The 'shrinking circle illusion' is a shrinking ellipse.

Twenty-four subjects judged the size of the horizontal and vertical extents of the path traversed by a circularly rotating target. The target was rotated at velocities ranging from 0.18 to 1.13 cycles s-1, and observed with smooth pursuit eye movements or by fixating a point in the center of the path. It was found that underestimation of the target path increased with velocity, and was more pronounced for the horizontal than vertical component of the target path. These effects were quite strong during smooth pursuit, but weak during fixation. In previous research on the 'shrinking circle illusion' the vertical component of the path was not measured. The present findings indicate that the apparent shape of the path is elliptical, and that the eccentricity of this ellipse increases with velocity during smooth pursuit.