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Effect of stimulus presentation rate upon visual threshold
Vision Rrs.Vol 17.pp. 379 to 383 Pergamon Press1977.Printed in GreatBritain.
EFFECT OF STIMULUS PRESENTATION VISUAL THRESHOLD HOWARD D. BAKER
and
RATE UPON
FREDERICK G. BARGOOT’
Psychology Department and Institute of Molecular Biophysics, Florida State University, Tallahassee, FL 32306, U.S.A. (Received 2 December 1974; in revised form 12 August 1976)
Abstract-Threshold measurements during dark adaptation are shown to be affected by the rate at which the threshold stimuli are presented. Thresholds for rods and for peripheral cones are raised by higher stimulus presentation rates, but thresholds for fovea1 cones are not. This difference for cones implies a previously unrecognized mechanism of visual sensitivity. The results are compared with the Troxler fading effect, and with earlier studies of light adaptation to intermittent stimuli.
When one employs the absolute threshold to measure visual sensitivity, one usually supposes that the threshold stimuli are so weak that they themselves have no effect on sensitivity. We have been surprised, therefore, to observe that regularly repeated threshold
flashes could sometimes be seen more clearly when the fixation crosshairs were lost briefly and the stimuli momentarily struck a fresh adjacent retinal area. If stimuli at absolute threshold do indeed affect sensitivity during dark adaptation, this poses a serious problem for all experimenters that test theories of visual sensitivity by reference to dark adaptation curves. It behooves us to establish whether repeated presentations of a threshold stimulus do appreciably alter sensitivity, and if so, to determine the characteristics of the effect. METHOD
The direct experiment is not possible: Threshold values cannot be compared with what they would be if they were not measured. The approach we chose was to compare dark adaptation curves measured by stimuli at different regular rates of presentation. If there is an effect on sensitivity due to threshold stimuli, then a faster presentation rate (more stimuli) should mean a greater loss of sensitivity. This paradigm has advantages. Presentation rate is a simple variable that permits parametric comparisons; also, the results have practical usefulness because a growing number of experiments use regular presentation rates with automatic recording of thresholds to measure dark adaptation. To avoid possible confusion arising from day-to-day variability in the subjects or the apparatus, the data weie collected as pairs of dark adaptation curves, one curve measured using one rate of presentation of the stimulus flashes, the other measured using a different rate. The hypothesis was that in each pair of dark adaptation curves, the threshold would be higher in the curve measured with the faster rate of stimulus presentation. Each pair of curves was taken on the same day, except I CL! ~IICII a l’ailure of the apparatus forced the second
1 Present address: Department of Anatomy, School of Medicine, University of Pennsylvania, Philadelphia, PA 19174, U.S.A. 379
of a pair to be delayed until the next day. The pairs were taken in a series that was counterbalanced for order of measurement of pairs and for sequence within pairs. The results eventually showed that the careful counterbalancing had been unnecessary, however, so the details of the counterbalance will not be reported. The apparatus was a two-channel Maxwellian-view adaptometer using glow-modulator tubes as light sources, and its optical system has been described before (Baker, Doran and Miller, 1958). For the present experiment the optical wedge in the stimulus channel was attached directly to a calibrated Moseley X-Y recorder, so the Y-axis of the recorder traced the value of the stimulus in log td. The X-axis of the recorder was connected to a clock. The recorder was fitted with a character printer, and pressing a switch plotted the retinal illuminance of the stimulus at that time. The preadapting field was 20” dia and gave 10,000 td of retinal illuminance. The stimulus flashes were 1” dia, and 20 msec duration. Both field and stimulus were white. The stimulus always appeared on the location of the center of the preadapting field. Fixation was on crosshairs that appeared either black against the preadapting field or dim red against darkness after the adapting field had been turned off. For most experiments the crosshairs were located 7” temporal to the center of the adapting field, hence to the stimulus, but for the fovea1 experiment the crosshairs were centered on the stimulus. An artificial pupil of 2.5 mm dia was used throughout the experiments. Procedure
The subject observed the 4.Olog td adapting field for 7 min to become light adapted, while fixating on the crosshairs. After a warning signal, the adapting field was turned off. The fixation crosshairs now appeared in dim red, and the subject used a potentiometer control to keep them appearing dim as his dark adaptation proceeded. The psychophysical procedure was essentially the method of adjustment, or average error. The stimulus flashes appeared automatically at a set rate, e.g. l/set or Z/set. The subject turned a control that moved a wedge to dim or brighten the stimulus flashes, and when he judged that the flashes were appearing exactly at threshold, he pressed the switch that printed a point representing the time of the threshold judgment and the retinal illuminance of the flashes at that time. A series of such judgments made up the dark adaptation curve.
MO
HOW~KII II. BAKI.R and
For the first experiment.
to cstahlish that the rate of presentation does have an clfect upon the threshold. four subjects were used. Two of them wcrc the authors: the other two wcrc students who did not know the purpose of the experiments. For the remaining experiments, intended to cxplorc the nature of the effect. the two authors alone were the subjects.
RESC LTS
There are considerable differences between thresholds taken on different days even with the same individual subject. As noted above, the data were taken as pairs of dark adaptation curves to minimize the effect of such fluctuations. The experimental observations were restricted to comparisons between the paired curves. Figure 1 shows a typical comparison between two dark adaptation curves measured 7” nasal to fixation, using different rates of stimulus presentation. Each point of Fig. 1 is a single measurement. In Fig. 1. the rates were 1 flash/set and 2 flashes/set. When lines are drawn through the final levels of the cone segments, and through the terminal levels of the rods thresholds, the Z/set curves are higher than the l/set curves for both rods and cones. A statistical comparison was made for all curves, as follows. Comparisons were made between curves taken at 1 flash/set and curves taken at 2 flashes/set; also between 2 and 4 flashesisec; and between 1 and 4/set; so there were three rate differences at which paired curves were compared. For the authors as subjects. each pair was repeated once in a program counterbalanced for order and sequence, to make six comparisons between pairs of curves at different rates. One of the naive subjects made a complete single series of three comparisons, but the other was able to complete only two. Since there were two terminal levels (rod and cone) for each comparison, there were In ever) i12 + 3 + 7) times 2. or 34 comparisons.
0
different
rates
case. both the cone plateau and the rod terminal level wcrc higher for the faster rate of presentation. The expectation that ?4 such comparisons will lit in one direction by chance alone is but one in 3”‘+. ZC)the finding is clearly significant. Frcquont regular threshold stimulus presentation dots indeed raise the threshold during dark adaptation.
-Ihscr~c~~ in the jixw. All of the cone segments of the curves measured 7 from fixation show an advantage for thresholds measured at slower rates of stimulus presentation. but the difference is often small, as in Fig. I. Since fovea1 pure cone dark adaptation curves generally show a greater total sensitivity increase during dark adaptation than do the cone segments of parafoveat curves. it might be expected that fovea1 curves would show the rate effect more clearly. To our surprise, no effect of stimulus presentation rate upon threshold could he seen at all in fovea1 curves. Figure 2 is typical of the four comparisons made using the authors as subjects. Although the total amount of change of the threshold is greater in Fig. 2 than in the cone segment of Fig. 1. thresholds measured at a rate of 1 flashisec show no advantage over thresholds measured even at 4 flashes/set. The rate of stimulus presentation therefore seems to affect the threshold only in the periphery, not the fovea. C’mtrol ,for rod--cone ir&rncfion. In the fovea there is only one class of receptor. the cone, so it is conceivable that the rate effect represents some kind of rod-cone interaction and is missing in the fovea because the fovea has only cones. If so. then pure rod dark adaptation curves might also be free of stimulationrate influences. To test the idea, four comparison pairs of dark adaptation curves were measured. again at 7 nasal to fixation, but following preadaptation with a field of but 100 td. Under these conditions only the rod segment of the dark adaptation curve is obtained. In all four comparisons the thresholds were clcarl\i lo\\cr when stirnull were prcscntcd at :I slower
I
I
I
5
IO
15 TIME
Fig. I. Comparison
FKtIIt-KI(‘KG. BAIWOOI
I
I
20
25
30
(MINUTES)
between parafoveal dark adaptation curves for subject HDB measured at two of stimulus presentation. t-measured at 2 flashes/set, O-measured at I flashjsec. showing difference in terminal levels for both cone and rod segnients.
Stimulus presentation rate
381
presentation increases. The fastest rate represented in Fig. 5, 5 flashes/set, yields a rod threshold that is about 10 times higher than the slowest rate of 1 flash every 2 sec. On the other hand, cone thresholds are much less affected. The change from slowest to fastest rate yields a cone threshold change of only two or three times. The data of Fig. 5 are taken from new curves measured specifically for the purpose of the graph, and are not drawn from the earlier part of data, so judgment of the reliability of the effect can be made not only from the vertical lines but also from comparing the separate data of Figs. 1 and 3 with Fig. 5. This considerable variability between individual curves was of course the reason why the matched-pair comparison method was used to establish the existence of the rate effect in the first part of the study. There is no evidence from the lower ends of the IO 5 curves of Fig. 5 that our slowest rate of stimulus presTIME (MINUTES) entation yield thresholds that are at a minimum. The Fig. 2. Fovea1 dark adaptation of subject FGB measured curves of Fig. 5 rise continuously right from the slowat 0 flashes/set and O-at 1 flash/set. No difference est rate. Apparently threshold-level stimuli that are in terminal levels. presented even as slowly as one every 2 set must be suspected of raising their own threshold. The rates of stimulus presentation represented in Fig. 5 probably cover the entire range of rates that rate. Figure 3 shows a typical pair of curves. We conare practical in dark adaptation experiments. Regular clude that (a) it does not require the function of both flashes that are less frequent than one every 2 set are receptors for presentation rate to influence the threshdifficult for a subject to attend to, while rates of presold, and that (b) the absence of a rate effect in the fovea seems to depend upon other characteristics of entation greater than S/set tend to fuse into a steady light toward the end of rod adaptation, because the the fovea. critical fusion frequency is not greater than 4 or 5 Time course. Inspection of the dark adaptation flashes/set at threshold intensities in the fully darkcurves reveals that the effect of a steady presentation adapted eye (Hecht and Verrijp, 1933). A steady fused rate upon the threshold develops gradually and conlight is likely to disappear even above threshold, and tinuously through the course of dark adaptation. To the result is a very serious variability in the measurehave an independent estimate of the course with ments. There are, as well, serious theoretical questions which the rate effect develops, the two authors dark about comparing the late fused thresholds with the adapted for 30min without making measurements earlier unfused ones. For example, should the intenand then made threshold measurements at regular sity of each 20msec flash be reported, or should the rates of stimulus presentation for 15 min in the dark. Talbot level of the fused light be reported? Four such series were measured for each subject, two It is interesting to note that the critical fusion rate at a rate of 1 flash/set and two at 4 flashes/set. Under these conditions, after dark adaptation was essentially of a threshold light depends on its physical intensity rather than its subjective brightness. High flash rates complete, the threshold drifted upward just as expected from the dark adaptation curves. Figure 4 at threshold are easily separated by a subject in the shows one comparison for one subject. At 1 flash/set early part of dark adaptation. Only when the subject the threshold drifts very slowly but continuously has become sensitive enough to see very weak flashes upward; at 4 flashes/set the threshold drifts condo the flashes tend to fuse. tinuously and more rapidly. The rates of stimulus increase from this experiment appear to be entirely 3ti comparable to the results of the dark adaptation iz curves. Apparently, the effect of steady stimulus presentation is a continuous decrease of sensitivity d 2; c throughout the time that the thresholds are measured. . Extent. When individual curves are inspected, as in Figs. I or 4. it seems that the advantage of a slower stimulus presentation rate is largest at the final levels of dark adaptation. Figure 5 shows the terminal levels for rods and cones at several rates of stimulus presentation for the two authors. The graph connects the mean final threshold level of one curve each from the two authors, while the vertical bars show the 5 IO I5 20 range from the lower to the higher of each pair at TIME (MINUTES) each rate. The effect of regular stimulus presentation rates Fig. 3. Parafoveal rod dark adaptation curves of subject upon the threshold clearly increases as the rate of FGB measured at W flashes/set and 0-l tlash/sec.
,t.----J
s t
iR1
HOWAKI)
and FKI:~E.KI~‘K G. B~IK;~OI
D. BAKEK
TIME
(MINUTES)
Fig. 4. Upward drift of parafoveal thresholds of subject HDB following complete dark ad~iptatioll, U--measured at 1 flashjsec and t-4 flashes/see.
9 P E ‘-
t-
: 2 z 8* ?
I 0.5
I 1.0
,
I
I
2.0 STIMULUS
I
I
3.0
PRESENTATION
I 4.0
,
I . 5.0
RATE
(FLASHES / SEC)
Fig. 5. Terminal levels of O--cone and +--rod segments of parafoveal dark adaptation curves that have been measured at various rates of stimulus pr~sen~tion. Data are averages from one curve each of subjects HDB and FGB.
DISCUSSION
The most important characteristic of the effect of stimulus presentation rate upon the threshold is that the effect appears to be absent in the fovea. Although the cones of the fovea show very extensive dark adaptation, their threshold was not affected by any rate at which the stimulus was presented in our experiment. It appears possible, then, that the rate effect may be separate from the regular adaptation processes by which fovea1 cones become sensitive in the dark. There is another example of loss of visual sensitivity that occurs in the periphery but not in the fovea: Troxler fading, under conditions of normal fixation In Troxler fading, the entire peripheral field of view fades away when simple fixation is maintained for Iong periods. Troxler fading does not occur in the fovea unless the image is stabilized by optical means. It appears to be a special case of complete light adaptation wherein fixation provides adequate stabilization for the peripheral retina (Moreland, 1972). According to this view, the response to any stabilized image tends to adapt completely away, but only in the periphery of the retina are the receptive fields of the receptors large enough so ordinary fixation stabilizes the image sufficiently to allow complete adaptation. If that is true, then the rate effect also could rcprcsent regular light adaptation. It would be
absent in the fovea only because normal fixation is inadequate to stabilize the edges of the image on the fine receptor mosaic of the fovea. If the rate effect upon the threshold is due to the simpie accumulation of light adaptation from the stimulus flashes, then the relationships in Fig. 5 are revealing. On the assumption that according to the Weber-Fechner law the threshold should be affected in direct proportion to the amount of light delivered by the stimulus, one might expect that the threshold would be raised by a factor of two when one goes from a stimulus presentation rate of l/set to 2/set, or by a factor of 3, at 3/set. Figure 5 should show therefore a 1O:l change from one end of the graph to the other. In the curve for rods, the prediction is met very well. The points show an increase with rate exactly proportional to the increase in light, and a total change of a factor of IO. On the other hand. the cone thresholds in Fig. 5 increase far more slowly than would be predicted from the increase in the amount of light. Regardless of what mechanisms are involved, it is apparent that the effect of the stimulus accumulates without loss from flash to flash with the rods. The effect is partially dissipated from flash to flash in the peripheral cones, while in the fovea1 cones the effect of one threshold flash is completely dissipated before the next. It should not be surprising that rod light adaptation effects can accumulate without apparent loss
383
Stimulus presentation rate over intervals as long as 1 sec. When Mote and Reed (1952) studied the light adapting effect of flickering lights upon subsequent dark adaptation, they found that the same total amount of light flux, delivered as flashes at l-see intervals, was usually a little less effective in raising the threshold during subsequent dark adaptation than was the same total amount of light delivered as a continuous light. But that was true only for intense light levels. A weak preadapting light, presented at a rate of 1 flash/set, could be equivalent to the same total amount of steady light for subsequent rod dark adaptation, and under some conditions, could be even more effective than a continuous light. Mote and Reed’s experiment concerned the effectiveness of preadapting luminances, while ours concerns inadvertent light adaptation from threshold stimulus flashes. But in our experiment, there is a similar accumulation of light adapting effect from flash to flash, spaced I set apart. Similarly, the stimuli are weak ones, and similarly the effect is upon the rods. The conclusion is the same from our experiment and from the Mote and Reed experiment: The rods integrate the effects of weak stimuli completely, over a 1-set interval. On the other hand, our experiment
also shows that peripheral cones do not integrate the complete effect of weak flashes over a full second, and fovea1 cones are able to dissipate the effects of threshold-level flashes completely within 1 sec. Acknowledgement-This investigation was supported by USPHS Research Grants No. R01 ET01394 and R01 EYOO550from the National Eye Institute.
REFERENCES
Baker H. D., Doran M. D. and Miller K. E. (1959) Early dark adaptation to dim luminances. J. opt. Sot. Am. 49, 106~1070. Hecht S. and Verrijp C. D. (1933) Intermittent stimulation by light. III. The relation between intensity and critical fusion frequency for different retinal locations. J. gen. Physiol. 17, 251-265. Moreland J. D. (1972) Peripheral colour vision. In Handbook of Sensory Physiology, VII/4, Visual Psychophysics (Edited by Jameson D. and Hurvich L. M.), pp. 517-536. Springer, Berlin. Mote F. A. and Reed E. C. (1952) The effect of extending the duration of various lightdark ratios of intermittent pre-exposure upon dark adaptation in the human eye. J. opt. Sot. Ant. 42. 333-338.
EFFECT OF STIMULUS PRESENTATION VISUAL THRESHOLD HOWARD D. BAKER
and
RATE UPON
FREDERICK G. BARGOOT’
Psychology Department and Institute of Molecular Biophysics, Florida State University, Tallahassee, FL 32306, U.S.A. (Received 2 December 1974; in revised form 12 August 1976)
Abstract-Threshold measurements during dark adaptation are shown to be affected by the rate at which the threshold stimuli are presented. Thresholds for rods and for peripheral cones are raised by higher stimulus presentation rates, but thresholds for fovea1 cones are not. This difference for cones implies a previously unrecognized mechanism of visual sensitivity. The results are compared with the Troxler fading effect, and with earlier studies of light adaptation to intermittent stimuli.
When one employs the absolute threshold to measure visual sensitivity, one usually supposes that the threshold stimuli are so weak that they themselves have no effect on sensitivity. We have been surprised, therefore, to observe that regularly repeated threshold
flashes could sometimes be seen more clearly when the fixation crosshairs were lost briefly and the stimuli momentarily struck a fresh adjacent retinal area. If stimuli at absolute threshold do indeed affect sensitivity during dark adaptation, this poses a serious problem for all experimenters that test theories of visual sensitivity by reference to dark adaptation curves. It behooves us to establish whether repeated presentations of a threshold stimulus do appreciably alter sensitivity, and if so, to determine the characteristics of the effect. METHOD
The direct experiment is not possible: Threshold values cannot be compared with what they would be if they were not measured. The approach we chose was to compare dark adaptation curves measured by stimuli at different regular rates of presentation. If there is an effect on sensitivity due to threshold stimuli, then a faster presentation rate (more stimuli) should mean a greater loss of sensitivity. This paradigm has advantages. Presentation rate is a simple variable that permits parametric comparisons; also, the results have practical usefulness because a growing number of experiments use regular presentation rates with automatic recording of thresholds to measure dark adaptation. To avoid possible confusion arising from day-to-day variability in the subjects or the apparatus, the data weie collected as pairs of dark adaptation curves, one curve measured using one rate of presentation of the stimulus flashes, the other measured using a different rate. The hypothesis was that in each pair of dark adaptation curves, the threshold would be higher in the curve measured with the faster rate of stimulus presentation. Each pair of curves was taken on the same day, except I CL! ~IICII a l’ailure of the apparatus forced the second
1 Present address: Department of Anatomy, School of Medicine, University of Pennsylvania, Philadelphia, PA 19174, U.S.A. 379
of a pair to be delayed until the next day. The pairs were taken in a series that was counterbalanced for order of measurement of pairs and for sequence within pairs. The results eventually showed that the careful counterbalancing had been unnecessary, however, so the details of the counterbalance will not be reported. The apparatus was a two-channel Maxwellian-view adaptometer using glow-modulator tubes as light sources, and its optical system has been described before (Baker, Doran and Miller, 1958). For the present experiment the optical wedge in the stimulus channel was attached directly to a calibrated Moseley X-Y recorder, so the Y-axis of the recorder traced the value of the stimulus in log td. The X-axis of the recorder was connected to a clock. The recorder was fitted with a character printer, and pressing a switch plotted the retinal illuminance of the stimulus at that time. The preadapting field was 20” dia and gave 10,000 td of retinal illuminance. The stimulus flashes were 1” dia, and 20 msec duration. Both field and stimulus were white. The stimulus always appeared on the location of the center of the preadapting field. Fixation was on crosshairs that appeared either black against the preadapting field or dim red against darkness after the adapting field had been turned off. For most experiments the crosshairs were located 7” temporal to the center of the adapting field, hence to the stimulus, but for the fovea1 experiment the crosshairs were centered on the stimulus. An artificial pupil of 2.5 mm dia was used throughout the experiments. Procedure
The subject observed the 4.Olog td adapting field for 7 min to become light adapted, while fixating on the crosshairs. After a warning signal, the adapting field was turned off. The fixation crosshairs now appeared in dim red, and the subject used a potentiometer control to keep them appearing dim as his dark adaptation proceeded. The psychophysical procedure was essentially the method of adjustment, or average error. The stimulus flashes appeared automatically at a set rate, e.g. l/set or Z/set. The subject turned a control that moved a wedge to dim or brighten the stimulus flashes, and when he judged that the flashes were appearing exactly at threshold, he pressed the switch that printed a point representing the time of the threshold judgment and the retinal illuminance of the flashes at that time. A series of such judgments made up the dark adaptation curve.
MO
HOW~KII II. BAKI.R and
For the first experiment.
to cstahlish that the rate of presentation does have an clfect upon the threshold. four subjects were used. Two of them wcrc the authors: the other two wcrc students who did not know the purpose of the experiments. For the remaining experiments, intended to cxplorc the nature of the effect. the two authors alone were the subjects.
RESC LTS
There are considerable differences between thresholds taken on different days even with the same individual subject. As noted above, the data were taken as pairs of dark adaptation curves to minimize the effect of such fluctuations. The experimental observations were restricted to comparisons between the paired curves. Figure 1 shows a typical comparison between two dark adaptation curves measured 7” nasal to fixation, using different rates of stimulus presentation. Each point of Fig. 1 is a single measurement. In Fig. 1. the rates were 1 flash/set and 2 flashes/set. When lines are drawn through the final levels of the cone segments, and through the terminal levels of the rods thresholds, the Z/set curves are higher than the l/set curves for both rods and cones. A statistical comparison was made for all curves, as follows. Comparisons were made between curves taken at 1 flash/set and curves taken at 2 flashes/set; also between 2 and 4 flashesisec; and between 1 and 4/set; so there were three rate differences at which paired curves were compared. For the authors as subjects. each pair was repeated once in a program counterbalanced for order and sequence, to make six comparisons between pairs of curves at different rates. One of the naive subjects made a complete single series of three comparisons, but the other was able to complete only two. Since there were two terminal levels (rod and cone) for each comparison, there were In ever) i12 + 3 + 7) times 2. or 34 comparisons.
0
different
rates
case. both the cone plateau and the rod terminal level wcrc higher for the faster rate of presentation. The expectation that ?4 such comparisons will lit in one direction by chance alone is but one in 3”‘+. ZC)the finding is clearly significant. Frcquont regular threshold stimulus presentation dots indeed raise the threshold during dark adaptation.
-Ihscr~c~~ in the jixw. All of the cone segments of the curves measured 7 from fixation show an advantage for thresholds measured at slower rates of stimulus presentation. but the difference is often small, as in Fig. I. Since fovea1 pure cone dark adaptation curves generally show a greater total sensitivity increase during dark adaptation than do the cone segments of parafoveat curves. it might be expected that fovea1 curves would show the rate effect more clearly. To our surprise, no effect of stimulus presentation rate upon threshold could he seen at all in fovea1 curves. Figure 2 is typical of the four comparisons made using the authors as subjects. Although the total amount of change of the threshold is greater in Fig. 2 than in the cone segment of Fig. 1. thresholds measured at a rate of 1 flashisec show no advantage over thresholds measured even at 4 flashes/set. The rate of stimulus presentation therefore seems to affect the threshold only in the periphery, not the fovea. C’mtrol ,for rod--cone ir&rncfion. In the fovea there is only one class of receptor. the cone, so it is conceivable that the rate effect represents some kind of rod-cone interaction and is missing in the fovea because the fovea has only cones. If so. then pure rod dark adaptation curves might also be free of stimulationrate influences. To test the idea, four comparison pairs of dark adaptation curves were measured. again at 7 nasal to fixation, but following preadaptation with a field of but 100 td. Under these conditions only the rod segment of the dark adaptation curve is obtained. In all four comparisons the thresholds were clcarl\i lo\\cr when stirnull were prcscntcd at :I slower
I
I
I
5
IO
15 TIME
Fig. I. Comparison
FKtIIt-KI(‘KG. BAIWOOI
I
I
20
25
30
(MINUTES)
between parafoveal dark adaptation curves for subject HDB measured at two of stimulus presentation. t-measured at 2 flashes/set, O-measured at I flashjsec. showing difference in terminal levels for both cone and rod segnients.
Stimulus presentation rate
381
presentation increases. The fastest rate represented in Fig. 5, 5 flashes/set, yields a rod threshold that is about 10 times higher than the slowest rate of 1 flash every 2 sec. On the other hand, cone thresholds are much less affected. The change from slowest to fastest rate yields a cone threshold change of only two or three times. The data of Fig. 5 are taken from new curves measured specifically for the purpose of the graph, and are not drawn from the earlier part of data, so judgment of the reliability of the effect can be made not only from the vertical lines but also from comparing the separate data of Figs. 1 and 3 with Fig. 5. This considerable variability between individual curves was of course the reason why the matched-pair comparison method was used to establish the existence of the rate effect in the first part of the study. There is no evidence from the lower ends of the IO 5 curves of Fig. 5 that our slowest rate of stimulus presTIME (MINUTES) entation yield thresholds that are at a minimum. The Fig. 2. Fovea1 dark adaptation of subject FGB measured curves of Fig. 5 rise continuously right from the slowat 0 flashes/set and O-at 1 flash/set. No difference est rate. Apparently threshold-level stimuli that are in terminal levels. presented even as slowly as one every 2 set must be suspected of raising their own threshold. The rates of stimulus presentation represented in Fig. 5 probably cover the entire range of rates that rate. Figure 3 shows a typical pair of curves. We conare practical in dark adaptation experiments. Regular clude that (a) it does not require the function of both flashes that are less frequent than one every 2 set are receptors for presentation rate to influence the threshdifficult for a subject to attend to, while rates of presold, and that (b) the absence of a rate effect in the fovea seems to depend upon other characteristics of entation greater than S/set tend to fuse into a steady light toward the end of rod adaptation, because the the fovea. critical fusion frequency is not greater than 4 or 5 Time course. Inspection of the dark adaptation flashes/set at threshold intensities in the fully darkcurves reveals that the effect of a steady presentation adapted eye (Hecht and Verrijp, 1933). A steady fused rate upon the threshold develops gradually and conlight is likely to disappear even above threshold, and tinuously through the course of dark adaptation. To the result is a very serious variability in the measurehave an independent estimate of the course with ments. There are, as well, serious theoretical questions which the rate effect develops, the two authors dark about comparing the late fused thresholds with the adapted for 30min without making measurements earlier unfused ones. For example, should the intenand then made threshold measurements at regular sity of each 20msec flash be reported, or should the rates of stimulus presentation for 15 min in the dark. Talbot level of the fused light be reported? Four such series were measured for each subject, two It is interesting to note that the critical fusion rate at a rate of 1 flash/set and two at 4 flashes/set. Under these conditions, after dark adaptation was essentially of a threshold light depends on its physical intensity rather than its subjective brightness. High flash rates complete, the threshold drifted upward just as expected from the dark adaptation curves. Figure 4 at threshold are easily separated by a subject in the shows one comparison for one subject. At 1 flash/set early part of dark adaptation. Only when the subject the threshold drifts very slowly but continuously has become sensitive enough to see very weak flashes upward; at 4 flashes/set the threshold drifts condo the flashes tend to fuse. tinuously and more rapidly. The rates of stimulus increase from this experiment appear to be entirely 3ti comparable to the results of the dark adaptation iz curves. Apparently, the effect of steady stimulus presentation is a continuous decrease of sensitivity d 2; c throughout the time that the thresholds are measured. . Extent. When individual curves are inspected, as in Figs. I or 4. it seems that the advantage of a slower stimulus presentation rate is largest at the final levels of dark adaptation. Figure 5 shows the terminal levels for rods and cones at several rates of stimulus presentation for the two authors. The graph connects the mean final threshold level of one curve each from the two authors, while the vertical bars show the 5 IO I5 20 range from the lower to the higher of each pair at TIME (MINUTES) each rate. The effect of regular stimulus presentation rates Fig. 3. Parafoveal rod dark adaptation curves of subject upon the threshold clearly increases as the rate of FGB measured at W flashes/set and 0-l tlash/sec.
,t.----J
s t
iR1
HOWAKI)
and FKI:~E.KI~‘K G. B~IK;~OI
D. BAKEK
TIME
(MINUTES)
Fig. 4. Upward drift of parafoveal thresholds of subject HDB following complete dark ad~iptatioll, U--measured at 1 flashjsec and t-4 flashes/see.
9 P E ‘-
t-
: 2 z 8* ?
I 0.5
I 1.0
,
I
I
2.0 STIMULUS
I
I
3.0
PRESENTATION
I 4.0
,
I . 5.0
RATE
(FLASHES / SEC)
Fig. 5. Terminal levels of O--cone and +--rod segments of parafoveal dark adaptation curves that have been measured at various rates of stimulus pr~sen~tion. Data are averages from one curve each of subjects HDB and FGB.
DISCUSSION
The most important characteristic of the effect of stimulus presentation rate upon the threshold is that the effect appears to be absent in the fovea. Although the cones of the fovea show very extensive dark adaptation, their threshold was not affected by any rate at which the stimulus was presented in our experiment. It appears possible, then, that the rate effect may be separate from the regular adaptation processes by which fovea1 cones become sensitive in the dark. There is another example of loss of visual sensitivity that occurs in the periphery but not in the fovea: Troxler fading, under conditions of normal fixation In Troxler fading, the entire peripheral field of view fades away when simple fixation is maintained for Iong periods. Troxler fading does not occur in the fovea unless the image is stabilized by optical means. It appears to be a special case of complete light adaptation wherein fixation provides adequate stabilization for the peripheral retina (Moreland, 1972). According to this view, the response to any stabilized image tends to adapt completely away, but only in the periphery of the retina are the receptive fields of the receptors large enough so ordinary fixation stabilizes the image sufficiently to allow complete adaptation. If that is true, then the rate effect also could rcprcsent regular light adaptation. It would be
absent in the fovea only because normal fixation is inadequate to stabilize the edges of the image on the fine receptor mosaic of the fovea. If the rate effect upon the threshold is due to the simpie accumulation of light adaptation from the stimulus flashes, then the relationships in Fig. 5 are revealing. On the assumption that according to the Weber-Fechner law the threshold should be affected in direct proportion to the amount of light delivered by the stimulus, one might expect that the threshold would be raised by a factor of two when one goes from a stimulus presentation rate of l/set to 2/set, or by a factor of 3, at 3/set. Figure 5 should show therefore a 1O:l change from one end of the graph to the other. In the curve for rods, the prediction is met very well. The points show an increase with rate exactly proportional to the increase in light, and a total change of a factor of IO. On the other hand. the cone thresholds in Fig. 5 increase far more slowly than would be predicted from the increase in the amount of light. Regardless of what mechanisms are involved, it is apparent that the effect of the stimulus accumulates without loss from flash to flash with the rods. The effect is partially dissipated from flash to flash in the peripheral cones, while in the fovea1 cones the effect of one threshold flash is completely dissipated before the next. It should not be surprising that rod light adaptation effects can accumulate without apparent loss
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Stimulus presentation rate over intervals as long as 1 sec. When Mote and Reed (1952) studied the light adapting effect of flickering lights upon subsequent dark adaptation, they found that the same total amount of light flux, delivered as flashes at l-see intervals, was usually a little less effective in raising the threshold during subsequent dark adaptation than was the same total amount of light delivered as a continuous light. But that was true only for intense light levels. A weak preadapting light, presented at a rate of 1 flash/set, could be equivalent to the same total amount of steady light for subsequent rod dark adaptation, and under some conditions, could be even more effective than a continuous light. Mote and Reed’s experiment concerned the effectiveness of preadapting luminances, while ours concerns inadvertent light adaptation from threshold stimulus flashes. But in our experiment, there is a similar accumulation of light adapting effect from flash to flash, spaced I set apart. Similarly, the stimuli are weak ones, and similarly the effect is upon the rods. The conclusion is the same from our experiment and from the Mote and Reed experiment: The rods integrate the effects of weak stimuli completely, over a 1-set interval. On the other hand, our experiment
also shows that peripheral cones do not integrate the complete effect of weak flashes over a full second, and fovea1 cones are able to dissipate the effects of threshold-level flashes completely within 1 sec. Acknowledgement-This investigation was supported by USPHS Research Grants No. R01 ET01394 and R01 EYOO550from the National Eye Institute.
REFERENCES
Baker H. D., Doran M. D. and Miller K. E. (1959) Early dark adaptation to dim luminances. J. opt. Sot. Am. 49, 106~1070. Hecht S. and Verrijp C. D. (1933) Intermittent stimulation by light. III. The relation between intensity and critical fusion frequency for different retinal locations. J. gen. Physiol. 17, 251-265. Moreland J. D. (1972) Peripheral colour vision. In Handbook of Sensory Physiology, VII/4, Visual Psychophysics (Edited by Jameson D. and Hurvich L. M.), pp. 517-536. Springer, Berlin. Mote F. A. and Reed E. C. (1952) The effect of extending the duration of various lightdark ratios of intermittent pre-exposure upon dark adaptation in the human eye. J. opt. Sot. Ant. 42. 333-338.