Visual pursuit over textured backgrounds in different depth planes.

Several investigators have reported that voluntary pursuit of a moving object is less smooth when it moves over a textured background compared with when it moves over a dark background. Furthermore, when a person fixates a stationary target on a moving background, microdrifts of the eyes occur in the direction of motion of the background. These two facts suggest that OKN is not completely switched off during voluntary pursuit or fixation. Howard and Simpson (1989) found that optokinetic nystagmus (OKN) is only weakly evoked by moving stimuli which are out of the plane of convergence. This led to the prediction that voluntary pursuit of a visual target should be disrupted more by a stationary background in the same depth plane than by a background which is nearer or further away than the pursuit target. Pursuit disruption was measured by the frequency and total amplitude of saccadic intrusions. For horizontal pursuit, disruption was reduced, compared with the coplanar condition, when the textured background was further away than the target but not when it was nearer. For vertical pursuit, disruption was reduced in both the far and near conditions compared with the coplanar condition. In both cases disruption was least when the background was dark.