Effects of stimulus size and eccentricity on horizontal and vertical vergence

We measured the gain and phase of horizontal and vertical vergences of five subjects as a function of stimulus area and position. Vergence eye movements were recorded by the scleral search coil method as subjects observed dichoptic displays oscillating in antiphase either from side to side or up and down with a peak-to-peak magnitude of 0.5° at either 0.1 Hz or 1.0 Hz. The stimulus was a central textured disc with diameter ranging from 0.75° to 65°, or a peripheral annulus with outer diameter 65° and inner diameter ranging from 5° to 45°. The remaining field was black. For horizontal vergence at both stimulus frequencies, gain and the phase lag were about the same for a 0.75° stimulus as for a 65° central stimulus. For vertical vergence, mean gain increased and mean phase lag decreased with increasing diameter of the central stimulus up to approximately 20°. Thus, the stimulus integration area is much smaller for horizontal vergence than for vertical vergence. The integration area for vertical vergence is similar to that for cyclovergence, as revealed in a previous study. For both types of vergence, response gains were higher and phase lags smaller at 0.1 Hz than at 1.0 Hz. Also, gain decreased and phase lag increased with increasing occlusion of the central region of the stimulus. Vergence gain was significantly higher for a 45° central disc than for a peripheral annulus with the same area. Thus, the central retina has more power to evoke horizontal or vertical vergence than the same area in the periphery. We compare the results with similar data for cyclovergence and discuss their ecological implications.