JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Color Recognition during Voluntary Saccades*t

VICTORIA LEDERBERGt

Brown University, Providence, Rkode Island 02912(Received 8 August 1969)

Color recognition during voluntary eye movements has been measured by presenting various luminancesof red, blue, or green test flashes superimposed on a white adaptation field. A 3-,usec test flash is deliveredat various times before, during, or after a saccadic eye movement that is recorded on film by use of ultra-violet light reflected from the cornea. Color recognition is found to be best when the flash is presentedduring steady fixation, somewhat poorer just before a saccade, and still poorer just after a saccade. Thepoorest recognition of red and green test flashes occurs when the flash arrives precisely in the middle of asaccade; with blue stimuli, the maximum inhibition occurs when the flash arrives 40 to 80 msec later.Recognition of colors is not impaired during a control procedure in which the subject moves his hand totrigger the stimulus flash during steady fixation. Hence it is concluded that a specific visual suppressionaccompanies eye movements but not other motor activities that might equally well divert attention fromthe visual task.INDEX HEADINGS: Vision; Color.

The impairment of vision during large rapid eye move-ments or saccades has been ascribed, on the one hand,to the smearing of the image on the retina,1 and, onthe other, to a central inhibitory mechanism that causesa blanking out of vision during saccades.2 In studyingthe parameters of this suppression, Volkmann3 deviseda system in which very brief flashes of light were pre-sented to the eye, thereby eliminating the possibility ofsmearing the stimulus image. Under these conditions,vision was suppressed during voluntary saccades, butnot to the extent of a complete blanking out of vision.Generally, the moving eye required about three timesmore light to see the same stimulus as the fixating eye.Volkmann concluded that while smearing of the imageacross the retina may impair vision in the course oftypical reading or scanning tasks, the suppression thatremained after smearing was eliminated might be at-tributed to central inhibitory processes, perhaps involv-ing attention.

Subsequent studies by Latour,4 Volkmann, Schick,and Riggs,5 and Richards6 provided further data con-cerning the magnitude and time course of suppression.The findings that the rise in threshold precedes thesaccade, that maximal suppression coincided for spots50 apart on the retina, and that rod and cone mechan-isms behave similarly imply that the retina variessynchronously over the whole retina and suggests acortical basis for the phenomenon.4 But saccades alsoproduce shearing of the retina, resulting in amplifica-tion of the background activity following eye move-ment.6 Saccadic suppression is minimal in the absenceof a background field, and saccadic elimination of thetransient elevations in threshold which occur duringfixation in which the background field is flashed on andoff suggests that saccades disrupt the retinal feedbackloop underlying adaptation, and that the saccadic effectoccurs at, or prior to, the site of activity bursts in theoptic nerve.6

It is conceivable that suppression during saccadesresults from the subject's attending to the task of

moving his eye, which might divert his attention fromthe stimulus. If a generalized attentional factor is in-volved, can a suppression be demonstrated when atask other than eye movement is performed? At leastthree different photopigments appear to be located indifferent classes of cone receptor 7.8; do all cones con-tribute equally to the suppression, or does the suppres-sion occur with only one or two colors? How doesvarying the luminance as well as the color of thestimulus flash affect the extent and duration of theinhibition? The present study attempts to answerthese questions.

APPARATUS AND PROCEDURE

A general description of the apparatus appears else-where3 ; differences will be mentioned. The equipmentwas arranged as shown in Fig. 1. The subject's head,with the right eye at E, was positioned using a head-rest and a biteboard. An eyepatch occluded the left eye.

Background Field and the Stimulus

When the subject looked straight ahead at eye level,he saw a bright background field reflected by a largebeam splitter (S2). The background was 18 cm wide by11 cm high and was located at B, at an optical distanceof 60.5 cm from the eye; it was seen as a milk-glassdiffuser fitted over one side of a metal box. The lightsource of the field was a tungsten lamp covered by ahemispheric Corning Daylight filter 12.7 cm in diameter.The arrangement produced a white light approximatingthat of an equal-energy spectrum. The luminance ofthe field, which was the sole light constantly viewed bythe subject, was 0.129 mL, measured from the positionof the subject's eye. The fixation marks that definedthe aim points for the saccades were black lines drawnon the background field. The left and right sets of lineswere separated by a visual angle of 6.74°, hereafterreferred to as 7°. To execute saccades, the subject firstfixated on the gap between the lines at one side, then

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FIG. 1. Schematic showing the arrangement of the apparatusthat presented and recorded the stimuli and the fixation field,the corneal reflection, and the "go" signal. Path A (-......records the beginning of each trial on film, path B ( )records the path to the film of the rays used to signal the time ofthe stimulus flash, path C (- ) records the path of the cor-neal reflection to film and to the photomultiplier (P), and path D(- -) records the view of the stimulus flash (D) superimposedon the fixation field (B). See text for further explanation.

rapidly moved his eye to the gap between lines on theother side.

The subject saw the test flash as an incremental fieldof light at D, superimposed on the background field.The test field was 5 cm high by 12.5 cm wide, or 40 by10° visual angle. The flash came from a General RadioCo. Strobotac type 1531-A and lasted 3 Asec beforedeclining to less than I peak luminance. The Strobotacflash unit (F) was housed in a black box, through oneend of which the stimulus was displayed through amilk-glass diffuser D (see Fig. 1, path D). Two circularfilter wheels, one containing wavelength-selective filtersand the other containing neutral-density filters, weremounted between the flash unit and the stimulusaperture. The selective filters were narrow-band inter-ference type (Baird-Atomic, Inc.). They peaked at(blue) 488 nm, (green) 549 nm, and (red) 632 nm. Themaximum 2 bandwidth of the three was 7 nm, or 1.5%of the peak wavelength. Wratten neutral-density filtersproduced a range of stimulus luminances such that thesubject's percentage of correct judgments ranged fromzero to 100%.

Recording System

Eye movements were recorded by use of a beam oflong-wave ultraviolet light reflected from the subject'scornea. The beam, whose source ([I) was a mercury arclamp (GE H1100 A38) fitted with heat filters, focusing

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lens, and a Kodak Wratten filter No. 38, was channeledby two achromatic lenses (L) to a beam splitter (SI).A portion of the light was reflected through a focusinglens onto film (F'). This reflected beam was invisibleto the subject. In order to check the alignment of thecorneal reflection in the recording system, the WrattenNo. 38 filter was replaced by a green Kodak gelatinfilter. This resulted in a temporary exposure of thesubject's eye to a field of green light of approximatelythree times greater luminance than the backgroundfield. A recording camera (Grass Instrument Co., modelC-4C) run at either 100 or 250 mm/sec recorded theeye movements (Fig. 1, path C). Two mirrors (MI, M2)directed a beam of light from the stimulus flash ontofilm (Fig. 1, path B). A small neon lamp (N), wired inparallel with the start bell, signalled the beginning ofeach trial (Fig. 1, path A).

Experimental Procedure

The subjects were three college students: two females,LLH, a Chinese student, and KLY, and one male,JEL. Each observed for a total of about 70 h. Eachsession contained 200 trials, and lasted about 75 min.Sessions of control trials were run separately from ses-sions in which saccades were made. For each trial, oneof three colors was presented at one of seven luminancelevels, in a predetermined, semirandom sequence. Thesubject's task was to observe the stimulus, and aftereach trial to write down one of four symbols: R, B,or G if he recognized a red, blue, or green flash, respec-tively, or a checkmark if he was unable to name thecolor. Subjects were able to detect the stimulus flashon all trials, but the color of the flash was recognizedonly when bright stimuli were presented. Sessions beganwith 5 min of adaptation to the background field.

During control trials in which no voluntary eye move-ments were made, the subject fixated at the center ofthe background field, midway between the two sets offixation lines, or at the gap between the left fixationlines, or at the gap between the right set of lines. Theexperimenter signalled "ready," and pressed a switchwhich rang the start bell. The stimulus flashed 1.0 seclater.

During trials in which the stimulus was presentedduring a saccade, the subject was instructed to fixateleft (or right), and warned "ready." After a delay of0.8 sec, a timing circuit rang the start bell and operatedthe camera for 0.5 sec. A Hunter decade interval timerwas preset to control the delay of the stimulus flashwithin each 0.5-sec trial. Each color at each luminancewas presented at 10 different times, with respect to thebell, by setting the timer at values between 0.8 and1.3 sec in 0.05-sec steps. This range of time was chosenin order to cover an appropriate range of actual timesof the flash relative to the saccades. Because of thevariable reaction time of the subjects, it was necessaryto measure the actual time between each saccade and

June1970 COLOR RECOGNITION DURING

stimulus flash on film; the use of 10 different times forthe flash helped to insure that an adequate number offlashes would occur at various times before, during,and after a saccade. Each combination of color, lumi-nance, and time was presented 25 times to each subject,a total of about 8000 trials for each subject, includingall conditions.

During trials that called for the stimulus flash tocome precisely during a saccade, the corneal reflectionwas adjusted to trigger the flash as it swept across thephotomultiplier tube.

The sequence of eye movements was L-R-R-L,R-L-L-R, L-R-R-L, etc.

Calibrations

The background-field luminance was kept at a con-stant level of 0.129 mL as measured by a Macbethilluminometer. For the colored test flashes, luminancecould not be measured directly because of their ex-tremely short duration. Hence, an approximate evalua-tion of their visual effectiveness was attempted by aprocedure of brightness matching. A bipartite field wasused to display simultaneously half the test field on oneside, and a reference field of comparable shape on theother. This reference field was illuminated by 0.077-secflashes provided by a rotary shutter that exposed aconventional tungsten light source. The combined fieldhad the same size as the colored test flashes presentedduring the experiments. The luminance of the referencefield was varied until a match was achieved, using themethod of limits, for a test flash of each color. Becausethe optical densities of all filters were known, theequivalent luminance values for the other test flashescould be calculated. This indirect measure of test-flashluminance was necessary because it is not possible togive a meaningful specification of the luminance pro-duced by a flash whose energy rises to a peak and thenfalls to near zero within a few microseconds.

The time relationships between the bell, stimulusflash, and saccade were measured by dividing lineardistances on the film by the velocity at which the filmmoved during the trials. The time course of the suppres-sion was measured in two ways: (1) as percent correctrecognition as a function of time from the test flash tothe middle of the saccade, and (2) as percent correctrecognition as a function of the time from the "go"signal to the middle of the saccade. Saccades thatoccurred within the first and last 5% of the duration ofeach trial (about 0.1% of the total number of trials)were not included, because they occurred outside theperiod during which the film was traveling at constantspeed.

Equal numbers of left and right saccades were made.The results from eye movements to the right and to theleft have been combined in the calculations becauseno significant differences were found 'between theseconditions.

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FIG. 2. Recognition of red flashes as a function of the log relativeluminance of the stimuli, for subjects KLY (top), LLH (middle),and JEL (bottom), while fixating straight ahead *, before X,during o, and after A, executing a 70 saccade, and duringfixation while performing a task, 0.

RESULTS

Recognition during Fixation Compared to Visionbefore, during, and after Saccades

The recognition of colors during fixation is comparedwith recognition before, during, and after saccades inFigs. 2, 3, and 4, where the results using red, green,and blue flashes, respectively, are shown. In thesefigures, the temporal interval over which responses areaveraged in order to obtain values for the "before"results are 400 msec (KLY, LLH) and 240 msec (JEL),for "during" results 40 msec, and for "after" results360 msec (KLY, JEL) and 440 msec (LLH). In allcases, percent correct recognition increases as the lumi-nance of the flash increases. In addition, recognition ismost accurate during fixation, next best just before asaccade, third best just after a saccade, and poorest

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FIG. 3. Recognition of green flashes as a function of the logrelative luminance of the stimuli, for subjects KLY (top), LLH(middle), and JEL (bottom) under the conditions described inFig. 2.

during the execution of a voluntary eye movement.This result holds without exception for red and bluetest flashes, but is not consistently true for green. Thefour test-flash conditions (during fixation, before sac-cades, during saccades, and after saccades) may beranked for all subjects, using as a criterion the relativeluminance values at 67% recognition. The color recogni-tion follows a reliable pattern of decreasing efficiencyfrom fixation, before, after, and during saccades. Gen-erally, the moving eye requires about twice as muchlight as the fixating eye.

Several control procedures were followed in order tomake sure that no aspect of the experimental designcould account for the inhibition of vision. First thepossibility was investigated that the ultraviolet record-ing beam might be having some effect, even though thesubject was never aware of it. Because the source of theultraviolet light beam was located 200 temporal relative

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FIG. 4. Recognition of blue flashes as a function of the logrelative luminance of the stimuli, for subjects KLY (top), LLH(middle), and JEL (bottom) under the conditions described inFig. 2.

to the center of the field, any effects of this source wouldbe most serious when the subject was fixating to theright. Results show, however, that for all subjects,there is no difference between the recognition of thecolored flashes with either right, left, or central fixation.

A second group of controls examined the possibilitythat the recognition of color was affected by threefactors: (1) the adaptation produced by the green filterused to check the alignment of the corneal reflectionbeam, (2) progressive changes of adaptation to thebackground fixation field, and (3) the interval betweentrials. The results of these three tests show (1) thatred and blue flashes were equally well recognized regard-less of whether the green alignment filter was used atevery sixteenth trial. The green flashes, however, needto have relative luminances approximately 1.7 timesas great if they are to be recognized as well as flashespresented in the absence of the alignment period. There

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June1970 COLOR RECOGNITION DURING VOLUNTARY SACCADES

was no evidence that this adaptation level to green waschanging in any systematic fashion during the 75-minsessions. The same number of correct and incorrectresponses to green stimuli occurred in the first half asduring the last half of the sessions, indicating thatthere was no cumulative decrease of sensitivity togreen because of increasing adaptation to green light.(2) A control experiment with subject JEL involvedclosing his eyes between trials instead of keeping themopen as was usually the case. No difference was found,however; evidently, the continual fixation of the back-ground field during the actual testing is sufficient tomaintain a constant level of light adaptation. (3) Therecognition of stimulus flashes presented at the rateof 4/min is about equal to the recognition of flashespresented at 1/min, suggesting that there is no measur-able loss of color discrimination because of adaptationproduced by the stimulus flashes.

Vision during the Performance of aCompeting Response

It might be argued that the inhibition of vision duringvoluntary saccades is not specific to eye movements,but would occur as a result of having to perform anytask in which attention is directed toward executingthat task. If a generalized attention factor were in-volved, an inhibition of vision might be demonstratedwhen subjects execute a task other than moving theeyes while fixating on the test object. The competingresponse chosen to test this possibility was operationof a lever switch. Subjects fixated straight ahead, asthey had during the fixation controls and, on signal,were required to push the lever through a 600 arc fromleft to right or from right to left. Using their left hand,subjects operated the lever in the same sequence inwhich the saccades were made during eye-movementsessions. As the lever crossed the mid-position in itsarc, the stimulus flash was triggered. This was analogousto the trials in which the corneal reflection was used totrigger the stimulus flash, as the eye crossed midwaybetween the fixation marks. Instead of being triggeredby the reflection, the stimulus was triggered by thelever at the moment when the subject pushed it throughits mid-position.

The results are presented in Figs. 2, 3, and 4 for red,green, and blue flashes, respectively. In comparing thecurves for control fixation and fixation during the, com-peting response, overlapping regions of the curves aredefined as lying less than 2o-, between conditions. Onthe basis of these ao, values, we may conclude thatrecognition is unimpaired by the performance of thecompeting task.

Analysis of the Time Course of Inhibition of Visionfor Red, Green, and Blue Stimulus Flashes

The recognition of brief colored flashes is generallypoorer immediately before, during, or after a 70 volun-

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FIG. 3. The time course of the inhibition with red stimuli forELY (top), LLH (middle), and JEL (bottom), where percentcorrect recognition of red is a function of the time before, during,and after the saccades at seven luminances of the red stimulus.The zero-abscissa value coincides with the midpoint of the saccade.The saccades occupy the time between -20 msec to +20 msec.The log relative luminance values of red stimuli are -0 86, *;-1.06, 0; -1.18, X; -1.36, *; -1.49, A; -1.78, E; -2.09, A.

tary saccade than during steady fixation. There wereno systematic changes of the appearance of colors andno consistent misreporting of colors during saccades.Colors were either reported correctly or were reportedas unrecognized. In order to trace the time course ofthis inhibition, the accuracy of recognition of each colorvs time was tallied. Recognition is plotted as a functionof the time of arrival of the flash in relation to the mid-saccade for each luminance level of red, green, and bluetest flashes. In Figs. 5-7, the abscissa value zero denotesthe time of a flash that occurs simultaneously with themiddle of the saccade. Negative values apply to flashesthat precede the saccade, whereas positive values applyto flashes that follow the saccades by the designatedinterval in milliseconds. Each curve in the figures shows

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FIG.J6. The time course of the inhibition with green stimuli,for KLY (top), LLH (middle), and JEL (bottom), where percentcorrect recognition of green is a function of the time before, during,and after the saccades at seven luminances of the green stimulus.Saccades begin at -20 msec on the abscissa and are completed by+20 msec; the zero value corresponds to the middle of the saccade.The log relative luminance values of green stimuli are -0.50, 0;-0.58, 0; -0.80, X; -0.91, A; -1.18, A; -1.37, m; -1.57, El.

how recognition varies at one of the seven luminancelevels at which each stimulus color was presented toeach subject. The duration of each 70 saccade was ap-proximately 40 msec ±t 10% for each subject (20 msecbefore to 20 msec after the zero abscissa value).

In the recognition of red stimuli (Fig. 5) there areseveral similarities among the three subjects. First,inhibition of vision is maximum when the flash occursduring saccades; each curve has its minimum at thezero abscissa value. It is apparent, also, that the declineof sensitivity begins when the test flash comes beforethe initiation of the saccade, and continues after thesaccade is completed. Second, the inhibition is mostapparent with a stimulus of intermediate luminance.The brightest stimuli are always seen correctly, or show

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FIG. 7. The time course of the inhibition with blue stimuli, forKLY (top), LLH (middle), and JEL (bottom), where percentcorrect recognition of blue is a function of the time before, during,and after the saccades at seven luminances of the blue stimulus.Saccades begin at -20 msec on the abscissa and are completedby +20 msec; the zero value corresponds to the middle of thesaccade. The log relative luminance values of blue stimuli are-0.75, *; -0.86, 0; -1.09, x; -1.21, A; -1.47, A; -1.i0, A;- 1.88, El.

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only a slight loss of color recognition. The dimmeststimuli are always seen, but their color is never success-fully identified. All intermediate flashes show some im-pairment of vision that lasts between 50 and 200 msec.Third, the curves show clearly that the recognition ofred just after a saccade is poorer than before a saccade.

The time course of the inhibition with green flashes(Fig. 6) is similar to that of red stimuli; maximuminhibition occurs when the flash comes at the middleof the saccade. In general, however, a more gradualdecrease and increase of sensitivity occurs. This mayconceivably result from adaptation to the green align-ment filter used during the sessions, although it wouldseem more likely that any such adaptation, if it wereto lower the sensitivity, would do so equally for all

COLOR RECOGNITION DURING VOLUNTARY SACCADES 841,

time intervals. This adaptation may account for LLH'sanomalous results that show better performance inbefore and after conditions than straight ahead.

The results with blue stimuli (Fig. 7) exhibit onestriking difference from the results with red; the maxi-mum inhibition with blue stimuli does not occur whenthe flash occurs during the middle of the saccade, butafter the saccade has been completed. For two subjects,this is 40 msec after the middle of the saccade, a timecorresponding to about 20 msec after the saccade iscompleted. For subject LLH, the maximum inhibitionoccurs with flashes coming 80 msec later than mid-saccades, or about 60 msec after saccades are completed.As with red stimuli, the inhibition of color recognitionis most evident with stimuli of intermediate luminance.

In order to learn whether the trials of maximal im-pairment displayed unique latencies compared to trialsgiving efficient vision before and after saccades, the mid-saccade trials for all luminances of red and green stimuliand the 40- or 80-msec trials for blue were examined.Comparing these to trials in which the stimulus arrived200 msec before or 200 msec after saccades revealed nodifferences of latencies. In other words, the trials thatdisplayed maximal inhibition follow a reaction-timedistribution similar to that for all trials combined.

In addition to analyzing inhibition during saccadesin terms of the relation between the saccade and thestimulus flash, the time between the "go" signal andthe mid-saccade was tabulated for each trial. The rela-tionship between this signal and a possible resultantinhibition was examined. These data yield two piecesof information. First, they give the distribution andmode of each subject's reaction time in executing eyemovements. For JEL, the mode was 200 msec. ForLLH, the distribution of reaction times was broaderand flatter; the mode also occurred at 200 msec. KLY'sreaction times were longer, and the mode of this dis-tribution was 320 msec. The latency of executing thesesaccades is within the range tvpical for reaction timesto visual stimuli.9 The second result of this analysisindicated that, in terms of its influence on inhibition,there is no orderly or apparent relationship betweenreaction time to the "go" signal and the impairment ofvision during the saccade.

DISCUSSION

The present experiments show that voluntary sac-cadic eye movements impair recognition of the color offlashes. The maximal suppression depends on the tem-poral relation between the test flash and the saccade.Maximal impairment occurs when a red flash or agreen flash is presented during the saccade or when ablue flash is presented 40-80 msec later. Why does thistime difference occur, and how do the present resultsrelate to earlier reports?

Suppose a flash arriving after a saccade has the samepsychophysical effect (maximal suppression) as one

arriving before or during an eye movement. In terms ofresponse latencies at the level of the occipital cortex,the response to the later flash must have a shorterlatency than one arriving earlier.5 Does this mean thatthe latency of a response to blue light is shorter, so thata blue stimulus may be presented later to have agiven effect? This does not seem probable, in view ofthe evidence' that latencies are longer for blue thanfor other colors. Recall that the subject's task is oneof color recognition rather than detection, and that theflash appears superimposed on a white background.The thresholds of colormetric purity'1 and of wave-length discriminations are low in the regions of 488 nm.Thus, the subject's task of distinguishing color againsta white background should be particularly easy in thecase of the blue flashes used in these experiments. Ifthe background wavelength is the same as the test-flash wavelength, the test flash would be hardest todistinguish; this might contribute to the maximal sup-pression that was observed using identical wavelengthsfor the background and test flashes.0

The present results for red and green flashes aresimilar to those for white light' in that maximal sup-pression occurs during the saccades. The maximalsuppression for red and green that occurs about 20msec before saccades4 may result from stimulation ofprimarily parafoveal receptors that require longer re-sponse times. The amount of color-dependent suppres-sion in the present study cannot be compared directlywith previous studies4' 6 because the exact amount ofsuppression depends on the luminance of the test flash(Figs. 5-7). The large test flash used here (40 by 100)gives results that agree with the prediction4 of a sharp-ening and deeper rise in threshold with large flashes.Numerous differences in the site of retinal stimulation,the luminance of adapting fields and test flashes, andtiming of test stimuli preclude further comparisons. 4"

Until more is known about the site of the impair-ment and the various latencies involved,' furtherspeculation seems unjustified. The present experimentshave emphasized that specifically visual factors deter-mine the time course of the impairment; the controlexperiments with a hand movement have shown thatno such impairment appears in connection with anonvisual motor task.

ACKNOWLEDGMENT

The author wishes to thank Professor L. A. Riggs forhis interest, help, and support during each stage of thiswork.

REFERENCES

* This research was conducted in the Hunter Laboratory ofPsychology at Brown University and was supported by U. S.Public Health Service Predoctoral Fellowships and research grant,and by the aid of a contract between Brown University and theOffice of Naval Research, Department of the Navy. The basicapparatus was constructed by Volkmann (Ref. 3), with differ-ences described below.

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VICTORIA LEDERBERG

t This report is based on a thesis that was submitted in partialfulfillment of the requirements for the Ph.D. degree in the De-partment of Psychology at Brown University.

I Present Address: Psychology Department, Rhode IslandCollege, Providence, R. I. 02908.

1 R. Dodge, Psychol. Rev. 7, 454 (1900).2 E. B. Holt, Harvard Psychol. Stud. 1, 3 (1903).3 F. C. Volkmann, J. Opt. Soc. Am. 52, 571 (1962).4 P. L. Latour, thesis, Institute for Perception RVO-TNO,

Soesterberg, The Netherlands, 1966.5 F. C. Volkmann, A. M. L. Schick, and L. A. Riggs, J. Opt.

Soc. Am. 58, 562 (1968).

6 W. Richards, J. Opt. Soc. Am. 59, 617 (1969).7 W. B. Marks, W. H. Dobelle, and E. F. MacNichol, Jr.,

Science 143, 1181 (1964).P P. K. Brown and G. Wald, Science 144, 45 (1964).V. J. Polidora, P. Ratoosh, and G. Westheimer, Percept.

Mot. Skills 7, 247 (1957).10 H. Pi6ron, Annee Psychol. 32, 1 (1931); H. deLange, J. Opt.

Soc. Am. 48, 784 (1958).11 I. G. Priest and F. G. Brickwedde, J. Opt. Soc. Am. 28, 133

(1938).12 W. D. Wright and F. H. G. Pitt, Proc. Phys. Soc. (London)

46, 459 (1934).

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