PRISM ADAPTATION OF REACHING MOVEMENTS - SPECIFICITY FOR THE VELOCITYOF REACHING

Accurate reaching toward a visual target is disturbed after the visual field is displaced by prisms but recovers with practice. When the pri sms are removed, subjects misreach in the direction opposite to the pr ism displacement (aftereffect). The present study demonstrated that th e severity of the aftereffect depends on the velocity of the movements during and after the visual displacement. Trained subjects were requi red to reach with one of four movement durations (<300, similar to 800 , similar to 2000, and similar to 5000 msec) from a fixed starting poi nt to a target that appeared at a random location on a tangent screen (400 mm away). The size of the aftereffect was largest when the moveme nt after the removal was performed with the same duration as that perf ormed with the prisms. It became smaller as the difference in velocity became larger. When the contralateral arm was used after visual displ acement, the aftereffect was never significant. Because the adaptation does not generalize across velocities or to the other arm, we infer t hat the underlying changes occur at a later stage in the transformatio n from visual input to motor output, in which not only the direction b ut also the time-dependent parameters of movements, such as velocity, acceleration or force, are represented.