Smooth eye movements and spatial localisation.

We asked subjects to align a target that flashed as their eyes rotated to the right in pursuit of a moving ring, with a target that flashed as their eyes rotated to the left in pursuit of the ring. Subjects systematically mislocalised the targets in the direction of pursuit. When the ring and flashes were the only structures that were visible, the alignment error was about 4 cm, corresponding to a timing error of about 100 ms. The timing error was independent of the position along the ring's path, but did depend to some extent on pursuit velocity. Adding a textured background reduced the mislocalisation considerably, presumably because it enabled subjects to localise the targets relative to the surrounding. There was almost no mislocalisation if the subject was not pursuing the ring. It is suggested that the mislocalisation arises because incoming retinal signals are combined directly with outgoing oculo-motor commands, with no attempt to account for any of the involved neuronal and muscular delays.