An anti-Hick's effect in monkey and human saccade reaction times

In order to execute movements to targets in the environment, we must first select a target in which to move, generally from an array of alternatives. Hick's Law states that reaction time (RT) increases as the number of response alternatives increases. Violations of this law, however, generally in the form of the absence of a relationship between response alternatives and RT have been reported in the literature. K. Kveraga, L. Boucher, and H. C. Hughes (2002), for example, found that saccades to visual targets violate Hick's Law. To examine this violation further, we measured saccade RTs in monkeys and humans and found that saccade RTs actually decreased as the number of potential target locations increased. We hypothesize that this arises because subjects must actively inhibit premature saccades, and that the required inhibition increases as the certainty of a movement to a particular location increases. With increased inhibition, saccade onset is delayed, resulting in an anti-Hick's effect.