Multiple mechanisms of response suppression to self-induced sensation during pursuit eye movements.

Eye movements generate a perceptual challenge, that of distinguishing self-induced sensations from movement in the world. We ask about the mechanisms involved in suppressing eye movements towards self-induced sensation, ensuring visual stability. When tracking with the eyes an object moving against a textured background, the background retinal image moves in the opposite direction to the smooth pursuit eye movement. Optokinetic responses, such as optokinetic nystagmus or ocular tracking to this reafferent signal, must be suppressed to sustain the pursuit of the object of interest. We varied the contrast of a brief background motion signal to tell apart two plausible accounts of the suppression of optokinesis during pursuit; a visuomotor gain modulation account, which predicts that ocular tracking of background motion is suppressed in the same proportion irrespective of contrast, and a sensory attenuation account, which predicts that larger contrasts are needed to elicit the same response. Unexpectedly, neither account fit ocular tracking in the reafferent signal direction. The combination of contrast-dependent gating, with maximal suppression observed with higher contrasts, and either contrast gain or visuomotor gain modulation, provides a good fit for most observers' data. Contrast-dependent gating promotes visuomotor stability in response to most salient signals, as a likely adaptation to the statistics of the environment.