Figure 1.

A trial started with the cursor (circle) held stationary in a target (dotted box). Then a new target (solid box) boundary was illuminated and subjects had to shift the cursor into this new position as quickly as possible. Once the cursor was positioned over a target, the latter became color filled. Movement direction and amplitude were randomly distributed across trials. In viscous challenge trials, a different viscous force was applied on each trial, and the target size was constant. In target challenge trials, there was no viscous force but the size of the target varied from trial to trial.