The precision of oculocentric direction and its role in the stability of fixation.

We compared stability of fixation midway between two targets (separated horizontally by from 1 to 12 deg) to precision in specifying these targets' oculocentric directions. Unsteadiness of fixation tended to parallel changes in the precision of direction for larger target separations, but was worse than predicted from an addition of estimated oculomoter and sensory variabilities. The data are consistent with the notion that oculocentric direction provides the sensory signals used to maintain fixation, but to account for the magnitude of fixational instability another source of variability, possibly related to attention, must also be considered.