that variants o f the n o r m a l V E P s a r e f o u n d m o s t l y w h e n c o a r s e p a t t e r n s t i m u l i a r e used, a n d d i s a p p e a r w i t h 2 o r 4 c p d p a t t e r n s . T h i s f i n d i n g has c l i n i c a l a p p l i c a t i o n s f o r t h e interpretation of abnormal VEP distributions. A few p r e v i o u s r e p o r t s ( O i s h i et al. 1985; O n o f r j et al. 1987; N o v a k et al. 1988) h a d a l r e a d y s h o w n t h a t a b n o r m a l VEP wave shapes were observed with coarse stimuli, while with finer patterns, normal wave shapes with i d e n t i f i a b l e c o m p o n e n t s w e r e o b s e r v e d in the s a m e s u b jects.
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