Adaptation of ocular vergence to stimulation with large disparities

Ocular vergence movements were measured with a scleral coil technique under stabilized viewing conditions for disparity. Crossed disparity steps, ranging between 0.25 and 10 deg, of three different targets were imposed. Ocular vergence responses consisted of converging movements with an initially constant velocity. This velocity increased with the magnitude of disparity up to about 4 deg and decreased for larger disparities. For disparities up to 2 deg the responses saturated at the limit of convergence. For larger disparities responses were transient, i.e. after large converging movements the angle of convergence gradually declined to about its initial value. For disparities larger than 5 deg amplitudes of the transient responses decreased and occasionally responses were completely absent. The transient character of responses was apparently due to adaptation of the vergence system to a specific disparity, since responses to different disparities could still be induced. Probing of the vergence system with two successive disparity steps of different magnitudes showed that adaptation was selective for a limited range of disparities around the adapting disparity stimulus. Stabilized disparity pulses with durations ranging from 100 to 800 ms induced ocular vergence movements following the time integral of disparity rather than momentary disparity. This indicated that the part of the vergence system sensitive to disparity has mainly integrative properties for large as well as small disparities. Stimulation with trains of shortlasting disparity pulses showed that adaptation also occurred under normal viewing conditions. They further revealed that adaptation is erased during a short period after each blink.