Two-pulse discrimination and rapid light adaptation: Complexeffects on temporal resolution and a new visual temporal illusion

Richard W. Bowen, Kathryn A. Markell, and Cecilia M. SchoonDepartment of Psychology, Loyola University of Chicago, Chicago, Illinois 60626

(Received 6 March 1980; revised 24 July 1980)

Temporal resolution during rapid light adaptation was studied using a suprathreshold two-pulsediscrimination paradigm with stimulus conditions modeled after the classic increment thresholdexperiments of B. H. Crawford. The two-pulse stimulus (1' diam, two 20-ms pulses separated by 40ms) was presented before, during, and after a 6' diam, 500 ms background conditioning field, anddiscriminability was measured as d' using signal detection methods. Discriminability is depressed atbackground field onset and offset, but increases during the exposure time of the background. Immedi-ately following background offset, two-pulse discriminability is dramatically enhanced: two pulsesare discriminated from one pulse with a d' of 4 or more. When the two-pulse stimulus follows back-ground offset by 80 to 240 ms, a novel temporal illusion occurs: a single pulse consistently appears asa double pulse. Several hypotheses for understanding these results are discussed.

INTRODUCTION

The sensitivity of the visual system is altered before, during,and after the presentation of a pulse of light. Visual sensi-tivity is typically measured as the increment threshold for asmall, brief test stimulus presented at various times relativeto a light pulse presented in the form of a large "conditioning"background field, a paradigm introduced by Crawford.'Crawford's original results showed that: (i) the incrementthreshold begins to increase about 100 ms before the back-ground field is turned on, an example of backward masking;(ii) threshold is sharply elevated (sensitivity is at a minimum)at background field onset and offset; (iii) threshold is signif-icantly elevated during the exposure time of the background;and (iv) following the offset of the background field, thresholddeclines slowly to a dark-adapted level over 200-300 ms.These effects may be referred to collectively as rapid lightadaptation.

Crawford's findings have stimulated considerable inter-est2 - 8 primarily because the observed changes in sensitivityare too rapid to be accounted for by photochemical factors,and hence seem to reflect the action of neural mechanismsresponding to the presentation of the conditioning back-ground field. Boynton and others,2t 5 for example, have em-phasized that the peaked elevations of threshold at back-ground onset and offset may be related to neural on- andoff-responses within the visual system.

Most studies of rapid light adaptation have studied themasking effects of the background with respect to thresh-old-level stimuli, and relatively few studies have examinedpossible effects of rapid light adaptation on suprathresholdvisual processing tasks.9"0 In the present study, we measuredsuprathreshold two-pulse discrimination during rapid lightadaptation. A typical measurement of suprathreshold two-pulse discrimination is to determine the interval between asequence of two brief spatially overlapping pulses for whichthe observer can just discriminate the presence of twopulses."",12 In the experiments reported here, rather thandetermining two-pulse temporal separation thresholds, wehave employed a variant of signal detection methods for de-termining two-pulse discriminability and, as discussed below,we measure suprathreshold discriminability as d' for a fixed

pulse pair sequence. Certain studies of two-pulse discrimi-nation have used forced-choice procedures' 2"13 but withoutexpressing discriminability with d' measures.

Our interest in studying two-pulse discrimination stemsfrom both empirical and theoretical factors. Empirically, thetwo-pulse task is a fundamental measure of temporal resolu-tion which has previously been studied only for dark-adaptedconditions'2"13 or under steady light adaptation.' 4 From atheoretical standpoint, the data we obtain could be related tothe proposition that, in general, light adaptation producesboth a decrease in sensitivity (elevation of the incrementthreshold) and an improvement in temporal resolution (asindexed by a decrease in critical duration for detectiontasks).' 5 In the context of the Crawford-type paradigm, itmight be assumed on one hand that since the background fieldproduces decreases in sensitivity, two-pulse discriminabilitywould be relatively poor in the presence of the backgroundbecause the visibility of the pulses to be discriminated wouldbe poorer than under dark-adapted conditions. On the otherhand, on the basis of the assumption that the adaptingbackground is producing better temporal resolution, the op-posite effect would be expected. The present paradigm hasthe potential for assessing whether equivalent changes insensitivity and temporal resolution occur under supra-threshold stimulus conditions.

METHOD

ObserversTwo of the authors (R.W.B. and C.M.S.) served as observ-

ers. Both had normal acuity with corrective lens.

ApparatusThe apparatus was a three-channel Maxwellian-view optical

system that used glow modulator tubes (Sylvania R1131C)as individual sources for each channel. The stimulus array(Fig. 1) was viewed monocularly through a 2 mm artificialpupil positioned one focal length from the exit lens of thesystem. One channel produced a steadily presented fixationreticle (40 td). The second channel produced the 6 deg diam"conditioning" background field, and a 1 deg diam two-pulse

1453 J. Opt. Soc. Am., Vol. 70, No. 12, December 1980 0030-3941/80/121453-06$00.50 © 1980 Optical Society of America 1453

FIXATION----- IRETICLE

TWO-PULSESTIMULUS(I DEG DIA)

FIG. 1. Stimulus array.

BACKGROUNDFIELD(6 DEG DIA)

target was derived from the third channel, which included atwo log-unit Inconel-coated neutral-density wedge (Kodak)positioned at a point conjugate to the artificial pupil. Thewedge was controlled by a motorized servomechanism topermit continuous adjustment of the radiance of the two-pulsestimulus.

Sequencing and presentation of stimulus events were con-trolled by electronic timers (Hunter Model 131C) and labo-ratory-constructed lamp drivers. The glow modulator tubeswere continuously irradiated with ultraviolet light to ensurestable triggering.'6 During an experimental session, the lightoutput of the optical channel generating the two-pulse stim-ulus was monitored continuously with a silicon photocell(PV100A, EG&G). Retinal illumination calibrations weremade on a regular basis with an Ilford photometer (S.E.I.)using the method described by Westheimer.'7

Stimulus conditions and experimental design

Suprathreshold two-pulse discrimination during rapidlight adaptation

Two-pulse discrimination was measured for a two-pulsestimulus consisting of two 20-ms pulses (retinal illuminanceof 1700 td) with an interpulse interval of 40 ins, presented attimes before, during, and after a 500-ms exposure of the 6 degbackground field. The two-pulse stimulus was presented atstimulus onset asynchrony (SOA) values ranging from -160ms (onset of two-pulse stimulus leads onset of background)to 820 ms (onset of two-pulse stimulus follows offset ofbackground by 320 ms and onset of background by 820 mis).The background field was set at a retinal illuminance of either1700 or 170 td.

Two-pulse discriminability at a given SOA value was as-sessed using signal detection methods. A "signal" trial con-sisted of the two 20-ms pulses separated by 40 mis, while a"noise" trial consisted of a single 20 ms pulse whose onset wasat the same time as the first pulse in a "signal" trial.' 8 Duringa daily experimental session, for a given background illumi-nance level, all SOA values were presented in a random order.For each SOA value, 50 trials, 25 "signal" trials, and 25 "noise"trials were presented. On each trial, the observer was re-quired to report whether he saw a single flash or a double (ormultiple) flash. Individual trials were separated by ap-proximately 8 s, an interval sufficient to eliminate most of thecumulative light-adapting effects of repeated presentationsof the background field.2 "9

For each condition data were collected during four dailyexperimental sessions so that final values of two-pulsediscriminability were based on 200 trials at each SOA value.

1454 J. Opt. Soc. Am., Vol. 70, No. 12, December 1980

Values of d'-two-pulse discriminability-were determinedfrom "hit" and "false alarm" rates using standard proce-dures.

It might be argued that our stimulus sequence constitutesa "probe" event that is too long in duration to detect withprecision any very rapid changes in discriminability inducedby the background field. However, Boynton and Siegfried3

have shown that a probe stimulus of nearly 20 ms durationreflects changes in sensitivity with the same precision as aprobe many times briefer, suggesting that the length of ourstimuli may not present limitations to observing rapid changesin visual function.

Increment threshold measurementsIn a control experiment, we determined the increment

threshold for a single 20 ms pulse presented at the same SOAvalues employed for the suprathreshold two-pulse experiment.The 500 ms background field was presented at an illuminancevalue of 1700 td. This experiment was designed to replicatethe original experiment of Crawford.

Increment threshold measures at a given SOA were deter-mined during a single experimental session using a double-random staircase procedure. Estimates of threshold werebased on a total of 20 response reversals between the twostaircases. The illuminance of the 20-ms pulse was adjustedwith a combination of Wratten neutral density filters and theneutral density wedge. A threshold measurement was alsomade without the background field and with only the lightedfixation reticle present in the otherwise dark field.

RESULTS

Figure 2 shows two-pulse discriminability d' plotted as afunction of time after onset of the background field for twolevels of background field retinal illuminance, 1700 td (filledcircles) and 170 td (open circles). The left-most data pointin each panel indicates discriminability of the two-pulsestimulus in darkness with no background field presented: forboth observers this value was near 1.0.

At an onset asynchrony of -160 ins, discriminability at bothbackground levels is close to the value for darkness. Beforethe onset of the background, from -80 to 0 ms asynchrony(indicated as region A), d' values fall to near 0.0, a depressionof discriminability. Here observers exhibited both low hit andlow false alarm rates, and consistently reported seeing onlya single pulse. In region B, extending from 40 to 460 ins,discriminability increases during the exposure time of thebackground. This is seemingly an effect of increasing lightadaptation, but the improvement in discriminability does notincrease directly with light level: overall, d' values are higherfor the 170 td background than for the 1700 td background.At the higher background level, a sharp decrease in discrim-inability occurs at an asynchrony of 420 ms (point C) the valueat which the two-pulse stimulus (30 ms long overall) and thebackground coterminate. A steep increase in discriminabilityto a d' value of 4.0 or more peaking is evident at 500-540 msasynchrony (region D). Thus, two-pulse discriminability isnearly perfect for conditions where the pulses begin near theoffset of the background field.

Following the offset of the background in a range extending

Bowen et al. 1454

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FIG. 2. Two-pulse discriminability d' as a function of time of onset (inmilliseconds) of the two-pulse stimulus relative to the onset of the back-ground field for two observers at two levels of background field retinal il-luminance, 1700 td (filled circles) and 170 td (open circles). Vertical dashedlines indicate onset and offset of background field. Letter designations A-Eare discussed in text.

from 580 to 740 ms asynchrony (region E) discriminability isdepressed to a level generally below that of the "no field"condition. This decrease in discriminability is a consequenceof a striking temporal illusion in which, throughout this rangeof asynchronies, a single 20-ms pulse of light is seen as a doublepulse. The d' values are low here because the observer gen-erates high rates of both false alarms and hits. Finally, fol-lowing the interval of the temporal illusion, discriminabilityreturns to near the values obtained in darkness at an onsetasynchrony of 820 ms.

We computed the logarithm of the ratio of the total number

1455 J. Opt. Soc. Am., Vol. 70, No. 12, December 1980

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FIG. 3. Log R (Ratio of one-pulse to two-pulse reports) as a function oftime after onset of background field (in milliseconds). Retinal illuminancelevels are as in Fig. 2.

Bowen et al. 1455

of responses of "one pulse" (from correct rejections andmisses) to the total number of responses of "two pulses" (fromhits and false alarms). The statistic, which we designate 20 logR (for ratio of one-pulse to two-pulse reports), will be at avalue of 0.0 if, for a given condition, the observer is as likelyto report one pulse as to report two pulses. Figure 3 gives logR as a function of time after the onset of the background fieldfor the 1700-td background (filled circles) and 170-td back-ground (open circles) for the two observers. Log R is generallyat a zero level except at two points: at asynchronies of -80to 0 ms, where log R is positive, and at asynchronies from 580to 740 ms, where log R is negative. In the first case, the ten-dency of the observer is toward saying "one pulse." In thesecond case, the interval of the temporal illusion, the tendencyis toward saying "two pulses."

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FIG. 4. Increment threshold (log trolands) as a function ofof background field (in milliseconds) for a background fienance of 1700 td.

The increment threshold data as a function ochrony (Fig. 4) are congruent with Crawford's' oralthough the local increases in threshold nearonset and offset are more pronounced in Crawforin our own. Generally, our stimulus conditionscharacteristic "masking" effects associated with t'adaptation paradigm.

DISCUSSION

Our results exhibit two phenomena that seemchanges in sensitivity measured in the Crawfonthreshold paradigm: enhancement of two-punability (at D, Fig. 2) and the temporal illusion IBut the enhancement effect may be related toperceptual phenomena.

Enhancement effectThe enhancement phenomenon resembles,

"retroactive contour enhancement" effect firstStanding and Dodwell.2 1 They found that thethreshold for a brief test target (a visual form)hanced if the target was presented 100 ms or leoffset of a background field. Discriminabilit,

greater than if the test target was presented on a steadybackground field.

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1456 J. Opt. Soc. Am., Vol. 70, No. 12, December 1980

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Enhancement in two-pulse discriminability may also berelated to "retroactive brightness enhancement" investigatedby Donchin and Lindsley.2 2' 23 In their paradigm, a brief (10ms) test stimulus is followed by a higher-luminance, equallybrief overlapping background pulse. If the interval betweenthe pulses is short (90 ms or less), the test pulse is masked bythe subsequently presented background pulse, but if the in-terval between the pulses is increased to 100-150 ins, the testpulse actually appears brighter than when presented alone.If it is the offset of the brief background pulse that is affecting

* the brightness of the test pulse, then this phenomenon isanalogous to the enhancement of temporal resolution we haveobserved.

RWB In our increment threshold data (Fig. 4), and in similar datafrom the Crawford paradigm,",2' 5-7 the background condi-tioning field has a desensitizing effect on the visual system atall asynchronies. Recently, however, Pulos and colleagues24' 25

have demonstrated that with a pulsed annular backgroundfield, transient sensitization (a decrease in threshold belowbaseline) can occur following background offset. The relationof this effect to the present results is unclear since our stimulusconditions are not comparable. However, the sensitizationeffect has been linked empirically to the occurrence of back-ground offset,25 and hence may share a common underlyingmechanism with the suprathreshold effects-retroactivecontour enhancement,2 1 retroactive brightness enhance-ment,23 and enhancement of two-pulse discriminability-discussed above. In all of these cases, enhancement of visual

OF function can be attributed to the offset of a conditioning(Ms) stimulus, and enhancement seems to consist of a transient

increase in the magnitude of the sensory response to testtime after onset stimuli.Id retinal illumi-

Two-pulse temporal illusionOver a substantial interval following the offset of the

,f pulse asyn- background field, a single brief pulse appears to be a doubleiginal results, pulse. This illusion is extremely compelling, and one needbackground not resort to the use of signal detection methods to reveal it:

d's data than the illusion was readily reported by over a hundred vision* produce the scientists when it was demonstrated using a portable opticalhe rapid light system at a meeting of the Association for Research in Vision

and Ophthalmology.26

It has been known for some time that a single pulse of lightmay be perceived as a double pulse. 27' 28 However, the con-

unrelated to ditions that produce that effect (e.g., large peripheral testI increment- stimulus fields) stimulate both rod and cone receptors, and

se discrim the effect is thought to result from differential latencies be-Ise EsFig2- tween the rod and cone systems.28-30 This phenomenoneatE, lg. othe probably has no relation to the illusion, since the illusion oc-

curs with a stimulus confined to the fovea.

The illusion is presumably the result of persisting activitygenerated by the background which interacts with the sensory

first of all, a response to the test pulse to produce the illusion of "double-reported by ness." The persisting influence of the background could, forrecognition example, "subtract" from the response to the test pulse to

could be en- produce a temporal gap in the response and generate thess before the double appearance. We have observed informally that theby was much illusion appears equally strong at closely spaced (20 ms) pulse

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Bowen et al. 1456

asynchrony steps within the range of its occurrence. Thissuggests that any such interacting event would have to bemultiphasic or oscillatory in nature, and simple explanationsbased, for example, on afterimages or on neurophysiological"off-responses" are not likely to apply since these classes ofevents could interact with the test pulse at some particularstimulus onset asynchrony, but not over a continuous rangeof asynchronies.

An alternative explanation for the illusion is that presen-tation of the background conditioning field drives the visualsystem into some (possibly unstable) state in which the re-sponse to a subsequent test pulse is itself an oscillating one.In other words, the test pulse may elicit additional phases ofsensory activity following exposure to the background field.This idea may have merit in view of another informal obser-vation concerning the illusion: a single test pulse appearsbrighter under the illusory conditions than it does when pre-sented without the background.

Other effects on two-pulse discriminabilityIn the Introduction, we suggested that two-pulse discrim-

inability might be governed directly by sensitivity changesduring rapid light adaptation. This may be the case for ef-fects A and C in Fig. 2, where it appears that the onset andoffset of the background field produce both poor discrimi-nability and decreased sensitivity. However, during the ex-posure time of the background (range B, Fig. 2) sensitivity issubstantially reduced but two-pulse discriminability is en-hanced; hence the hypothesis linking discriminability tosensitivity cannot be applied.

The effects in range B replicate work by Shickman 3 l whomeasured traditional two-pulse thresholds" during rapid lightadaptation and found an increase in temporal resolving powerduring exposure to a 1-s background field. The present data,and those of Shickman,3 ' could be attributed to the fact thattemporal resolution, as measured by critical duration, im-proves with increasing light adaptations Presentation ofthe background may shorten the time constant of visual pro-cessing and improve two-pulse discrimination. But the factthat discriminability is better for the 170-td background thanfor the 1700-td background (Fig. 2) is not entirely consistentwith this hypothesis. However, combining the ideas con-cerning sensitivity and time constant just discussed, we mayspeculate that the results in range B of our data are due tojoint but nonequivalent changes in temporal resolution andsensitivity during the background presentation. Specifically,observed effects on discriminability could occur if the changesin sensitivity and the time constant of visual processing withlevel of light adaptation followed different functions acrossour experimental conditions instead of being precisely re-ciprocal, as is often assumed.'5 We cannot confirm thispossibility from the present data, but further measures ofdiscriminability over a much wider range of rapid adaptationlevels might permit some estimation of the function governingchanges in sensitivity and time constant.

It is also possible that the effects at point B depend uponan effect first studied by Craik.32 He showed that differentialbrightness discrimination is optimal at the light level to whichthe visual system is adapted, a fundamental characteristic ofthe process of light adaptation. If the system is adapted es-sentially to the dark field present between trials, the presen-

1457 J. Opt. Soc. Am., Vol. 70, No. 12, December 1980

tation of the 170-td field represents a smaller departure fromthat state of adaptation than does the presentation of the1700-td field. If two-pulse discriminability is correlated withdifferential brightness discrimination of the successive pulses,Craik's result could predict better performance during theexposure of the lower-luminance background field.

We have demonstrated that suprathreshold temporal res-olution varies in a complex fashion during rapid light adap-tation and that many phases of this variation cannot be relatedto a corresponding increment threshold function. Furtherwork will be necessary to establish the sensory basis for thetemporal illusion as well as the other effects on two-pulsediscriminability.

ACKNOWLEDGMENTS

We thank D. C. Hood, J. Pokorny, V. C. Smith, and W. A.Yost for several helpful suggestions and a critical reading ofthe manuscript. This work was supported by NationalScience Foundation Grant No. BNS 78-17779 to R. W.Bowen.

'B. H. Crawford, "Visual adaptation in relation to brief conditioningstimuli," Proc. R. Soc. London Ser. B 134, 283-302 (1947).

2R. M. Boynton and G. Kandel, "On-Responses in the Human VisualSystem as a Function of Adaptation Level," J. Opt. Soc. Am. 47,275-286 (1957).

3R. M. Boynton and J. B. Siegfried, "Psychophysical Estimates ofOn-Responses to Brief Light Flashes," J. Opt. Soc. Am. 52, 720-721(1962).

4R. T. Kintz and R. M. Boynton, "Temporal Summation DuringBackward Visual Masking," J. Opt. Soc. Am. 59, 212-216 (1969).

5H. D. Baker, "Initial Stages of Light and Dark Adaptation," J. Opt.Soc. Am. 53, 98-103 (1963).

6W. S. Battersby and I. H. Wagman, "Neural limitations of visualexcitability. I. The Time Course of Monocular Light Adaptation,"J. Opt. Soc. Am. 49, 752-759 (1959).

7G. Sperling, "Temporal and spatial visual masking. I. Masking byImpulse Flashes," J. Opt. Soc. Am. 55, 541-559 (1965).

8L. Ganz, "Temporal factors in visual perception," in Handbook ofPerception, Vol. 5, Seeing, edited by E. C. Carterette and M. P.Friedman (Academic, New York, 1975), Chap. 6.

9L. H. Theodor, "The detectability of a brief gap in a pulse of light asa function of its temporal location within the pulse," Percept.Psychophys. 12,168-170 (1972).

'0E. Matin, "Light adaptation and the dynamics of induced tilt,"Vision Res. 14, 255-265 (1974).

"A. Mahneke, "Foveal discrimination measured with two successivelight flashes," Acta Ophthalmol. 36, 3-11 (1958).

"M. L. Kietzman, "Two-Pulse Measures of Temporal Resolution asa Function of Stimulus Energy," J. Opt. Soc. Am. 57, 809-813(1967).

13M. F. Lewis, "Two-Flash Thresholds as a Function of Luminancein the Dark-Adapted Eye," J. Opt. Soc. Am. 57, 814-815 (1967).

' 4D. G. Purcell and A. L. Stewart, "The two-flash threshold: Anevaluation of critical-duration and visual-persistence hypotheses,"Percept. Psychophys. 9, 61-64 (1971).

15 L. Matin, "Critical Duration, the Differential Luminance Threshold,Critical Flicker Frequency, and Visual Adaptation: A TheoreticalTreatment," J. Opt. Soc. Am. 58, 404-415 (1968).

16L. Matin, "Use of the glow modulator tube for visual research," Am.J. Psych. 77, 650-651 (1964).

17G. Westheimer, "The Maxwellian view," Vision Res. 6, 669-682(1969).

tA 920-ms pulse was selected as the "noise" stimulus rather than anequal-energy 40-ms pulse because observers could easily discrim-inate a single 40-ms pulse on the basis of its greater brightness. Thesingle 20-ms pulse and the pair of 20-ms pulses separated by 40 msdid not differ in brightness, so observers were compelled to base

Bowen et at. 1457

their judgments on temporal properties of the stimuli, rather thanother phenomenal aspects.

19R. M. Boynton, W. R. Bush, and J. M. Enoch, "Rapid changes infoveal sensitivity resulting from direct and indirect adaptingstimuli," J. Opt. Am. Soc. Am. 44, 56-60 (1954).

2 0The R measure is equivalent to the traditional measure f3 (responsebias) when the a priori probabilities of signal trial and noise trialpresentation are equal. R was chosen over i3 because accurate es-timates of : could not always be derived on the basis of 200 exper-imental trials. See W. A. Yost, "A forced-choice adaptive proce-dure for measuring auditory thresholds in children," Behav. Res.Methods Instrum. 10, 671-677 (1978) for a similar use of log R.

2 1L. G. Standing and P. C. Dodwell, "Retroactive contour enhance-ment: A new visual storage effect," Quart. J. Exp. Psych. 24, 21-29(1972).

2 2E. Donchin and D. Lindsley, "Retroactive brightness enhancementwith brief paired flashes of light," Vision Res. 5, 59-70 (1965).

2 3E. Donchin and D. Lindsley, "Visually evoked response correlatesof perceptual masking and enhancement," EEG Clin. Neurophysiol.19, 325-335 (1965).

2 4E. Pulos, J. E. Raymond, and W. Makous, "Transient sensitization

by a contrast flash," Vision Res. 20, 281-288 (1980).25E. Pulos and W. Makous, "Sensitization following offset of an an-

nulus," J. Opt. Soc. Am. 69, 1452(A) (1979).26R. W. Bowen, K. A. Markell, V. Pappageorge, and F. Alfano,

"Temporal resolution during transient light and dark adaptation,"Invest. Ophthalmol. Visual Sci. Suppl. 18, 248-249 (1979).

27K. Dunlap, "The shortest perceptible time interval between twoflashes of light," Psychol. Rev. 22, 226-250 (1915).

28S. H. Bartley and F. W. Wilkinson, "Certain factors in producingcomplexity of responses to a single pulse of light," J. Psychol. 9,299-306 (1953).

2 9R. M. Boynton, "Discrimination of homogeneous double pulses oflight," in Visual Psychophysics, Vol. VII/4, Handbook of SensoryPhysiology, edited by D. Jameson and L. Hurvich (Springer-Verlag,Berlin, 1972), Chap. 9.

30 R. M. Springer, J. A. Deutsch, and G. Stanley, "Double flashes fromsingle pulses of light," Percept. Psychophys. 18, 398-400 (1975).

31 G. M. Shickman, "Brief illumination and visual temporal resolvingpower," Dissertation, Harvard University, 1960 (unpublished).

3 2K. J. W. Craik, "The effect of adaptation on differential brightnessdiscrimination," J. Physiol. 92, 406-421 (1938).

1458 J. Opt. Soc. Am., Vol. 70, No. 12, December 1980 0030-3941/80/121458-14$00.50 © 1980 Optical Society of America 1458