Visually evoked response correlates of perceptual masking and enhancement

ELECTROENCEPHALOGRAPHY AND CLINICAL NEUROPHYSIOLOGY

VISUALLY

EVOKED

PERCEPTUAL

RESPONSE

MASKING

AND

CORRELATES ENHANCEMENT

325 OF 1

E. DoNcmN, PH.D. s a n d D. B. L~NDSLE¥, PH.D.

Departments of Psychology and Physiology, and the Brain Research Institute, University of California, Los Angeles, Calif. (U.S.A.) (Accepted for publication: March 29, 1965)

INTRODUCTION

The perception of some characteristic of a brief test flash (TF) such as brightness, form, or orientation, can be influenced by the subsequent presentation of a second closely following and overlapping brighter flash (BF). If the interflash interval ([FI) is brief (90 msec or less) the TF will be masked by the BF (see review by Raab 1963). It the IFI is longer than that required for masking of the TF (100-150 msec), the brightness of the TF can be enhanced (Donchin and Lindsley 1965). The IFI values for both masking and enhancement effects depend upon a difference in luminance between the TF and the BF; in general the higher the luminance of the BF relative to the TF, the higher the IFI at which masking and enhancement will occur. The development of new techniques has made it possible to follow the changes induced in at least one aspect of neural activity in human subjects as stimulus conditions are varied (see Whipple 1964 for numerous studies of sensory evoked response in man). Using the computer averaging technique, we have recorded evoked potentials elicited by paired flashes which gave rise to the perceptual phenomena described above and it was found (Donchin et al. 1963), by visual inspection of the records, that average evoked potentials elicited by paired flashes can be classified into three groups as a function of the interval between the two flashes. In the present study the responses obtained to paired flashes have been further analyzed by attempting to separate the components contributed by each flash of the pair. In addition, in order to compare t Supported by NSF grant GB-1844, NASA contract NsG-623, and aided by Navy contract Nonr 233 (32). 2 Present address: Division of Neurology, Stanford University Medical School, Palo Alto, Calif.

the observed modifications of the TF evoked potential to the observed perceptual changes, the analysis was performed for three different values of TF luminance. Specifically, average evoked potentials were computed from EEGs recorded from the surface of the scalp over the visual area in human subjects when paired flashes were presented with different IFls and also when the two different flashes comprising the pair were presented separately, Then the average evoked potential of the BF was subtracted from the combined re.sponse to the pair of flashes allowing a delay corresponding to the IFI. The remainder was assumed to be the contribution of the TF to the paired response. It was predicted that the remaining evoked potential associated with the TF would vary in relation to the way the TF was perceived, thus reflecting the retroactive effect of the second flash upon the first when the interval between the flashes was varied. MATERIAL A N D METHODS

Five adult subjects were used, three males and two females, all with normal or corrected vision. The EEG was recorded with a Grass model 6 electroencephalograph; bandwidth was limited to 1-35 c/sec. The amplified EEG was recorded on frequency-modulated magnetic tape (bandwidth of 0-625 c/sec). Grass silver cup electrodes were applied by means of bentonite paste, their resistance being 5000 [2 or less and remaining stable throughout a prolonged session. All electrodes were positioned with respect to the inion and the vertical plane of the auditory meatuses. Ground was through a vertex electrode. All data reported in this study are based upon potentials recorded between the Electroenceph. clin. NeurophyffoL, 1965, 19:325-335

326

I~. D O N C H I N

AND

D. B. LINDSLE¥

ate Wratten Neutral Density filters (2.0, 3.0 or 4.0 log unit) in the TF beam. Each of the three T F luminances was presented to a subject with a 9000 mL BF. From 9 to 12 IFIs were used for eachTF-BFcombination. Each series was composed of 120 stimuli but only the first I00 were averaged; a series consisted of either single or paired flashes. For any given series of flashes the luminance was held constant. In each experimental session there were 13-15 such series. All subjects served in a number of sessions. To insure maximal alertness and proper fixation at the instant of stimulus presentation, the subject triggered all stimuli, when ready, by pressing a key. The subjects were inst~ructed not to trigger the flashes unle~3 they were properly aligned in the system and could see four dim red lines converging upon the stimulus area. A minimum interval of 1.6 sec between any two successive stimulus pres0entations was determined by the control system. The room lights were extinguished when a series of flashes was presented. Between series room lights we~'e turned on and the subject could relax. Single and paired flashes were presented during every session. While in

occiplzal area (2.5 cm above the inion and 2.5 cm to the right of the midline) and the left ear lobe. The light flashes were produced by two sylvania RII31C glow modulator tubes. The light I:eams we[e collimated and superimposed concentrically, and p~e~ented in biaxwellian view to the right eye of the subject. Stimulus duration and IFIs were controlled by two Grass $4 stimulators (timing .controls supplemented by decade capacitance boxes) and were monitored with a cathode ray oscilloscope and a HewlettPackard electronic counter. The TF was presented as a semicir¢:le, through a semicircular stop, defining a visual angle of 1°22 ' along its diameter; the BF as a full-circle, through a circular stop, defining a visual angle of 206' along its diameter. Thus the BF completely overlapped the area of the retina upon which the TF was projected. Both stimuli were 10 msec in duration. The unfiltered luminance of both flashes was 9000 mL but due to unequal areas their respective bfightaesses expressed in log units above threshold'was 4.2 for the TF and 4.7 for the BF. The BF luminance was held constant throughout the study. Three different TF luminances (90, 9.0 and 0.9 mL) were achieved by introducing appropriS.Y.

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Fig. 1 Average evoked potentials from visual area for each of four subjects (SY, SZ, AK, JW). In this and subsequent figures each average based on 100 stimulations; all flashes 10 msec in duration; stimulus onset at beginning of trace; negativity upward. Top row: Response to second or brighter flash (BF), full-circle, presented alone. Rows 24: Response to first or test flash (TF), semi-circle,presented alone at decreasing luminances. Eiectroenceph. clin. NeurophysioL, 1965, 19:325-335

EVOKED RESPONSE CORRELATES OF PERCEPTION

the observing posture two control series were obtained from each subject in which the EEG was recorded but the subject received no flashes. In one control condition, the subject pressed the switch 120 times but received no flashes. In another control condition the experimenter placed 120 trigger pulses on the tape at the same rate used by the subject. Average evoked potentials were obtained "off line" with the Mnemotron Computer of Average Transients (CAT). The averaging epoch was 1000 msec and the sampling rate 400/see. Calibration was performed by recording a 10 c/see 10/~V sine wave on the tape and summing this sine wave 100 times. The output of the CAT was obtained both in analog form as an X-Y plot and in digital form on punched paper tape from which digital magnetic tape was prepared in the Data Processing Laboratory of the Brain Research Institute; subsequent data analysis was performed on an IBM 7094 digital computer in the Health Sciences Computing Facility.

327

the dimmer flashes consist mainly of a single diphasic wave with a latency to the negative peak of about 160 msec. The latency of this peak increases by about 10 msec for each log unit reduction in the luminance of the TF (see also Tepas ~nd Armington 1962; Clynes et al. 1964; Diamond 1964; Efron 1964). Thus the stimuli are associated with two distinctly different patterns of the average evoked potential. The [~F; because of its greater luminance, has a double-peaked pattern whereas the dimmer TFs have a single-peaked pattern. ~In spite of the inter-subject variability in response, the same basic differences between the TF and BF evoked potentials were preserved for all four subjects.

Average evoked potentials obtained with paired flashes Fig. 2 shows the average evoked potentials obtained when flashes were presented in pairs. These tracings are from one subject, but similar results were obtained from the other four subRESULTS jects. In the average potentials obtained with Average evoked potentials obtained with single pairs of flashes separated by 250 msec the two different patterns (the single- and double-peaked stimuli Four stimuli were used in this part of the waves of the TF and BF, respectively) can be study, a 9000 mL circular flash (BF) and three easily distinguished and are similar to the patdifferent luminance values of a semicircular flash terns shown in Fig. I. In all cases the perceptual experience of (TF). In Fig. 1 are presented the average evoked brightness enhancement of TF (which was found potentials elicited by these stimuli in four subto occur at different IFIs for different values of jects. In this and in all subsequent records negativity at the occipital electrode is represented TF luminance, Donchin and Lindsley 1965) was associated with an overlapping of the evoked by an upward deflection. The pattern of the~¢ average evoked potentials potential patterns of the TF and the BF. The is quite consistent with results reported by other potentials evoked by the paired flashes with investigators (see Kooi and Bagchi 1964, for a IFIs in the brightness enhancement range, review). Two major diphasic waves can be ob- resemble neither the T F nor the BF pattern served in the response elicited by the brightest when these stimuli are presented alone (see Fig. flash (top row, Fig. 1). The latencies to the two 1), suggesting an electrophysiological interacnegative peaks are approximately 80 and 160 tion. The two bottom rows in Fig. 2 illustrate the msec; those to the positive peaks are about 115 and 200 msec. A number of later deflections can results with IFIs sufficiently short for the TF to be observed in some cases (see Cigfinek 1961; be retroactively masked by the BF: the average Contamin and Cathala 1961; Walter 1964). evoked potential being quite similar to that As reported previously 0Vicke et al. 1964), the elicited by the BF presented alone. It appears first diphasic component decreases in amplitude that displacement of the TF evoked potential is as the luminance of the flash is decreased and associated with the perceptual masking of this flash (see also Donchin et aL 1963). Note that eventually disappears. As shown in Fig. 1, the potentials elicited by this occurs at different IFIs as the luminance of Eiectroenceph. clin. NeurophysioL,1965, 19:325-335

328

E. DONCHIN AND D. B. LINDSLEY

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Fig. 2 Average evoked potentials from the visual area of subject JW to paired flashes (TF followed by BF, small vertical lines), separated by different IFIs (indicated before each trace and measured from onset of TF to onset of BF). Luminance of second, or brighter, flash 9000 mL in all cases; luminance of TF indicated at top of each column. Top row: Responses to paired flashes with interflash interval at which no perceptual interaction occurs; two flashes seen. Row.~' 2-4: Responses to paired flashes with interflash intervals at which brightness of first flash is enhanced; two flashes seen. Bottom 2 rows: Responses to paired flashes with interflash intervals at which the first flash is masked by the second flash; only one flash perceived.

the TF is varied: the longer the intervals over which masking takes place, the longer the IFIs at which the displacement of the TF evoked potential occurs.

The additivity of average evoked potentials The potentials obtained with paired flashes with a brief IFI represent a combination of the effect of the TF and BF. This intermingling of the two responses does not permit direct observation of the TF response. However, if the response to one of the two stimuli is known, and the rules under which the two responses combine are known, it is possible to obtain from the response to the pair the response to the other stimulus. Such a procedure has been used in studies of excitability cycles (Allison 1952; Shagass and Schwartz 1962; Cigfinek 1964). In these studies the assumption was made that the neural re-

sponse to a pair of stimuli is a linear sum of the responses to each of the two stimuli. It was further assumed that when a pair of stimuli of equal intensity is presented, the response to the first stimulus is not affected by the presentation of the second ::timulus. Under such conditions, a simple subtraction of the response to the first stimulus from the paired response should leave as a residual the response to the second stimulus. In the present work, we wanted to obtain the response to the first stimulus from the response to the pair, since the perceptual changes are associated with the first stimulus. As the second flash is not modified perceptually we assumed that the corresponding average evoked potential is not modified by the presentation of the TF. The response to the TF could then be obtained from the paired response by the subtraction of a BF response recorded alone. This procedure is valid Electroenceph. clin. Neurophysiol., 1965, 19:325-335

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Fig. 3 Comparison of average evoked potentials to paired stimuli, TF and BF (solid lines), with average evoked potentials synthesized from responses to TF and BF stimuli presented singly (dotted lines). Solid lines based on same data as in Fig. 2; dotted lines obtained by summing responses to TF alone with responses to BF alone. BF record is displaced by the appropriate interltash interval.

only if the two responses summate linearly to produce the response to the pair. In order to test this assumption, average evoked potentials were synthesized on the IBM 7094 computer. Each synthetic evoked potential represented a linear summation of the response to a TF presented alone with a response to a BF presented alone, the latter being shifted on the time axis to correspond to a given IFI. A TF average evoked potential and a BF average evoked potential recorded during the same experimental session were used in the synthesis. The BF trace was shifted on the time axis so that the onsets of the two stimuli for the two average evoked potentials were separated by a particular IF[. The two traces were then added algebraically. The ~ynthesized evoked potential was plotted with an average evoked potential recorded during the same session using a pair of flashes separated by the same IFI. The correlation (Pearson product-moment) be-

tween the two re:ords was calculated and was used as a measure of the similarity of the synthesized to the actually recorded evoked potential (see also John et al. 1964). Fig. 3 presents, for one subject, the comparison of evoked potentials to a synthesized pair of flashes (responses t o TFs and BFs recorded separately and put together as a pair with an appropriate IFI by means of the computer) with the average evoked potentials to an actual TFBF pair presented together at three different TF luminance levels (the plots are digital plots by the computer, based on 100 points only and therefore slightly dissimilar to the plots in Fig. 2 from which the solid iine was derived). For interflash intervals when there was no perceptual interaction, and also when brightness et,hancement occurred (first four rows), the actual and the synthetic evoked potentials are quite similar, both in amplitude and in latency, for all three levels of Electroenceph. clin. Neurophysiol., 1965, 19:325-335

330

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TF luminance. However, when the IFls are short and within the masking range (last two rows), the two evoked potentials are discrepant in both amplitude a1~d latency of the major components. The above results suggest that within certain ranges the brain operates on the two stimuli in a linearly additive manner; thus the response to the paired flashes is a sum of the responses to the two flashes as long as they are perceived as two flashes. If we now assume that the BF response is not affected by the presentation of the TF when the TF is dimmer than the BF, the response to the TF can be obtained by subtracting the properly shifted BF potential from the average evoked potential to the paired flashes.

Modifications in the test flash evoked potentials Subtractions such as were described in the previous section were performed on the IBM

7094 and the results for subject JW are presented in Fig. 4. When the TF is not masked, no appreciable change can be observed in its evoked potential. The potentials obtained during brightness enhancement conditions appear to contain the response to both the TF and the BF, neither of them modified to any great extent. On the other hand, in all three columns, as the masking stage is reached (two bottom rows), the TF potential is markedly reduced in amplitude and no order can be observed in its fluctuations. In other words, the residual left after subtracting the BF from paired evoked potentials obtained during masking suggests that no potentials were actually elicited by the masked TF. For the three T F luminance levels this blocking of the TF occurs at different IFIs. The residuals for all evoked potentials in each of the columns was correlated with the average Electroenceph. clin. NeurophysioL, 1965, 19:325-335

EVOKED RESPONSE CORRELATES OF PERCEPTION

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Fig. 5 Correlations (for three subjects at three luminance values for the TF) between the average evoked potential for the TF when presented alone and the residual attributed to the TF when the response to the BF has been subtracted from the response to the TF-BF pair. Pearson product-moment correlations are plotted against the interflash intervals (delays) at which the paired ,]ashes were presented. Note increase in correlations as delays increase. NO STIMULUS PRESENTED ANALYSIS SWEEP TRIGGERED BY

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Electroenceph. clin. Neurophysiol., 1965, 19:325-335

332

E. DONCHIN AND D. B. LINDSLEY

evoked potential obtained with the TF alone shown at the head of the column. A high correlation indicates that the TF potential was not greatly modified by the presentation of the BF. These correlations, plotted against delay or IFI, are presented in Fig. 5 for three subjects. The data are plotted separately for each TF luminance to show the degree of intersubject similarity. The trends discussed above for JW are apparent for the other subjects. Because the subject pressed a switch to present the stimuli it was important to determine the extent to which the act of pressing had an effect on the results. In Fig. 6 are shown the results obtained m the two control conditions. The averaging was initiated with respect to a trigger placed on the tape by either the subject or the experimenter without any flashes presented to the subject. In neither case was there any systematic pattern resembling the average evoked potentials obtained when flashes were presented. Thus, the pressing of the stimulus switch in itself had no effect on the potentials recorded in this study. The records of average evoked potentials presented here are not instances of myogenic potentials reported by Bickford et al. (1964). When the averaging technique is applied to EEG records obtained from relaxed subjects and in which high frequency "muscle activity" is absent no myogenic potentials will be obtained (Bickford, personal communication). In order to confirm this we have recorded and averaged potent!als from an electrode over cervical muscles of the neck to the ear reference ,,lectrode in this visual stimulation situation without indication of muscle response. DISCUSSION

This study has been concerned with the correlation of electrocortical events as registered by average evoked potentials and psychological events as represented by perception of stimuli studied by psychophysical methods. When two light flashes (the second more intense than the first) are presented in sequence, three psychological or perceptual events may be distinguished. (1) Twoness: Two flashes perceived as two without interaction. (2) Brightness enhancement: Two flashe3 which interact in such a way as to make

the first appear brighter than when presented alone or than when separated from the second by a greater IFI. (3) Perceptual masking: The characteristic form, pattern or orientation of the first stimulus is obliterated perceptually by the second when the IFI is sufficiently short. It has been possible to demonstrate that these three perceptual effects are paralleled by characteristic electrocortical events. When there is no perceptual interaction and the two flashes are perceived separately, the corresponding average evoked potentials are distinctly separate and typical of the pattern when each is recorded alone. When there is perceptual interaction and the TF is retroactively enhanced in brightness, there is clearly an electrophysiological interaction since the evoked potential pattern now resembles neither that of the TF nor the BF alone. Nevertheless when the evoked potential for the BF recorded alone is subtracted from that of the evoked potential to the pair of flashes presented together there remains as a residual the essential pattern of the TF. Finally, when perceptual interaction takes the form of masking of perception of the first flash by the second the form of the resulting average evoked potential is clearly that of the response to the BF or second flash, which appears to have displaced completely the response to the first flash. In attempting to understand the brightness enhancement effect it is of interest to consider whether the two flashes have separate representations beyond the receptor cell level. We have suggested (Donchin and Lindsley 1965), on psychophysical grounds, that the enhancement is an increase in the brightness of the TF and that the two flashes maintain separate representations beyond the receptor cell layer. This suggestion is confirmed by the results presented above. The two flashes elicit two separate evoked potentials and as wa~ shown in Fig. 4 the evoked potentials obtained under this condition are similar to those elicited by the two flashes when presented singly. Thus the data presented here suggest that the interaction producing the brightness enhancement effect is between the neural representations of the two flashes and this is reflected at the cortical level by the average evoked potentials. It has been suggested that retroactive masking Electroenceph. din. Neurophysiol., 1965, 19:325-335

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is due to the different rates at which the neural messages are generated and conveyed centripetally when the BF luminance is greater than that of the TF (Crawford 1947; Baker 1963). The latency of the ERG and the neural response in the visual pathways is inversely related to the intensity of stimulation (Hartline 1938; Bernhard 1940; Auerbach et al. i961; Granit 1962; Hughes 1964). Since the BF is more intense than the TF, messages produced by the BF are assumed to "overtake" and interfere with the reception of messages evoked by the TF. The processes generated by the BF thus are assumed to interact with those of the TF, producing either the masking or the enhancement effects. Such an account of these perceptual phenomena implies that at some point in the neural mechanism associated with visual perception the neural response to the TF is modified as a result of the BF. This modification is assumed to be a neural correlate of the perceptual phenomena described. No such modification, associated either with perceptual masking or with brightness enhancement, has heretofore been directly observed. This report has described an investigation of average evoked potentials over the visual area in which electrocortical correlates of these perceptual effects have been observed. A question of considerable importance is the localization of the stage in the visual system at which the neural effect produced by the second flash (BF) interferes with the effect elicited by the first flash (TF). At least four different loci in which these interactions may take place can be identified. These are the retina (receptor cell layer to ganglion cell layer), lateral geniculate body (relay and other functions), cortical structures (primary and secondary) and nonspecific sensory systems (reticuI~r and thalamic). The data presented in this report, although certainly not conclusive in this respect, provide information which has a possible bearing on the alternatives. The evidence presented indicates quite clearly that the interactions take place at or preceding the point at which the evoked potentials are recorded from over secondary or association cortex (areas 18 and 19). Thus the same processes that are involved in the perceptual masking effect seem to be involved in the blocking of the

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average evoked potential. Whereas the sot:rce c~f the evoked potentials has not been identified it is quite probable that these potentials are elicited in cortical structures. The integrity of these processes seems to be a necessary condition for the perception of the stimulus eliciting them. It would not be appropriate to conclude at this time that the masking is a "subcorticar' effect. Since the average evoked potentials could represent activity of the association areas of the visual cortex (see also Allison 1962; Ebe and Mikami 1962; Goff et al. 1962; Donchin 1964), the masking interaction might take place at the primary receiving area or between primary and secondary areas. In the last few years numerous investigations have established that average evoked potentials can be recorded rather reliably and that their configurations show a fairly consistent pa! g,~rn. The relationship of evoked potentials to per~ptuai and behavioral phenomena should be carefully investigated. The results of the present investigation, as well as other recent studies (Dustman and Beck 1963; Vaughan et al. 1963; Garcia-Austt et al. 1964; Haider et al. 1964; Walter 1964; and others in Whipple 1964; Callaway et al. 1965), suggest that the evoked potentials are intimately related to the data processing activity of the cortex and that they covary with the perception of stimuli and with the arousal and attentive state of the subject. SUMMARY

Average evoked cortical potentials to pairs of flash stimuli have been studied in five subjects under conditions which give rise to three perceptual effects. When the flashes are relatively far apart two distinct flashes are seen and there is po perceptual interaction. As the flashes are brought closer together there is a retroactive brightness enhancement of the first flash by the second. When the flashes are still closer together a stage is reached where only one flash is seen and the characteristics of the first flash which the subject is required to report are masked. The parameters of the flash stimuli determine the critical point at which twoness and brightness enhancement cease and where masking of the first flash by the second begins. These parameters are the luminance level, Electroenceph. clin. NeurophysioL, 1965, 19:325-335

334

I~. DONCHIN AND D. B. LINDSLEY

the ratio of the luminances of the two flashes, and their durations. For all interflash intervals in the brightness enhancement range the response to paired flashes was approximately a linear sum of the responses to the two flashes when presented alone. Thus when the response to the second, brighter, flash (BF) was subtracted from the response to the pair, the residual represented the response to the first, or test flash (TF). For interflash intervals in the masking range, tho residual shows no detectable response to the T F after subtracting the response to the BF. These results suggest that retroactive brightness enhancement represents an interaction between the neural representations of the two flashes, while the masking phenomenon is due to a displacement of the neural response to the T F by the response to the BF and that this interaction occurs prior to the stage at which the average evoked potential is elicited. REFERENCES

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