Fixation and the stereokinetic phenomenon

One of two circles on a rotating disk appears to execute a planetary motion about the other circle. It is shown that the fixated circle serves as the center of rotation for the nonfixated circle. The effect of fixation is absent when polygons replace the circles. However, when one corner of an isolated square is fixated, the remaining corners rotated about the fixated corner. These planetary effects are consistent with the retinal paths followed by the elements of a display during fixation. This is not unlike the failure of position constancy associated with smooth pursuit of linearly moving targets in environments lacking a stationary visual frame of reference. In the present instance, however, the “retinal” responses occur during tracking of circularly moving targets, even in the presence of a visual frame of reference. These results are discussed in relation to the stereokinetic phenomenon. It is also shown that there is a strong interaction between the effects of fixation and the configurational features of the display. When a circle and a square are overlapping on a rotating disk, fixation of the square does not produce the perception of planetary motion. However, when the circle is fixated, the square is readily perceived as executing a planetary path about the circle. The possibility that position constancy in general is attributable to the geometry of the scene rather than to a “discounting” of information about eye movements is mentioned.