Reaction-time asymmetry covaries with shifts in hand choice in macaques

How primates choose which hand to reach with remains poorly understood. Humans prefer the hand on the target's side of the body midline, and that preference shifts with biomechanical cost and with head and gaze direction. Earlier monkey studies used only two targets, one per side, too few to fit a psychometric function. Reaction time may also reflect the choice; in humans it is longest at the point of subjective equality (PSE), the target position at which either hand is equally likely to be chosen. We measured hand choice at ten horizontal target positions in two male rhesus macaques (Macaca mulatta) that chose freely on each trial. For peripheral targets, both animals used the hand on the same side of the body midline, with a well-defined transition in between. Displacing the hands, rotating the head, or displacing gaze to one side each moved the PSE toward that side. Reaction time was longest near the PSE. Hand displacement slowed the hand on the displaced side, and gaze displacement slowed the hand opposite the gaze. In every condition, the farther the PSE lay toward one side, the slower that side's hand was relative to the other. Macaque hand choice thus resembles human hand choice, making the macaque a tractable model for studying how the brain chooses a hand.