Perceived movement of the afterimage during eye movements' ARlEN MACK2 AND JANET BACHANT NEW SCHOOL FOR SOCIAL RESEARCH The question ofwhether an afterimage viewed in a dark field appear stationary, for there is neither efferent information about appears to move during eye movement lWlS studied by comparing eye movement nor image displacement.3 recordings of eye movements with recordings of reports of It has recently been demonstrated that abnormal image perceived movement. The correlation was found to be quite good displacement coupled with head movement invariably leads to the even under conditions where the eye movements were perception of movement, at least prior to adaptation to this spontaneous rather than specifically directed. The results were unusual state of affairs (posin, 1966; and described in Rock, taken to support the hypothesis that the behavior of the retinal 1966; Wallach & Kravitz, 1965). If the image displaces at a image is "interpreted" by taking into account information different rate or in a different direction than the head, it will concerning what the eyes are doing. appear to move. Only if it displaces at the same rate and in the normal direction wnI it appear stable. This has not, however, been "When the eyes move, the after-image moves in the same way unequivocally demonstrated with abnormal image displacement [Helmholtz, 1924]." Since Helmholtz, if not before, it has accompanying eye movement. Wallach and Lewis (1965) report generally been assumed that an afterimage would appear to move that a mismatch between image displacement and eye movement if the eyes were moving while it was viewed. The general basis of most often does notlead to reports of movement. By a technique this notion is the phenomenon of position constancy: the fact entailing the perception of the shadow cast by the pupil of the that despite movements of the 0 that cause the image to move eye, these investigators achieved a state of affairs in which the over the retina, a stationary scene will continue to look movement of the shadowed disc was controlled by the movement stationary. It is plausible to believe that position constancy is the of the eye, so that it moved only when the eye moved. The result of a process in which image displacement that is concurrent technique made it possible to produce a situation in which the with movement does not lead to the perceived movement of the target disc moved across the retina at either a faster or slower rate objects whose images are displacing, because under these than the rate at which the eye was moving. This experiment led conditions the displacement is taken into account and movement Wallach and Lewis to conclude, "Our results are incompatible is attributed to the 0 himself. Thus, image displacements with the view that the apparent rest of visual objects whose dependent on 0 movement are taken into account so that the images shift due to eye movements is to be explained by a objects in the visual field appear stationary and the 0 experiences compensating process which takes the eye movement into himself or some part of himself to be moving. Now, if it is the account [po 28]." Instead they argue that any shifting of the case that a correspondence between image displacement and 0 image during movements of the eye is simply disregarded or movement is a necessary condition for position constancy, then suppressed. Yarbus(1967), however, reports that abnormal image the absence of such a correspondence should lead to the loss of shifts coupled with eye movements do, in fact, lead to the position constancy and the appearance of movement in the visual appearance of object movement. field. Since the afterimage does not displace, the afterimage "If the direction of the retinal image shift in step with the should appear to move when the 0 moves. saccade was not directly opposite to the direction of One might summarize this view of position constancy by movement of the retina, the visual object also appeared to saying that retinal image displacements are evaluated in terms of be moving jerkily [po 207] ." the available information about 0 movements so that a Yarbus's results, which differ from the Wallach and Lewis correspondence between image displacement and 0 movements findings, appear to be based on the use of specially designed will always lead to position constancy and conversely a lack of contact lenses that enabled him to vary the direction of image correspondence, retinal displacement in a noncorresponding displacement accompanying eye movements. direction or at a noncorresponding rate, or the absence of any Thus, while an investigation into the relationship between eye retinal displacement at all coupled with 0 movement will always movements and the apparent behavior of the afterimage might lead to the appearance of object movement. The perception of an have seemed like an investigation of the obvious, there now afterimage during eye movement is quite analogous to Emmert's appears to be reason to question whether in fact the absence of Law in the case of size perception. If the diminishing size of the image displacement matched with eye movement necessarily must image is taken into account in perceiving size as constant at lead to the perception of movement. If it is true that there increasing distances, then a nonchanging image must be seen as generally is no perception of movement when the magnitude or changingsize with increasing distance. direction of displacement of the retinal image no longer Some qualifications: The evidence now indicates that the corresponds to the magnitude or direction of movements of the information about eye position and eye movement is derived eye, then one might reasonably predict that a condition of from signals corollary to those going to the eye muscles rather nondisplacement of the image accompanying eye movement than from proprioceptive feedback from the eye muscles might also fail to yield the perception of movement. "No (Brindley & Merton, 1960). Thus, where "there is no efferent displacement" is, after all, a special case of abnormal or information that the eyes have been directed to move and there is nonmatching behavior of the image when coupled with an eye image displacement-as in the case of a passively pushed eye movement. On the other hand, ifthe taking-into-account process viewing a normal scene-no transference of the displacement that is effective in maintaining position constancy during head "energy" occurs-and the scene will appear to jump. An movements also operates during eye movements, we should afterimage, however, viewed under the same condition should predict that an afterimage will be seen to move. Perception & Psychophysics, 1969, Vol. 6 (6A) Copyright 1969, Psychonomic Journals, Inc., Austin, Texas 379 It is, of course, also possible to argue that no displacement of in the base of the box along the arc in which the pointer moved the retinal image accompanying eye movement is a special case at the 0-, 45-, 90-, 130-, and 180-deg marks. The 0 was able to and that it is only necessary that there be some unspecified feel these as he moved the pointer. The zero position of the retinal image displacement for position constancy to obtain. On pointer coincided with the O's straight ahead. The arm could be this view the correlations between eye movement and retinal moved through an arc of about 200 deg (I00 deg either side of image movement signifying position constancy would be straight ahead). A telegraph key was placed to the left of the completely nonspecific so that any direction or rate of image pointer on the base of the box. It was wired to one of the two displacement during an eye movement should be accounted for event markers in the recording system. The 0 was instructed to by the eye movement. The difficulty with this view is that it fails depress the key whenever the light (target or image) appeared to to account for the ability to perceive movement during an eye be stationary. movement. A second potentiometer was secured to the top of the box Since a distinction can be made between the perception of a directly above the center of the bite board. From the shaft of this change in visual direction and the perception of movement per se potentiometer a metal arm extended I ft just beneath and parallel (the hour hand of a clock regularly changes its visual direction to the upper surface of the box. At the end of this arm was a without our normally being aware of any movement), and visual I~-V light bulb behind a milk-glass plate bearing a ~·in.-diam direction certainly must be a function of eye position and retinal hole. The housing of the bulb was light-proof so that, while the location, the operation of a taking-into-account process can only light was on, the 0 saw only a small disc of diffuse light be established by reports ofafterimage movement, not simply by insufficiently bright to illuminate any features of the box that reports of shifts in the visual direction of the afterimage, as the had been painted matte black to minimize reflectance. At the top eyes move. of the shaft and outside the box a knob was installed that allowed The experiment to be described is an attempt to determine the E to move the light. This light, which was very close to the whether in fact the afterimage does appear to move when the O's eye level, was visible through an arc of about 200 deg. This eyes move. The experiment is, to our knowledge, the first light was the target light and was used only during the training actually to investigate this matter.