Asymmetrical hand use in rhesus monkeys (Macaca mulatta) in tactually and visually regulated tasks.

Asymmetrical hand use by rhesus monkeys (Macaca mulatta) was investigated in a series of tactually and visually guided tasks. The 1st experiment recorded manual preferences of 29 monkeys for solving a haptic discrimination task in a hanging posture. There was a left-hand population bias: 21 monkeys had a left-hand bias, 4 a right-hand bias, and 4 no bias. The 2nd experiment, 4 tasks with 23 to 51 monkeys, investigated the critical components of the 1st experiment by varying the posture (hanging, sitting, or tripedal) and the sensory requirements (tactile or visual). Posture influenced hand bias, with a population-level left-hand bias in hanging and sitting postures, but an almost symmetrical distribution in the tripedal posture. A left-hand bias was found for both sensory modalities, but the bias was stronger in the tactual tasks. Results suggest a possible right-hemisphere specialization in the rhesus for tactile, visual, or spatial processing.