Contrast evoked responses in man

P’ision Res. Vol. 13. pp. 1577-1601. Pcrgamon

Press 1973. Printed in Great Britain.

H. SPEKREIJSE, L. H. VAN DERTWEEL and TH. ZUIDE~MA Laboratory of Medical Physics, University of Amsterdam, Amsterdam (Received 1 Januur~ 1973)

THE YISUALLYevoked occipital responses in man to sinewave modulated light often show considerable harmonic distortions. One of the most striking effects observed for subjects with pronounced a-activity (i.e. preponderance at about 10 Hz in the spontaneous EEG) is the appearance of a strung second harmonic component for stimuIus frequencies of about half the a-frequency. This frequency doubling occurs even for stimuli with modulation depths as low as I per cent, so that it cannot be due to curvature of the amplitude characteristic (e.g. a saturation or a logarithmic characteristic). We therefore suggested (SPEKREIJSE, 1966) a quite different explanation for the frequency doubling (second ha~o~ic generation). Our hypothesis was that there is a rectifier-like stage iu the human visuai pathway (see also CLYNES, KOHN and LIFSHITZ,1964). Such a stage would have the property of generating signals at twice the stimulus frequency, even for very weak stimuli. This property is lost, however, if the stimulus is contaminated by noise (SPE~~~JS~ and ~~s~~~G, 1970). Due to the quanta1 character of light, noise is inherent to a visual stimulus. Estimation of the magnitude of the quanta1 noise per receptor indicates that it overpowers the modulation at depths, where there is still considerable distortion in the response. Hence it was also necessary to postulate that a kind of spatial summation precedes the r~ct~er-cake stage which improves the signal to quanta1 noise ratio sufficiently to allow the appearance of second harmonics for modulation depths as low as 1 per cent (VAN DER TWEEL, 1961). Without such spatial summation quanta1 noise and also noise of neural origin, feeding in at a site preceding the rectifier, would have strongly reduced the second harmonic component even at higher modulation depths (SCHELLARTand SPEKREIJSE,1973). With the aim of estimating the average size of these “electrophysiological” spatiat summation fields, experiments were performed with a ch~kerboa~d patterned stimulus. If the checks are considerably smaller than the hypothesized peripheral summation fields no response at all should be found when the two sets of checks are modulated in counterphase. In this case the flicker signals from adjacent checks would cancel before reaching the rect~ying stage (first column Fig. 1). On the other hand, fur compa~a~vely large checks, the second harmonic should resemble that elicited by an unpatterned stimulus field. Note that with this kind of counterphase stimulation the average illumination of the eye remains constant. As can be seen in the second col~rn~ of Fig. 1 this approach proved fruitful in the analysis of spatial interaction in the goldfish retina (SPEKREIJSE and VANDENBERG, 1971). Counterphase checkerboard stimulation within the receptive field of a ganglion cell in this retina did not result in a spike discharge at all, This means that the goldfish retina treats each patch of light in the same way imspective whether neighbou~~g patches are modulated in phase 1577

1578

H. SPEKREIISE,L.H.VA.V DERTWEELASDTH.ZLJIDEMA

Model

Goldfish

Man

FIG. 1. First column: schematic representation of spatial summation between counterphase signals generated by the two sets of counterphase modulated squares of a che&&oard pattern. No response will be obtained for the depicted case with spatial summation preceding the rectifying element. Second column: period averaged spike data for green-off centre process of a gangfion cell in goldfish retina. The top figure gives the average spike response to sine wave modulation (50 per cent) of one set of squares (0.35 mm width in the plane of the retina). The other set of squares is not modulated at all but has a steady intensity identical to the mean intensity of the modulated set. Reversing the condition and modulatingthe other set ofsquareswith the same depth, but in counterphase, results, of course, in the same rectified sinusoid but shifted 180” in phase (mid figure). Simultaneous counterphase modulation of both sets of squares does atit result in a spike discharge at all (bottom figure); an observation which suggests a location of the summing point at a stage preceding the rectifier. The mean intensity of the stimulus light passing through an Ealing F‘TP interference filter (530 nm) is approximately 4rW/cmz. The number of summations is 100; the modulation frequency is 5 Hz. The patterned spot focused on the retina has a dia 2 mm. Third column: period averaged occipital responses in man to in-phase (homogeneous field) and counterphase (switching of spatial contrast) modulation of the two sets of squares. If both responses would have the same origin, then the size of response B should be equal or smalkr than that of response A. The reverse is the case, which indicates that the two responses originate from different systems. The st~ulus field with a mean luminance of 5000 asb extends 6’; the check size is 15’; inion-vertex derivation.

(homogeneous field stimulation) or in counter phase (pattern reversal). The aIgebraically summed reactions to al1 patches falling within a receptive field determines the strength of the ganglion cell discharge. In its most simple form this luminance model predicts that the response to counter phase stimulation can never exceed the response to homogeneous field stimulation. However, for the visually evoked responses in man this seems not to be the case since counter phase stimulation resulted in a much larger evoked response than homogeneous fidd stimulation (third column Fig. I). Furthermore the characteristics of this response differ in many aspects from those to homogeneous field stimulation. This &ding indicates directly that the luminance model, which was so successful in goldfish, cannot be applied on the human EP’s to counterphase stimulatiun. The latter responses are spec5c to changes in spatial contrast. They were for the first time reported by MAWL and HARDEN(1952) and many interesting properties have since been established. A recent review can be found in the report of the workshop in Paris devoted to Progress atid Future of Spatial Contrast Research (Es~&z and R.&xJD, 1972). In many previous studies contrast stimuli were contaminated by overall changes in luminance (SPEWLMAN, 1965; REWELD, TORDOIR, HAGENOUW, LUBBERSand SPOOR,

1379

Contrast Evoked Responses in hIan

1967; HARTERand WHXE, 1968, 1970; H?LRTER,1970, 1971). This in addition to the large variety of conditions used makes it difficult to compare the results of different investigators. “Theproblem is rendered more disrupt since no way has yet been found for predicting shape and size of responses to spati~~~~structured stimuli. For these reasonswe systematically varied a number of parameters under well defined conditions with the aim of finding how contrast evuked responses originate. Although we did not succeed in building a general model, we were able to establish a number of points. METHODS Twenty healthy male subjects with normal visual acuity were used in our exponents. through most of the experiments reported in this paper.

Four of them went

The light st~u~ator is depicted in Fig. 2. It consis:s of two seciions which are combined through the ~~s~~~tter BS. Each section contains two ~depe~d~~t welt co~tr~~~ed (withy O-I “/,>~uo~es~ent tube light sources (S, and Sr or S3 and S,) which are viewed through an oblique patterned mirror (M)* The spatial contrast stimulus is formed by the reflecting and transparent elements of this mirror. Most of our experiments are performed without the beamsplitter i.e. only half the stimulator is used. Simultaneous use of both sections was restricted to the experiments with superimposed contrasty patterns: e.g. a high contrast steady pattern outIi~ng the borders of a modulate bar or ch~kerbo~d pattern. We prefer a checkerboard pattern as spatial contrast stimulus because an analysis of e.g. the influence of the size of the pattern elements is best performed if the shape of the elements fits the generally assumed

Apwarance Dqxxarance

FIG.2.

Apcwam

i:appearance

eke for generating spatial contrast stimuli_ $4, and MI patterned mirrors; BS ~amspIitter; S1, Sr, Sj, Sr ei~tro~~a~iy controlled fluorescent tube light sources; FS fieldstop. Figure t{a)-(d): various spatial contrast stimuli as a function of the temporal square-wave modulation of one set of light sources (S, and S, or S, and S4).

1580

H.

SPEKREUSE,

L.

H. VANDERTWEEL ASD TH. &~.D.EM.~

Circular shape of the intelpative or receptive fieIdsL In a study of two cases of defective vision this concept PrOVGd fruitfui ~.U?i DER -h%?x and SPEKREI.BE, 1973; SE’E~XEE, &-lOE and VAN DER TWFEL, 1972). Ehr patterns for example behave as a contrast stimulus iu only one direction. Along th& lengths thy are subject to integmtian. Qn the other hand, bar patterns provide a more simpk rituatiun if concepts of dmdy

spatial Fourier ~~~~ with a ~~k~~d

are ~rn~~~~d. pattern, dlfTerent types of spatial contrast stimuli can be obtained depending oo the

mean intensities and modulations (generally square wave) of the two tight sources. If both s&s of checks are mad&&d

equally and in counterphase, the total amount of light falling on the eye remains constant. However, this constant Iuminance flux stimulus can produce the foik~wing two spatial coxmast stimuli:

60 ~~~~@r~retxtsaj is obtained when both sets of checks have the same mean intensity. The bright and dark checks interchange rhythmically at twice the modulation frequency (Fig. ?a). Hence .oaly response

components can be obtained that are even multiples of the modulation frequency regard&s their luminance or contrast origin. (b) ~~~~~c~j~~~~~c~ is achieved by adjusting the intensities of the two sets of che&s in such a way that they equal do half the stimulus period frzig. 2b)_ The ch~k~~~d pattern first appears tbeen disappears to leave a blat& field once during each stimulus cycie. Sometimes it is useful to modulate only one light source, in which situation the net luminance of the In the app~~~i~p~~~ situation twoconditions can ~~t~gu~s~d :

stimulus changes rh~~~~~y.

The mean luminance (Lo) of the checkerboard is kept at 5000 ash, except where otherw& noted in the figures. For the constant 1~~~~ flux cond~t~~s (Fig. 2a and b), relative contrast [CT) is de&red as: x XUO%.Note that this measure is the ~qui~1~~ of sedation ttr tid half the C = {&6,X - L&,) conventional measure for steady contrast. To avoid confusion for ah &her conditians,.such as of Fig, Zc and d, the percent huninance changes are depicted in the figures. The individual checks of the eheckerbosrd subtended from 25’ to 100’. The presentation time of the pattern was usuahy 250 mxc and the cycle time about 500 msec. Subjects were situated at SOcm from the patterned mirror, and hated upon either the center or top of the stimulus field. The extension of the patt~med field could be varied between 1 and 8 deg of WC by the Reid stop (FS). If not indicated by the type of experiment no artihciai pupil was used because of the considerable effort required of the subject,

The visually evoked poteutials were detected by scalp electrodes in the occipital and parieu%lregions. Usually three electrodes were placed 4-5 cm apart on the midline with the lowest electrode I cm above the inion. A reference electrode was on the left ear lobe and a point on the midline was ground@. The VJW’s and the output from a photocell monitoring the lightsource were fsd into conventional di@erential EEG otro13 ‘~~~rnpute~ of spaders with a ~dwid~ of O-5-75 Hz The respanses were avertiged with a A~era~ Transients” (CAT). The number of counts or averages was generally 5o-400. Most ~~~~nts were repeated many times in various sub&cts. A &al record was made with an Jr’-Y recorder. When there was any doubt with regard to artifacts, recordings were taken from the subject with his eyes closed.

RESULTS PrrStern reversai us appewance-dimppearance The simplest way of minimizing luminance contamination of spatial cmtrast evoked ~~t~~tia1~ wsutd seem to be as fo~ows, The mean l~~~~nce of the two sets of spatiai elements (bars, checks, etc.) are rn~i~tain~~ identical but their i~stant~~~s values are modulated equally in counterphase (see methods). In this situation the pattern reverses two times during each stimulus cycle. Examples of contrast W’s obtained with abru#Iy reversing, low contmsty ch~k~~b~~ds~ are given for four subjects in the left column of Fig. 3. This figure shows that, logically, at each reversal of the pattern an identical response originates which is rather complicated in waveform and which varies between subjects. This identity forms a sensitive control for the quality of the stimulus. i Polygons of higher order are of course more disk-like, but such polygons wolf have more than four so that they are not feasible for counterphase experiments.

neighbours

Contrast

Evoked Responses in Milan

1581

FIG. 3. Responses of four subjects to pattern reversal (first column) and appearance-disappearance (secondcolumn) of a checkerboardpattern of 3’ sith 10’checks. The subject tixates with both eyes at the center of the stimulus field. In this and all subsequent figures: Positivity

is upward.

One of the striking features of pattern reversal is the subjective appearance of pattern This is, however, not a necessity for obtaining contrast EP’s, as follows directly from Fig. 4, where the luminances of the two sets of checks are triangularly modulated in counter phase (see also SPEKREIJSE,1966). The mean luminance of one set of checks was gradually reduced until the luminances of the two sets of checks were equal only once during each stimulus cycle. In this final condition the response was found to occur only once per stimulus cycle, thus excluding a luminance origin for these responses. In this situation, however, there is no sensation of motion although the contrast EP is present (VAN DER TWEEL, REGAN and SPEKREIJSE, 1969). In the stimulus condition of Fig. 4 the pattern can be considered to disappear for a short time and reappear either in the same or in a reversed position. However, this condition does not allow a study of possible differences between the signals generated by the appearance and disappearance of a pattern. These can best be separated by keeping the luminances of the two sets of checks equal for a sufficiently long time. For example, square wave counterphase modulation of the two sets of checks will then produce appearance and disappearance of the pattern for equal duration. Each appearance and disappearance were indeed found to elicit a characteristic but different response (right column Fig. 3). These responses exhibit a complicated and individual relation both to the timecourse of the luminance modulation and to the pattern used. Because of these differences, the reversal condition where both types of response interact (EST~VEZ and SPEKREIJSE, 1973), seems less suited for a general study of contrast EP’s. motion.

1582

H.

SPEKREIJSE,

L. El. VAVDER TWEEL AXI TH. ZUIDEMA

FIG. 4. Schematic luminance t@Ie course of the two sets of checks f$rst c&m@ and the occipital responses (secondcolumn).In the top row the pattern reverse every 2 jo msec; in the bottom row the pat&% can be reg&rded to disap@ear~fora short time +ZW~ j@l msec and fitxt to reappear in the same position. The respozxe frequent fotkrws the c frcqmcy of spatial ccmtrast. The ~te~ed~te steps a05 obtain& by ~&S&Z c luminance levels of bath sets of checksinopposite tire&on. No& that ia ail si frequency of lumir~ance modulation remains the same. The respons&shifts in accorda;nce with the difference in time of trigger and zero contrast point. COITeWXld~g

By combining analysis of the distribution of contrast EP’s over the skull with knowledgg about r~tino~o~~a1 prQj~o~ HALLDAY and MICHAEL (1970) and M~CHBL and HALLIBAY (1971) were able to show that the ~orn~~~~t atro~g~y tOOm~ec in the ~~~ reversal response that they used in their study, does not originate in the calcarine &sure, one of the cortical areas that according to JEFFREYS (19713 and JEFFXEYSand AXWRII (1972) ~~~tribut~ to the response to the batter appearance. In the most commonly used occipital de~vat~o~s three ~orn~~~~~ts (Fig. 5) can be distinguished which may have different cortical origins and which exhibit difkent relations to stimulus parameters. Ihe fust ~orn~on~~t fI) consists of a positive d~~t~o~ with a Ilatency of 65-80 rns~~

The-latency of this component increases with decreasing contrast and mesa- uminar~. According to Jeffreys this companent originates from surface negative dipoles oriented ~~~di~ul~ly to the cortex in and around the calcarine fissure. On the o&z hand the s~~~d~ ~ega~ve ~ornp~~e~t @I) stems f’om ~x~~~~~~ areas; its ~~~t~de is ~~~u~ariy reduced at more lateral electrode positio% This component, w&h is often the most prominent deflection, has a latency of approximately 90-1-10 msec and exhibits comparable dependency on contrast and mean luminance to component I. At low cont~&+a~~d/or luminances the wavefo~ rises aad ~fa& more sloppy (Fig. 6J. The o~ig~ of the Tad component (III) with a latency sf about 160 msec has not yet been est&%hed_ It i% a

Contrast Evoked Responses in Man

1583

positive deflection that depends strongly on contrast and which is favored by binocular stimulation. The disappearance response consists of a sharp positive deflection followed by a decay which is, however, not always monotonous (e.g. subject O.E. in Fig. 3). In most subjects this response is preceded by a weak negative deflection. It should be noted that if square size is decreased the disappearance response becomes relatively stronger compared to the appearance response (Fig. 5).

panernsix 5’

75’ 10’ 15’ 20’ 40’

FIG. 5. Occipital responses to the appearance and disappearance of a checkerboard of 3.8 per cent (first column) and 15 per cent (second column) contrast as a function of checksize. Center fixation.

FIG. 6. Occipital responses to an appearing-disappearing checkerboard as a function of contrast and mean luminance level. The pattern was present for 150 msec.

H. SPEKREIISE,L. H. VAN DER TWEELASD TH. ZLIDE.SIA

1j0-4

The data in Fig. 5 show also that a simple comparison of the appearance responses holds only over a restricted range due to a large variation in shape and size of the response with check size. Moreover, other types of patterns produce completely different waveforms as has also been shown by RIET~ELD et al. (1967) and JEFFREYS(1969). But for checks between 7’ and 20’ there is not much change in shape; moreover a constant stimulus field with checks of these sizes evokes the largest response. Therefore in most of our experiments we have chosen checks ranging between 7’ and 20’. Another complication forms the different representation of upper and lower half field stimulation (JEFFREYS,1971; VAN DER TWEEL et al., 1969). Both the distribution and the polarity of pattern EP’s depend on retinal site of stimulation. In particular, stimulation of the upper half of the visual field can give EP’s which are generally of smaller amplitude and sometimes of opposite polarity than the EP’s elicited by stimulation of the lower half of the visual field. Except for these differences the EP’s both to upper and lower half field stimulation seem to show the same dependence on stimulus parameters such as contrast, checksize, etc. For these reasons we used mostly lower half field stimulation. Contrast dependency

Saturation is one of the most striking features of spatial contrast EP’s. At high luminances some of the components in the appearance response saturate at contrasts of less than 10 per cent. The level at which saturation becomes obvious has a rather complicated relation With other stimulus parameters such as the type of pattern and the size of pattern elements. For example, the response to a checkerboard with checks exceeding 20’ saturates at a lower contrast than to one with smaller checks. For a given checksize, the luminance and contrast dependence of the appearance response is exemplified in Fig. 7, where contrast is plotted on a logarithmic scale, The main difficulty in deciding which type of relation exists between the amplitude of the appearance response and contrast, is the lack of a valid quantitative measure for the response. For sinusoidal grids, modulated at a relatively high frequency (8 Hz) CAMPBELL and MAFFEI (1970) found a good correlation between the to zero amplitude extrapolated

7. occipital responses to an appearing-disapp checkerboard with 20’ checksas a function of contrast. The left column responsesshow the change in wav&Cm wit&contrast. me c&t hand graph shows that the peak to peak value of component‘I-II safuratm at IOWCr contrasts for higher mean luminances. FIG.

Contrast Evoked Responsesin Man

1585

response (on linear-log scale) and the psychophysical threshold. However, the exclusive use of high stimulus frequencies can obscure the contributions of various components, e.g. appearance and disappearance, since a practically pure sinusoidal response is obtained and only two parameters-amplitude and phase remain. Another complication forms the fact that the properties of contrast EP’s may differ in various frequency ranges, as has been’shown for the homogeneous field responses (VAN DER TWEEL and VERDUYNLUNEL, 1965; SPERREUSE, 1966; REGAN, 1970). Furthermore the properties of contrast EP’s may differ for various harmonic components even when their absolute frequencies are the same (REGAN, 1973). Figure 7 gives a characteristic example of the relation between the amplitude of the appearance response and contrast at a low presentation rate. Whereas initially the response may increase logarithmically with contrast, there is nevertheless saturation at low contrast levels. The contrast at which saturation becomes obvious is lower the higher the mean luminance level. It is also evident from this figure that the third component in the appearance response is only present if comparatively high contrasts are used. Steady contrasts

Figure 8 illustrates an interesting phenomenon that fits in with the notion of a saturation mechanism. The left side of the figure shows the response to an appearing-disappearing checkerboard of 20 per cent contrast. In the right side, spatial contrast was varied between 10 per cent and 30 per cent. Also in the latter situation there is no net change in luminance. An initial contrast of 10 per cent was already sufficient to reduce considerably the responses to a further increase of contrast. It should be noted that the response to the decrease of contrast is not affected. At lower luminances the influence of a steady contrast upon the appearance response is weaker, in accordance with the saturation curves of Fig. 7. The size of the response to an increase in contrast can to a first approximation be predicted from the responses to a real appearance of a pattern, if it is assumed that the initial

500msec

T

*

500 msec

FIG. 8. Occipital response to an appearing-disappearing checkerboard of 20 per cent contrast (left side) and to the same checkerboard but with a contrast varied between 10 and 30 per cent. The response to the increase is affected strongly by the standing contrast.

1586

H. SPEKRELJSE, L. H. VAN DER TWEEL A&D TH. ZUTDEMA

contrast sets the system at a starting point. This starting point is the size of the responw to the appearance of a pattern with the same contrast (Fig. 9). Hence a given increase will elicit a smaller response the larger the initial contrast, since the response to the appearance of a pattern saturates. On the other hand for small initial contrasts the response may be larger than the response to the appearance of a pattern_ This on tist view surprising finding, which is especially evident at low luminance levels, can be understood as foilows. The saturation curve of Fig. 7 shows that the to zero extrapolated amplitude of the appearance response crosses the horizontal axis at a higher contrast for progressively lower iuminances. Suppose that the intersection is at C, then the response to the appearance of a pattern with contrast C will be negligible. However, if an initial contrast C is appIied then the response to a subsequent increase of contrast from C to 2C will evoke a response similar to that to the appearance of a pattern with contrast 2C. This larger response is obtained because the initial contrast takes account for the “dead zone” in the amplitude vs contrast curve. This seemingly enhancement of the response by small initial contrasts is in accordance with the psychophysical findings of SHAPLEY and TOLHURST (1973) who showed that (sub threshold) standing contrasts of proper sign lower the psychophysical threshold for contrast detection. It should be noted that the above description for both the reduction of response by high and the enhancement by small initial contrasts implies that a steady contrast does not produce appreciable adaptation.

&

fl-’ 1

htial COnfraSt

---------

4

+

I

I

I

I

I

I

10

20

30

40

50

Contrast, % FIG. 9. Amplitudeof the appearance response (component I-II) as a function of contrast for three levels of initial contrasts of 0,5 and 10 per cent. To a first approximation the responses can be described by addition; the initial contrast sets the baseline.

On the other hand the response to a decrease in contrast remains much more unaffected by a steady contrast (Fig. 8). This indicates that the main criterion for the decrease response is the step in contrast, irrespective of the absolute values of the initial and final contrast levels reached. This holds only to a first approximation since for small contrast reductions the response for low initial contrasts is larger than for higher ones (Fig. 10). De obvious difference between the response to an increase and that to a decrease in contrast is that for the decrease response it is of less importance whether the final level is a blank field or not. The above data indicate that the tenlninology of appearance and disappeamnce needs to be used with caution. The responses are primarily produced by the change of contrast. Since,

Contrast Evoked Responses

1587

in Man

however, even this terminology is not unambiguous we will continue to follow the conventional usage of “appearance” and “disappearance”.

’ I

’

60

,‘I

50

.’ 40

’

30

2Q

IO

0

Contrast, % FIG. 10. Amplitude of the decrease response as a function of reduction in spatial contrast for three levels of initial contrast:

80, 50 and 20 per cent.

Simultaneous changes in luminance and contrast 1n all the stimulus conditions, described up till now, half of the receptors receive an increase in illumination whereas the illumination of the other half is simultaneously decreased to the same extent. We found that it depends purely on the initial contrast present and the final contrast reached, whether these practically identical conditions in luminance result in appearance, disappearance, or reversal responses. This finding suggested that luminance and contrast responses are independent; a suggestion which accorded with subjective observations. In order to carry out a more quantitative study of the relation between luminance and contrast EP’s, checkerboards of constant absolute contrast were produced in the following ways : 1. The luminance of one set of squares was kept constant and the luminance of the other set was increased by 10 per cent. .7. The luminance of one set of checks was kept constant, that of the other set was decreased with 10 per cent. The subject was unable to distinguish condition 1 from condition 2; also the responses were identical. 3. Half of the checks were increased 10 per cent in luminance, the other half 20 per cent. 4. As in 3, but now the checkerboard was produced by reductions in luminance of the two sets of checks: one set by a reduction of 10 per cent, the other by a reduction of 20 per cent in luminance. Although there were certainly differences between the responses obtained in these four conditions, the overall constancy was striking (Fig. 11). This leads to the general conclusion that these responses are mainly caused by the change in spatial contrast, independently of the way in which this contrast is reached.

1588

H. SPEKREIJSE, L. H. VAN DER TWEEL AXD TH. ZIIDE.MA

FIG. 11. Contrast EP’s to an appearing-disappearing checkerboard with 20’ checks of 10 per cent absolutq modulation (this is a relative contrast of about 20 per cent), superim$osed on simultaneous changes in luminance. The luminance changes do not have muck intluence.

In experiments of this type there is the problem whether the final contrast should be treated as absolute or relative. The data of Fig. 12 demonstrate that the instantaneous relative contrast is more relevant for the response than the absolute contrast.

FIG. 12. Contrast EP’s to an appearing-di.%apWtring checkerboard (top&se). Asimui~yzeous jump in luminance does not a&ct this rnp~~% as 10% a~ the relative contrast feplains St (bottom left figure). To a similar IumiaanGs jump, but with a eoilstant absolute d3iI~.S&, the response becomes smalkr and broader (bottom right figure).

Contrast Evoked Responses in Man

1589

D_vnamics

From pattern reversal experiments it was concluded that the speed of crossover of the spatial contrast was of importance for the shape of the response (Fig. 13). This dependence was complicated by our finding that the shape of the response is also determined by the final contrast reached and by the varying time lapse between appearance and disappearance.

FIG. 13. Pattern reversal responses as a function of speed of crossover of spatial contrast,

By using trapezoidal modulation we were able to show that the size of response to the appearance depended less on the slope of modulation than did the response to the disappearance of a pattern (VAN DER TWEEL et al., 1969). This pointed to an integrative property for the appearance response. To obtain a quantitative estimate of the integration time involved we modulated the light sources with pulses of variable duration. The amplitude of the appearance response (component I-II) as a function of presentation time is shown for various contrasts in Fig. 14. These data indicate an integration time-constant of about 50 msec. Such a behaviour, comparable to that of a leaky integrator, supports our earlier

$15

c

;---

-

Contr. [%I .

30

-

MOn-5

Pres. time, FIG. 14. Amplitude

m’sooec

of the appearance response (componentI-11)as a function of presentation time of a checkerboard with 10’ checks and a contrast of 30, 15 or 7.5 per cent.

1590

H.

SPELREIJSE,L.

H. VAXDER TWEEL AND TH.

ZUIDEMA

conclusion (Fig. 9) that for contrast EP’s contrast as such is not much adapted, i.e. there is no low frequency attenuation. Moreover the data in Fig. 14 demonstrate that most of the saturation is preceded by integration, since at higher contrasts saturation was reached at progressively shorter stimulus durations. If contrast saturation would have occurred before the integration then all curves should have become saturated at the same presentation time. The data in Fig. 15 give a typical demonstration of the applicability of the contrast equivalent of Bioch’s law. Identical responses were obtained for presentation times of respectively 20 and 40 msec with contrasts of resp. 20per cent and 10 per cent. However, for short disappearances, time and contrast are certainly not interchangeable as is shown in the bottom half of Fig. 15. By taking into account the disappearance duration it became clear that the most prominent part in this response is due to the reappearance of the pattern. The longer the time the pattern has disappeared the larger the (re-appearance) response, even if the contrast is lowered proportionally. 500

msec

&

_

FIG. 15. Top half figure gives the responses to short appearances of a checkerboard pattern with 10’ checks. As long as the product of contrast and presentation time remairis constant identical responses are obtained (right hand responses). Bo!tom half figure gives theresponses to short disappearances of the same checkerboard. In this condition presentation time and contrast arc not interchangeable.

The close correspondence between the psychophysical threshold and the zero (extrapolated) amplitude of the appearance response~as a function of log presentation time is evident from the data in Fig. 16. The graphs obtained~at contrasts of 30, 15 and 7:5 per cent are separated on the time axis by about a factor of 2, which implies full integration. For the lowest contrast used (3.8 per cent) this no longer holds because presentation durations were involved that apparently exceeded the integration time. At the right side of~Fig. 16 the data are replotted as a function of the product-of contrast and presentation time. -For the upper three contrasts the data points coincide on a straight line. Extrapolation of this line gives an intersection with the horizontal axis which is near the psychophysically determined threshold in accordance with the findings of CAMPBELLatid KULlKOWsr (1972). Since for

Contrat

Evoked Responses in Man

1591

contrast (3-8 per cent) the presentation times for a sizeable response exceed the integration time, these points deviate. the lowest

JEFRCEYS (1971) has made a careful study of the various components in the appearance response as a function of the retina1 area stimulated. He could demonstrate that for upper field stimulation the G.rst component (I) originates from the peripheral area, the second (If) from the fovea1 area. Pa a~~o~da~~~with the retinot~~~~ re~rese~ta~i~~ on the occipital cortex, a rather complicated McClureensues. Whereas the overall appearance response seems to be the largest for stimuli in the central fovea1 area, responses to disappearance are relatively larger for stimuli in the near central region between 313’ and 90’.

y* 5

J

1

10

20

1

20

Pi-es.time,

1

80

msec chres”ord~ofltr~~x msec

Fro, 16, Left hand graph gives the amplitude of the appearance response ~c~rn~o~e~t I-II> to IO’ checks with various ~~ut~~ts as a function of ~~~s~~ta~i~~ time. These data art: replotted in the right hand graph as a function of the product of presentatian time and contrast. For the upper three contrasts the data points coincide on a straight line which crosses the horizontal axis near the psychophysically determined threshsld. Note that the horizontal axes have a logarithmic scale.

Figure 17 gives the amplitudes of the appearance and disappearance responses as a of checksize. Note the difference between the graphs as a function of contrast, For example, the response ratio as a function of checksize depends on contrast. Jn many publications it issuggested that the maximal amplitude of the occipital response in man to checks of 15-20’ is related to average (retinal) receptive field size. The findings of REGAN and RICHARDS (1373) show that a fink with subjective field size under various ~o~ve~~e~~e ~o~~~~~onsis lacking since the check size which gives the Iargest contrast EP is dissociated from the size measured psychophysically, To evaluate the electrophysiological data account must be taken of the number of checks contributing to the response. Since a given stimulus field ~on~~~s nine times as many checks of lo” than of 30’, the peak in the response curve as a function of checksize is not surprising. After correctionfur the number of checks or even for the total Iength of the edges, the curves show an ever increasing function

H. SPEKREIJSE, L. H. VAN DER TWEEL ASD TH. ZLXDEMA

1592

Appearance

response

27 -

Smppearms

Conrrast _‘j’$

response

~ 2’. -

. 36",

1

> -15

,’Y-

2 i

-

/'\ 1')-

A'

== -/

:/

./

*,I

2 z !C9

\

/='-' \

'\::j_

.'

.r.-.I>:>;

.' I

/

5

10

I 2cl Pattern 40

size,

-00011

' 5

I 10

I

20 40 Pattern we,

80 mm

/

I

5

10

t

I

I

40 80 29 Pattern we, mm

I

80 mm

FIG. 17. Top graphs depict the amplitudes of the appearance response and of the disappearance response as a function ofchecksize for twulevels of contrast. In thebotiorngraph~og-bgscale) the appearance data are replotted with the amplitudes corrected for the number of checks in

the stimulus field.

response magnitude, as can be seen in the bottom half of Fig. 17, where the appearance-data of the top half figure are replotted on a logarithmic scale. With regard to the type of pattern used it is difficult to predict which configuration will produce the largest response (fEFFREYS,1969). Furthermore, the waveform of the regonse to the appearance of a pattern depends strongly on the type of pattern chosen, A demonstrative example is given in Fig. 18. Horizontal or vertical bar patterns give a ra@er sluggish type of response (Fig. 18a and b). When the two bar mirrors are superimposed perpendicularly, a trellis is obtained which produces a response with many aspects in common with that to a checkerboard (Fig. l&). The change of the angle of intersection has a profound effect on the shape of the appearance response and on the amplitude of the disappearance response, an observation which seems in accordance with the tiding that smaller checks are favorable for the disappearance response (Fig. 5; see also HARTER, 1971). This may be related to the finding of RUZVnD et al. (1967) that the response to contrast flashes is larger for diamonds than for checks. It should be noted that the pattern depicted in Fig. IS(e) appeared whenever the lines became brighter, whereas for the pattern in Fig. 18(f) this occurred when the diamonds increase in luminance. The responses to the appea&ce of these two patterns of identical spatial configuratian are, however, Tather d&rent. T&is difference is unlikely to be due to the ix&erent change ip overall luminance since the subject has a negligiile luminance response and the disappearance responses are identicaf. Suppres.Gon

In a recent paper (SPEKREIJSE, VANDERTWEELa& REGAN, 1972) wedeebed a method for sustained binocular suppression. In these experiments, the appearing-disappea&g

Contrast Evoked Responses in Man

1593

5c1v Appearance Disawearance v v 500 msec

(b)

FIG. 18. Responses to an appearing-disappearing pattern as a function of pattern COnfigUratiOU. All configurations were made by superimposing two bar mirrors. For decreasing an&s of intersection the disappearance response increases in amplitude, without change in waveform. For the appearance response the situation is more complicated; each configuration elicits a different response.

pattern was presented to one eye whereas the other eye received a high contrast black and white steady pattern of comparable or identical spatial configuration. With the two stimulus fields fused, the subject perceived only the steady pattern with some residual flicker from the appearing-disappearing pattern. Concurrent with this observation, the contrast EP was suppressed strongly (Fig. 19). In this figure use has been made of the different topological representation of upper and lower half fields. Whether the identical half of the field was occluded physically or by means of a high contrast steady pattern in the upper field of the consensual eye made no difference for the contrast EP. This observation immediately excludes ocular disaccommodation as a possible cause of suppression. The relation with perceptual phenomena was very strong as can be seen from Fig. 20 in which the steady pattern was stabilized by an optical-lever method (for a description see KOENDEIUNK, 1972). In the phases that the retinal stabilization was effective there is no influence on the response, whereas in the intervals when the pattern could be seen the steady pattern suppressed effectively. Although the identity of the two patterns proved not to be a stringent condition, fusion of the field was a necessity. A contrasty pattern just to the side of the stimulation field presented to the same or the other eye, had practically no influence on the contrast EP. On the other hand, a properly positioned trellis within the stimulus field had a strong suppressing effect (SPEKREIJSE, 1966). In Fig. 21 the effect of the presence of steady lines of

1594

H. SPEKREIJSE, L. I-i. VAN

DER

TWEEL AXD TH. ZLIDE.W

I’ on the edges of an a~~ar~~g~isap~ar~ng checkerboard with 20’ checks is depicted for bright and dark lines. Irrespective of the luminance of the lines, a steady contrast of 20 per cent is sufhcient to half the response to an appeasing-disappearing checkerboard of 20 per cent contrast. At the same time the disappearance response has been ~o~pIete~y abolished, indicating again a diRerent origin of the two. The suppressing effect is weaker when the steady lines are positioned across the centre of the checks, and the more SO the larger the checks (SPEKREUSE,1966), although heaq contrasts wherever in the tieid reduce the response. Left eye

a?.ght We

FIG. 19. IInft hand Column shows the stikity

Righr eye

in the e&x% of p&&dly

IJCC~U&Qf&etap_

mod&ion cannot account for the intercxukr suppression phenomenon. Reface on left ttu lobe.

kctrode

Particularly with pattern reversal we were ~o~ro~ted time and again wit& the problem whether the responses were solely due to chaqes in spzttial contrast or whether t&y were contaminated by changes in lutinance. To Wperiodic stimulation, the respor~e_ in this situation can ~o~ta~u only even harmonics i~s~~~i~e of their ~~~a~~e or cotitmst ori@tin. A criterion often used to establish the origia of the EP’s is the dependence of-t&e~respoase on blurring of the pattern (SPE~CWWE,1366). In a number ofcases the r&u&m in the size of the response as a fP;inction of bi~~g is indeed so strong that it forms an a~~~~t for a true contrast origin of the response. On the other hand it would be expe@d -that under

Contrast Evoked Responses in Man

Left eye

Right eye

@ Stabiked

5COmsec

FIG. 20. Suppression experiments in which the steady pattern is presented as a stabilized retinal image. When, under stabilized viewing conditions the steady pattern is no longer visible, there is no apparent suppression of the contrast EP to the appearing-disappearing checkerboard presented to the other eye (top and bottom figures). In the non-stabilized condition (mid figure) interocular suppression is present.

Appearance Disappearance v

v

500 msec

Without

lines

:..:: :::. pj :j,

@

1 ‘$

1:

Dark lines20%contr.

Bright lines20%contr.

FIG. 21. Steady contrast (20 per cent) lines of 1’ on the edges of an appearing-disappearing checkerboard with 20’ checks of 20 per cent contrast are sufficient to half the contrast EP’s. Note that this effect is the same irrespective whether the lines are bright or dark. vISlON13/8-N

1595

1596

H.

SPEKRHJSE, L. El. V.ei

DER TWEEL

an

TH. ZUIDEMS

counterphase conditions physical blurring by d&accommodation w&d always provoke a decrease both for pure luminance and for spatial contrast responses (GA#LPB~SJ. and GREEN, 1965). The decrease for pure luminance responses by blurring is due to the fact that diffracted (scattered) tight from one set of checks reduces the effective modulation depth of the other set and vice versa. This reduction is the stronger the smaller the checks and with enough defocusing a zero response will be obtained irrespective its origin. Since accommodation has in general a stronger effect for small grids than for large ones even for low rn~d~~a~io~s, blurring effects do not give an uneq~~v~a1 proof of a contrast origin of response. REGANand RICHARDS(1972) reported that under their conditions a disaccommodation of between 1 and 5 dptr+ even increased the response to large (40’) checks. ~~t~~n~ngexperjments offer another WriteNowfor estab~is~~g a contrast origin of the EP. For luminance processing one would expect that the position of the fines is not criticaf. However, a trellis outlining the borders of large checks has been shown to a&ct the response more than other positions of the trellis. With an appea~~g~isap~ar~n~ pattern of &onstant average iumi~an~e a more principal distinction can be made. With regard to luminance this stimuhts resembles the reversal. condition; hatf of the elements receive a luminance increase and the other half an equal luminance decrease. Therefore a fundamental component in the response is likely to be due to the change in ~on~ast” ~owever~ esp~ia~ly at high education depths a ruminates origin cannot be excluded since the two sets of checks are modulated at diRerent mean Iuminances, From these changes in luminance two fundamentals of di&rent amplitude can emerge resulting in a fundamental ium~ance component in the response to tfatl:appeariagdisappearing ~he~kerbuard. This ~~d~st~nctnesscan be abdished by k~~ng the ~urni~a~~e of one set of checks constant and square wave moduiating that of the other set. By appropriate choice of the steady luminance level we arranged that the pattern appeared either at the ~~rninan~~ increase or at the ~urn~nan~e decrease. If this change produces a 180” phase shift of the fundamental component, a contrast contribution is certain, ff not then the fundamental is due to changes in luminances which have identical phases in both conditions. This technique would not work if the response to an appearing-disappearing pattern were symmetrical i.e. ~~nta~~ed only even ha~~~~~~s as is found for instance in the doubfetriggered ERG. One way of deciding whether such a symmetrical response is due to Iuminance response distortion or is of contrast origin is to compare the responses to pattern reversal with those to a homogeneously illuminated field, modulated to the same extent. The even harmonics in the response can be easily selected by trigg~r~~~ the avera~~r twice per period, so that the fundamental and ali odd harmonics are cancelled (Robsoa, persotiaf communication). As can be seen in Fig. 22 highly similar ERG’s were obtained to counterphase checkerboard and to pure luminance stimulation. Furthermore the dependence on contrast and modulation depth looked much the same. Therefore most if sotaI1of Ihe ERG to counterphase stimulation can be considered as the net result of addition-of Iumina~ee increase and decrease responses. If in the double triggered cun~tio~ the responses to reversal are much larger or differ from those to homogeneous field st~m~ia~o~ a contrast co~~ibut~o~ is warrmtdA~~~~i~~ the above criteria the Ef’s presented in this paper can be ascribed for the bigger part if not totally as reactions to changes in spatial contrast. mother d~~ctio~ ~tw~~ contrast and lum~nan~ EP’s can be found in their relation to psychophysics. For contrast EF’s, as for EP’s of other mod&ties, a corre&ion has been reported between psychophysical threshold and the extrapolated zero response ampfibde

Contrast Evoked Responses in Man

25Hz

Luminance

Pattern reversal

/---?,

,fi

1597

105~

FIG. 22. ERG’s to counterphase checkerboard (right hand column) and pure luminance stimulation (left hand column). In the double triggered condition the responses to both stimulus conditions are the same irrespective of the temporal stimulus frequency. in a logarithmic plot. For a checkerboard pattern one example is given in Fig. 16. STERNHEIM and CAVONIUS (1972) extended the method to the frequency domain. In contrast, the evoked responses to pure luminance stimulation do not show such good correlations with psychophysics. Under favorable conditions the fundamental as well as the second harmonic can be

obtained at modulations far below the critical modulation depth and the shape of the MTF (flicker frequency) plots for EP and flicker perception are quite dissimilar (VAN DER TWEEL, 1964; SPEKREIJSE, 1966; REGAN, 1968 a,b). In the previous paragraphs various methods are discussed to distinguish between contrast and luminance origin of an EP. In those conditions where net luminance changes were presented simultaneously with changes in spatial contrast, a pure contrast response could sometimes be obtained by the subtraction of the response to a pure change in luminance. Although this method, which has also been used by other investigators, would therefore seem acceptable, it is in fact not so. Caution is necessary since the presence of spatial contrast can result in enhanced luminance responses. An example is given in Fig. 23, where the responses to a 500 msec flash superposed on an already projected checkerboard are

presented. In this way luminance is added. Paradoxically this stimulus gave a much larger response than the same flash projected on a dark field. The response, however, had no relation to the appearance response as can be seen from the first row of Fig. 23. Moreover, the relative contrast is diminished at the moment that the superimposed fiash is presented. The complicated dynamic interaction found for the pattern reversal situation was the main reason for studying the contrast responses in the appearance-disappearance situation. Both appearance and disappearance give rise to a typical response with different relations to varying stimulus conditions. For instance, whereas integration as expressed in Bloch’s law for contrast holds for the appearance response, for the disappearance response a more differentiative behaviour is found (VANDERTWEELet al., 1969). Also the coarseness of the pattern has a different effect on size and shape of either response. These findings suggest that the appearance and disappearance responses originate from different populations of cortical cells. Such difference in origin can also be a factor in the explanation of the different topological data of JEFFREYS(1971) and HALLIDAYand MICHAEL(1971). Jeffreys studied

1598

H. SPEKREUSE,L. H. VAN DER TWEEL AND TH. ZUIDEMA

mainly the appearance response to short presentations of small elements (14’) in the configuration of Fig. 18(c) whereas Halliday and Michael worked with high contrasty reversals of relatively large checks (50’). It has been made plausible (Esriv~z and SPEKREUSE, 1973) that the component in the reversal response measured by Ha&day and Michael, is mainly related to the decrease of contrast. In this view the different results of JEFFRIXS (1971) and HALLIDAY and MICHAEL (1971) seem reconcilable to a large extent.

125 170mrec

FIG.23. The constant presence of spatial

contrast can result in an enhanced luminance response to a superimposed fIa.sh, as follows from the two lower responses. These responses bear no relation to the appwvance response as can be seen from the top figure. The lktekie~ of component I and II in the appearance response are relatively long due to the IoiNlumti6ncCievel (200 ash) used in this experiment.

The study of the dynamics of the appearance and disappearance responses led naturally to the recording of responses to short appearances and disappearances. Whereas the responses to short appearance obey the contrast equivalent of Bloch’s law, those to short disappearances deviate strongly because the re-appearance response proved to be dominant (Fig. 15). This response becomes larger for increasing disappearance durations at proportionally decreasing contrasts. Contrast sensitive elements which loose their excitation gradually at the offset of spatial contrast could account for this observation. If the starting condition consisted of a high spatial contrast and if the disappearance duration was short, then the system’s representation of the contrast had not yet reached the zero position at the moment of re-onset of the pattern. This representation acts in the same way as a steady contrast so that the saturation prevented fuil development of the re-appearance response. Figure 15 shows that indeed a larger response is obtained for a 40 msec, 2Oper cent contrast than for a 20 msec, 40 per cent contrast disappearance. The time constant~seems of the same order as found for the leakyintegrator describing Bloch’s law for the appearance response. As already mentioned the study of contrast responses is troubkd by the absence of a quantitative description. For instance the same pattern gives a more slug&h response at low than at high contrasts. Therefore completely different results are obtained when the amplitude or e.g. the area of the response is taken as measure. Another measure could be

Contrast Evoked Responses in Man

1599

the “energy” of the response, although a physiological basis for such is not clear. It depends on the model one has in mind which measure is preferred. Notwithstanding these complications, the size of the response to large squares seems too large if one takes into account the limited number of borders or checks in the stimulus field. It is conceivable that a kind of spatial integration within borders takes place in a sense as proposed by LAND (1971). In such a situation also elements far away from comers and borders could contribute to the contrast EP (see also REG.&N and RICHARDS, 1971). It requires a more definite model, however, to reconcile this with the relatively short distance effects as demonstrated in the outlining experiments. .Probably the most important consequence of our study regards the distinction between luminance and contrast processing. Supported by our psychophysical experiments the conclusion seems warranted that EP’s to contrast stimulation cannot be derived from luminance responses. Contrast responses have a quality of their own as is so clearly demonstrated in Fig. 11. The apparent lack of a spatial contrast component in the ERG (Fig. 22) indicates that the contrast pathway separates from the luminance pathway after the structures that generate the ERG. Since the counterphase checkerboard and homogeneous field ERG’s of Fig. 22 are practically identical, the retinal elements behave quantitatively the same in both situations. Hence for the relatively large checks used (60’) lateral interactions seem not to influence the ERG. Acknowledgemenfs-We

are indebted to Mr. A. B. DE GRAAFFof the lighting design and engineering centre of Philips, Eindhoven and to Mr. P. TH. I. PIREEof the fluorescent tube research centre of Philips, Rozendaal for providing the fluorescent tubes with the required properties of brightness and rise time (< 2 msec). Part of this work was supported by the Organization of Health Research (TNO), The Hague.

REFERENCES CAMPBELL,F. W. and GREEN, D. G. (1965). Optical and retinal factor affecting visual resolution. J. Physiol. Lend. 181,576-593. CAMPBELL,F. W. and KULIKOWSKI,J. J. (1972). The visual evoked potential as a function of contrast of a grating pattern. J. Physiol. Lend. 222, 345-356.

CAMPBELL,F. W. and MAFFEI, L. (1920). Electrophysiological evidence for the existence of orientation and size detectors in the human visual system. J. Physiof., Land. 207,635-652. CLYNES, M., KOHN, M., and LIFSHITZ,K. (1964). Dynamics and spatial behavior of light evoked potentials, their modification under hypnosis, and on-line correlation in relation to rhythmic components. Ann. N. Y. Acnd. Sci. 112,468-509.

HALLIDAY,A. M. and MICHAEL,W. F. (1970). Changes in pattern-evoked responses in man associated with the vertical and horizontal meridians of the visual field. J. Physiol., Lond. 208, 499-513. ESTB~EZ,O., and RFMOND, A. (editors). Proceedings of the workshop: Progress and Future of Spatial Contrast Research (January 1972, Paris) Trace 1972. ESTBVEZ,0. and SPEKREIJSE,H. (1973). Relationship between pattern appearance-disappearance and pattern reversal responses. Exp. Brain Res. (In press). HARTER,M. R. (1970). Evoked cortical responses to checkerboard patterns: effect of checksize as a function of retinal eccentricity. Vision Res. 10, 1365-1376. HARTER,M. R. (1971). Visually evoked cortical responses to the on- and offset of patterned light in humans. Vision Res. 11,685-695. HARTER,M. R. and WHITE, C. T. (1968). Effects of contour sharpness and check-size on visually evoked cortical potentials. Vision Res. 8, 701-711. HARTER, M. R. and WHITE, C. T. (1970). Evoked cortical responses to checkerboard patterns: effect of checksize as a function of visual acuity. Electroenceph. clin. Neurophysiol. 28, 48-54. JEFFREYS,D. A. (1969). Evoked brain potentials as indicators of sensory information processing (edited by D. M. MACKAY). Neurosciences Res. Progr. Bull. 7, 211-218. JEFFREYS,D. A. (1971). Cortical source locations of pattern-related visual evoked potentials recorded from the human scalp. Nature, Lond. 229, 502-504. JEFFREYS,D. A. and AXFORD,J. G. (1972). Source locations of pattern-specific components of human visual evoked potentials--I. Component of striate cortical origin. Exp. Brain Res. 16, 1-21.

1600

H. SPEKREIJSE, L. H. VAN DERTWEELAND TH. ZUIDLW

KOEND~K, J. J. (1971). Contrast enhancement and the negative afterimage. J. opt. Sot. Am. 62,685-690. LAND, H. E. and MCCANN, J. J. (1971). Lightness and ret&x the0ry.J. opt. Sot. Am. 61, l-1 I. MARSHALL,C. and HARDEN.C. (1952). Use of rhythmically varying patterns for photic stimulation. Electroenceuh. clin. Neurookvsiol. 4. 283-287. MI&L, W. F., &AY, A. &L (1971). Differences between the occipital distribution of upper and lower field pattern-evoked response in man. Bruin Res. 32,311-324. REGAN, b. (1968a). Evokeh potentials and sensation. Percept. & Psycho&s. 4, 347-350. REGAN, D. (1%8b). A high fi~ucncy mechanism which underlies visual evoked potcntiais. Electroenceph. clin. Neurophysibl. 25,231-237. &GA% D. (1970). Evoked potential and psychophysical correlates of changes in stimulus colour and intensity. Vision Res. 10, 163-178. REGAN, D. and RICHARDS,W. (1971). Independence of evoked potentials and apparent size. &/isionRes. 11, 679-684. REGAN, D. and F&CHARDS,W. (1973). Brightness and evoked potenti+, J. opt. Sm. em. (in press). RIETVELD,W. J., TORDOIR,W. E. M., HAGENOUW.J. R. B., LUBBERS,J. A. and SPOOR,Th. A. C. (1967). Visual evoked responses to blank and to checkerboard patterned flashes. ActaPhysiol. Neeri. 14,259-285. ~CHELLART,N. A. M. and SPEKREIJSE, H. (1973). Origin of the stochastic nature of ganglion cell activity in isolated goldfish retina. Vision Res. 13, 337-345. SHAPLEY.R. M. andToL?nJm, D. J. (1973). Edge detectors in human vision. J. Physiol, Lond. 229! 165-183. SPEHLMAN,R. (1965). The averaged electrical responses to diffuse and to patterned light in the human. Electroenceph. clin. Neurophysiol. 19, 560-569. SPEKREIJSE,H. (1966). Analysis of EEG responses in man, evoked by sine watre modulated light. Thesis, Dr. W. Junk, Publ. The Hague. SPEKREIJSE,H. and VAN DERBERG,T. J. T. P. (1971). Interaction between colour and spatial coded processes converging to retinal ganglion cells in goldfish. J. Physiol., Lo& 215, 679-695. SPfKREtJsE, H., KI~OE,L. H,, VAN DfR TwfEL, L. H. (1972). A case of arnblyopia; Electrophysiotogy and Psychophysics of Luminance and Contrast. In: The Visual System: Neurophysiologv, Biophysics ami Their Clinical Applications. (Edited by C. B. ARDEN), Plenum Press. SPEKafusE, H. and Oosmrc, H. (1970). Linearizing: A method for analysing and synthetizing nonlinear systems. Kybernetik 7,23-3 1. SmcrwsE, H., VAN DERTWEEL,L. H. and Roan, D. (1972). Interocular sustainedsuppression: correlations evoked with potential amplitude and distribution. Vision Res. 12, 521-526. STERNHEM, C. E. and CAVONIVS,C. R. (1972). Sensitivity of the human ERG and VECP to sinusoidally modulated light. Vision Res. 12, 1685-1695. VAN DfR TWEEL,L. H. (1961). Some problems of vision with regard to linearity and frequency response. Ann. N. Y. Acad. Sci. 89,829-856. VAN DERTWEEL,L. H., RfGAN, D. and SPEKREIJSE, H. (1969). Some aspects of potentials evoked by changes in spatial brightness contrast. Proc. 7th Int. Symp. ISCERG, Istanbul. 1-12. VAN DfR T-EL, L. H. and SpfKRwSE, H. (1968). Visual evoked responses. The ctinical value Of ekctroretinography, ISCERG Symp. Gent 1966. 83-94. VAN DERTWL, L. H. &nd SPEICIWJSE, H. (1973). Psychophysics and electrophysiology of a rod-achromat. Documenta. Ophth. (in press). VAN DERTWL, L. H. and VERDWN LuNfL, H. F. E. (1965). Human visual responses to sinusoidatly modulated light. Electroenceph. clin. Neurophysioi. 18, 587-598.

Ah&act--In man visually evoked responses (EP’s) can be recorded that are specific to changes in spatial contrast and cannot be derived from luminance responses. No spatial contrast component could be demonstrated in the ERG. Contrast EP’s depend on various parameters such as acuity, retinal location, size and contiguration of spatial elements, time course of luminance change. They are affected by overlapping steady contrasts presented either monocularly or dichoptically. For a given condition the contrast EP is mainly determined by the instantaneous relative contrast irrespective whether this contrast is reached by an increase or decrea&n luminance. The EP’s to the appearance and disappearance of a pattern btar different relations to stimulus parameters and seem to originate from differenf populations of cortical cefis. The responses to short appearances obey the contrast equivalent of

Bloch’s law and correlate with psychophysics. R&m&-On pout enregistrer sur l’homme des rkponses visuelles tvoqti (EP) qui Sollt stiifiqnes des changements de contraste spatial et ne peuvent pas Ure d&iv&s des r&ponses de luminance. On ne peut pas d&ekr dans I’ERG de composante de contraste spatial. La EP de contraste dCpendent de divers param&es (acuitC, emplacement sur la r&ine, dimension et

Contrast Evoked Responses in Man

configuratlondes ClCments spatiaux, evolution

1601

temporelle des changements de luminance). 11s sont affect& par les contrastes stables se chevauchant, prCsent& B un oeil ou aux dem yem. Dans des conditions don&es, l’EP de contraste est surtout ditermint par le contraste relatif mstantani, independamment de son obtention par augmentation ou diminution de la luminance. Les EP relatifs g l’apparition et la disparition d’une figure ont des relations diffirentes avec les paramktres dustimulus et semblent provenir de populationsdiffkrentes de cellules corticales. Les rCsponsesI des stimulibrefsobeissentB une loi de contraste iquivalente B celle de Bloch et sont en accord avec la psychophysique. Zusammenfassung-Beim Menschen kiinnen visuell evozierte Potentiale (EP’s) abgeleitet werden, die fiir hderungen des rsumlichen Kontrastes typisch sind und sich nicht aus der Antwort auf Leuchtdichtetiderungen herleiten lassen. Keine solche Komponente konnte im ERG gefunden werden. Kontrast-EP’s h&ngen von verschiedenen Parametem ab, wie SehschSrfe, Netzhautort, Grijsse und Zusammensetzung der Testzeichen, zeitliche &derung der Leuchtdichte. Sie werden durch iiberlagerte, monokular oder binokular dargebotene, zeitlich konstante Kontrastmuster beeinfluJ3t. Unter gegebenen Bedingungen wird das Kontrast-EP hauptslchlich durch den momentanen relativen Kontrast bestimmt, gleichgiiltig ob dieser Kontrast durch eine Zunahme oder Abnahme der Leuchtdichte erhalten wird. Die EP-s bei Erscheinen und Verschwinden eines Musters hangen in unterschiedlicher Weise von den Reizparametem ab und scheinen aus venchiedenen kortikalen Zellen zu kommen. Die Antwort auf kurze Darbietungen entspricht dem Bloch’schen Gesetz und ist mit der Psychophyslk korreliert. PeZitOMe--Y YeROBeKa MOryT 6bITb 3aperHCTpHpOBaHbI 3pliTenbHbIeBbDBaHHbIe IlOTeHUHaAbI(B~),KOTOpbleIIBjIlOTCRCneu~~~eClur~OTBeTa~~Ha~3MeHeH~npO~paHcTBeHHb~X.uapaKTepacTEx KOHTpaCTa; OHU He MOryT 6bITb BbIBeAeHbIM3 peaKI& Ha RpKOCTb. B 3PlKOMnOHeHT npOCTpaHcTBeHHOr0 KOHTpaCTa He 06HapymBaeTCR. BbI3BaAwre KOHTpaCTO\l nOTeHu&.iajIbI (Bn) 3aBECRT OT pa3JIFtHblXnapaMeTpOB,TaKHX KaKOCTpOTa 3peHUSt,IIOKUE3aAm4 Ha cemame, Benawia s4Koii@rypaunrr!npocrpaHcraeHHblx 3neMeriToB~E3MeHeHHR 5IpKOCTliBOBpeMeHU. Hamrx BO3AetiCTByeTHaJIaraKWHZiCRnOCTOIIHHbtiKOHTpaCT,npeL-IaraeMbrZiw60 MOHOK~JIJI~HO,nw60 ~AHOK~IIII~HO.&~~~~HH~~~~CJIOB~UI“K~H~~~CTE~~~~"BIYI onpeAe.?rteTC~,rnaBHbMo6pa3oM," CHEOMHHyTHbIM"OTHOCKTelIbHbIM KOHTpaCTOM,He3aBACUMO OT Tore Aocc5iraeTcRn5i KompaCr yBena~eewesrujreyMeHbmeHEie,M npsocru. Bll Ha noflBneH3ieH wzye3HoBemie narrepaa H~IXOASITCR B pa3nwmbIx 0womemwX~ napaMeTpa5r CTHMy;ra 8, IIO-BKiXiMOMy, B03HHKaiOT B pa3JIFiHbIXnOlIyJI~mU7XKOpTHKaJIbWcC KJIeTOE. PeaKIIHH, IIpH KPaTKOk 3KClI03WLIEH,lTOA'5iHRIOTCRnpaBHJIy 3KBHBaneElTa KOHTpaCTa nO3aKOHy~rocHWKOppe~UpyIOTCnC~XO~~3~~eCK~~AaHHbI~.