Interactions between lateralized choices of hand and target.

Few studies have examined interactions between hand preference and target laterality preference when subjects are permitted unrestricted choices of both hand and target at the same time. We therefore trained three monkeys to perform a task in which different types of choices were presented in different trials. In some trials the monkey chose between targets presented simultaneously to the right and left, while being required to use a specific hand. In these trials, the monkeys preferentially chose the target ipsilateral to the required hand. In other trials, the monkey chose between its right or left hand to retrieve a single target presented either on the right or on the left. In these trials, each monkey showed a consistent pattern of hand preference whether the target was presented on the right or left. In still other trials, the monkey chose both which hand to use and which target to retrieve. In these trials, the choices made by each monkey reflected its individual pattern of hand preference, plus a tendency to choose the target ipsilateral to the preferred hand, with little additional evidence of preference for target laterality. These patterns of laterality preference were similar regardless of whether a cue instructing which hand(s) the monkey could use preceded or followed a cue indicating on which side(s) the target(s) would appear. If the monkeys were permitted to use a strategy in which retrieving the target on a given side could increase the amount of food retrieved per trial, however, the choices made were biased strongly by that strategy. We conclude that when lateralized targets are equivalent, lateralized choices are biased primarily by hand preference and secondarily by a preference to retrieve a target ipsilateral to the preferred hand.