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 repro- duced 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 oculo- motor 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 pre- sent 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 follow- ing 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, re- sults 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 con- vergence 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 con- ditions: binocular or monocular observa- tion, 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).