The impact of head direction on lateralized choices of target and hand.

We examined choices made by monkeys performing a task in which two food-well targets were positioned on either side of the monkey, and LEDs provided instructions on hand use and food target availability. We have previously reported that when gaze and head direction were unrestricted, lateralized choices were biased primarily by hand preference and secondarily by a preference to retrieve a target ipsilateral to the preferred hand. Here, we used a similar behavioral paradigm, but now during trial instructions the monkeys were required to maintain head direction aimed toward a left, a center, or a right fixation LED. When a lateralized head direction was required during presentation of the instructional cues, monkeys were more likely to choose the hand and target ipsilateral to the head direction. Lateralized head direction more strongly biased the monkeys' choice of hand than their choice of target, but hand preference produced even stronger bias on target choices than did head direction. Although target cues were presented before hand cues, our data indicate that target and hand choices were made interactively. We also found that the monkeys' choices were better correlated with their success rate for particular combinations of hand and target than with movement times.