RAPID-VELOCITY AND SLOW-VELOCITY VERGENCE EYE-MOVEMENTS

Previously we have measured rapid-velocity vergence responses to targets at different distances that provided no disparity or accommodative stimulation. To evaluate the possibility that this rapid-velocity vergence occurs during saccades, the latencies of eye movement between two long dim luminous rods were compared under two conditions. Rapid-velocity vergence with an average latency of almost-equal-to 300 ms was elicited when subjects alternately viewed horizontal rods at distances of 38 and 78 cm, and with a vertical separation of 5.2-degrees. Horizontal saccades with a comparable latency were measured when the rods were equidistant and oriented vertically. The correlation between the mean latencies of vergence and saccadic movements was 0.97, sugesting that the two movements occurred together. In a second experiment vegence responses were measured when the subject looked between a bright vertical line on a screen at 76 cm and a second pair of lines (vertically displaced between 0.15-degrees and 3-degrees) with crossed disparity to simulate a target at 38 cm. Slow-velocity vergence often occurred alone when the vertical separation between targets was small; rapid-velocity vergence intruded when the separation between the targets was larger. The results can be accounted for if proximity and disparity stimulation act through a single vergence controller, the output of which produces slow- or rapid-velocity vergence depending upon whether the saccadic system is concurrently active.