Electroencephaiography and Clinical Neurophysiology, 1976, 41 : 387--398

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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

AMPLITUDE CHANGES IN T H E V I S U A L EVOKED CORTICAL POTENTIAL WITH BACKWARD MASKING * J.L. ANDREASSI, J.J. DE SIMONE and B.W. MELLERS

Baruch College, City University of New York, 155 East 24th Street, New York, N.Y. 10010 (U.S.A.) (Accepted for publication: March 5, 1976)

Previous studies (e.g., Andreassi et al. 1971) have shown that when a horizontal string of five successive (single) stimuli are presented at certain input rates and at given locations, the first t w o in the sequence will n o t be perceived. However, visual evoked potentials (VEPs) occurred to the first stimulus in the string indicating a situation in which a cortical response occurred to a stimulus which was n o t perceived b y subjects, i.e., there was a dissociation b e t w e e n psychological experience and the VEP recorded from over the occipital cortex. Further investigation revealed that if the third, fourth and fifth stimuli were of greater luminant intensity than the first two, then not only did perceptual suppression occur, b u t a partial physiological inhibition occurred in the form of a delayed VEP to the first stimulus in the series. In addition, the a m o u n t of delay in the occurrence o f the VEP to the stimulus which was not perceived depended on the ratio o f the intensity differences between the first t w o stimuli and the next three, i.e., the greater the difference the longer the delay in the appearance of the VEP to the first stimulus in the string (Andreassi et al. 1974). The perceptual interference observed, in which later stimuli prevented perception o f earlier ones, is a t y p e of "backward masking" (see Kahneman (1968) * This study was made possible through a grant to t h e first author by the Environmental Physiology s e c t i o n o f the Physiology Program of the Office of Naval Research under ONR Contract N00014-72-A-

0006, and ONR Contract Authority NR-201-053.

and Turvey (1973) for thorough discussions of visual masking, and Mayzner and Tresselt (1970) for a discussion of masking involving single sequential stimuli). Other investigators have explored the VEP correlates o f p h e n o m e n a such as metacontrast and visual masking. For example, Schiller and Chorover (1966) investigated the question of whether or n o t the brightness reduction observed under metacontrast conditions (where brightness changes b u t intensity does not) is correlated with changes in the VEP. They reported no changes in the VEP associated with the reduction of brightness induced b y metacontrast and concluded that the VEP does n o t necessarily reflect changes in subjective brightness. However, Vaughan and Silverstein (1968) report attenuation o f VEPs to foveal stimulation b u t n o t parafoveal, during metacontrast suppression. They conclude that the reason for the failure of VEPs to reflect metacontrast suppression in the Schiller and Chorover study was because parafoveal stimulation may have given rise to the VEPs generated largely b y stray light impinging on the fovea. The attenuation found by Vaughan and Silverstein occurred to the VEP c o m p o n e n t appearing a b o u t 200 msec after the first stimulus. Donchin et al. (1963) studied VEPsunder the visual masking paradigm in which the second, brighter flash (BF) masks perception of the initial test flash (TF), at interstimulus intervals (ISis) of 0--25 msec. At the 20 msec ISI, when visual masking occurred, VEPs were like those to the BF at the 5 0 0 msec ISI, or when

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presented alone, i.e., the VEP to the TF was completely suppressed. A similar result obtained by Donchin and Lindsley (1965) led to the conclusion that the interference with TF by the BF takes place at or preceding the point at which VEPs are recorded (occipital cortex). They expressed the opinion that the same processes which are involved in perceptual blanking seem to be involved in the blocking of the VEP. It must be noted that in these two studies of Donchin and his colleagues the BF was many times more intense than the TF (ranging from 100 to 10,000 tumes, in miUilamberts) probably accounting for the complete suppression of the VEP to the TF. In a related study, Fehmi et al. (1969) recorded evoked potentials from three sites in the visual system of monkeys during backward visual masking. Masking occurred when a test flash (T) o f 15 mL was followed 20 msec or less by a blanking flash (B) of 15,000 mL. This masking was accompanied by a complete lack of response to T at all recording sites which included the optic nerve or tract, lateral geniculate and visual cortex. The result indicated that the interaction underlying masking obtained with this method occurred at the retinal level as well as at the other locations. Again, the blanking flash was very intense and this could be the reason why the evoked potential to T was completely obliterated at all sites, compared to the metacontrast situation of Vaughan and Silverstein (1968) where the VEP was merely attenuated. To explain the results of the previously mentioned studies (Andreassi et al. 1971, 1974) it was suggested that when blanking stimuli (i.e., the later appearing ones) were more intense than the blanked stimuli (first two) the action of the visual cortical inhibitory fields generated by the blanking stimuli upon the excitatory fields generated by the blanked stimuli was of sufficient magnitude to delay the VEP to the first two stimuli. However, when all stimuli were of equal intensity, the inhibitory action was less and only a perceptual response difference occurred. Although the more intense, later appear-

J.L. A N D R E A S S I ET AL.

ing, stimuli affected the time of occurrence of the VEP, at no time did they affect the amplitude of the VEP to the blanked stimuli. The present experiments were conducted to determine the effects of varying spatial and configurational factors upon the perception of, and VEPs to, the initial stimuli in a series o f stimuli. This account describes three experiments in which, instead o f presenting single sequential stimuli, multiple stimuli were presented in sequence. It would be useful at this point to define several terms in accordance with current practice in masking studies (see e.g., Turvey 1973). The term Target refers to the stimulus or stimuli which the subject is required to identify. In backward masking paradigms it is the earlier stimulus or stimuli. The Mask is the stimulus pattern which comes after the Target and is expected to change the perception of the Target in some manner. Stimulus onset asynchrony (SOA) indicates the time between the onset of the Target and the onset of the Mask. The interstimulus interval (ISI) refers to the time between offset of a Target and onset of a Mask.

Method

Subjects The total number of subjects for all three experiments was 9 males and 6 females associated with Baruch College o f the City University o f New York (3 o f the males participated in t w o of the experiments). None had known neurological or visual defects other than myopia (corrected to at least 20/30). All were tested in three experimental sessions, on 3 separate days, at approximately the same time of day.

Apparatus and procedure The subjects were seated in an electrically shielded sound-attenuated (IAC) Chamber.

VEP C H A N G E S W I T H B A C K W A R D

MASKING

All experimental sessions were c o n d u c t e d with the light dimmed. In order to obtain the averaged cortical evoked potential, the electroencephalogram (EEG) of each subject was recorded from Oz ( " T e n - - T w e n t y " System, Jasper ( 1 9 5 8 } ) w i t h Grass silver cup electrodes referenced to a silver clip electrode on the subject's left ear lobe. A Beckman t y p e RM Dynograph Recorder was used to record the EEG and a Mnemotron C o m p u t e r o f Average Transients (CAT 1000) was used to obtain the averaged evoked potential. Electrode resistance was 5000 ~2 or less. The subject was grounded b y means of an electrode attached to the right ear lobe leading to "patient g r o u n d " of the Beckman Dynograph. The 9806A coupler of the Dynograph was used to condition the EEG signal (bandpass set at 0.5--32.0 c/sec). The filtered and amplified signal was than fed into the CAT. A " s t a r t " signal from a PDP8/E digital computer triggered the CAT to take EEG samples of 500 msec duration following the presentation o f each stimulus to the subject. After 100 stimulus presentations, the summated VEP responses from CAT m e m o r y were plotted b y a Hewlett--Packard X--Y Plotter. On-line monitoring o f the EEG was accomplished with a Tektronix 502A oscilloscope. The electro~culogram (EOG) was measured b y a separate channel of the Beckman Dynograph and averaged by the CAT to check on possible distortions o f the VEP due t o eye m o v e m e n t or eye blink. Inspection for eye m o v e m e n t contamination involved a comparison of the averaged EOG trace with the VEP trace. A straight line EOG trace indicated very little or no eye movement. A rise then a dip in the EOG trace would indicate consistent eye movement, and if the EOG trace was superimposed over the VEP trace, the effect would be noticeable b y rises and dips in the VEP at the same temporal positions. None o f our trials had to be repeated because o f eye m o v e m e n t contamination. The basic element for the stimuli in all three experiments was a 5 × 7 matrix array

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(grid) consisting of 35 yellow-green points of light. Subjects were presented with conditions in which either grids, letter B's, or a combination of grids and B's were presented. Stimuli were displayed on a VR-14 Display CRT (Digital E q u i p m e n t Corp.) m o u n t e d at the subject's eye level outside the Chamber at a distance of 99 cm (39 in.) from the subject's nasion. The VR-14 CRT displays were under program control of the PDP-8/E digital computer. A small fixation point, 3 mm in diameter, was used to maintain the subject's line of vision at the center of the CRT. The fixation point was a dim (0.001 mL) red neon light source located 6 mm above the center of all stimulus arrays presented on the CRT. The VR-14 CRT electronics are such that the total luminous energy appearing at any one time upon the screen is constant. This energy is equally distributed among simultaneously presented stimuli. Thus, the intensity of a given individual character decreases as the number of similar elements displayed simultaneously on the screen increases. F o r example, a single grid stimulus produced an intensity of 5.50 mL; when two grids appeared, each had an intensity of 2.75 mL. Another character, with fewer light points, would result in a lower luminous intensity. Luminance measurements of individual stimuli were made at a distance of 2.54 cm from the display screen surface with a Tektronix J-16 Digital Photometer, with the VR-14 intensity control set to maximum, i.e., the setting used throughout all of the experiments reported here. F o r all experiments, there were three general conditions: Condition A: T w o stimuli appearing on the screen for 20 msec (ON time o f 20 msec). Condition B: Two stimuli for 20 msec ON, followed by three stimuli 40 msec later (ON time of 20 msec, O F F time of 40 msec, i.e., SOA of 60 msec). Condition C: T w o stimuli, followed b y three stimuli 40 msec later, followed b y six additional stimuli 40 msec later (ON time o f 20 msec, O F F time o f 40 msec, i.e., SOA of 60 and 120 msec b e t w e e n the first and second

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and first and third stimuli). There were always 1000 msec between the end of each stimulus set and the beginning of the new set. Fig. 1 indicates the arrangement of grids as they were presented on the CRT display for Conditions A, B and C of Experiment I. This same general arrangement of stimuli was used in Experiments II and III, the only variation consisting of the actual characters used. The numbers indicate the order of appearance, e.g., the two grids labelled " l " appeared first. The spatial arrangement of the clear, dotted and black grids illustrate how the first two were bounded by the second three in the horizontal dimension, while the second set of three grids was spatially overlapped, 2/7ths on top and bottom, by the later-appearing six grids. The grids were coded in Fig. 1 merely to distinguish between the sequentially presented sets of grids and in no case did more than one set of grids (either 2, 3 or 6) ever

A

iHii

iHil

@ Fig. 1. Schematic of spatial and temporal arrangement of stimuli as they appeared on CRT in Experiment I. Numbers indicate order of presentation. The same general arrangement for conditions A, B and C was also used in Experiments II and III.

appear on the screen at the same time. The grids were 0.95 cm square and at a distance of 99 cm produced a visual angle at the eye of 0.55 ° for a single grid and 2.75 ° for a horizontal array of 5 grids. Under the fixation condition of this study, a portion of the horizontal array in B and C were parafoveal, since most estimates of foveal extent are 2 ° of visual angle. In those experiments which employed letter B's, the display orders and locations of the stimuli presented on the CRT were identical to those in Fig. 1. The instructions asked subjects to focus just below the fixation point between and prior to the start of presentations. They were asked to count silently the number of presentations and say " 1 0 0 " aloud when this number was reached. The counting procedure was used to help insure subject concentration in this tedious task. The possible influence of a language function (counting) upon the VEP should be minimal since O~ is over the juncture of left and right hemispheres. The subjects were asked to avoid excessive movement or eye blink during presentations of stimuli. After the presentation of each condition, subjects were asked to report the number of elements seen in any single presentation and these responses were recorded by the experimenter. The subjects were familiarized with the various displays by 10 sample presentations of each condition. Further testing was continued only if subjects evidenced backward masking. None of the subjects screened n this manner failed to experience masking. The three conditions were completely counterbalanced across the 6 subjects, over a period of 3 days. Each subject was presented with each condition six times during the course of three experimental sessions, for a total of 18 VEP traces from Oz with each VEP based on 100 presentations. The data analysis was accomplished by computing the mean amplitudes (/~V) and latencies (msec), for each subject, for the primary X--Y Plotter tracings. The N1 comp o n e n t was considered to be the first negative dip in the trace which occurred 50 msec after

VEP CHANGES WITH BACKWARD MASKING

the stimulus. The amplitude o f the N1 compon e n t was measured as the vertical distance from "baseline" to the trough of this first depression. The baseline was determined by the horizontal X--Y tracing. The P1 c o m p o n e n t was measured as the vertical distance from N1 to the peak of the first positive component, while N2 was measured as the vertical distance from the peak of P1 to the trough of the second major depression and so on for P2, N3 and P3. Latencies (or time after stimulus presentation) were measured to the midpoints of each positive and negative peak. If the " p e a k " was fiat and appeared more as a plateau, the midpoint of the plateau was taken as the latency measurement. According to our criteria, the N3 and P3 components did n o t occur frequently enough to be included in the analysis for A and B, i.e., t h e y occurred in less than 50% of the VEP traces. While data for N1 and P1 are shown, the main analyses are for N2 and P2, the largest amplitude and most consistent components of the VEP in this experiment.

Results

Experiment I: Effects of sequential sets of like stimuli (5 X 7 grid squares) In this experiment, 3 male and 3 f e m a l e subjects were presented with a varied sequence of 5 × 7 grid squares in the three conditions. The ~esearch questions posed were: 1. Will backward masking occur under the stimulus conditions used? 2. If backward masking does occur, will it be accompanied by changes in the VEP? The perceptual reports indicated that backward masking did occur. To summarize the reports for the three conditions: Condition A: All 6 subjects saw 2 grids (2 grids presented). Condition B: Five subjects saw only 3 grids, while one subject saw 3 grids plus fragments of 2 other stimuli (5 grids presented).

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Condition C: All subjects saw only the last 6 grids (11 grids presented). There was evidence for complete masking in 5 o f 6 subjects, and partial masking for another, under Condition B, and for all 6 subjects in Condition C. I n response to our questioning regarding how m a n y elements were seen in each presentation, subjects reported seeing two "squares" or " b o x e s " in Condition A, three in B and six in C. Though we have referred to the stimuli used as "grids", our subjects reported them as varying numbers of "squares" or "boxes". Analysis of the major VEP components (N2 and P2) indicated that this backward masking effect was accompanied by VEP changes. Table I shows the average amplitudes of the various VEP components under the three conditions and shows that the major components (N2 and P2) were highest in amplitude under Condition A (no masking.}. Table I also reveals that additional VEP components occurred under Condition C. The N3 and P3 responses s~ggest that the last stimulus set (6 grids) was sufficiently removed in time from the first set (2 grids) to allow additional VEP components to occur (the SOA was 120 msec). The amplitude differences were evaluated by t tests foT correlated data, using a two-tailed criterion, and the following values were obtained: For N2 amplitude, A vs. B (t = 2.72, P < 0.05, 5 df); A vs. C (t = 3.87, P < 0.02, 5 df); B vs. C (t = 2.15, P > 0.05, 5 df);

TABLE I Mean aplitude (~V) of VEP componehts under Conditions A, B, C, for Experiment I (N = 6). VEP components

N1 P1 N2 P2 N3 P3

Conditions A

B

C

1.98 3.80 6.72 12.73

1.83 3.86 5.76 10.43

2.73 4.28 5.00 9.82 9.01 7.12

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and for P2 amplitude, A vs. B (t = 3.07, P < 0.05, 5 df); A vs. C (t = 3.09, P < 0.05, 5 df); and B vs. C (t = 1.17, P > 0.05, 5 df). Thus, the major VEP components showed significant amplitude attenuation when perceptual masking occurred {Conditions B and C) as compared to when it did not occur {Condition A). The latency data, presented in Table II, were also analyzed b y t tests for correlated data, two-tailed criterion, and none of the comparisons resulted in significant t values (P > 0.05, 5 df). Fig. 2 shows the superimposed traces of one person (B.W.M.) for the 3 days combined. This subject had a mean P2 amplitude of 15.58 /iV for Condition A, 12.42 under B, and 11.42 for C. The results were in the same direction for the other 5 subjects.

Experiment H: Effects of sequential sets o f like stimuli (letter B 's) A replication of Experiment I was completed in order to determine the reliability of the effect with a new stimulus configuration and 3 new subjects. Three of the subjects were also tested in Experiment I. The stimuli used in all conditions consisted of letter B's. The illumination of 20 of the available 35 points of light produced the letter B. The reduction in number of light points resulted in a luminance of 3.20 mL for a single B, while each of 2 B's

TABLE II Mean latency (msec) of VEP components under Conditions A, B, C, for Experiment I (N = 6). VEP components

N1 P1 N2 P2 N3 P3

Conditions A

B

C

88 118 137 209

89 123 139 213

90 120 145 207 267 323

A

I IO,pV ( TIME

IN

MILLISECONDS

Fig. 2. Six VEP traces (Oz) superimposed for one subject (B.W.M.) under conditions A, B and C. Each trace is based on 100 stimulus presentations. Negativity is downward.

were 1.60 mL. Other than the stimulus change and 3 new subjects, the procedure was the same as Experiment I. The visual angle produced at the eye was 0.55 ° for the B's, the same as for the grids. The results of Experiment II confirmed those of Experiment I. The major VEP components (N2 and P2) were o f significantly lower amplitude when masking occurred (P < 0.05, 5 df) than when the first t w o sets of stimuli were clearly perceived. Backward masking occurred for all persons under Conditions B and C. That is, under Condition A, two B's were seen and reported, while under Condition B, the two B's were masked by the 3 B's and in Condition C, the 2 B's and the succeeding 3 B's were all masked by the last set of 6 B's. To summarize what the subjects saw under the three conditions: A: 2 B's (2 presented).

VEP CHANGES WITH BACKWARD MASKING

B: 3 B's (5 presented). C: 6 B's (11 presented). Fig. 3 shows the superimposed traces for one of the subjects in Experiment II. This person (P.A.G.) produced a mean P2 amplitude of 19.25 gV for A, 15.75 for B, and 15.25 for C. The results were in a similar direction for all others w h o participated in Experiment II. Questions raised b y the results of Experiments I and II are the following: 1. Would backward masking with the sequential sets of stimuli be as effective if the shape or configuration o f the earlier stimuli were different from those o f later sets? 2. If there is no masking with stimuli of different shape, will this perceptual effect be reflected in the VEP b y a lack of change? Experiment III represents an attempt to answer these questions.

A

B

¢

I IOjJV I

0

I

I00

I

I

I

I

200 300 400 500

TIME IN MILLISECONDS Fig. 3. Six VEP traces (Oz) superimposed for one subject (P.A.G.) under conditions A, B and C. Each trace is based on 100 stimulus presentations. Negativity is downward.

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Experiment III: Effects of sequential sets of unlike stimuli (B's and grids) Six new subjects, 5 male and 1 female, participated in this experiment. Although new stimulus combinations were used, the display orders and locations of the stimuli and the time sequence were the same as in Experiments I and II. It should be noted, however, that the total luminance when B's appeared on the screen was 3.20 mL while for grids it was 5.50 mL. The conditions were: Condition A: 2 B's. Condition B: 2 B's followed b y 3 grids. Condition C: 2 B's followed b y 3 grids, followed b y 6 grids. The perceptual data obtained from subjects's reports indicate the following perceptions under the various conditions: Condition A: All 6 subjects reported 2 B's (2 B's presented). Condition B: All 6 reported 2 B's and 3 grids (no backward masking). Condition C: All 6 reported 2 B's and 6 grids (backward masking of 3 grids only). These perceptual reports suggest that dissimilar shapes were n o t as effective in masking Target stimuli as were similar shapes, this despite the fact that the grids were slightly higher in luminance than the B's. The selectivity o f this t y p e of masking is indicated in t h e result for Condition C in which the 3 grids did n o t mask the earlier presented B's, b u t were themselves masked b y the 6 grids which followed. The raw VEP traces were analyzed with respect to amplitudes and latencies o f major c o m p o n e n t s as in the previous experiments. The amplitude and latency differences o f N2 and P2 were tested b y t tests for correlated data (two-tailed criterion}. The tests indicated that none o f the amplitude or latency comparisons, A vs. B, A vs. C, and B vs. C, were significant ( P > 0.05, 5 df). The amplitude data for the 6 subjects under the three conditions are presented in Table III. The trend shown in Table III is towards a higher amplitude VEP under Condition A, a trend which

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T A B L E III

Mean amplitude (pV) of VEP components under Conditions A, B, C, for Experiment III (N = 6). VEP components P1 N2 P2 N3 P3

Conditions A

B

C

3.46 5.90 11.25

3.70 5.52 9.07

4.17 5.37 10.01 8.17 5.26

did not reach significance as indicated by the t tests results. The latency data are shown in Table IV and indicate very little difference between conditions. The superimposed traces for one of our subjects are presented in Fig. 4. This individual (M.A.F.) had a P2 amplitude o f 14.25 #V for Condition A, 15.08 for Condition B, and 14.92 for Condition C, exhibiting very little VEP amplitude difference between conditions. In Experiments I and II, with like stimuli, it was f o u n d t h a t when backward masking occurred, there were significant decreases in VEP amplitude. However, in Experiment III, with unlike stimuli, backward masking did not occur and there was no corresponding decrease in VEP amplitude.

TABLE IV Mean latencies (msec) of major VEP components under Conditions A, B, C, for Experiment III (N = 6). VEP components P1 N2 P2 N3 P3

A

Conditions A

B

C

110 151 221

109 155 225

111 147 222 288 340

B

IO.uV

TIME IN MILLISECONDS Fig. 4. Six VEP traces (Oz) superimposed for one subject (M.A.F.) under conditions A, B and C. Each trace is based on 100 stimulus presentations. Negativity is downward. Discussion

The three experiments just described indicated t h a t when backward masking occurred there was a significant decrease in VEP amplitude and when display conditions did not produce masking, VEP amplitude was not attenuated. Thus, with regard to the research questions posed, the results o f Experiments I and II provide similar answers: (1) backward masking o f the first stimulus set was produced under the conditions examined; (2) under conditions which produced backward masking a significant decrease in VEP amplitude occurred. Our results are similar to those of Vaughan and Silverstein ( I ~ ) w h o reported attenuation of VEP amplitudes to foveal stimulation during metacontrast suppression. On the other hand, our findings differ from those of Schiller and Chorover ( 1 ~ ) who did n o t find VEP changes under conditions of metacon-

VEP C H A N G E S WITH B A C K W A R D

MASKING

trast. However, Vaughan and Silverstein (1968) have already pointed o u t that a possible reason for the failure of VEPs to reflect metacontrast suppression in the Schiller and Chorover study was because parafoveal stimulation may have given rise to VEPs generated largely b y stray light impinging on the fovea. The VEP attenuation reported b y Vaughan and Silverstein occurred to the c o m p o n e n t which appeared at approximately 200 msec after the first stimulus. This P 200 c o m p o n e n t corresponds to our P2 wave which we found significantly reduced with the backward masking effect reported for Experiments I and II. The results of the present experiments are n o t like those o f the visual masking experiments reported by Donchin et al. (1963), Donchin and Lindsley (1965), and Fehmi et al. (1969), in which VEPs were found to occur in response to the Mask rather than the Target stimulus. However, in these studies, the Mask was of much greater intensity than the Target (from 100 to 10,000 times) while in our Experiments I and II the Masks were the same luminance as the Targets. In addition, the Masks in our experiments were in either adjacent or slightly overlapping locations as the Targets, while the diffuse masking flashes used in the three studies mentioned above were at the same spatial location as the Targets. Finally, the SOA which produced visual masking and complete VEP suppression was 20 msec or less in the Fehmi et al. (1969) study compared to 60 and 120 msec in our experiments. These differences in stimulus intensities, locations and SOA are believed to underly the different VEP effects obtained in the metacontrast and our multiple stimuli masking experiments on the one hand, and those o f Donchin et al. (1963), Donchin and Lindsley (1965) and Fehmi et al. (1969), on the other. In the latter studies the very intense Masks (light flashes) completely obliterated the VEP to the Target flash at the appropriate SOA. In fact, the Fehmi et al. (1969) study indicated that this complete suppression o f the evoked potential to the first stim-

395 ulus occurred at the retinal as well as the lateral geniculate and visual cortical areas. Fehmi and colleagues acknowledge that masking effects have been observed for SOAs which were long enough to preclude the possibility that masking stimuli could overtake the masked ones at the retinal level, when patterned rather than diffuse blanking flashes were used. Metacontrast suppression becomes maximal at SOAs b e t w e e n 40 and 100 msec, and in our experiments, the SOA was 60 msec b e t w e e n Target and Mask onset for Condition B, and 120 msec b e t w e e n Target and second Mask onset in Condition C. The VEP attenuation observed in our experiments could n o t have been completely due to inhibition at a retinal level since the receptors there would have had time (60 msec) to respond to the initial stimuli before the second set appeared. It is possible that a certain degree o f inhibitory interaction occurred at various subcortical sites (e.g., lateral geniculate body, ascending reticular formation) as well as at the visual cortex, in producing the V E P amplitude attenuation observed in the metacontrast and our experiments reported here. However, we do not have direct evidence regarding possible subcortical interaction sites and, therefore, we currently prefer an interpretation which places emphasis on cortical interactions rather than subcortical. It is believed that the explanatory mechanism proposed for earlier findings (Andreassi et al. 1971, 1974) m a y at least partially account for the findings of the present experiments. That is, it would appear that the excitatory fields produced in the visual cortex by the early stimuli in a series are decreased in their activity by the inhibitory action produced by later presented stimuli in areas adjacent to, or slightly overlapping, the excited areas. The complexity of various results obtained from masking studies using V E P correlates makes it difficult to ascribe backward visual masking effect to any particular mechanism or area in the visual system. For example, it has been found that when Masks were of greater intensity than Targets, the V E P to the

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initial stimulus was completely eliminated (Donchin et al. 1963; Donchin and Lindsley 1965; Fehmi et al. 1969). These results might be explained in terms of an integration theory of masking which says that the first and second stimuli are summed as the Mask overtakes the Target, and the response to their presentation in sequence is the same as that to their simultaneous presentation (Kahneman 1968). In another study, it was reported that when masking stimuli were of equal intensity and presented in single adjacent locations, sequentially, perceptual masking was not accompanied by VEP alterations (Andreassi et al. 1971). This dissociation between perceptual masking and the VEP may be due to the fact that recordings were made from over the occipital cortex, since it is possible that cortical areas other than the visual cortex may be involved in this kind of masking effect. This interpretation would be consistent with Turvey's (1973) discussion of central and peripheral factors in visual masking in which he suggests that Targets and Masks of equal intensity are processed centrally rather than peripherally, with "central" referring to those areas concerned with processing of visual information after the striate cortex. Under conditions of single sequential presentation of stimuli, in which later stimuli were of greater intensity than earlier ones, there was a delay in time of appearance of the VEP generated by the initial {masked) stimulus (Andreassi et al. 1971, 1974). Further, under metacontrast and multiple sequential stimuli conditions in which Targets and Masks were equally intense, but later stimuli spatially surrounded early ones on at least two sides, VEPs to the Targets were reduced in amplitude (Vaughan and Silverstein 1968; and Experiments I and II reported here). These latter findings of VEP alterations with masking could be interpreted within the framework of an interruption theory of masking in which processing of the Target stimulus is terminated, or interfered with, by the Mask, the interference in this case taking the form of either delayed latencies or re-

J.L. ANDREASSI ET AL.

duced amplitudes of VEPs generated by the Target stimuli. This interpretation would not be incompatible with the explanation offered earlier in this discussion since excitatory--inhibitory effects may be characterized as a form of interruption in visual processing and, in effect, the earlier excitatory effects produced at the cortex may be altered by the interference of later-arriving stimuli. Thus, it would appear from the various possibilities proposed that different visual system mechanisms might be involved in different types of backward masking phenomena. Our final observation is that neither VEP changes nor perceptual masking occurred when later stimuli were different in configuration from earlier ones (Experiment III reported here). As a consequence of his concurrent-contingent model of masking, Turvey (1973) proposes that the more similarities between Target and Mask the greater the probability that masking will occur. This similarity includes both geometric and semantic features, and the geometric aspect would be supported by the results of Experiment III: This selectivity in backward masking, based on configuration of Targets and Masks, has also been reported by Mayzner and Tresselt (1970). To explain their findings that varying the shape of successive inputs eliminates masking effects, they suggested that " . . . i n p u t s with different geometric properties are 'routed' to different locations in the visual cortex, sufficiently distant from one another so that the inhibitory fields of these later.arriving inputs do not interact with the excitatory fields of the earlier inputs arriving in the system" (1970, p. 608). Mayzner and Tresselt draw upon the work of Hubel and Wiesel (1959, 1962, 1968) as bases for their explanation. Thus, the factors which appear to be involved in producing VEP changes and backward masking are Mask and Target intensity, spatial orientation of the Mask with regard to the Target, 8OA between Target and Mask, and relative configurations of Target and Mask. When the VEP to the Target stimulus is compIetely suppressed, as when the Mask is

VEP CHANGES WITH BACKWARDMASKING of much greater energy than the Target, many levels of the visual system may be involved, including the retina, superior colliculus, lateral geniculate body, reticular formation and visual cortex (in fact, Fehmi e t al. showed complete suppression of response to the target at three of these sites}. W h e n VEP changes do not occur with masking by equally intense stimuli, the visual system sites subject to electrophysiological change may be other than those which have been explored by surface recording techniques to date. When VEP changes do or do not accompany perceptual masking accomplished by subtle parameters involving contour, spatial, configurational, and timing aspects, perhaps only areas of the visual system known to respond to shape and orientation of stimuli, such as the visual cortex, may be involved. In addition, there may be "higher" cortical areas, other than the visual, involved in the final processing of certain types of visual stimuli. Hubel (1963), describing his work with cats, has expressed the view that the visual cortex is only an early stage of the visual pathway and points out that little is known regarding pathways leading from the visual cortex. This could point to the importance of recording from over many cortical sites, in addition to the occipital, when conducting visual masking experiments, as well as the desirability of using subtle masking techniques in animal studies exploring responses to Target stimuli at subcortical sites.

Summary A series of three experiments examined backward visual masking effects and visual evoked cortical potential correlates of such masking. In Experiment I, which was concerned with the effects of sequential sets of like stimuli (grids}, it was found that when backward masking occurred, it was accompanied by decreased visual evoked potential (VEP) amplitudes. Experiment II replicated the first experi-

397 ment using different stimuli (letter B's) and several new subjects. Again, backward masking was accompanied by decreased VEP amplitudes. In Experiment III, which examined the effects of sequential sets of unlike stimuli (B's and grids), it was found that when earlier stimuli differed in configuration from later stimuli, there was an absence of backward masking and VEP amplitude changes also failed to appear. Thus, when backward masking did not occur, no changes in the VEP were observed. The results are discussed in terms of interactions between excitatory and inhibitory activities produced at the visual cortex by the earlier and subsequently presented stimuli. Various concepts regarding the visual system mechanisms involved in backward masking, including integration and interruption hypotheses, m a y help to explain the complexity of findings obtained thus far in studies using V E P correlatesof backward visual masking.

R~sum~ Modifications d'amplitude du potentiel dvoqug visuel cortical avec masquage rdtroactif Une s~rie de 3 experimentations examine les effets de masquage visuel r~troactif et les eorr~lats d'un tel masquage au nlveau du potentiel ~voqu~ visuel cortical. Dans l'exp~rimentation I qui concerne les effets de s~ries s~quentielles de stimuli semblables (barres) on observe que lorsque survient le masquage r~troactif, il s'accompagne d'une diminution d'amplitude du potentiel ~voqud visuel (VEP). L'exp~rimentation II r~p~te la premiere experimentation en utilisant des stimuli diff~rents (lettres B) er plusiers sujets nouveaux. A nouveau, le masquage r~troactif s'accompagne d'une diminution d'amplitude du potentiel ~voqu~ visuel. Dans l'exp~rimentation III, qui examine les effets de s~ries s~quentielles de stimuli non semblables (lettres B et barres) on ob-

398

serve que lorsque les premiers stimuli different en configuration des stimuli ult~rieurs, il y a une absence de masquage r~troactif et les changements d'arnplitude du potentiel ~voqu~ visuel n ' a p p ~ n t pas. Ainsi, quand le masquage r~troactif ne survient pas, il n'y a pas de modification du potentiel ~voqu~ visuel. Ces r~sultants sont discut~s en terme d'interactions entre les activit~s excitatrices et inhibitrices produites au niveau du cortex visuel par les premiers stimuli et les stimuli ult~rieurs. Diff~rents concepts concernant les m~canismes du syst~me visuel impliqu~s dans le masquage r~troactif, incluant les hypotheses d'int~gration et d'interruption, peuvent aider ~. expliquer la complexit~ des donn~es obtenues jusqu'~ present dans les ~tudes qui utilisent les corr~lats VEP du masquage visuel r~troactif. The authors wish to thank J.A. GaUichio, M.A. Friend, and P.A. Grota for their assistance in data collection and analysis.

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