The role of exocentric reference frames in the perception of visual direction

Abstract

One classic piece of evidence for an efference copy signal of eye position is that a small, positive afterimage viewed in darkness is perceived to move with the eye. When a small stationary reference point is visible the afterimage appears to move relative to the reference point. However, this is true only when the afterimage is localized to a small 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 move, even with large changes in eye position. When subjects were instructed to maintain their direction of gaze, we observed small saccades (typically < 1 deg) and slow drift movements often totalling more than 10 deg over a 30 sec period. When the instructions were to simply inspect the extended afterimage, subjects made larger saccades (up to 5 deg) which were not accompanied by afterimage movement. The smaller movements observed under the first instructions are greater than those observed in the dark or with small afterimages. When a visible reference is present with these large afterimages, the afterimage appears stationary, while the reference point appears to move. Eye position was monitored following the generation of such afterimages. In general, the perceived motion of the stationary reference point was in a direction opposite to the motion of the eye. Similar drift movements of smaller magnitude were observed with localized afterimages, but the motion was attributed to the afterimage. This suggests that with whole scene afterimages, extraretinal information about slow movements and small to moderate saccades is absent or suppressed, and that stationarity is assigned to the complex scene. This indicates a perceptual disposition to rely on visual information (when it is available) for maintaining constancy of visual direction, despite substantial changes in eye position. We found that changes in gaze as large as 13 deg produced no change in reported position of the large scene afterimage. When a small, stationary reference light was present it appeared to move in the afterimage scene in a way consistent with the recorded eye movements. When observers attempted to maintain their direction of gaze, most of the eye movements were slow drift, with occasional small saccades. When the instruction to hold gaze was relaxed, larger saccades up to about 5 deg were observed which were not accompanied by afterimage movement.