Fusional vergence response to local peripheral stimulation.

Horizontal, convergent disparities were introduced between dichoptic stimuli confined to small regions of the peripheral retina. Stimuli were presented at 32 locations: radial positions varied in 5-deg increments up to 20 deg, while angular position varied in 45-deg steps. The stimulus size and disparity were scaled in accordance with the cortical magnification factor. Eye movements were objectively measured, and the relative contributions of the motor and nonmotor components to the fusional response were evaluated as a function of stimulus eccentricity and angular position. Vergence responses elicited by peripheral disparities had longer latencies and durations and were more asymmetric than the movements elicited by foveal disparities. The composition of the fusional response changed with the position of the stimulus. The largest percentage of motor compensation was observed for stimuli located either near the line of sight or directly above it. The variations in the size of the motor response with increasing eccentricity could not be explained by the cortical magnification factor.