A Visual-Field Anisotropy of the Filehne Illusion

Abstract

During pursuit eye movements stationary objects are perceived to move in the direction opposite to the ocular pursuit (the Filehne illusion). The strength of this illusion, which indicates a loss of position constancy, was quantified by a cancellation method. Subjects pursued a small target that moved horizontally at 6 deg s⁻¹ over 18 deg. When the target reached the midpoint of its trajectory, a test spot was exposed for 200 ms, either 0.5 or 1 deg above or below the target. Subjects reported the direction in which the test spot appeared to move. Test-spot speed was varied according to subject response (by means of an adaptive program Best PEST) to determine the point of subjective stationarity. This test speed was divided by the target speed to derive a percent measure of the illusion (eg 0% if no cancellation was needed, 100% if cancellation speed equalled that of pursuit). Whereas the magnitude of the Filehne illusion did not depend on the separation of the target and the test spot, it depended strongly (p<0.01) on whether the test stimulus was in the upper (26% illusion) or lower (36.5% illusion) visual field. The present anisotropy is compared to corresponding anisotropies in motion sensitivity (Ehrenstein et al, 1991 Pflügers Archiv 418 R39) and pursuit velocity (Schlykowa et al, 1996 NeuroReport 7 409 – 412) and conforms with a general view of a differentiation of visual and oculomotor functions between upper and lower visual fields, serving far-space and near-space orientation, respectively (Previc, 1990 Behavioral and Brian Sciences 13 519 – 575).