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Visually evoked cortical responses to the on-and off-set of patterned light in humans
VSion Rex Vol. 11, pp. 685-695.
Pergamon Press 1971. Printed in Great Britain
VISUALLY EVOKED CORTICAL RESPONSES TO THE ONAND OFF-SET OF PATTERNED LIGHT IN HUMANS’ M. RUSSELL HARTER Department of Psychology,Universityof North Carolina, Greensboro, North Carolina 27412,U.S.A. (Received 15 July 1970; in revisedfortn 22 August 1970)
INTRODUCTION SINGLEunits in the visual system of animals are particularly sensitive to temporal and spatial gradients in visual stimuli falling on their receptive fields-the on- and off-set of light containing edges or contours (see GLEZER, 1965 ; JACOBS,1969; THOMAS,1970 for recent reviews). Visually evoked cortical responses (VERs), recorded from the surface of the human scalp, similarly are sensitive to the sharpness of contours (COPENHAVER and PERRY, 1964; LIFSHITZ, 1966; CLYNESand KOHN, 1967; HARTERand WHITE, 1968), the distance between or density of contours (SPEHLMANN, 1965; SPEKREIJSE,1966; REITVELD, TORDOIR, HAGENOUW,LUBBERSand SPOOR, 1967; LOMBROSO,DUFFY and ROBB, 1969; MACKAY, 1969; HARTERand SUITT, 1970; HARTER and WHITE, 1970; HARTER, 1970), and the onand off-set of diffuse light (CLYNES,KOHN and LIFSHITZ, 1964; EFRON, 1964; ELLINGSON, 1964; WHITE and EASON,1966; PEACOCK,1970). These effects typically were attributed to physiological processes (lateral inhibition, receptive field size, and on- and off-cells observed at the single unit level of animals). If this assumption is correct, it may be predicted that VERs to both the on- and off-set of patterned light should reflect the sharpness and density of contours in the evoking stimulus. In previous studies, the effects of patterned stimuli on VERs have been investigated with brief light flashes wherein the on- and off-set of the stimulus are virtually simultaneous. On- and off-response thus were confounded and could not be observed independently. In the present study, VERs to the on- and off-set of patterned light are investigated as a function of the sharpness and density of pattern in the evoking stimuli. A secondary purpose is to investigate the effects of variations in the distance between the elements of the stimulus independently of the size of the elements. In previous studies where the effects of contour density were systematically investigated (REIT~ELD et al., 1967; HARTER and WHITE, 1970; HARTER,,1970), the distance between stimulus elements was manipulated by varying the size of checks in checkerboard patterns. It could not be determined, therefore, whether the relevant aspect of the stimulus was the size of checks or the distance between checks, these variables being confounded. 1 This study was presented at the Southeastern Psychological Association, Louisville, Kentucky, U.S.A., 1970. It was supported by funds received from the National Science Foundation under Grant GB-8053. I wish to thank Mr. RONALD HUGH= and Miss MILDRED MOORE,graduate research assistants employed under NSF Grant GB 7324, for their helpful assistance in the collection and analysis of the data. 685
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M. Russ~~ HAR’IER
METHOD sUlJ&cts Six subja%s participated in the expariment: three fan&es (MM, CS, and PI) and three males (RH, RGH, and IL). Subjects ranged from 22-29, had binocular visual acuitics of batter than 20/20, and were working in the author’s labotatory. Stimuli
Polka-dot stimulus patterns (white dots on black background) ware found most suitable for the purpose of the premnt axpuim& t. This cunBgui&on lands itself to the hrdapandent manipulation of the sixe of dots and tha distanca batwcen dots. It also contains a =aunsidarable amount of contour, a factor we have found prarequisite to obtaikg appmciable pattern a&&s on VERs. Subjacts viawed two aarias of polka-dot @tarns. each diffaring in the overall distance between dots. The ratio of bstwean_dot-d&anca and dot-& for s&s I and II raspaCtively was 1.59 and @85.The bstwbsn dot-distance of a given pattern in sach series, themfore, may be obtainad by multiplying dot-size by a factor of 159 and 0.85 raspactively. Six dot&ass ware usad in both series (21.7, 11.2, 8.1, 57, 4.1 and 3.5 min of arc subtansa), a savanth difiusa “pat&m” (3.5 mln dotkxa diffused with -t5 diopter refractive lens) serving as a control. Theaa valm wcru se&ad for inveatigativn since previous data indicate VERs are particukly san&iiy~ to va&&ns in the dhuanca batwaen stimulus elements between 0 and 40 min of arc (Hm~xa and Wurrx, 1970; ‘tirlRleR. 1970). Two patterns for aach series. am illustrated in Fig. 1. It should be noted
Ratio:bshreen-dot-distance/dot-airs
Ro. 1. Illustration of four of the stimulus tmnspamncies viewed by subjects: the two largest dot-sixes for between-dotdistanca/dot& ratios of 139 (saries I) and 0.85 (sarias II). that ~~~t~~ was manipulated i&panda&y of dot-size whereas changes in dot-sins always were cnnfmmdad witbchangas in batw The stimuli wem pNJduccd phi type 146L P&raid tranapamnckn ~~~plrrcadoveratranslumntscran~~46cmfronnt)rc*u~ (thcddspirys~~in~~~ar~~s~ut7”). contoura@pst@awa8varladbyhauing yiaw ona stimuh~ pattarn (dot-&a = 8.1 mitt of arc; ~~=69rninofam~thraughnsariaaofrrdracthnr lanaas (0, -1, -3, and -5 dioptam). This procedure had the advantage of prugmasivaly transforming the stimulus from patternad to diffuse light.
Visually Evoked Cortical Responses to the On- and Off-set of Patterned Light in Humans
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Apparatus and recording procedure Subjects were situated in an electrically-shielded, partially soundproof cubicle and requested to binocularly tixate a l-mm white dot situated in the center of each display. The fixation point was dimly illuminated (0.01 ml) and visible at all times. The polka-dot displays were visible only when the stimulus light was on, otherwise they were darkened. The stimulus transparencies were back-illuminated once every 1300 msec for a period of 600 msec by a perceptually fused (presented at 70 Hz) train of lO+sec light flashes. Luminance transmittance was constant for a given pattern series, although it was twice as great for series II as I. A Grass S8 Laboratory Stimulator was used td drive a Grass PS2 Photostimulator set on intensity “2” (cu. 3.5 log units above threshold for pattern series I of the experiment). A Grason-Stadler 9OlB noise generator provided sticient white noise to mask auditory stimuli generated by the experimental equipment. Electroencephalograms were recorded monopolarly from the surface of the scalp (active and reference electrodes placed respectively 2.5 cm above the inion on the midline and on the right earlobe) and amplified with a Grass 7WC polygraph (4 amplitude high and low frequency filters set at 35 and 1 Hz respectively). A Fabri-tek 1062 Instrument Computer was set to sample and store 1280 msec of activity following stimulus on-set. Thus, VERs to stimulus on- and off-set (which occurred 600 msec after on-set) were recorded in a single channel of the computer. Averaged VERs (based on 64 summations) were recorded with a HewlettPackard 7035B X-Y plotter and Fabri-tek 201 Decimal Printer. Individual VERs were monitored on a Hewlett-Packard 141A variable persistance oscilloscope. Experimental design The experiment was divided into three parts: pattern series I, pattern series II, and refractive lens series. A series consisted of the consecutive presentation of all the dot-sizes or refractive lens. The three series were presented to a subject in a given l+hr recording session, four such sessions (replications) being conducted on diierent days. The order of presenting the three series, dot-sizes, and refractive lenses was counterbalanced across subjects, recording sessions, and days or replications.
RESULTS
The effects of refractive lens value (induced refractive error or degree of blurring)2 on VERs to the on- and off-set of patterned light are illustrated in Fig. 2. The wave-form and amplitude of both on- and off-responses varied as a function of refractive lens value. In general, greater amplitude VERs were elicited by stimulus on-set and by sharply focussed pattern (0 diopter lens condition). Consistent with data based on checkerboard patterns (HARTER and WHITE, 1968), induced refractive error resulted in a progressive reduction in VER amplitude, particularly at latencies of approximately 100 and 180 msec after stimulation. When the image was sharpened (refractive error reduced), the amplitude of activity at IO0 msec latency became more negative, in some subjects (e.g. Fig. 2, JL) showing a progressive transition from a surface-positive to surface-negative component; whereas the amplitude of activity at approximately 180 msec latency became progressively more positive. To be consistent with the terminology used in previous studies (SPEHLMANN, 1965 ; RIETVELD et al., 1967; HARTER and WHITE, 1968), these potentials will be termed “surfacenegative” (SN) and “surface-positive” (SP). They are respectively most negative and positive when VERs are elicited by patterned light as compared to diffuse light. It may be noted that CLYNE~ et al. (1964) and EFRON (1964) reported on- and off-responses to diffuse light similar in wave-form to those shown here, although alpha-like afteractivity was not associated with stimulus off-set as they observed. Pronounced after-activity was evident in VERs obtained from only two of the eight subjects : in one case (subject MM) z The refractivelensesemployeddo not necessarily reflect the absolute degree of induced refractive error. Since a dim fixation point was visible to subjects, some accommodation possibly occurred when viewing the patterns through the refractive lens. The subjective reports indicated that the -1 diopter lens resulted in little blurring whereas the -3 and -5 diopter lenses virtually diffused the pattern, although the form of the pattern was discernable with the -3 diopter lens. We are presently investigating the possibility of measuring accommodation with this technique.
lo0 200
300 400
Lofency,
500 600
msec
Fro. 2. Effects of induced refractive error or blurring on VERS to the on- and off-set of the pattern with a between-dot-distanceldotsize ratio of 6-9/&l min of arc (subjects RH and JL). Each of the four superimposed tracings is a replication based on 64 summations. Arrows and dotted lines indicate latencies at which the amplitude of the surface-negative (SN) and surface-positive (SP) components
100 200 300 400 500
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Visually Evoked Cortical Responses KOme On- and Off-set of Patterned Light in Humans
Ratio between-dot-distance/dot-size 0-85
s
-6
f0n
Stimm
Rti
Stimillus
Diffuse
100zoo300 Latency,
400
500
msec
Fro. 3. Effects of dot-size and ~tw~n-dot-distaa~~dot-size ratio (t.59 and 045) on VERs to the on- and off-set of patterned light (subject RH). Rest of aption same as in Fig. 2.
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to the on-set of patterned light, and the other case (subject RGH) to the off-set of diffuse light. On- and off-responses to patterns varying in distance between stimulus elements are illustrated in Fig. 3. The amplitude of the SN and SP components of on- and off-responses of ah subjects was rneasured,3 averaged across subjects and replications, and plotted as a function of dot-size and between-dot-distance/dot-size ratio (Fig. 4). Variance analyses appropriate for the repeated-measures design used in this study were performed to aid in Component Surface -negative B64-
BDWDS: Id9 l -. 0.85b--0
2-
(Diffuse)
(Diffuse) Dot-size
(min of arc)
Fro. 4.
Effects of dot-size and betwcundotdistanm by dot-size ratio (BDD/DS) optha amplitude of the surfac+negativc and sur&c-positive components in VJ3Ksevoked by the on- and off-set of patterned light. Data have been averaged across subjcczs and rcplim&ns, each piotted point being an awxage of 24 scores. See text for further expfsnation.
the interpretation of data. Dot-size (confounded with between-dot-distance) signikantly intluenced the amplitude of the SN and SP components of both on- and off-responses (P < @OOl). All measures indicated a decrease in VER amplitude as dot-size was reduced below 8.1 min of arc. Although the main effect of between-dot-distance, as reflected by the two stimulus ratios, did not approach statistical significance at the w)5 level, this variabie interacted siggificantly the amplitudes of the SN and SP components has been darctibad in ‘ThcproccdumuscdtomcasW previous studies (HARTERand Wnrrz, 196%:I%msu, 1370]. Sri&y, the amplitude of the SN and SP ~meuUreain~~Othc~~voltrgelevelat~owst).~Mz)lftudo zzzz?s coqmnant was mcasuzed at a Wmcy dstcrrmnsd by the puak of this componaatllndortbs 21~7&t~~~Thislotoncyw#dftom9oto128Illsrc~onthesubjsct.Tbs~of tbaSPcompomntwo3always~utrdatitspePkwhichvaric#1Inlrrtsncyfrom155to~~depcndhrO on the wbjcct.
VisuallyEvokedCorticalResponsesto the On- and Off-setof Patterned Lightin Humans
691
with dot-size for the SP component of the on-response and SP and SN components of the off-response (P < O-01). The effects of reduction of between-dot-distance depended on the initial distance between stimulus elements. A reduction in between-dot-distance resulted in increased amplitude as long as it was not reduced below approximately 9 min of arc-for example, given the 1l-2 min of arc dot-size, reducing between-dot-distance from 17.8 (l-59 x 11.2) to 9.52 (0.85 x 11-2) min of arc resulted in an increase in amplitude. If between-dot-distance was reduced below approximately 9 min of arc, then amplitude was reduced-for example, given the 8-l min of arc dot-size, reducing between-dot-distance from 12.88 (l-59 x 8.1) to 6.89 (0.85 x 8-l) min of arc resulted in a decrease in amplitude. This trend was evident with the SN component of the on-response although the critical between-dot-distance appeared to be greater (between 11.2 and 21.7 min of arc). Perhaps if larger dot-sizes had been employed, this interaction also would have been statistically significant. The functions shown in Fig. 4, particularly the amplitude of the off-responses, appear to have an inverted “U” shaped form, VER amplitude first increasing and then decreasing as dot-size was reduced. The peaks of these functions, reflecting the most effective stimuli, occurred in response to different between-dot-distance/dot-size combinations depending on the measure considered: 18*5/21-7 and 17*9/l l-2 for the SP component of the on-response, and 9*5/l l-2 and 12-g/8*1 for the SN and SP components of the off-responses. Although the functions reflecting the amplitude of the SN component of the on-response were not inverted “U” shaped, it may be assumed that they would have been if a sufficient range of dot-sizes had been employed. Previous studies employing checkerboard light flashes indicated the amplitude of the SN component peaked in response to checks subtending 20-30 min of arc (RIETVELDet al., 1967; HARTERand WHITE, 1970; HARTER, 1970). In summary, the most effective stimuli appear to be those where the distance between stimulus elements (both dot-size and between-dot-distance) was between approximately 10 and 30 min of arc subtense, depending on the measure considered. The most effective stimuli were smaller in subtense for the off-responses than on-responses.
DISCUSSION
The data support the prediction that VERs to both stimulus on- and off-set should reflect the sharpness and density of contours in the evoking stimuli. In general, reducing the sharpness of the patterns as well as the distance between stimulus elements (below approximately 9 min of arc) resulted in a decrease in amplitude of the SN and SP components of both on- and off-responses. The presence or absence of polka-dot pattern influenced the wave-form of on-responses in the same manner as did checkerboard patterned light flashes in previous studies (SPEHLMANN,1965; RIETVELDet al., 1967; HARTERand WHITE, 1968). Perceptual and physiological processes have been reported which may be related to the effects of contour sharpness and distance between contours on VERs. Contrast effects, which are greatest when the light and dark elements of the stimulus are separated by a sharp boundary (THOMASand KOVAR, 1965; WESTHEIMER,1966), possibly account for the effects of refractive error. Numerous perceptual variables have been shown to vary with the spatial extent of stimulus elements: efficiency of detecting stimulus orientation (ANDREWS, 1967), perceived contrast (BRYNGDAHL, 1966; WESTHEIMER,1966; BLAKEMORE
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and CAMPBELL,1969), visual acuity (FLOM, HEATH and TAKAHASHI,1963), figural aftereffects (GANZ, 1964), and visual threshold (GLEZER, 1965; RATLIFF,1965). Critical distance of approximately 10-30 min of arc were reported in these studies; reduction of distance between stimulus elements below these values inhibited the perceptual phenomenon. The inverted “U” shaped function between VER amplitude and the distance between stimulus elements possibly reflects the interaction between perceived amount of edge (which increases with reduction of distance) and contrast between elements (which decreases with reduction of distances starting at IO-30 min of arc). WIESELand HUBEL(1966) have shown that lateral geniculate cells in macaque monkeys (whose visual system is particularly similar to the human’s) have retinal receptive fields arranged in a manner which enhances their sensitivity to white (and black) spots of light. As suggested in earlier ‘studies (HARTER‘and WHITE, 1968, 1970; HARTER, 1970), the modal size of receptive field centers and the inhibitory interaction between antagonistic parts of fields may account for the effects of stimulus element size and contour sharpness on VER amplitude. VERs to stimulus on-set (or off-set) possibly reflect the combined effect of excitation of on-center (or off-center) cells and inhibition of off-center (or on-center) cells, the most effective stimulus size being determined by the modal size of the receptive field center of the cells activated. HUBEL and WIESEL(1966), have reported modal sizes varying from 7.5 to 30 min of arc, depending on the type of cell, which supports this suggestion. The majority of cells observed at the lateral geniculate and cortical levels of monkeys have on-centers (WIESELand HUBEL, 1966; HUBELand WIESEL,1968 ; WURTZ, 1969) which may account for the fact that greater amplitude on- than off-responses were obtained in the present study. Blurring of contours possibly resulted in a reduction in VER amplitude since stimulation was spread to the antagonistic part of the receptive field. THOMAS(1970) has formulated a model which also deals with the effects of blurring and stimulus size in terms of lateral inhibition and receptive field size. If the most effective stimulus sizes in the present study, in terms of evoking the greatest amplitude VERs, reflect the modal size of receptive field centers, the question may be raised as to why the critical stimulus size combinations varied for on- and off-responses. Possibly, the most effective stimulus size was smaller for the off- than on-responses because: (a) receptive field size was reduced as a function of light adaptation (GLEZER, 1965) and thus would be smaller at stimulus off- than on-set; or (b) contours may have been slightly blurred at stimulus onset, due to accommodation error, which would result in a shift in peak sensitivity to larger element sizes (HARTER and WHITE, 1970). The latter alternative appears unlikely-accommodation appeared to remain stable during the course of stimulation since the effects of refractive error were similar on both on- and off-responses (Fig. 2). The first alternative is complicated by the fact that the size of receptive field centers apparently is smaller for on- than off-center cells (WIESELand HUBEL, 1966). It may be noted that the SN and SP components differed in terms of how pronounced they were in the on- and off-responses to patterned light (Figs. 2 and 3), the SN component being primarily an off-response and SP component an on-response. This was less apparent in responses to diffuse light. Possibly, the amplitude of SN and SP components primarily reflect the activity of off- and on-center cells respectively. It is of interest to note that the activity of on- and off-center cortical cells of cats showed increased activity at latencies which correspond respectively to the latencies of the SP and SN components discussed in this study (CREUTZFELDT,ROSINA,ITO and PROBST,1969; MACKAY, 1969). In conclusion, it is acknowledged that the proposed relationship between various com-
Visually Evoked Cortical Responses to the On- and Off-set of Patterned Light in Humans
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ponents of on- and off-responses, recorded at the surface of the human scalp, and activity at the single unit level, based on animal studies, is speculation and at best an over simplification of the relationship. The investigation of on- and off-responses to patterned light, however, appears to be a step in the direction of isolating the physiological basis and significance of individual components of the complex VER. REFERENCES ANDREWS,
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Abbrt--Averaged evoked cortiatl potentials were investigated as a function of the on- and off-set of patterned light (white polka-dots on biack background) and the density and +arpnassof~~.sixadulthumrneu~viawsdpattcrwdstimuliwhichwsnillumrnrrtbd once~1~3sec:for6oomsec.contour~tyand~swas~~byvpryiasthe size and distance between polka-dots (O-35 min of arc) and induced refractive error (O-5 dioptcl8). AlIalyses of va&wx!cindicrrwdthatdotain:and bwee+dot-bistprrcsi~tly componrmts of evoked reaponscato both the oninfiuencedthe8mplitudeofsarlyandlate and~attofIwttern(P<~i).hgamral,~~hldeproqsssinslydagarsedasa function of doacased d&bUXObf&VC&kokmpalsd~rrdraGtVOarroi.Thsdts wero~intennsol~iahibitionandthesizeofraaptivefieidantmofon-and off-center cc& in the visual system of animals.
-Es wurden evo&rte Rindenpotcntiale als eine Funktion des An und Ab einea Lichtmuaten3 (n&a&% v&se Po&f4~111kteauf einem schwarzcn HinturgRtnd-)und $x umriasdiite und -sch&f8 untsrsucht. Se&s erwachwne Vp betrachteten MuWw4ze+ wekhe jade 1.3 Sek w&end 600 J&e& befeuchtet w&en. Die Umriasdichte amd -aeh&rfo wurden ve&dert, indem F die Gr%se und JWf’emung zwischen den Polkapunkten (av&chen 0 wd 35 a&n) veriindcrte amd Refraktionsf&hler(zwischen 0 und 5 Dioptrien) eh&hr& Die Vpdraaasrfylc @te ark,&assPur&tgrBse und wwg die Amplitude der fr%heranund &ten Ant&o der ev&vtenAntwortenbeimAnundAbdes Musters stark be&&awn (P < 0.01). Im AI&ameinen ver&ndertesich die Antwert fortschreitend, ah rich die Ent&mtm~ zwiscbeti den Re@&rmmten w&r&ate und der RefrakWel&IimShUledertatarathanmuafl&der tionsfehler sich e&&h&. Die Gr&sseder RezeptivWdmitten der on- und of%Mittenzdkn der Gcsichtsapparate der Tiere beSProchen.
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Pergamon Press 1971. Printed in Great Britain
VISUALLY EVOKED CORTICAL RESPONSES TO THE ONAND OFF-SET OF PATTERNED LIGHT IN HUMANS’ M. RUSSELL HARTER Department of Psychology,Universityof North Carolina, Greensboro, North Carolina 27412,U.S.A. (Received 15 July 1970; in revisedfortn 22 August 1970)
INTRODUCTION SINGLEunits in the visual system of animals are particularly sensitive to temporal and spatial gradients in visual stimuli falling on their receptive fields-the on- and off-set of light containing edges or contours (see GLEZER, 1965 ; JACOBS,1969; THOMAS,1970 for recent reviews). Visually evoked cortical responses (VERs), recorded from the surface of the human scalp, similarly are sensitive to the sharpness of contours (COPENHAVER and PERRY, 1964; LIFSHITZ, 1966; CLYNESand KOHN, 1967; HARTERand WHITE, 1968), the distance between or density of contours (SPEHLMANN, 1965; SPEKREIJSE,1966; REITVELD, TORDOIR, HAGENOUW,LUBBERSand SPOOR, 1967; LOMBROSO,DUFFY and ROBB, 1969; MACKAY, 1969; HARTERand SUITT, 1970; HARTER and WHITE, 1970; HARTER, 1970), and the onand off-set of diffuse light (CLYNES,KOHN and LIFSHITZ, 1964; EFRON, 1964; ELLINGSON, 1964; WHITE and EASON,1966; PEACOCK,1970). These effects typically were attributed to physiological processes (lateral inhibition, receptive field size, and on- and off-cells observed at the single unit level of animals). If this assumption is correct, it may be predicted that VERs to both the on- and off-set of patterned light should reflect the sharpness and density of contours in the evoking stimulus. In previous studies, the effects of patterned stimuli on VERs have been investigated with brief light flashes wherein the on- and off-set of the stimulus are virtually simultaneous. On- and off-response thus were confounded and could not be observed independently. In the present study, VERs to the on- and off-set of patterned light are investigated as a function of the sharpness and density of pattern in the evoking stimuli. A secondary purpose is to investigate the effects of variations in the distance between the elements of the stimulus independently of the size of the elements. In previous studies where the effects of contour density were systematically investigated (REIT~ELD et al., 1967; HARTER and WHITE, 1970; HARTER,,1970), the distance between stimulus elements was manipulated by varying the size of checks in checkerboard patterns. It could not be determined, therefore, whether the relevant aspect of the stimulus was the size of checks or the distance between checks, these variables being confounded. 1 This study was presented at the Southeastern Psychological Association, Louisville, Kentucky, U.S.A., 1970. It was supported by funds received from the National Science Foundation under Grant GB-8053. I wish to thank Mr. RONALD HUGH= and Miss MILDRED MOORE,graduate research assistants employed under NSF Grant GB 7324, for their helpful assistance in the collection and analysis of the data. 685
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M. Russ~~ HAR’IER
METHOD sUlJ&cts Six subja%s participated in the expariment: three fan&es (MM, CS, and PI) and three males (RH, RGH, and IL). Subjects ranged from 22-29, had binocular visual acuitics of batter than 20/20, and were working in the author’s labotatory. Stimuli
Polka-dot stimulus patterns (white dots on black background) ware found most suitable for the purpose of the premnt axpuim& t. This cunBgui&on lands itself to the hrdapandent manipulation of the sixe of dots and tha distanca batwcen dots. It also contains a =aunsidarable amount of contour, a factor we have found prarequisite to obtaikg appmciable pattern a&&s on VERs. Subjacts viawed two aarias of polka-dot @tarns. each diffaring in the overall distance between dots. The ratio of bstwean_dot-d&anca and dot-& for s&s I and II raspaCtively was 1.59 and @85.The bstwbsn dot-distance of a given pattern in sach series, themfore, may be obtainad by multiplying dot-size by a factor of 159 and 0.85 raspactively. Six dot&ass ware usad in both series (21.7, 11.2, 8.1, 57, 4.1 and 3.5 min of arc subtansa), a savanth difiusa “pat&m” (3.5 mln dotkxa diffused with -t5 diopter refractive lens) serving as a control. Theaa valm wcru se&ad for inveatigativn since previous data indicate VERs are particukly san&iiy~ to va&&ns in the dhuanca batwaen stimulus elements between 0 and 40 min of arc (Hm~xa and Wurrx, 1970; ‘tirlRleR. 1970). Two patterns for aach series. am illustrated in Fig. 1. It should be noted
Ratio:bshreen-dot-distance/dot-airs
Ro. 1. Illustration of four of the stimulus tmnspamncies viewed by subjects: the two largest dot-sixes for between-dotdistanca/dot& ratios of 139 (saries I) and 0.85 (sarias II). that ~~~t~~ was manipulated i&panda&y of dot-size whereas changes in dot-sins always were cnnfmmdad witbchangas in batw The stimuli wem pNJduccd phi type 146L P&raid tranapamnckn ~~~plrrcadoveratranslumntscran~~46cmfronnt)rc*u~ (thcddspirys~~in~~~ar~~s~ut7”). contoura@pst@awa8varladbyhauing yiaw ona stimuh~ pattarn (dot-&a = 8.1 mitt of arc; ~~=69rninofam~thraughnsariaaofrrdracthnr lanaas (0, -1, -3, and -5 dioptam). This procedure had the advantage of prugmasivaly transforming the stimulus from patternad to diffuse light.
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Apparatus and recording procedure Subjects were situated in an electrically-shielded, partially soundproof cubicle and requested to binocularly tixate a l-mm white dot situated in the center of each display. The fixation point was dimly illuminated (0.01 ml) and visible at all times. The polka-dot displays were visible only when the stimulus light was on, otherwise they were darkened. The stimulus transparencies were back-illuminated once every 1300 msec for a period of 600 msec by a perceptually fused (presented at 70 Hz) train of lO+sec light flashes. Luminance transmittance was constant for a given pattern series, although it was twice as great for series II as I. A Grass S8 Laboratory Stimulator was used td drive a Grass PS2 Photostimulator set on intensity “2” (cu. 3.5 log units above threshold for pattern series I of the experiment). A Grason-Stadler 9OlB noise generator provided sticient white noise to mask auditory stimuli generated by the experimental equipment. Electroencephalograms were recorded monopolarly from the surface of the scalp (active and reference electrodes placed respectively 2.5 cm above the inion on the midline and on the right earlobe) and amplified with a Grass 7WC polygraph (4 amplitude high and low frequency filters set at 35 and 1 Hz respectively). A Fabri-tek 1062 Instrument Computer was set to sample and store 1280 msec of activity following stimulus on-set. Thus, VERs to stimulus on- and off-set (which occurred 600 msec after on-set) were recorded in a single channel of the computer. Averaged VERs (based on 64 summations) were recorded with a HewlettPackard 7035B X-Y plotter and Fabri-tek 201 Decimal Printer. Individual VERs were monitored on a Hewlett-Packard 141A variable persistance oscilloscope. Experimental design The experiment was divided into three parts: pattern series I, pattern series II, and refractive lens series. A series consisted of the consecutive presentation of all the dot-sizes or refractive lens. The three series were presented to a subject in a given l+hr recording session, four such sessions (replications) being conducted on diierent days. The order of presenting the three series, dot-sizes, and refractive lenses was counterbalanced across subjects, recording sessions, and days or replications.
RESULTS
The effects of refractive lens value (induced refractive error or degree of blurring)2 on VERs to the on- and off-set of patterned light are illustrated in Fig. 2. The wave-form and amplitude of both on- and off-responses varied as a function of refractive lens value. In general, greater amplitude VERs were elicited by stimulus on-set and by sharply focussed pattern (0 diopter lens condition). Consistent with data based on checkerboard patterns (HARTER and WHITE, 1968), induced refractive error resulted in a progressive reduction in VER amplitude, particularly at latencies of approximately 100 and 180 msec after stimulation. When the image was sharpened (refractive error reduced), the amplitude of activity at IO0 msec latency became more negative, in some subjects (e.g. Fig. 2, JL) showing a progressive transition from a surface-positive to surface-negative component; whereas the amplitude of activity at approximately 180 msec latency became progressively more positive. To be consistent with the terminology used in previous studies (SPEHLMANN, 1965 ; RIETVELD et al., 1967; HARTER and WHITE, 1968), these potentials will be termed “surfacenegative” (SN) and “surface-positive” (SP). They are respectively most negative and positive when VERs are elicited by patterned light as compared to diffuse light. It may be noted that CLYNE~ et al. (1964) and EFRON (1964) reported on- and off-responses to diffuse light similar in wave-form to those shown here, although alpha-like afteractivity was not associated with stimulus off-set as they observed. Pronounced after-activity was evident in VERs obtained from only two of the eight subjects : in one case (subject MM) z The refractivelensesemployeddo not necessarily reflect the absolute degree of induced refractive error. Since a dim fixation point was visible to subjects, some accommodation possibly occurred when viewing the patterns through the refractive lens. The subjective reports indicated that the -1 diopter lens resulted in little blurring whereas the -3 and -5 diopter lenses virtually diffused the pattern, although the form of the pattern was discernable with the -3 diopter lens. We are presently investigating the possibility of measuring accommodation with this technique.
lo0 200
300 400
Lofency,
500 600
msec
Fro. 2. Effects of induced refractive error or blurring on VERS to the on- and off-set of the pattern with a between-dot-distanceldotsize ratio of 6-9/&l min of arc (subjects RH and JL). Each of the four superimposed tracings is a replication based on 64 summations. Arrows and dotted lines indicate latencies at which the amplitude of the surface-negative (SN) and surface-positive (SP) components
100 200 300 400 500
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Visually Evoked Cortical Responses KOme On- and Off-set of Patterned Light in Humans
Ratio between-dot-distance/dot-size 0-85
s
-6
f0n
Stimm
Rti
Stimillus
Diffuse
100zoo300 Latency,
400
500
msec
Fro. 3. Effects of dot-size and ~tw~n-dot-distaa~~dot-size ratio (t.59 and 045) on VERs to the on- and off-set of patterned light (subject RH). Rest of aption same as in Fig. 2.
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to the on-set of patterned light, and the other case (subject RGH) to the off-set of diffuse light. On- and off-responses to patterns varying in distance between stimulus elements are illustrated in Fig. 3. The amplitude of the SN and SP components of on- and off-responses of ah subjects was rneasured,3 averaged across subjects and replications, and plotted as a function of dot-size and between-dot-distance/dot-size ratio (Fig. 4). Variance analyses appropriate for the repeated-measures design used in this study were performed to aid in Component Surface -negative B64-
BDWDS: Id9 l -. 0.85b--0
2-
(Diffuse)
(Diffuse) Dot-size
(min of arc)
Fro. 4.
Effects of dot-size and betwcundotdistanm by dot-size ratio (BDD/DS) optha amplitude of the surfac+negativc and sur&c-positive components in VJ3Ksevoked by the on- and off-set of patterned light. Data have been averaged across subjcczs and rcplim&ns, each piotted point being an awxage of 24 scores. See text for further expfsnation.
the interpretation of data. Dot-size (confounded with between-dot-distance) signikantly intluenced the amplitude of the SN and SP components of both on- and off-responses (P < @OOl). All measures indicated a decrease in VER amplitude as dot-size was reduced below 8.1 min of arc. Although the main effect of between-dot-distance, as reflected by the two stimulus ratios, did not approach statistical significance at the w)5 level, this variabie interacted siggificantly the amplitudes of the SN and SP components has been darctibad in ‘ThcproccdumuscdtomcasW previous studies (HARTERand Wnrrz, 196%:I%msu, 1370]. Sri&y, the amplitude of the SN and SP ~meuUreain~~Othc~~voltrgelevelat~owst).~Mz)lftudo zzzz?s coqmnant was mcasuzed at a Wmcy dstcrrmnsd by the puak of this componaatllndortbs 21~7&t~~~Thislotoncyw#dftom9oto128Illsrc~onthesubjsct.Tbs~of tbaSPcompomntwo3always~utrdatitspePkwhichvaric#1Inlrrtsncyfrom155to~~depcndhrO on the wbjcct.
VisuallyEvokedCorticalResponsesto the On- and Off-setof Patterned Lightin Humans
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with dot-size for the SP component of the on-response and SP and SN components of the off-response (P < O-01). The effects of reduction of between-dot-distance depended on the initial distance between stimulus elements. A reduction in between-dot-distance resulted in increased amplitude as long as it was not reduced below approximately 9 min of arc-for example, given the 1l-2 min of arc dot-size, reducing between-dot-distance from 17.8 (l-59 x 11.2) to 9.52 (0.85 x 11-2) min of arc resulted in an increase in amplitude. If between-dot-distance was reduced below approximately 9 min of arc, then amplitude was reduced-for example, given the 8-l min of arc dot-size, reducing between-dot-distance from 12.88 (l-59 x 8.1) to 6.89 (0.85 x 8-l) min of arc resulted in a decrease in amplitude. This trend was evident with the SN component of the on-response although the critical between-dot-distance appeared to be greater (between 11.2 and 21.7 min of arc). Perhaps if larger dot-sizes had been employed, this interaction also would have been statistically significant. The functions shown in Fig. 4, particularly the amplitude of the off-responses, appear to have an inverted “U” shaped form, VER amplitude first increasing and then decreasing as dot-size was reduced. The peaks of these functions, reflecting the most effective stimuli, occurred in response to different between-dot-distance/dot-size combinations depending on the measure considered: 18*5/21-7 and 17*9/l l-2 for the SP component of the on-response, and 9*5/l l-2 and 12-g/8*1 for the SN and SP components of the off-responses. Although the functions reflecting the amplitude of the SN component of the on-response were not inverted “U” shaped, it may be assumed that they would have been if a sufficient range of dot-sizes had been employed. Previous studies employing checkerboard light flashes indicated the amplitude of the SN component peaked in response to checks subtending 20-30 min of arc (RIETVELDet al., 1967; HARTERand WHITE, 1970; HARTER, 1970). In summary, the most effective stimuli appear to be those where the distance between stimulus elements (both dot-size and between-dot-distance) was between approximately 10 and 30 min of arc subtense, depending on the measure considered. The most effective stimuli were smaller in subtense for the off-responses than on-responses.
DISCUSSION
The data support the prediction that VERs to both stimulus on- and off-set should reflect the sharpness and density of contours in the evoking stimuli. In general, reducing the sharpness of the patterns as well as the distance between stimulus elements (below approximately 9 min of arc) resulted in a decrease in amplitude of the SN and SP components of both on- and off-responses. The presence or absence of polka-dot pattern influenced the wave-form of on-responses in the same manner as did checkerboard patterned light flashes in previous studies (SPEHLMANN,1965; RIETVELDet al., 1967; HARTERand WHITE, 1968). Perceptual and physiological processes have been reported which may be related to the effects of contour sharpness and distance between contours on VERs. Contrast effects, which are greatest when the light and dark elements of the stimulus are separated by a sharp boundary (THOMASand KOVAR, 1965; WESTHEIMER,1966), possibly account for the effects of refractive error. Numerous perceptual variables have been shown to vary with the spatial extent of stimulus elements: efficiency of detecting stimulus orientation (ANDREWS, 1967), perceived contrast (BRYNGDAHL, 1966; WESTHEIMER,1966; BLAKEMORE
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and CAMPBELL,1969), visual acuity (FLOM, HEATH and TAKAHASHI,1963), figural aftereffects (GANZ, 1964), and visual threshold (GLEZER, 1965; RATLIFF,1965). Critical distance of approximately 10-30 min of arc were reported in these studies; reduction of distance between stimulus elements below these values inhibited the perceptual phenomenon. The inverted “U” shaped function between VER amplitude and the distance between stimulus elements possibly reflects the interaction between perceived amount of edge (which increases with reduction of distance) and contrast between elements (which decreases with reduction of distances starting at IO-30 min of arc). WIESELand HUBEL(1966) have shown that lateral geniculate cells in macaque monkeys (whose visual system is particularly similar to the human’s) have retinal receptive fields arranged in a manner which enhances their sensitivity to white (and black) spots of light. As suggested in earlier ‘studies (HARTER‘and WHITE, 1968, 1970; HARTER, 1970), the modal size of receptive field centers and the inhibitory interaction between antagonistic parts of fields may account for the effects of stimulus element size and contour sharpness on VER amplitude. VERs to stimulus on-set (or off-set) possibly reflect the combined effect of excitation of on-center (or off-center) cells and inhibition of off-center (or on-center) cells, the most effective stimulus size being determined by the modal size of the receptive field center of the cells activated. HUBEL and WIESEL(1966), have reported modal sizes varying from 7.5 to 30 min of arc, depending on the type of cell, which supports this suggestion. The majority of cells observed at the lateral geniculate and cortical levels of monkeys have on-centers (WIESELand HUBEL, 1966; HUBELand WIESEL,1968 ; WURTZ, 1969) which may account for the fact that greater amplitude on- than off-responses were obtained in the present study. Blurring of contours possibly resulted in a reduction in VER amplitude since stimulation was spread to the antagonistic part of the receptive field. THOMAS(1970) has formulated a model which also deals with the effects of blurring and stimulus size in terms of lateral inhibition and receptive field size. If the most effective stimulus sizes in the present study, in terms of evoking the greatest amplitude VERs, reflect the modal size of receptive field centers, the question may be raised as to why the critical stimulus size combinations varied for on- and off-responses. Possibly, the most effective stimulus size was smaller for the off- than on-responses because: (a) receptive field size was reduced as a function of light adaptation (GLEZER, 1965) and thus would be smaller at stimulus off- than on-set; or (b) contours may have been slightly blurred at stimulus onset, due to accommodation error, which would result in a shift in peak sensitivity to larger element sizes (HARTER and WHITE, 1970). The latter alternative appears unlikely-accommodation appeared to remain stable during the course of stimulation since the effects of refractive error were similar on both on- and off-responses (Fig. 2). The first alternative is complicated by the fact that the size of receptive field centers apparently is smaller for on- than off-center cells (WIESELand HUBEL, 1966). It may be noted that the SN and SP components differed in terms of how pronounced they were in the on- and off-responses to patterned light (Figs. 2 and 3), the SN component being primarily an off-response and SP component an on-response. This was less apparent in responses to diffuse light. Possibly, the amplitude of SN and SP components primarily reflect the activity of off- and on-center cells respectively. It is of interest to note that the activity of on- and off-center cortical cells of cats showed increased activity at latencies which correspond respectively to the latencies of the SP and SN components discussed in this study (CREUTZFELDT,ROSINA,ITO and PROBST,1969; MACKAY, 1969). In conclusion, it is acknowledged that the proposed relationship between various com-
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ponents of on- and off-responses, recorded at the surface of the human scalp, and activity at the single unit level, based on animal studies, is speculation and at best an over simplification of the relationship. The investigation of on- and off-responses to patterned light, however, appears to be a step in the direction of isolating the physiological basis and significance of individual components of the complex VER. REFERENCES ANDREWS,
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Abbrt--Averaged evoked cortiatl potentials were investigated as a function of the on- and off-set of patterned light (white polka-dots on biack background) and the density and +arpnassof~~.sixadulthumrneu~viawsdpattcrwdstimuliwhichwsnillumrnrrtbd once~1~3sec:for6oomsec.contour~tyand~swas~~byvpryiasthe size and distance between polka-dots (O-35 min of arc) and induced refractive error (O-5 dioptcl8). AlIalyses of va&wx!cindicrrwdthatdotain:and bwee+dot-bistprrcsi~tly componrmts of evoked reaponscato both the oninfiuencedthe8mplitudeofsarlyandlate and~attofIwttern(P<~i).hgamral,~~hldeproqsssinslydagarsedasa function of doacased d&bUXObf&VC&kokmpalsd~rrdraGtVOarroi.Thsdts wero~intennsol~iahibitionandthesizeofraaptivefieidantmofon-and off-center cc& in the visual system of animals.
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