Effect of a local ibotenic acid lesion in the visual association area on the prelunate gyrus (area V4) on saccadic reaction times in trained rhesus monkeys.

Two rhesus monkeys were trained to make saccadic eye movements from a central fixation point towards a peripheral target. Saccadic reaction times (SRTs) were measured in the gap paradigm (200 ms pause between offset of fixation point and onset of peripheral target). Target position for extensive training (SRTs of 150 to 250 saccades were collected per day) was four degrees eccentric in the lower quadrant of the visual field contralateral to the intended lesion site in area V4. For control the monkeys were also trained for target positions either in the lower quadrant ipsilateral to the intended lesion site or in the upper visual half field. After several weeks of training a bimodal distribution of saccadic reaction times, one peak at 85 ms (express saccades) and the other around 160 ms (regular saccades) was obtained for each target position. Local injection of ibotenic acid into the 4 deg representation of area V4 resulted in a unimodal distribution of saccadic reaction times (over 90% express saccades) towards the corresponding target position, leaving the distribution of reaction times for the control position unchanged. Recovery began after 5 days and was complete 8 to 10 days after the injection. From these results we conclude that V4 is involved in the generation of the longer latency peak in the distribution of saccadic reaction times by delaying the initiation of visually guided saccades.