THE ROLE OF EXOCENTRIC REFERENCE FRAMES IN THE PERCEPTION OF VISUAL DIRECTION

One classic piece of evidence for an efference copy signal of eye posi tion is that a small, positive afterimage viewed in darkness is percei ved to move with the eye, When a small stationary reference point is v isible the afterimage appears to move relative to the reference point, However, this is true only when the afterimage is localized to a smal l area, We have observed that when an extended afterimage of a complex scene is generated by a brief, bright flash it does not appear to mov e, even with large changes in eye position, When subjects were instruc ted to maintain their direction of gaze, we observed small saccades (t ypically <1 deg) and slow drift movements often totalling more than 10 deg over a 30 sec period, When the instructions were to simply inspec t the extended afterimage, subjects made larger saccades (up to 5 deg) which were not accompanied by afterimage movement, The smaller moveme nts observed under the first instructions are greater than those obser ved in the dark or with small afterimages. When a visible reference is present with these large afterimages, the afterimage appears stationa ry, while the reference point appears to move, Eye position was monito red following the generation of such afterimages. In general, the perc eived motion of the stationary reference point was in a direction oppo site to the motion of the eye, Similar drift movements of smaller magn itude were observed with localized afterimages, but the motion was att ributed to the afterimage, This suggests that with whole scene afterim ages, extraretinal information about slow movements and small to moder ate saccades is absent or suppressed, and that stationarity is assigne d to the complex scene, This indicates a perceptual disposition to rel y on visual information (when it is available) for maintaining constan cy of visual direction, despite substantial changes in eye position, W e found that changes in gaze as large as 13 deg produced no change in reported position of the large scene afterimage. When a small, station ary reference light was present it appeared to move in the afterimage scene in a way consistent with the recorded eye movements, When observ ers attempted to maintain their direction of gaze, most of the eye mov ements were slow drift, with occasional small saccades, When the instr uction to hold gaze was relaxed, larger saccades up to about 5 deg wer e observed which were not accompanied by afterimage movement.