Sensitization by annular surrounds: sensitizaton and masking

Virion Rex Vol. 11. pp. 1445-1453.

Pcrgamon Press 1971. Printed in Great Britain

SENSITIZATION BY ANNULAR SURROUNDS: SENSITIZATON AND MASKING DAVIDAY. TELLER,CHARLESMATTER,’ W. DANIELPHILLIPSI and KENNETHALEXANDER’ Departments

of Psychology and Physiology/Biophysics; University of Washington,

Department

Seattle, Washington

(Received 3 June 1971; in revisedform

of Psychology,

98105

19 July 1971)

INTRODUCTION

INCREMEXthreshold for a small test spot at a point P on the retina is influenced not only by the background illuminance at P but also by the pattern of illuminance faliing on surrounding regions of the retina. In a particularly clear demonstration of such spatial interactions, WESTHEIL~ER (1965) showed that the increment threshold for a small test spot located at the center of an illuminated disk varies with the diameter of the disk. For rod vision, the threshold for the test spot rises with increasing diameters of the disk, reaching a maximum when the disk diameter is about 45’ of arc (the peak diameter) for a typical subject. For disk diameters larger than the peak diameter, the threshold falls agaic, dropping as much as a log unit as the disk diameter increases to 1.5 or 2 deg. Thus, light at a distance from the center of the disk can lower the threshold for the centrally located test spot. The effect may be rephrased to say that a 45’-2” annulus concentric to a 45’ disk lowers the threshold at their common center. This psychophysical sensitization effect is often attributed to the existence of an antagonistic surround in the receptive fields of neural units within the retina. It is interesting to note that a close parallel to the psychophysical sensitization effect has been reported recently in cat ign cells (NAKAYALIA,1971). In the present experiments, we relate the spatial sensitization effect to the properties of early or neural adaptation (CRAWFORD,1947; BAKER, 1963; KAHNEMAN,1968). In studies of early light (or dark) adaptation, one typically turns a large adapting field on (or off) abruptly at a point in time, t, ; and measures the increment threshold for a very brief superimposed test spot, presented at various times before and after to. The results of such an experiment on early adaptation are usually of the form of the upper curve presented in Fig. 8. The threshold begins to rise 100 or more msec before the onset of the adapting field, reaches a maximum value at or within 100 msec after the onset of the adapting field, and then drops a few tenths of a log unit to a more or less steady level. Near the offset of the adapting field a short-lived elevation of the threshold is sometimes found; after which the threshold drops rapidly before leveling off into the slower “pigment” adaptation curve. The large adapting and test fields typically used in studies of early adaptation cover regions of retina large enough to include both the center and the sensitizing annulus revealed in psychophysical studies of sensitization. The question posed in this paper is whether early light and dark adaptation, traced with a very small test spot, will vary systematically with variations in the size of the adapting field. In particular, it seemed possible that early adaptation to fields of sizes less than the peak diameter for sensitization might reflect a THE

’ Department

of Psychology,

University

of Washington, 1445

Seattle, Washington

98105.

1446

DAVIDA

Y.

TELLER

single physiological process; while early adaptation to larger fields might reveal the interof two spatiaily distinct processes in their influence upon the psychophysical thresholds.

action

iMETWODS The view presented to the subject was composed of three concentrically arranged fields, Iocated 7 deg temporal to a fixation cross. The smallest iield was a test spot 5’ in diameter and 8 msec in duration. The test spot was presented against the second Beid, an ilhrminated disk the diameter of which couid be varied between 12’ and 3 deg. The disk could be cycled on and off repeatedly with a recycling time of 2 set (1 set on, 1 set off) or 4 set (I set on, 3 set off). The test spot could be presented once per cycte at any desired temporal location (delay) relative to the onset of the disk. The test spot and the recychng disk were superimposed upon the center of a continuously illuminated 10 deg background field. The illuminance of the 10 dcg field was typically set 1.4 I.u. below that of the disk. The IO dcg field was included to reduce stray light artifacts (W~~THEIM ER, 1965). The test spot was green (Kodak No. 74) and the disk and 10 deg fietds red (Kodak No. 29), in order to maximise the range over which rods determine the threshotd (Aeurwt and STKU, 1954). Apparatus

Stimuli were presented via three channels of a four channel Maxweilian view instrument, described previously (TELLER and G-N, 1%9; TILLER and LINDSEY,1970). A &cation cross was presented at optical infinity in non-Maxwellian view. Speaker c&i shutters at the first filament images of two of the channels controlled the exposure of the disk and the test spot. Rise and fall times of the disk and the test spot were 2 msec. All times and time delays were calibrated by means of a photocell and CRO. Intensity calibrations were identical to those of TELLERand LI?JXX!~Y (1970). Two retinal ilfuminances of the disk were used: O-0 and 0.5 log scat. td. Thresholds measured upon disks of all sixes at these two illuminan~s (CM) and upon the lower illuminance (DT) can be attributed to rods (TIUER and LINDSEY, I970). In addition, on subject DT, a few curves (Figs. 7 and 8) were run with the illuminance of the recycling disk increased to 1-Olog scat. td. For these curves, the illuminance of the 10’ field remained at -1.4 log Scot. td. The disk diameters used in these supplementary experiments were 12’ and 2 deg. Dark adaptation runs of the form described in TELLERand LINDSEY (1970) co&rmed that for Df threshohis upon disks of these diameters at this ihuminance level can also be attributed to rods. Procedure

Prior to the day’s run a random sequence was selected for a set of 9 (CM) or twelve (DT) delays to be run that day, The set of delays spanned either the onset or the offset of the disk. The ilhrminances of the disk and 10 deg field, and the diameter of the disk were set to the appropriate values for that day. At the begi&ng of the run, the subject &ated the fixation point and preadapted for ten minutes to a co~t~on of concentric fields in his temporal retina, These were the 10 deg field presented steady; thediskcycling on and off* and the test spot, Bashing once per cycle at one randomly chosen delay. During the 10 min of preadaptaion, the subject made repeated adjustments of the test flash to threshold. At the end of 10 min, one of the other delays, selected at random, was presented. The subject made five practice adjustments of the test spot to threshold. This set of judgmentswas discarded. The delay was then changed to the first delay in the preset random sequence; the subject made five judgments, and so on. A session lasted about 1) hr. In the next session the same experiment was repeated with the previous random order of the (9 or 12) final conditions revemed. Most of the data points presented in Figs. l-8 are the means from two such counterbalanced sessions, so that Nper point = 10. For all offset conditions for subject CM, the entire experiment was repeated with new random orders, so for these conditions, iV per point = 20. Vertical bars indicate f 1 SE. of the mean.

RESULTS

The main results of the 2-set cycle of the disk (1 set on, 1 set off) are presented in Figs. l-6. Figures 1, 3 and 5 show early adaptation measured upon disks of relatively smaller diameters, while Figs. 2, 4 and 6 show early adaptation measured upon disks of relatively larger diameters.

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FIG. 1. Changes in the threshold for a brief S’ test spot at the center of a disk of light, at various times relative to the onset of the disk. Recycle time 2 set (1 set on, 1 set off). The parameter on the curves is the disk diameter. Subject DT. The illuminances of the fields in this and subsequent Figs. are given in log Scot. td. Disk illuminance 0.0; 10 deg field illuminance -14.

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1448

DAVIDA Y. TELLER

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1450

DAVIDAY. TELLER

Disk onset For the larger disk diameters for each subject, the curves bear a general resemblance to earlier data on early light adaptation (e.g. CRAWFORD, 1947). However, for the smaller disk diameters the curves show no indication of a maximum within the first 100 msec after the onset of the disk. Rather, the curves rise smoothly (except for noise) to a steady threshold level which is then maintained. The transition from smooth to peaked curves appears to occur at about 36’ of arc for DT, and at about 1 deg for CM. TELLER and LINDSEY (1970) have shown that, in the sensitization paradigm, the peak diameter-the disk diameter producing the highest threshold-varies from one subject to another. For the two subjects used in the present study, the peak diameter was about 36’ of arc for DT, and 48’1 deg for CM. Hence, the point of transition from a flat to a peaked curve agrees reasonably well with the details of the sensitization curves for these two subjects. A number of additional features are worth pointing out. For all disk sizes, the threshold (plotted in stimulus time) begins to rise before the onset of the disk, as expected from many earlier studies. This effect (subject DT) is 100 msec or less for the larger disk sizes, but increases to perhaps 150 msec for the smaller disk sizes. Within the on-cycle of the disk, the curves for disks of different diameters flatten out at different levels. This spread of points corresponds to the existence of a sensitization effect in the middle of the on-cycle-the threshold is highest upon disks of 36’ (DT) or 45’ (CM) of arc. However, it is somewhat surprising that even for negative time delays-i.e. before the onset of the disk-the curves do not lie at the same level. That is, as much as 785 msec after the offset of a disk of light (i.e. -215 msec with respect to disk onset in a 2 set cycle) the threshold varies with the diameter of the previousiy exposed disk. Disk offset The basic offset data are presented on the right-hand sides of Figs. 1-6. These data are not well-behaved in that the shapes of the early dark adaptation curves vary with the subject and the illuminance level of the adapting disk. However, despite these problems it seems clear that within each data set the shape of the early dark adaptation curves varies with the diameter of the adapting disk. For example, for DT (Fig. l), the threshold begins to drop at least 100 msec before the offset of the 36’ disk; drops by about O-8 I.u. by the instant of disk offset; and then maintains a constant plateau over the entire course of the 1-set offcycle. For larger disk sizes (Fig. 2) the change in the threshold begins at about the instant of disk offset, and extends over 100 msec or more after disk offset. The threshold then stabilizes at a level 0.5 I.u. or more below the plateau reached after the offset of the 36’ disk (Fig. 1). For CM at 0.5 log td. (Figs. 3 and 4), a marked elevation of the threshold occurs near the instant of offset of the 2’ disk (Fig. 4); no such elevation occurs for the 48’ disk (Fig. 3). For CM at O-0 log td., the drop in the threshold again appears to be greater and more prolonged for larger (Fig. 6) than for smaller (Fig. 5) disks, although the data are flawed by the failure of the two segments of the 48’ curve to meet each other during the off-cycle. Variations of illuminance and duration The results of two additional experiments on subject DT are shown in Figs. 7 and 8. For the data of Fig. 7, the illuminance of the disk was increased to 1-Olog td., in order to increase the total change of the threshold occurring at the onset of the disk. Two disk sizes were used, 12’ and 2 deg. The experiment succeeded in the sense that the thresholds

Sensititatioa

by Am&x

Surrounds:

Sensitization

1451

and ?h.skhg

in the middle of the on-cycle for both disk sizes are roughly the same and are approximately the same as the mid-cycle threshold upon the 36’ disk in the main experiment (Fig. 1). Hence, it is possible to compare 12’, 36’ and 2 deg disks under conditions such that the thresholds are roughly matched in the middle of the on-cycle. Figure 8 shows the same experiment as Fig. 7, but with the re-cycling time of the disk increased to 4 set (I set on, 3 set off).

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1452

D~~IDA Y. TELLER

Both sets of data are quite similar, and both clear@ confirm the results of the main experiment. First, at the onset of the 12’ disk, the threshold rises monotonically with time, while at the onset of the 2 deg disk, the threshold rises to a pronounced maximum shortly after the onset of the disk, and then decreases. This maximum is enhanced with a Csec re-cycle rate. Second, the threshold begins to drop welI before the offset of the 12’ disk, and drops smoothly to a surprisingly high plateau. For the 2 deg disk, a small rise of the threshold occurs near the disk offset in these data, increasing the difference between the threshoId curves measured upon the two sizes of the disk. This rise resembles earlier data from CM (Fig. 3) and is similar to the rise often reported in the literature on eariy dark adaptation (e.g. BAKER, 1963). It will be discussed further below. And third, for the 12’ disk the high plateau during the off-cycle persists as Iong as 785 msec or even 2785 msec after the offset of the disk (1785 msec and 3755 msec on the abscissas of Figs. 7 and 8 respectively).2 FORCED-CHOICE

EXPERIMENTS

Most of the interesting features of the present data, as described above, depend either upon small differences in the shapes of early light and dark adaptation curves, or upon comparisons of the absolute levels of points from separate curves. The differences are small enough, and the experiments subjectively difficult enough, to warrant confirmation of the results by forcedchoicepsychophysical techniques(Green and SWETS,1966). Accordingly, a series of supplementary two-interval forced-choice experiments was performed. PROCEDURE A 4-set recycle time was used for ah forced-choiceexperiments.Three 4-set cycles in sequence formed a trial, The first two cycles formed the two temporal intervals, within one of which the test spot wes presented. The third cycle was “dead time,” used for the subject’s response and for feedback from the experimenter. The ilhrminance levels used were l*Olog td. for DT (as in Figs. 7 and 8) and 0.5 log td. for CM (as in Figs. 3 and 4). Prior to the day’s run a set of (typic&y) 4 conditions were chosen for testing together. For each condition, 5 teat spot ilIuminance levets were chosen, spanning the range from just above the threshold value found eariier by the method of adjustment to about I I.u. beIow that ievef. After the standard preadaptation procedure, the 20 subconditions (4 conditions x 5 test ihuminances) were presented in a preset random sequence, for a block of 16 trials each, A session took about 2 hr. In the next session the same experiment was repeated with the previous random order of 20 conditions reversed, yielding four psychometric functions with 32 trails per point (as in Fig. 9B.) In some cases (Figs. 9C and 9D), only 2 conditions (rather than 4) were being compared. In these cases, the 10 subconditions (2 conditions x 5 test illuminances) were run in a random order, and then counterbalanced within the same session. In some cases the psychometric functions generated with 32 trials per point were not smooth. In these cases the experiment was replicated with a new random order, to yield 64 trials per point (Figs. 9A and 9D). RESULTS ‘I%e resuits of the forced-choice experiments are presented in Fig. 9. The results in Fig. 9A and 9B co&m most of the salient features shown in Figs. 7 and 8; a detailed comparison

of the two kinds of data is left to the reader. A failure of confirmation of one feature of the earlier data occurred with the 2 deg disk (Figs. 9C and 9D). For subject DT, using the method of adjustment, it will be recalled that 2 It would be profitabk to replicate these findings with a recycling rate long enough that the 12’ and 2 deg curves would come together between presentations of the disk. PreEminary investigations with longer recychng rates suggest that the two curves may remain separated for as mu& as 10 set a&r disk offset.

~nsi~iza~ion by Annular Surrounds:

Sensitization

and Masking

1443

no elevation of the threshold occurred at the offset of a disk of 0.0 log td. (Fig. 2); a small elevation occurred when the illuminance of the disk was increased to 1-Olog td. (Figs. 7 and 8). When the forced-choice method is used, the psychometric functions measured at 280 msec before the offset of the disk and at 25 msec before the ofTset of the disk are virtually identical {Fig. 9C). In the forced-choice data, then, no elevation in the threshold is seen at disk offset, even under the most favorable set of stimulus conditions. A similar result occurs for subject CM. Although a large elevation in the thresboId near the instant of offset of a 2 deg field is apparent for CM in Fig. 4, no such difference occurs with the forcedchoice method (Fig. 9D).

LOG

RELATIVE

\NTENSlTY

FIG. 9. Psychometric functions obtained for selected combinations of disk size and time delay with a two-interval forced-choice procedure. Illuminances were the same as in Fig. 7 for DT, and as in Fig. 3 for CM. Four set re-cycle time. A-Subject DT. The test spot was presented either coincident with disk onset (closed circles) or 5iO msec after disk onset {open circles). Disk diameter 12” (upper) and 2 deg (lower). B-Subject DT. Same as in A except delays are with respect to disk offset (i.e. closed squares for test spot coincident with disk offset; open squares for $10 msec after disk offset). Disk diameter 12’ (upper) and 2 deg (lower). C-Subject DT. The test spot was presented either 25 msec (closed triangles) or 250 resee (open triangles) before the offset of a 2 deg disk. D-Subject CM. The disk diameter was 2 deg and the test spot occurred either 510 msec after

disk onset (open circles) or concurrent with disk offset (closed squares). Data taken on CM with the method of adjustment immediately subsequent to the forcedchoice data of Fig. 9D showed a much reduced effect at the offset of the adapting field.

These data confirm the observations of HhLLE’rT (1969), and support his suggestion that data taken at the offset of adapting fields are particularly prone to subject bias in the psychophysical sense (GREEN and SWETS, 1966). This fact may help to explain some of the variations between subjects in published accounts of early dark adaptation (e-g. BA~RSBY, OESTERREICH and STURR, 1964). It is, however, interesting to note that in the present study elevations of the threshold near the offset of the adapting disk were never seen with smaller disk diameters. Whatever the factor is that leads some subjects to use a high or unstable criterion in making jud,oments near the offset of larger adapting fields, this factor does not occur when smaller fields are used.

1454

DAVIDA Y. TELLER

DISCUSSIOK The results may be discussed from three points of view: (1) Comparison to classical studies of early adaptation or masking, (2) spatial interactions between a dark light disk and a dark light annulus and (3) neural mechanisms. Comparison to earlier studies For the larger disk sizes used in the present study, the data confirm the results of a variety of earlier studies. For example, the 2 deg disk data presented in Figs. 7 and 8 bear a remarkable resemblance to the data presented by CRAWFORD(1947); this is the more remarkable because Crawford’s data almost certainly reflect cone thresholds, while the present data reflect rod thresholds. Previous studies of early light and dark adaptation at mesopic or high scotopic illuminance levels have usually included most of the features of the present data, including a threshold maximum at or within 100 msec after the onset of the adapting field, a possible but variable secondary maximum at the offset of the adapting field, and negative temporal influences of 100 to 150 msec (e.g. BOYNTON and TRIED~~AN, 1953; BATTERSBY and WAGMAN,1959, 1962; BAKER,1953; HALLETT,1969; but c.f. BAKER, DORANand MILLER, 1959). The dependence of the height and shape of masking functions upon the size and shape of the masking field has also been noted previously (for references see FRUMKESand STURR, 1968; BAKER,1965; STURRand FRUMKES,1968; HALLETT,1971). Several recent investigators have studied early light adaptation upon disks of different sizes. MA~EWS (1971) has shown that in cone vision, early light adaptation curves upon disks of the peak diameter for cone spatial sensitization are smooth, while early light adaptation curves upon larger disks have the standard on-transient-a result entirely similar to the present results for rod vision. MARKOFFand STURR (1971) have studied the increment threshold at the centers of adapting fields of different diameters, for both steady and pulsed adapting fields. They found that the peak diameter for sensitization varies from one stimulus condition and retinal region to another; in each case the disk diameter yielding the highest threshold in the steady state case also yields the highest threshold (at zero delay) when the field is pulsed. HALLETT (1971) has also found the analog of the sensitization effect for pulsed adapting fields. Vertical cuts through the present data at each of a series of times, however, serve to emphasize the fact that, particularly with adapting fields presented for more than 100 msec, the shape (and maximum) of the threshold vs. diameter curve will sometimes vary with the delay chosen. Hence the peak of the steady-state sensitization curve cannot necessarily be inferred from experiments in which only one or a small number of delays are investigated. Does dark light sensitize dark light ? The points at -215 msec with respect to disk onset in Figs. l-8 show that as long as 785 msec (or even 2785 msec) after the offsets of disks of various diameters, the threshold at the center of the afterimage left by the disk varies with the diameter of the previously exposed disk. In particular, the threshold is higher upon afterimages of disks of smaller diameters than upon afterimages of disks of which the diameter was iarge enough to allow light to fall upon the sensitizing annular region of retina observed by WSTHEIMER(1965). This result is consistent with the idea that in the early stage of rod dark adaptation the aftereffect remaining from the light in the annulus may indeed lower the threshold for the

Sensitization by Annular Surrounds: Sensitization and Masking

1455

test light at the center of the afterimage. Although dark light does not enter into sensitizing interactions over the long time course of classical dark adaptation (WESTHINER, 1968; TELLER and GESTRIN,1969), the present data suggest that it may do so for the first couple of seconds after the offset of illumination. That is, in at least the iirst seconds of dark adaptation, dark light appears to sensitize dark light, as it appears to sensitize real light (TELLER, 1971).

Relation to neural mechanisms When an adapting light is large enough to cover both the center and surround processes revealed by the psychophysical sensitization effect, a threshold maximum occurs near the time of onset of the disk. But when the adapting light is confined to a disk of a diameter less than the peak diameter for the sensitization effect, the threshold rises smoothly At the onset of the adapting light. The fact that no maximum occurs in the case of smaller adapting fields implies that the threshold maximum found with larger fields cannot be caused solely by signals generated by the receptors underlying the position of the test spot. It must be caused by the interaction of these signals with signals generated by light falling on more distant regions of the retina. The possible neural mechanisms underlying our results will be discussed in the context of one of the most completely known vertebrate retinas, that of the mudpuppy, Nec.rlirtrs. In particular, in the mudpuppy retina, the receptors, horizontal cells, and bipolar cells show graded responses to light (WERBLINand DOWLIKG, 1969). Center-surround anlagonism occurs at the bipolar cell level, in the sense that a maintained change in the membrane potential of the cell, away from the resting potential, is caused by light falling in a central region of the receptive field of the cell; and light falling in the surround of the receptive field changes the membrane potential back toward the resting potential. The membrane potential of the cell would then, say, first increase and then decrease as the diameter of a stimulating spot was increased. On this basis, MCKEE and WESTHEIMER (1970) ha1.e suggested that the thresholds measured in the basic sensitization paradigm reveal the membrane potentials of bipolar cells. To put the argument another way: the known center-surround antagonism of the bipolar cell, plus the assumption that the threshold is higher the greater the deviation of the cell from its resting level, provide a model sufficient to account for the steady-state sensitization effect. The temporal properties of the sensitization effect to some extent are consistent with this model. In particular, the smooth rise and fall of the psychophysical threshold upon small adapting disks nicely resemble the smooth changes in bipolar membrane potential accompanying the onset and offset of small disks of light confined to the center of the cell’s receptive field. The threshold maximum at the onset of larger fields finds a close parallel in the response of the bipolar cell to disks covering both the center and the surround of its receptive field (WERBLINand DOWLING, 1969; WERBLIN, 1971). Similar comparisons

can be made to the b-wave of the cat erg (STEINBERG,1969) and to cat ganglion cell responses (BARLOWand LEVICK, 1969). It is tempting to suggest that the threshold maximum which occurs near the onset of larger adapting disks is caused by the interaction of spatially opponent processes with different time courses. That is, the initial elevation in the threshold might be caused by processes arising from light falling in the central disk; and the subsequent drop in the threshold might be caused by a delayed or less rapidly developing input from light falling

DANDAY. TELLER

1456 on the sensitizing receptive

annulus.

Such a delay of the signal from the antagonistic

fields is in fact seen in mudpuppy

WERBLIN, 197 1; and personal However,

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sensitizing

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field simultaneously.

correlate

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the physiological

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with respect

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in the cell that provides

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that,

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the latency

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1971)

The latency for the influence

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One possibility

of

upon the larger range of disk sizes must be ruled out,

from the center is too simple to account adaptation

annulus.

the idea that a delayed

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study one of us (TELLER,

and studied the time course of the change

was at least as short as the Iatency

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communication).

psychophysical

or 45’ disk of light on continuously, in the annulus

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will be

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Acknawle&ements-This research was supported by USPHS grant EYOO421.The optical system was originally provided by a Fight-for-Sight grant from the National Council to Combat Blindness, Inc., New York, N.Y. We thank Bryant Lindsey for assistance in the running of the experiment, and Joseph Starr for valuable comments on the manuscript.

REFERENCES AGUILAR,M. and STILES,W. S. (1954). Saturation of the rod mechanism of the retina at high levels of stimulation. optica Acfu 1, 59-65.

BAKER,H. D. (1953). Instantaneous thresholds and early dark adaptation. J. opt. Sot. Am. 43, 789-803. BAKER,H. D. (1963). Initial stages of dark and light adaptation. J. opt. Sot. Am. 53, 98-103. BAKER, H. D. (1965). Area eiTects and the fluctuations of sensitivity in early dark adaptation. J. opt. Sot. Am. 55,614. BAKER,H. D., DORAN,M. D. and MILLER, K. E. (1959). Early dark adaptation to dim luminances. J. opt. Sot. Am. 49, 1065-1070. BARLOW,H. B. and LEVICK,W. R. (1969). Three factors limiting the reliable detection of light by retinal

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Sensitization

by Annular Surrounds:

Sensitization and Masking

1457

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WESTHEIMER, G. (1965). Spatial interaction

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Abstract-WESTHEIMER (1965) has shown that the rod threshold at the center of a 45’ disk of light is as much as a log unit higher than it is upon disks of larger diameters. In the present paper, we explore the influence of such spatial interactions on the rapid changes in the threshold which occur in early light and dark adaptation (masking). That is, early light and dark adaptation curves were traced with a 5’ test spot centered upon adapting disks of various diameters, from 12 min to 3 deg of arc. Standard early light and dark adaptation curves were found on the larger sizes of disks, with a clear maximum in the threshold occuring near or slightly after the instant of onset of the disk, and (under some conditions) a secondary maximum at or near the instant of offset of the disk. For the smaller range of disks, however, both the rise and the fall of the threshold were monotonic, with no maxima near disk onset or offset. These data suggest that the threshold maximum of early light adaptation results from the interaction of center and surround processes of neural units.

R&UU&WESTHEIMER (1965) a montre que le seuil des bitonnets au centre d’un disque de lumikre de 45’ est au moins %une unite logarithmique plus Clev6 que sur des disques de plus grands diamttres. Dans le present travail, on recherche l’influence d’interactions spatiales de ce genre sur les changements rapides du seuil qui se produisent au debut de l’adaptation $ la lumitre et B l’obscurit8 (masquuge). On trace done le dtbut des courbes d’adaptation B la lumiere et d I’obscuriti avec un test de 5’ centri dans disques d’adaptation de diamttres variCs (de 12’ g 3”). Four les grands disques, les courbes classiques du dCbut de l’adaptation B la lurnikre et B l’obscurit6 presentent un net maximum du seuil au voisinage ou juste apres le debut de I’iclairage du disque et (sous certaines conditions) un maximum secondaire & la cessation du disque ou juste apr&s. Pour les disques petits au contraire, la mont& et la descente du seuil &aient monotones, sans maximum prb de I’apparition ou de la disparition du disque. Ces don&es suggtirent que le maximum du seuil au debut de I’adaptation g la 1umitre risulte d’une interaction entre les processus du centre et du bord des unit& nerveuses.

DAVIDA Y. TELLER ZUSNWW~~SSUD~-W~~EIMER (1965) zeigte, da0 die Schwelle fiir Stabchensehen im Mittelpunkt einer 45’ grol3en Scheibe urn eine Log-Einheit hoher liegt als bei Scheiben grii&ren Durchmessers. In der vorliegcnden Abhandlung untersuchen wir den EinAul3 solcher raumlichen Wechselwirkungen auf die schnellen Schwellen~derungen, die bei der Sofortadaptation auftreten. Das heigt Sofortadaptationskurven wurden mit einem 5’ T’estreiz bei Hellund Dunkeladaptation gemessen. Dieser Testreiz wurde in der Mitte kreisformiger Adaptations-felder-Durchmesser von 12’ bis 3”-dargeboten. Bei den proberen Adaptationsfeldern erg&en sich die iiblichen Hell- und Dunkeladaptationskurven mit einem ausgeprggtem Maximum beim oder kurz nach dem Eiihalten des Adaptationsfeldes und (unter einigen Bedingungen) einem zweiten Maximum beim Ausschalten des Adaptationsfeides. Fiir die kleineren Adaptationsfelder waren die Schwellenkurven monoton ohne Maxima beim Em- bsw. Ausschalten. Diese Werte weisen darauf hin, da13 das Schwellenmaximum durch eine Wechselwirkung zwischen Zentrum und Umgebung neuronaler Einheiten entsteht.

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