The effect of size, retinal locus, andorientation on the visibility of

a single afterimage*

JANETI'E ATKINSONtThe Psychological Laboratory, University of Cambridge, Cambridge, England

Unitary and fragmentary visibilities of the afterimage of a small rectangularbar were measured at three retinal loci. Unitary visibility was higher for fovealthan for parafoveal viewing, whereas fragmentation was greater the longer thebar. A model for estimating the size of receptors has been applied and gives themean size of a "functional unit" to be 1.5 deg in the fovea and 3-4 degparafoveally. Unitary visibility has been shown to be higher for vertical andhorizontal bars than for obliquely oriented bars. Explanations for this resulthave been suggested in terms of the distribution, size, and selectivity of corticalorientation selective units.

Stabilized images undergo irregularbut nonrandom fluctuations in theirvisibility (Riggs, Ratliff, Cornsweet, &Cornsweet, 1953; Ditchbum &Fender, 1955; Pritchard, Heron, &Hebb, 1960; Evans, 1965).Examination of these fluctuations hasled to the general conclusion thatregeneration of the entire image, orsome specifiable part of it, cannot beentirely explained in terms of lensslippage (Ditchbum & Fender, 1955;Byford, 1962; Barlow, 1963;Rig~ &Schick, 1968). Certain structureddisappearances and reappearancesoccur without any apparent changes inretinal input, and may represent theactivity of perceptual units within thevisual cortex.

In the study described here, thevisibility of a stabilized image in theform of an afterimage of a singlerectangular bar was measured as afunction of stimulus size, locus, andorientation. The first experiment is anextension of an earlier one by Evans(1966, 1967), in which he deduces thesize of receptive fields within thevisual system from the properties of"functional units." A "functionalunit" is considered as a cluster of cellswith synchronous firing, so that if astabilized vertical line is projectedonto one of these units, it will tend toappear as a whole. Also, when itdisappears, it will do so as a unit andnot in fragments. A line which islonger than the functional unit willnot be linked for synchronous firing

*This work was supported by a ScienceResearch Council scholarship and theGeorge Henry Lewes Studentship(Physiological Laboratory, Cambridge,England). The author wishes to thank Dr. O.J. Braddick, Dr. F. W. Oampbell, and Dr. L.A. Riggs for their helpful criticisms of themanuscript.

"tAddress: Physiolocical Laboratory,Univemity of CambriciJle, Downing St.,Cambridge, England.

and will be likely to appear anddisappear in fragments. As image sizeapproaches unit size, the probabilityof fragmentary appearances rises andunitary appearances decreases. Anattempt is made in the present studyto interpret the visibility andfragmentation of an afterimage interms of functional unit size, using theabove model.

METHODApparatus

The afterimages were produced by aBraun F800 electronic flashgun. Theflash duration was approximately4 msec as calibrated by displaying aphotocell output on an oscilloscope.The light output, according to themanufacturer's specification, was160 J. A diffusion filter was placed infront of the flash tube. The flash wasgiven in a room dimly illuminated atapproximately 0.3 nits. Patterns wereproduced by cutouts in a black mask,placed in front of the diffusion filter.

SubjectsAll Ss were students at the

University of Cambridge; they werebetween 17 and 30 years of age andwere naive as to the purpose of theexperiments. All had normal orcorrected to normal vision.

ProcedureThe S fixated, with his right eye, a

white spot attached to the mask. Hisleft eye was covered throughout thetrial by an eyepatch. His eyes were28.5 em from the flash andhorizontally aligned with the fixationspot. Immediately after the flash, theS closed his eyes and kept them closedthroughout the trial time, thusmaintaining a constant backgroundluminance for viewing the afterimage,

The trial time was always 60 secimmediately following the flash. The Salways had 15-min adaptation before

the first experimental trial and 5-minadaptation between trials. Afterimageslasted on average for 2-3 min, althoughonly the first minute of recording wasanalyzed. Certain artifactual factors,known to affect visibility ofafterimages, were kept as constant aspossible. The S was asked to avoidblinking during the trial, to keep hishead still, and to make as fewvoluntary eye movements as possible.

The S used small, two-way switchesto record the visibility of theafterimage. The switches we-re alwaysdepressed by S when he could see theimage and released when the imagedisappeared. The switches controlledcounters which were separated from Sby a screen to prevent observation ofthe results during testing. Visibilitywas measured in seconds of trial timefor which the afterimage was visible, asrecorded on the counters at the end ofeach trial.

Orders of presentation were usedwhich balanced for practice effectsbetween different stimuli and viewingconditions.

Experiment 1To establish the afterimage, the S

fixated the center of the target (fovealcondition) or 2 or 7 deg to the right.Four different lengths of bar wereused: 45 min, 1, 2, and 3 deg. Ten Sscompleted three trials on each of the12 viewing conditions. The first trial ineach condition was treated as apractice trial, and the mean of theother two trials was used in the results.Two and a half sessions, each of 2 hduration, were required to completethe test trials for each S.

The S used two switches on eachtrial; he depressed the right-handswitch whenever the afterimage of theentire bar was visible (U) and theleft-hand switch whenever anythingwas visible, either the whole bar orfragments of it (U + F). The mean forthe visibility of the bar as a unit, infragments, and in either form werecalculated for each S on eachcondition.

Experiment 2The total unitary visibility (U) of

the afterimage of a single bar 1.5 degin length was measured for sevendifferent orientations of the barbetween 0 deg vertical and 90 deghorizontal in i5-deg steps. Two Sswere tested with foveal viewing andthree Ss tested using parafovealviewing, 2 deg to the right of fixation.

RESULTSExperiment 1

Mean visibility is plotted as afunction of the image for the threemeasures calculated (U, F, and U + F)in Fig. 1. An analysis of variance was

Perception &; Psychophysics, 1972, Vol. 12 (2B) Copyright 1972, Psychonomic Society,lnc., Austin, Texas 213

eo carried out on the results.For the afterimage as a unit (U),

there was a decline in visibility withincrease in eccentricity [F(2,18) ..48.5, P < .01]. There was also anEccentricity by Length of the Barinteraction [F(6,54) = 14.3, P < .01).For foveal viewing, the small barsappeared as a unit for more of thetime than did long bars. For parafovealviewing, unitary visibility was lowerfor small bars than for longer ones.For all three loci, fragmentation wasincreased by Increase in length.

Experiment 2A graph of unitary visibility as a

function of orientation is shown inFig. 2. An analysis of variance revealeda significant difference betweendifferent orientations of the bar[F(6,6) = 12.3, P < .01, for foveal andF(6,12) = 6.4, p < .01, for parafovealviewing). Comparison of the meansfor each orientation using theNewman-Keuls test (see Winer, 1962)revealed a significant differencebetween visibility of a bar at 0 and30 deg and between 90 and 60 deg,but no significant difference between30, 60, and 45 deg,

The effect was small but slightlylarger for foveal than for parafovealviewing. .

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DISCUSSIONExperiment 1

In order to compare the results ofExperiment 1 with those of Evans(1967), the means for the threemeasures were converted into the twoscores used by him and are shown inFig. 3. These were the disappearanceof the bar as a unit (which is equal totrial time with the time anything isvisible subtracted from it) and thedisappearance of the bar in fragments(F).

Evans suggested a model to explainhis results in which he predicts that asimage size is increased, the probabilityof fragmentary disappearances risesand unitary disappearances decreases.

If line length is X and the unitlength, L, if it is assumed that all unitsare the same size and do not overlap,and if it is assumed that X is smallerthan L, then the expected number ofboundaries that X will cross will beX/L. If the unit is inactive for fractionf of trial time and two units are neverinactive at the same time, thenfragmentation time = fX/L and thetime of complete unitarydisappearance = f(l- X/L). Whenthese two functions are equal,X=0.5L.

Using this model, the point at whichthe curve representing unitarydisappearance crosses that representingfragmentary disappearance willindicate the size of the functional unit

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214 Perception & Psychophysics, 1972, Vol. 12 (2B)

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Two neurophysiological modelswould meet the requirements of thepsychophysical results. The first oneproposes that detectors for verticaland horizontal lines are morenumerous than detectors for otherorientations, all detectors being ofequal sensitivity. The alternativehypothesis assumes an evendistribution of line detectors, but withthose sensitive to vertical andhorizontal orientations having anarrower bandwidth (higherselectivity) than those tuned tooblique orientations.

Campbell, Cleland, Cooper, andEnroth-Cugell (1968), looking at thedistribution of orientation-selectiveunits in cat visual system, did not findany difference between the numberselective to oblique orientations andthose to vertical or horizontalorientations. Hubel and Wiesel (1962,1965, 1968) also found no asymmetryin the distribution for cat and monkeyvisual cortical units. However, in anymicroelectrode study, as the numberof units sampled is small compared tothe total number of orientation unitspresent, it would be quite possible forthe expected real, but small,unevenness in the distribution to beobscured.

The effects found in the presentstudy have been measured in terms ofincrease or decrease in unitaryvisibility of an afterimage, which is asomewhat different measure from thatused in other psychophysical orphysiological studies. If it is assumedthat higher visibility reflects thesimultaneous firing of a larger numberof cells or single units forming afunctional unit, then the findings herewould reflect an asymmetricaldistribution of receptors. In contrast, if

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meridional variations in slantperception. Emsley (1925), Leibowitz(1955 ), Ogilvie and Taylor (1958),Campbell, Kulikowski, and Levinson(1966), and Mitchell, Freeman, andWestheimer (1967) have all measuredmeridional variation in acuity andconcluded that the threshold washigher for a 45-deg line than for onethat was horizontally or verticallyoriented. Campbell's study eliminateddioptrics as the possible cause of thevariation. Jastrow (1892), Gibson andRadner (1937), Muller, Sidorsky,Slivinske, Alluisi, and Fitts (1955),Andrews (1967a, b), and Bouma andAndriessen (1968) have all measuredthe accuracy of absolute settings of aline to a particular orientation.Accuracy was found to be higher atthe vertical and horizontal than atintermediate orientations. Recentmasking experiments (Campbell &Kulikowski, 1968; Houlihan &Sekuler, 1968; Gilinsky, 1968) andmeasurements of the evoked potentialalso support the hypothesis thatdiscrimination is finer at vertical andhorizontal orientations than at 45 deg,

A study similar to the presentexperiment was carried out by Craigand Lichenstein (1953), whoexamined the disappearance ofsteadily fixated lines as a function ofthe orientation of the lines. Minima inthe curve relating frequency ofdisappearance to orientation of thelines were found at the vertical andhorizontal, with maxima at 45 deg.The authors concluded that perhapsthe components of nystagmus weremaximal in the direction of fewestdisappearances. This explanationseems unsatisfactory in the light of thepresent experimental results onstabilized images.

Experiment 2There is an extensive literature on

that is involved. This peint is shownon the graphs in Fig. 3. Evans 8I8umeathat the eatimated size for functionalunits also indicates the aize ofreeeptiw fielda used in perception ofthe ban. 'ntus, from the results ofExperiment I, the fowaI receptivefield size would be 1.6 deg and theparafowal 3 or 4 deg. 'nteae sizeestimates are much larger than thosefound by Evana.

There are two main .differencesbetween the two experiments-one isthe difference in response made by theS in each study and the other is thebrightness of the flash used-rbutneither of these facton would beexpected to give such largediscrepancies between results. Onlyfour Sa were tested by Evans, and largevariabiltiy among Sa was noticed.Different receptiw field sizes fordifferent Sa would haw to bepostulated to fit his results, whichwould not be likely for such a basicneurophysiological property asreceptive field size. The differences areprobably due to differences in criteriaused by different Sa in judgingappearances and disappearances.

It seema likely that at least one ofthe assumptions that has to be madeto accept Evans's model is not correct,namely, that functional units do notoverlap. If there is overlap, thenfragmentation time cannot beaccurately specified without knowingthe amount of overlap.

The differences in visibilityrecorded for foveal and parafovealimages suggest that the patterndetectors involved have somewhatdifferent properties, particularlyinvolving their functional size. There isa great deal of evidence from otherstudies to suggest that the size of unitschanges with shift in retinal locus.Hillman (1958) and Hallet (1963)measured the relationship betweenstimulus size and threshold luminancewithin the retina. They found thatthere was an increase in summationarea with increase in eccentricity. It iswell established that in man, acuityfalls monotonically with increasingeccentricity of the stimulus (see Dor,1873; Burchardt, 1893). Histologicalstudies of the human retina haverevealed a decreasing density of rods,cones, and bipolar and ganglion cellswith increase in eccentricity (Vilter,1949). What is more, anatomicalevidence from Osterberg (1935) andWeymouth (1958) show that fovealand parafoveal units differ in densityand dimensions. It is not surprisingthat visibility of the afterimage varieswith eccentricity, but the preciselocation of the units determining theseeffects remains unresolved.

Perception & Psychophysics, 1972, Vol. 12 (2B) 215

Fig. 3. Unitary and fragmentarydisappearances (in seconds) as afunction of bar length and retinallocus (in degrees).

the same assumption holds and thesecond hypothesis is correct (i.e., thereis a difference in selectivity withorientation), then the total visibilityfor vertical and horizontal bars wouldbe lower than for' bars at obliqueorientations. Where the bandwidth wasnarrow (near the vertical andhorizontal), a given bar would fallwithin the bandwidth of fewer cellsthan where the bandwidth was wide(at 45 deg), At present, it is impossibleto say whether the above assumption

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A third possible alternativeexplanation of the result, which can betested in a future experiment, is thatthe difference in visibility noted forthe various orientations reflectsdifferences in the size of functionalunits; those sensitive to vertical andhorizontal lines are larger than thosesensitive to oblique lines. Unitary andfragmentary visibilities for differentlengths at different orientations will becompared to test the hypothesis. Therelative difference in functional unitsize would necessarily be in the samedirection at different eccentricities, forthe meridional effect was found bothat foveal and parafoveal viewingpositions.

While the exact reason for thevariation cannot as yet be decided, it isclear that the functional unitsresponding must be highly sensitive toorientation and as such must becortical, for units lower in the visualsystem do not show the samesensitivity to this feature.

SUMMARY AND CONCLUSIONThe main conclusion from

Experiment 1 is that the total visibilityof an afterimage of a small rectangularbar is higher when the image is viewedfoveally than when it is viewedparafoveally. As the length of the baris increased, the amount of time theafterimage is seen in fragments is alsoincreased. A difference has beensuggested in the size of functionalunits detecting the afterimage forfoveal and parafoveal viewing.

Experiment 2 has shown that totalvisibility of an afterimage also dependson the orientation of the bar.Horizontal and vertical bars are visiblefor longer than bars at intermediateorientations. Three possibleexplanations for this result have beenput forward: asymmetry in thedistribution of orientatioh-selectiveunits, differences in bandwidth forunits selective to differentorientations, and differences in thesize of units for different orientations.It is not possible in the light of theseexperimental results to decide amongthe three explanations.

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(Accepted for publication March 27. 1972.)

Perception & Psychophysics, 1972, Vol. 12 (2B) 217