HOW IS A SENSORY MAP READ-OUT - EFFECTS OF MICROSTIMULATION IN VISUALAREA MT ON SACCADES AND SMOOTH-PURSUIT EYE-MOVEMENTS

To generate behavioral responses based on sensory input, motor areas o f the brain must interpret, or ''read out,'' signals from sensory maps . Our experiments tested several algorithms for how the motor systems for smooth pursuit and saccadic eye movements might extract a usable s ignal of target velocity from the distributed representation of veloci ty in the middle temporal visual area (MT or V5). Using microstimulati on, we attempted to manipulate the velocity information within MT whil e monkeys tracked a moving visual stimulus. We examined the effects of the microstimulation on smooth pursuit and on the compensation for ta rget velocity shown by saccadic eye movements. Microstimulation could alter both the speed and direction of the motion estimates of both typ es of eye movements and could also cause monkeys to generate pursuit e ven when the visual target was actually stationary. The pattern of alt erations suggests that microstimulation can introduce an additional ve locity signal into MT and that the pursuit and saccadic systems usuall y compute a vector average of the visually evoked and microstimulation -induced velocity signals (pursuit, 55 of 122 experiments; saccades, 7 0 of 122). Microstimulation effects in a few experiments were consiste nt with vector summation of these two signals (pursuit, 6 of 122, sacc ades, 2 of 122). In the remainder of the experiments, microstimulation caused either an apparent impairment in motion processing (pursuit, 4 7 of 122; saccades, 41 of 122) or had no effect (pursuit, 14 of 122; s accades, 9 of 122). Within individual experiments, the effects on purs uit and saccades were usually similar, but the occasional striking exc eption suggests that the two eye movement systems may perform motion c omputations somewhat independently.