Afterimages, Despite the Invariance in Their Physical Size on the Retinae

Japanese Psychological Research 1986, Vol,28, No.4, 168-175

Effects of oculomotor cues on the apparent size of afterimages

KOTARO SUZUKI' Depnrtmenl of Psychology, I acully of Lepers, Uniuersily of Tokyo2

The effects of oculomotor cues on the apparent size of afterimages was examined. Five students served as subjects. In Experiment (a), subjects projected afterimages monocularly and binocularly onto the projection field in a lit room or onto the plane imagined at a point- light in a dark room. The distances of projection ranged from 25 to 300 cm. Subjects reproduced the apparent sizes of these afterimages. The result was that, whereas in the lit room the apparent size of afterimages changed proportionally with the distance of projection, it changed little beyond about 200 cm in the dark room. It suggests that the oculomotor cues are available within 200 cm for size and distance perception. In Experiment (b), subjects reproduced the apparent sizes of entoptic afterimages as well as those of afterimages seen in complete darkness. The result was that the former corresponded to the size of afterimages localized at about 30 em and the latter to that at 60 cm. These distances are thought to be related to the resting positions of accommodation. In Experiment (c), subjects tried to alter the apparent size of afterimages voluntarily. Whereas all the subjects could alter it in the lit room, none of them could do so in complete darkness, probably since accommodation and convergence were locked in their resting positions. Key words: afterimages (projected), apparent size, oculomotor cues, accommodation, resting positions of eyes.

Afterimages, despite the invariance in amined. their physical size on the retinae, change At least in a small space such as a their size proportionally to the distance laboratory room, Emmert's law of ap- of projection-the further the distance of parent size holds good (Boring, 1940; projection, the larger the size of after- Furedy & Stanley, 1970; Helson, 1936; images. This relationship is stated in Price, 1961). These researchers exam- Emmert's law. Emmert's law originally ined Emmert's law of apparent size formulated the linear relationship be- mainly in terms of its relationship to the tween the physical size of afterimages and size-constancy. In these studies numer- the physical distance of projection (Em- ous cues for size and distance perception mert, 1881). From the psychological were present, and when these cues were point of view, it has been referred to the reduced, the apparent size of afterimages relationship of the apparent size of after- changed little with the change of the dis- images to the perceived (or often physical) tance of projection (Edwards, 1953; distance of projection. In the present Irwin, 1969). These studies, however, paper this law of apparent size is ex- did not deal with the effects of the oculomotor cues, accommodation and con- 1 The author thanks Dr . Yuko Kimura for her vergence, on the apparent size of after- careful comments on an earlier version of the present manuscript and Prof. Shuko Torii of Uni- images. The present experiments deal versity of Tokyo for his valuable comments on the with it through the answers to the following three questions: present experiments and manuscript. 2 Present address: Department of Psychology , a) Does Emmert's law hold when only Faculty of Humanities, Niigata University, 2- accommodation and convergence are Ikarashi, Niigata-shi, 950-21. available as cues for size and distance Apparent size of afterimages and oculornotor cues 169

perception? Is there any distance limit (20 ern •~10 cm •~5.5 cm) to which a beyond which it does not hold? it so, it strobe light (Sunpak Corporation's SP-

might reflect the distance where the in- 140) was attached on one side was used. formation given by the oculomotor sys- In the other side of the box two holes

tem is available for size and distance were bored as viewing apertures (5 cm perception. in diameter and 1.5 cm apart each Even in the dark we can project after- other). On the surface of the strobe images at a given clioance by gazing at light was fastened a piece of black paper, a small point-light placed at that distance. from which a ring piece was cut out.

That is, we can project them onto the They were covered by a red filter (Kodak plane imagined at the point-light. In Wratten #25). Therefore, the strobe this situation only the oculomotor cues light transmitted light in the form of red are available for size and distance percep- ring. Outside and inside diameters of tion. Therefore, by comparing the re- the ring were 1.5 cm and 0.7 cm, results obtained in this situation to those spectively. When subjects were watch- obtained in the lit room where various ing at a center of the ring on the strobe cues are available, w hat role the oculo- light through two apertures, the experi- motor cues play in the determination of menter flashed it for 0.2 ms. The dis- the apparent size of afterimages can be tance from the subjects' eyes to the shown. original stimulus (the ring) was 29 cm. b) How large is the apparent size of The outside diameter of the produced entoptic afterimages (that is, afterimages ring afterimages subtended 2.96•‹ in vis- seen with eves closed) as well as of after- ual angle. The red filter was used be- images seen in complete darkness (that cause it enabled the subjects to observe is, afterimages seen without any projec- afterimages easily because of their color. tion point-light in darkness)? The ring configuration of the stimulus We can infer the localized distances of was used in order for a projection point these two kinds of afterimages by com- in darkness to be clearly visible in the paring their apparent sizes to those of center of the ring afterimages. afterimages obtained in the lit room. Inferred distances might give us an in- Procedure and Conditions dication of the state of the oculomotor Subjects, sitting on a chair with their system in complete darkness and with head and chin rested, projected after- eyes closed. images onto a sheet of black paper in the c) Can subjects alter the apparent size lit room or a plane imagined at a small

of afterimages voluntarily such as by con- point-light (the projection point) in the trolling their oculomotor system in the dark room, or saw them in complete lit room and in complete darkness? darkness and/or with eyes closed. Sub-

If convergence and accommodation are jects were required to reproduce the ap-

involved in the determination of apparent parent size of the outside diameter of the size of afterimages, subjects must be able ring of the afterimages by adjusting the to alter apparent size of afterimages length of a small steel tape measure voluntarily by controlling their own convergence and accommodation. 3The apparatus for producing afterimages and the measuring method used here were the modified Method3 ones that were originally used in Fujisawa (1973). The author thanks Prof. Shinsuke Fujisawa of Productionof Afterimages Atomi-Gakuen Women's College for his valuable In order to produce afterimages, a box advices on the use of the apparatus and the method. 170 K. Suzuki

(1.3cm in width), using its ungraduated images binocularly or monocularly seen back surface. They were asked to make in complete darkness, i.e., without any the adjustment on their knees with their projection point. They were also asked elbows extended. In darkness, its sur- to reproduce the apparent size of after- face was plastered with a luminous tape images binocularly seen with eyes closed to be visible. in the dark room as well as in the lit room. Experiment (a). Subjects projected They reproduced them four times under afterimages onto a sheet of black paper each condition. (the projection field) of 30 x 30 cm in the Experiment (c). Subjects were asked to lit room or onto the small point-light (the manage to alter the apparent size of after- projection point) in the dark room. The images binocularly seen in the lit room as projection field or the projection point well as in complete darkness. They were was placed on the bench (310 cm in asked to reproduce the smallest size as length) at subjects' eye level. In the lit well as the largest one. room, there was a 100 W diffused over- head daylight lamp which made various Subjects cues for size and distance perception Subjects were three undergraduate and available. In the dark room, subjects two graduate students. Three were na- saw nothing but the point-light. The ive to psychological experiments and the following seven distances were taken as other two knew of Emmert's law. Three the distances of projection: 25, 50, 100, of them had visual acuity of 1.0 with 150, 200, 250, and 300 cm. Under each naked eyes and the other two had the condition the distances of projection were acuity of 0.8 with correction contact arranged at random. Experiment was lenses. conducted under the following four conditions: binocular or monocular observation, and with the projection held in the Results lit room or with the projection point in The mean apparent sizes of afterimages the dark room.' These conditions were for each subject are shown in Table 1 and executed in turn and the sequence was those for five subjects are plotted in Fig. 1. repeated four times. Under the monoc- General results are as follows: ular condition, one eye was occluded Experiment (a). In the lit room, the ap- with a sheet of thick paper. This paper parent size of afterimages changed pro- was applied to the eye lightly so as not to portionally with the distance of projection press it. and Emmert's law was found to hold Experiment (b). Subjects were asked to good.In the dark room Emmert's law reproduce the apparent sizes of after- was found to hold within 200 cm but beyond 200 cm the apparent size of 4 1t has been rcported that the size of afterimages afterimages changed little and seemed to changes with the time-course (Obonai & Kawa- level off. A two-way analysis of variance, shima, 1961). In the present experiments, how- light conditions x subjects, showed that ever, it was assumed that a possible influence of the the differences of means between the lit time-course on the size estimation of afterimages and dark room conditions were not signi- would be little since subjects made size estimation ficant at 200 cm (F(1, 39)=.75, p>.05 at nearly constant time (10 to 15 s) . 5Literally ," the dark room"should be com. for binocular condition and F(1, 39)=3.38, pletely dark. But, to distinguish it from the condi- p>.05 for monocular one) but were signi- tion without a projection point in darkness (that is, ficant at 250 cm (F(1, 39)=19.34 , p<.01 the condition of complete darkness) , the name of for binocular condition and F(1 " the dark room "i , 39)= s used here. 28.58, p<.01 for monocular one). Apparent size of afterimages and ucnlouiotor curs 171

M e a n ap p a r en t s iz e s ( c m ) o f a f t e r im a g es

Tab le 1 172 K. Suzuki

Experiment (b). The apparent size of lit room (1.4 cm) was smaller than that entoptic afterimages corresponded in all in the dark room (1.8 cm). The ap- the subjects to the size of afterimages parent size of afterimages seen in complete localized at about 25 to 35 cm in both darkness corresponded to the size of after- the lit and dark rooms. The mean ap- images localized at 50 to 70 cm. It was parent size of entoptic afterimages in the two times larger than that of entoptic afterimages. Experiment (c). In the lit room all the subjects could alter the apparent size of MONOC. afterimages voluntarily, despite the ab- sense of projection field, by controlling their accommodation and convergence. In all the subjects except S5, the ranges of size-change were nearly the same as those obtained with a projection field in the lit room in Experiment (a). But, in complete darkness, none of the subjects were able to alter the apparent size of afterimages.

Discussion Emmert's Law and OculomotorCues

BINOC. The results suggest that the information given by the oculomotor system is available within approximately 200cm as cues for size and distance perception but beyond that distance it is not available. It has been suggested that, though convergence responds correctly to objects presented even at 10 m (Iida & Shiina, 1979), the distance where the information given by the oculomotor system is available for distance perception is limited to within 200 cm (Wallach & Floor, 1971). A recent neurophysio- logical evidence in the monkey as well supports this distance limit of the oculomotor cues (Sakata, Shibutani, & Kawa- no, 1980). In the present situation, only convergence can be thought to have an effect on the apparent size of afterimages, Fig. 1. Mean apparent sizes of afterimages as a function of physical distance of the projection since in darkness the eyes accommodate fieldlpoint. White and black dots represent the at near distance irrespective of distance sizes obtained in the lit room and the dark room, of the projection point (as shall be cited respectively. Vertical fine lines are SDs. Mean below; Owens & Leibowitz, 1976). apparent sizes of afterimages seen in complete Therefore, it can be thought that the darkness and of entoptic ones are shown on the information provided by convergence de- left. termined the apparent size of afterimages Apparent site of afterunages and oculornotor cues 173 directly, or that it determined their shorter than Gogel's specific distance". perceived distances cinch determined The implication of the localization of subsequently their apparent size. afterimages at 50 to 70 cm in complete This critical distance of 200 cm is darkness may be as follows. In com- quite similar to the distance obtained in plete darkness, the eyeballs relax and Stanley and Furedy's experiment on Em- they accommodate and converge at near mert's law (Stanley & Furedy, 1966). distance. It is called the resting posi- They had subjects project afterimages tion of the eyes (Schober, 1954). Leibo- onto a sheet of paper at various distances witz and Owens measured the resting within 126 in. (i.e., 315 cm) and found positions of the eyes and obtained the that subjects' ability to project them average distance of 116 cm for conver- broke down beyond 90 in. (i.e., 225 cm). gence (Owens & Leibowitz, 1976) and of However, they did not take this distance 58cm for accommodation (Leibowitz & as an indicator of the limit of the oculo- Owens, 1975). The inferred localized motor cues for size and distance percep- distance of afterimages seen in complete tion. Instead, they took it as reflecting darkness in the present experiment is near the method used in their experiment to the latter distance for accommodation. since other methods gave different re- Therefore it is likely that their apparent sults (Furedy & Stanley, 1970). Their size is determined by the state of ac- experiment was conducted under dim il- commodation. lumination and the observational situa- The state of accommodation can ex- tion might be thought to be similar to plain as well the difference in apparent that of the present experiment. There- size between entoptic afterimages and fore their 90 in. (i.e., 225 cm) might re- afterimages seen in complete darkness. present the upper limit of available lVestheimer and Blair (1973) measured oculomotor cues for size and distance three kinds of resting positions of ac- perception in the same way as the 200cm commodation (that is, its focal distance in the present experiment does. in relaxing state) by means of retinoscopy Little difference in the results between in the monkey: those in complete dark- the monocular and binocular observa- ness, in sleep, and under anesthesia. tions is consistent with Holway and Bor- They obtained 67 cm in complete dark- ing (1941)'s classical findings on size- ness and 33 to 40 cm both in sleep and constancy. Therefore, at least in the under anesthesia. The former is near to present experiment, the information pro- the localized distances of afterimages seen vided in the monocular observation is in complete darkness (50 to 70 cm) and supposed to be equivalent to that in the the latter to that of entoptic afterimages binocular observation. (25 to 35 cm). If the state of accommodation of closed eyes can be thought Apparent Size of Afterimages Seen in Com- to be similar to that of eyes in sleep and plete Darkness or with Eyes Closed under anesthesia, the apparent size of Gogel (1969) showed that objects tend- afterimages may be dependent on the ed to be localized at near distance (4 to state of accommodation. 8 ft., i.e., 120 to 240 cm) under reduced Therefore the following assertion is cues condition such as complete darkness. possible. In darkness, if there is a pro- This phenomenon is termed the specific jection point, the apparent size of after- distance tendency. In the present ex- images is determined by the information periment, however, the inferred localized provided mainly by convergence. But distance of afterimages seen in complete in complete darkness afterimages have darkness was about two to four times definite apparent sizes dependent on the 174 K. Suzuki

resting position of accommodation, not tarily by controlling accommodation and on that of convergence. convergence in the lit room. However, it is not the case in complete darkness VoluntarySizc-Change of Afeerimagcs since convergence and accommodation The result that the apparent size of are locked in their resting positions. afterimages could not be altered voluntarily in complete darkness supports the References fact that both convergence and accommodation cannot be controlled volun- Boring, E. G. 1940 Size constancy and Emmert's tarily in complete darkness since they law, AmericanJournal of Psychology, 53, 293-295. are locked in their resting positions. Edwards, W. 1953 Apparent size of afterimages This result agrees as well with the finding under conditions of reduction. American Journal that the apparent size of afterimages of Psychology, 66, 449-455. changes little under the condition of Emmert, E. 1881 Grusscnvcrhaltnissc der Nach- bilder. filinischc Monatbldtter far Augenheilkunde, reduced cues for size and distance percep- 19, 443-450. tion (Edwards, 1953; Irwin, 1969). Fujisawa, S. 1973 On perceived size and dis- Even in complete darkness, according tance of projected afterimages. Unpublished to Hoist's efference-copy theory (Hoist & master's thesis, Keio University. (In Japanese) MIittelstaedt, 1950), the brain might in- Furedy, J. J., & Stanley, G. 1970 The apparent lbrm the efferent system of its order to size of" projected" afterimages under conditions change the oculomotor muscles. Even where size-constancy holds. Perception Psycho- if the oculornotor system did not actually physics, 7, 165-168. cork, this efference-copy might well in- Gogel, W. C. 1969 The sensing of retinal size. duce the change in the apparent size of Vision Research, 9, 1079-1094. Helson, H. 1936 Size-constancy of the projected afterimages. However, it was not the afterimage. American Journal of Pschology, 48, case. Rather, the results suggest that the 638-642. extrinsic visual information (that is, real Hoist, E. von, & Mittelstaedt, H. 1950 Das visual input) is always necessary for ob- Realfercnzprinzip. Naturwissenschaften, 37, 464- servers to change the apparent size of 476. afterimages. Holway, A. F., & Boring, E. G. 1941 Determi- nants of apparent visual size with distance variant. American Journal of Psychology, 54, 21- Conclusions 37. From the results described above, the Iida, T., & Sliiina, K. 1979 Relationship between information on visual field and conver- following conclusions can be drawn: a) gence (II). Proceedings of the 43rd Annual Conven- Whereas Emmert's law holds good in the tion of the Japanese Psychological Association, 147.(In lit room, it holds good only within about Japanese) 200 cm in the dark room. Therefore Irwin, R. J. 1969 Emmert's law as a sequence of the information given by the oculomotor size constancy. Perceptual and 3lotor Skills, 28, system (mainly by convergence) is avail- 69-70. able within 200cm as cues for size and Leibowitz, H. W., & Owens, D. A. 1975 Anoma- distance perception; b) With respect to lous myopias and the intermediate dark focus of their apparent sizes, entoptic afterimages accommodation. Science, 189, 646-648, can be said to be localized at about 30cm Obonai, T., & Kawashima, K. 1961 Contribu- tions to the study of psychophysical induction and afterimages seen in complete dark- (65). Temporal characteristics of size-change ness at 60cm. These distances are of colored afterimage. Japanese Psychological thought to be related to the resting posi- Research, 3, 138-145. tions of accommodation; c) The apparent Owens, D. A., & Leibowitz, H. W. 1976 Ocular size of afterimages can be altered volun- adjustment in darkness and the specific distance Apparent size of afterimages and oculomotor cues 175

tendency. P,,,, i3 Pxychophysicx,20, 2-9. Journal of Psychology, 18, 266-270. Price, G. R. 191;1 On Emmert's law of apparent allach, II.,& Floor, L. W 1971' Fhe use of size 11, 145-151. ro.ttching to demonstrate the effectiveness of Sakata, H., Shihutani, 11.,& Kawano, h, 1980 accommodation and convergence as cues for Spatial proi, erti, s of vrsual fixation neurons in distance, Perception s P.c_ychoplysica, 10, 423- posterior 1, u tet.11 association cortex of the 128. nxitikcy'. Jaurual of.Neurophysiology, 43, I67.,-1 Wcstheitncr, G.,& Blair, S. M. 1973 Accom- 1672 modation of the eye during sleep and anesthesia. Schober, H. 19-, 1 Ober die Akkommodations- Vision Research, 13, 1035-1040. ruhelage. QptiA, 6, 282--290. (Received Sept. 22, 1984; accepted Stanley, G.,& Furecly, J. J. 1966 Size constancy July 12, 1986) and Emmert's law of apparent sizes.. lustralian