Smooth Visual Tracking and Fixation

This chapter summarizes the properties and neural substrate of smooth eye tracking movements, including visual fixation, smooth pursuit, the ocular following response (OFR), and optokinetic nystagmus (OKN). Fixational eye movements, including microsaccades and drifts, and the role of the OFR in stabilizing gaze are discussed. The properties of pursuit, OFR, and OKN are summarized, including anticipation, prediction, and target selection. The ability of pursuit adaptation to respond to new visual demands is reviewed. Pertinent cortical areas (MT+) and their projections to brainstem and cerebellum are discussed, as well as the accessory optic pathway, and nucleus of the optic tract. Current models for smooth pursuit that incorporate efference copy (corollary discharge), prediction, and Bayesian operators are summarized. Clinical and laboratory evaluation of fixation and visual tracking are reviewed, and the pathogenesis of disorders of these movements discussed, including latent nystagmus accompanying failure to develop binocular vision and infantile nystagmus syndrome.

Ocular tracking responses to background motion gated by feature-based attention

Involuntary ocular tracking responses to background motion offer a window on the dynamics of motion computations. In contrast to spatial attention, we know little about the role of feature-based attention in determining this ocular response. To probe feature-based effects of background motion on involuntary eye movements, we presented human observers with a balanced background perturbation. Two clouds of dots moved in opposite vertical directions while observers tracked a target moving in horizontal direction. Additionally, they had to discriminate a change in the direction of motion (±10° from vertical) of one of the clouds. A vertical ocular following response occurred in response to the motion of the attended cloud. When motion selection was based on motion direction and color of the dots, the peak velocity of the tracking response was 30% of the tracking response elicited in a single task with only one direction of background motion. In two other experiments, we tested the effect of the perturbation when motion selection was based on color, by having motion direction vary unpredictably, or on motion direction alone. Although the gain of pursuit in the horizontal direction was significantly reduced in all experiments, indicating a trade-off between perceptual and oculomotor tasks, ocular responses to perturbations were only observed when selection was based on both motion direction and color. It appears that selection by motion direction can only be effective for driving ocular tracking when the relevant elements can be segregated before motion onset.