background light falling not only on the tested region, but also on surrounding stimuli, whereas here we were dealing with an adaptation rather

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1 J. Physiol. (197), 26, pp With 1 text-figures Printed in Great Britain DISTANCE EFFECTS IN HUMAN SCOTOPIC RETINAL INTERACTION BY GERALD WESTHEIMER AND R. W. WILEY* From the Department of Physiology-Anatomy, University of California, Berkeley, California 9472, U.S.A. (Received 7 July 1969) SUMMARY 1. Using the threshold for a small, brief test flash as a measure, the effect of light stimulation of small adjacent retinal regions of the human retina was investigated during adaptation to a medium scotopic illuminance. 2. Within about I' of the tested area, light causes a desensitization gradually decreasing with distance. Beyond that there is a sensitizing zone which in turn gives way to an outer desensitizing zone. The sign of these effects is reversed if the peripheral stimulus is a patch of darkness. 3. Evidence for non-linearity of summation of individual patches was found. Occlusion of each other's sensitizing influence is observed if two regions are closely adjacent to each other. INTRODUCTION The increment threshold of a patch of human retina depends on the background light falling not only on the tested region, but also on surrounding regions. This spatial interaction of adapting stimuli may be either threshold-raising or threshold-lowering. In an earlier experiment (Westheimer, 1965a), the threshold changes were studied when the background area was concentrically increased. The effect of successively adding annuli to the background seemed to a first approximation to conform to Ricco's law of area-intensity reciprocity, although Ricco's law was originally formulated for thresholds of incremental stimuli, whereas here we were dealing with an adaptation rather than an increment-excitation phenomenon. In a detailed study on single units of the goldfish retina, Easter (1968 a, b) showed important differences between the two; in particular, that the spatial contribution to the adaptation state of a single unit, did not fit the 'top hat' concept of Ricco's law, i.e. unweighted linear summation of light within the recognized summation * Present address: Forest Glen Section, Walter Reed Army Institute of Research, Walter Reed Army Medical Center. 5-2

2 1313GERALD WESTHEIMER AND R. W. WILEY area. Rather, the effectiveness of light in contributing to the adaptation state of the single unit falls off with distance and there is evidence for nonlinear summation. We here report on the relative effectiveness of retinal stimuli at various distances from the test site in contributing to the adaptation state of a given retinal region in human scotopic vision and also concern ourselves with the question whether these adaptation contributions add linearly. The investigation is conducted by means of the probing spot testing procedure which allows the analysis of the contribution of excitatory (i.e. threshold-raising or desensitizing) and inhibitory (i.e. threshold-lowering or sensitizing) signals as they influence the responsiveness of a given small region of retina. The indicator is the subjectively determined threshold for a small brief increment flash of light. Occasionally, a null experiment can be designed in which the invariance of the increment threshold for such a probing spot under differing background illumination conditions defines equivalence in the net confluence of excitatory and inhibitory signals being spread laterally along the retina. The close analogy between the findings of this technique and those of electrical recordings from individual retinal ganglion cells and more recently, even more distal cells such as bipolar cells, suggests that one is here reporting on functional and perhaps even structural relationships between individual elements in the outer nuclear layer of the human retina. METHODS The optical apparatus has been described previously (Westheimer, 1965a). Briefly, it consisted of a three-channel Maxwellian view system (Fig. 1). Channels I and II were identical in all respects and had compact filament tungsten lamps as their light source. Channel III differed only in that its light source was a General Radio Stroboscope whose flash (- 1 /ssec) occurred every 1*5 sec. A lens (L1) imaged the lamp filament on a circular aperture which was then imaged by the succeeding components (L2 and L3) onto the pupil of the subject's eye. Beam-splitting pellicles (P) superimposed the three beams which were then seen by the subject through the same field lens. Field stops (T) were placed in each channel and adjusted in size and position according to the requirements of the particular experiment, and neutral density filters (N) were used to control luminance levels. In addition, a neutral density wedge (W) was positioned in either Channel II or III. The subject adjusted this wedge to vary and control the luminance of the background field or test stimulus. This wedge was coupled to the wiper of a calibrated potentiometer which in turn was in circuit with a Beckman digital voltmeter and printer. When the subject had adjusted the brightness of the parameter of interest to criterion (see later), he merely pressed a switch which sent a print command to the printer. Thus, a record of the luminance setting was obtained without interrupting the observations or introducing any possible stray light into the apparatus while recording results. Procedure. The subject was seated comfortably in front of the apparatus, his head was positioned and held firmly in that position by using a dental bite bar and forehead rest. After preliminary dark adaptation, the left eye was occluded, the subject

3 SCOTOPIC RETINAL INTERACTION viewed a dim red fixation light, and the various stimulus configurations were presented to the temporal retina of the right eye approximately 1 from the fovea. All of the data here reported were obtained on an experienced observer who had participated in a similar experiment previously. The major findings were validated on two additional experienced observers. The method of adjustment was chosen for the psychophysical procedure. In the first experiment to be described, the subject was presented with a uniform circular field, 12 diameter. The flashing test stimulus, consisting of a circular patch of light, 6 min of arc diameter, was located in the centre of this field. There was also P L2 N Li 131 F2 N LI L F, ~~x Fig. 1. Schematic diagram of the optical stimulating apparatus. For description, see text. a 16' x 22' elliptical field (Fig. 2a) whose position could be accurately varied. This field was created by having the beam in Channel II illuminate a first-surface mirror with a small hole in it. Channel I illuminated the hole, which thus could be made brighter, equal to, or darker than the uniform field in which it was embedded. The observer adjusted the wedge in Channel III controlling the brightness of the flashing test stimulus which initially was always well above threshold and then reduced until the flash could just no longer be seen. This was repeated four times. The elliptical field was initially centred on the flashing test stimulus and then moved 15 min temporal to the test stimulus and the procedure repeated. This was continued until data for a range of positions of the elliptical field were obtained. The threshold for the incremental stimulus was obtained on a completely uniform large background field at the beginning and then again at the end of each run to insure that the observer's threshold or criterion had not changed. The difference in the log thresholds obtained on the uniform field and in the presence of the elliptical field was taken as a measure of interaction resulting from such change in stimulation. The stimulus configurations for the remaining experiments were slightly different. In these experiments, the observer was presented with a circular background of fixed diameter continuously exposed at a medium scotopic luminance. The flashing incremental stimulus whose intensity remained constant throughout the whole

4 132132GERALD WESTHEIMER AND R. W. WILEY experiment was placed in its centre. The appropriate background diameter for the circular background varied somewhat for each observer and defines the central summation area of the receptive field for the particular observer. It was found by the technique described by Westheimer (1965a). The results of such a procedure are shown in Fig. 3. The background intensity at which a constant test stimulus is just at threshold diminishes with increasing background diameter up to a critical diameter, beyond which the adapting background must then be increased. Surround (12 diameter) *Background field Background (-96 diameter) Fixation mark X Fixation mark IElliptical satellite Test flash Satellite Test flash (a) Fig. 2. (a) Subject's view of stimulus. The test flash was.1 in diameter and a few,sec in duration, and occurred every 1 sec. It was superimposed on a steady background, illuminance of about 2 quanta (57 nm equivalent) absorbed rod-' sec-', several degrees in diameter. An elliptical satellite, either *7 log units brighter than the background, or completely dark, was placed at various distances from the probing flash whose threshold was measured. (b) Experimental arrangement for the null experiments. A background disk was chosen to cover the desensitizing region of the adaptation receptive field of the probing flash. The retinal illuminance of the probing flash remained constant and it was placed at threshold by varying the background disk's illuminance. The contribution of various kinds of steady surround illumination, as, for example, the satellite disk illustrated, to the adaptation state was measured by the change in the background disk's illuminance. A circular 'satellite' patch, 15 min diameter, was presented at various distances temporal to the circular background (Fig. 2b). For these experiments, the sliding filter wedge was placed in Channel II. To measure the contribution of the distant satellite to the adaptation state at the test site, the observer adjusted the intensity of the circular background to maintain the constant-intensity incremental flash at threshold. Five settings of the background luminance were made for each satellite position both in the presence and absence of the eccentric satellite during each run. On the assumption that the excitation level of the test region is the same whenever the constant incremental stimulus is just detectable, the difference in the required background luminance in the presence and absence of the eccentric satellite is a measure of the interaction resulting from the satellites' stimulation. Experimental conditions. Since the purpose of these experiments was to study (b)

5 SCOTOPIC RETINAL INTERACTION 133 spatial interaction properties of the human rod retina, stimulus conditions were chosen to maximally demonstrate these properties at the expense of cone function. The method of rod-cone separation reported by Aguilar & Stiles (1954) was duplicated. It achieves this separation by selecting a background illumination at which cones are desensitized very much more than rods, i.e. the red portion of the visible spectrum, and a test flash for which the rods are much more sensitive, i.e. the bluegreen region of the spectrum. Accordingly, a red glass filter (F1) was placed in Channels I and II. This filter fully transmits light of all wave-lengths beyond 63 nm and none below 62 nm. For the test stimulus, an Ilford 623 filter (F.) was placed in Channel III. This filter passes only a narrow band centred on 5 nm. Throughout the experiments reported in this paper, the retinal region whose excitation state was being influenced by surround illumination and tested by the probing flash was u.@ 1~.. foos._ -- *' -1* U) -J Diameter of background (degrees) Fig. 3. Area-intensity relation for circular background. The retinal illuminance of backgrounds of various sizes necessary to maintain a small incremental stimulus in their centre was found. Blue-green incremental stimulus (.1), red background, dark-adapted right eye, 1 temporal to the fovea. receiving steady illumination of about 2 quanta (57 nm equivalent) rod-' sec-1. This is an intermediate scotopic adaptation state; when lower, surround sensitization drops out; when higher, there is a risk of cone contamination. During the null experiments in which the target configuration was that of Fig. 2b, all stimuli were superimposed on a large field, 1-12 in diameter, which remained about 1-5 log units below the background disk's illumination. It served to mask any possible light scatter from the test flash. Fixation was maintained by reflecting the image from a small, red fixation lamp into the eye. The angle of inclination of reflecting mirror could be controlled to insure testing of the same retinal area.

6 134 GERALD WESTHEIMER AND R. W. WILEY RESULTS Empirical weighting functions. The hypothesis common to all of the various theories of retinal receptive field organization (reviewed by Ratliff, 1965) is that the magnitude of interaction produced by lightinduced activity in the peripheral portions of the receptive field on the centre of the field is a function of the distance separating the origin of that activity from the centre. Thus, a weighting function can be used to describe the strength of interaction. The magnitude of interaction depends on the stimulated area, the amount of stimulation to that area, and a weighting term which, in turn, is a function of the distance between the interacting elements. If there is no further second order or recurrent interaction, the final response would be given by the integral of these factors over all the active elements involved in the response... Satellite eccentricity (min of arc) Fig. 4. Change in increment threshold for a small, brief test flash superimposed on a uniform field with an elliptical satellite at various distances from the test flash (stimulus configuration shown in Fig. 2a). Co-ordinates on ordinate refer to threshold on background field alone (no satellite). Test flash: blue-green,.1 in diameter, a few,utsec in duration; background: large, red, 2 quanta (57 nm equiv) rod-1 sec-1; satellite elliptical 16' x 22', displaced horizontally from position of test flash. O, satellite 7 log units brighter than field; *, satellite completely dark. Rod vision, 1 from fovea. In the first experiment, the increment threshold was determined for the brief, small test flash presented first on a large uniform background and then in the presence of an added elliptical patch of light (16' x 22'), 7 log units brighter than this background, at various distances from the test

7 SCOTOPIC RETINAL INTERACTION 135 flash along the horizontal temporal meridian of the retina. The results are shown in Fig. 4. Several features of Fig. 4 are of interest. First, for zero eccentricity, i.e. when the test flash is superimposed on the satellite, the Fechner fraction log (AI/I) is considerably larger than 1. This is consonant with the steepness of the t.v.i. curves for highly localized background stimulation (Westheimer, 1965 a). Secondly, within the central desensitizing zone, there is a fall-off of effect, much as found by Easter (1968 b) in the goldfishretina ganglion cell. Thirdly, while there is a surrounding, sensitizing zone, as previously described, it is somewhat narrower than earlier experiments might have led one to believe, and it gives way in turn to a third zone, which is desensitizing. It is seen again in the next set of experiments and is quite a replicable observation. We will return to a discussion of this outer zone. A variety of arguments about the origin of these findings, e.g. some involving scattered light, border effects, change-over from one thresholddetermining mechanism to another, may be eliminated by the following simple experiment. The satellite was placed at a few selected spots and the lamp in its beam extinguished, giving a dark satellite instead of a light one. (Since the satellite was produced by transillumination of a hole in a mirror, the total illumination of the satellite region was separably controllable, up or down, from that of the background in which it was embedded.) The resulting threshold changes were in a direction opposite to that for a bright satellite (Fig. 4). A detailed quantitative study of this phenomenon with relative satellite luminance as the parameter has yet to be attempted, but there is no doubt about the qualitative observation of sign reversal of interaction signals in all three zones when the satellite is made in turn brighter and dimmer than the larger background. The experiment just reported utilizes the change in the probing flash threshold as the quantitative expression of the interacting signal introduced by the light satellite. While the change in incremental sensitivity is a time-honoured, widely and successfully employed measure of retinal adaptation state, it is possible to free the investigation from even this constraint by designing a null experiment, as follows. A null experiment. After preliminary determination of the critical background within which adaptation signals are wholly desensitizing, the subject was presented with a steady light disk of this diameter with the test flash in its centre. The test flash remained at a fixed intensity and the subject's task was to keep it at threshold by varying the neutral wedge setting controlling the background intensitv. Then a satellite stimulus of fixed size (15 min in diameter), 7 log units brighter than the background, was added to the surrounding field. Its contribution to the adaptation state

8 136 GERALD WESTHEIMER AND R. W. WILEY u *16 21.c 12_ E 8 _ 2 4 -o ~.' ~~~6 84/ u -4 Eccentricity of c t satellite (degrees) --8 bo *o J -12 Fig. 5. Change in background effectiveness with simultaneous peripheral stimulation. (Stimulus configuration shown in Fig. 2b.) Ordinates: difference in log background illuminance necessary to maintain the test stimulus flashing in its centre at threshold, without and with the I' circular satellite, 1 quanta absorbed rod-' sec-1, at various eccentricities from the test centre. Negative values indicate an increase in background, and positive values, a decrease compared to that without the satellite stimulation. Each point is an average of 1 observations. Brackets indicate + 1 S.E. of mean. u *1 *14 R n / o e P,- ~~~Eccentricity of satellite (degrees) C --1 _ -14 / bo/ o / -22 Fig. 6. Change in background effectiveness with simultaneous peripheral stimulation. This experiment was the same as that shown in Fig. 5, except that a.48 satellite was used and displaced through various distances vertically (open symbols and dashed line) and horizontally (filled symbols and continuous line). Sign convention and all other parameters remain the same as in the previous experiments. Each point is an average of twentyfive observations. Brackets indicate + 1 S.E. of mean.

9 SCOTOPIC RETINAL INTERACTION 137 of the circular background field was measured by that change in the latter's retinal illuminance which was necessary to keep the test flash at threshold. Constancy of probing flash threshold may be regarded as the indicator of constancy of the adaptation state of the background on which it is superimposed. The measurements, then, give the changes in illumination of the fixed background area that have to be made to compensate for the presence of the interacting satellite; they give, in other words, the background light equivalents contributed to the local adaptation state by interaction signals from the satellite illumination. Taken as a function of distance of the satellite, this constitutes a plot of the weighting function, though by the nature of the experiments only for distances beyond the central desensitizing zone. The results in Fig. 5 show for each point the mean of 1 observations and are highly reliable. In Fig. 6 the results are also plotted for a vertical displacement of the satellite with respect to the identical background field. The interaction zones are somewhat wider vertically than horizontally. Higher order interaction. So far we have examined only the interaction signals contributed to the adaptation state of one retinal region by illumination of an adjoining region. The question that follows naturally, concerns the effect of illumination of yet another region on the capacity of given retinal regions to send out interacting signals. Potentially this is a complex subject because even if such regions do not interact with each other, the combination of their effects on the test area may not obey simple laws such as algebraic addition. The following experiments are designed to give beginning insight into these phenomena. The interacting capacities of individual satellites in a range of positions relative to the tested area are shown in Fig. 7. The numerical values relate to the change in log background illumination to place a fixed incremental stimulus at threshold, and, while precise, are small enough that questions about summation of numbers or their logarithms are rendered moot. What happens when two satellites are presented simultaneously? Figure 7 illustrates the geometrical relationship of the three satellites used. Each satellite was first presented alone and its influence on the background was measured. The values given are means of twenty-five measurements which have a standard error of 1 units. Simultaneous presentation of the three satellites in various combinations of two is seen to produce effects that are approximately equal to the sum of the effects each produces separately. This applies also for equal effects of opposite sign, when two satellites may just cancel each others' interacting signals. So far, the observations relate to the additive effects of two discrete satellites rather remote from each other. Suppose two satellites are brought closer to each other. The experiment shown in Fig. 8 illustrates what

10 138 GERALD WESTHEIMER AND R. W. WILEY occurs then. One satellite remained fixed immediately adjacent to the background. It was then exposed successively in conjunction with a satellite in positions removed in.25 steps successively further peripheralwards andthe effects of such a joint exposure oftwo satellites were measured in the usual way. They are shown in the Figure by the filled circles. The open circles show the expected effect if there were simple addition of interacting A -96 B 2.16 C 2u 6 f Predicted: --48 Actual: ±-8 A B 2-16 C II 1.44c +-88 Predicted: Actual: Fig. 7. Diagrammatic representation of the stimulus conditions used to examine spatial additivity of interacting influences of two satellites presented first separately (first two columns) and then simultaneously (last column). The numbers below each Figure are the log change in background luminance to mask the test stimulus, with negative numbers indicating an increase in necessary background luminance. Each value represents the average of twenty-five observations. effects. The simple addition found previously is still found to operate for larger separations, but when two satellites are close together their effects do not sum. Two closely adjacent satellites interfere with each other's capacity to sensitize a remote retinal area.

11 SCOTOPIC RETINAL INTERACTION 139 Another expression of this phenomenon is seen in the following experiment. It involves the effect of a bar of light which starts at the edge of the background disk and extends further and further into the periphery. One might expect the change of threshold to be the integral of the contribution function of Fig. 5 and this has been included in open circles in Fig. 9 for purposes of comparison. In fact the change of threshold that is actually found does not have such a shape. The difference between the observed effect and that predicted from linear summation of the effects of added individual line segments, is most easily described as a reduced sensitizing contribution from adjacent regions when they are present simultaneously, as compared with their individual actions. 4._:" _-4 - < Li _ Distance between satellites (degrees) Fig. 8. Additivity of influence on the background field. The change in background brightness required to keep the incremental stimulus at threshold was found with simultaneous peripheral stimulation by two satellites ( 24 ). The results are shown as a function of the separation of the two satellites in the temporal meridian. -24 on abscissa means that first satellite was 6 and the second 84 from the centre of background; -48 means - 6 and 1-8, respectively, and so on. For comparison, the results to be expected on a linear additivity hypothesis are shown by the dashed line. These were obtained from Fig. 5 and were take'n under the same experi - mental conditions. See legend of Fig. 5 for sign convention. Each point represents a mean of 1 observations. The brackets give the 95 %/ confidence lirnits for each mean. The question follows whether a possible explanation is not to be sought in a saturation of the capacity of the tested area to respond to sensitization influence. This has been tested in the following experiment: a circular sensitizing satellite was divided into two equal halves. Each half

12 14 GERALD WESTHEIMER AND R. W. WILEY had approximately the same sensitizing influence when tested by itself (Fig. 1). Both together sensitized less than each by itself, but when one of the halves was exposed with double its previous luminance, the sensitizing influence was doubled. The total light reaching the sensitizing region of the retina is the same in the last two conditions, but the sensitizing action is vastly different; the conclusion previously reached that there is an occlusive tendency operating between adjacent sensitizing regions is therefore confirmed. u (_._ -o E IV C._ C. -J o 5o 8 4 h Length of horizontal slit (degrees) o N - 36 %1 "I 1% 1-d --4 Fig. 9. Influence of a horizontal slit of varying length. The necessary increase in the background brightness to maintain the flashing test stimulus in its centre at threshold was found when a horizontal slit was added temporal to the background field. The slit was contiguous with the background disk and 48 wide. Abscissa values refer to distance between centre of background disk and outer edge of slit. Other conditions remained the same as listed previously. The results expected on a linear additivity hypothesis are shown by the open circles and dashed line. Each point is an average of twenty-five observations. The brackets indicate the 95 % confidence limits for the mean values obtained with the slit. DISCUSSION I 212' /i j92*4 t-1 y, ~2-88 One of the interesting aspects of this investigation is the demonstration of lack of linear summation of adaptational signals in the human rod retina. We have been able to show this quite clearly in the sensitizing surround, where there seems to be an interference by closely adjoining regions / / I. I I /

13 SCOTOPIC RETINAL INTERACTION 141 with each other's capacity to lower the threshold for a test flash 12 or ;i' away. This lack of linear summation is probably also present in the central desensitizing zone. The empirical weighting function of Fig. 4 does not conform to the curve derived by differentiation of the area-intensity adaptation function (Westheimer, 1965b). Whether that discrepancy is due to a facilitative rather than an occlusive interaction is not immediately apparent from the available data a Expected ccu.8~~~~ on additive stimulus flashing inthecentreofhebackgrhypothresis. boh4 be o.x Stimulus condition :t4-i 4J += Fig. 1. Histogram showing the results of different peripheral stimuli acting with the background field. Each peripheral stimulus necessitated an increase in background field luminance to maintain an incremental stimulus flash-ing in the centre of the background at threshold. The peripheral stimulus configuration consisted of a circular patch of light (n24n diameter) immediately adjacent to the background field. It was bisected so that each half could be presented separately and both together. The fourth peripheral stimulus condition was achieved by doubling the amount of light in one half of the eccentric patch of light. The lower histogram shows the relative quantity of light available in the peripheral stimulus for each experimental condition. The presence of a third concentric component of the adaptational receptive field organization could not have been suspected from previous psychophysical investigations. The evidence for it is excellent and it cannot be written off as being caused by scattered light falling on the tested

14 142142GERALD WESTHEIMER AND R. W. WILEY area from the distant satellite. If one makes the most pessimistic and quite unrealistic assumption that all the light contained in the satellite is equally spread over a disk just including the central desensitizing area, the light falling within the latter is still not enough by a factor of about 4 to cause the observed elevation of threshold using any reasonable Fechner fraction. The conclusion demanded, then, is that neural signals from the far-distant satellite cause desensitization of the tested area. They may do so either directly, through direct, long distance interaction, or they may do so indirectly, by having an influence on an intermediate area whose own interacting signal to the tested area is modified by the outlying satellite. This kind of recoursive interaction, sometimes called disinhibition, is somewhat unlikely because the phenomenon may be observed even when the region intervening between the satellite and the primary target area is dark; but then this does not entirely rule out interference by the satellite of spontaneous activity in an unstimulated region, this changed spontaneous activity then in turn causing lateral interaction in the tested area. Another view of the origin of the outer desensitizing zone might involve the notion of superimposed excitatory and inhibitory activities each having a bell-shaped spatial distribution, an idea advocated by Rodieck & Stone (1965) and by Enroth-Cugell & Robson (1966). One may fit our finding into such a framework by giving the excitatory systems a larger, albeit tapering, spatial distribution that outdistances that of the inhibitory system. Such long-range desensitizing interference is not unknown in retinal physiology, and forms the basis of the periphery effect discovered by McIlwain (1964) in cat retinal ganglion cells. Why is there no indication of this third, desensitizing zone in experiments in which the threshold is tested in the centre of a circular background whose diameter is being concentrically increased (Blachowski, 1913; Fry & Bartley, 1935; Crawford, 194; Ratoosh & Graham, 1951; Westheimer, 1965a; Teller, Andrews & Barlow, 1966; Enoch, Sunga & Bachman, 1969; Fig. 3, this paper)? It is tempting to ascribe this absence to the lack of linear summation already demonstrated here. However, it may have a simpler explanation in the horizontal/vertical inequality seen in Fig. 6 of this paper. Added circular annuli of the appropriate diameters are likely to include both sensitizing and desensitizing sectors. To exclude this possibility, annuli would have to be constructed with shapes to fit the receptive fields individually mapped under the identical stimulus conditions. This work was supported in part by Grant EY-22 from the National Eye Institute, U.S. Public Health Service.

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