Effects of postural stability and age on behavioral laterality in squirrel monkeys.

Abstract

Six young and six aged Squirrel monkeys (Saimiri sciureus) made unimanual food reaches from a posturally stable, solid base platform, a posturally unstable fenced ceiling and an extremely posturally unstable chain link tightrope. Although population level right hand biases were present in the solid base and tightrope testing conditions, the monkeys were best described as consisting of two subpopulations; 9 strongly, consistently dextral subjects and 3 strongly, consistently sinistral subjects. The strength of lateral hand preferences, which were weakly expressed in the solid base condition, intensified during the posturally unstable ceiling and tightrope testing conditions. Right or left turning after a manual response was not affected by handedness of the monkeys or directly affected by manual task type. However, young monkeys turn and hand bias directions increased in congruence as manual task stability decreased. Aged subjects showed weak turn and hand bias direction congruence in the posturally stable task and this congruence decreased in the posturally unstable conditions. All monkeys expressed a right turn preference during free movement and this preference was positively correlated to strength of hand biases. Each subject also expressed a left hindlimb preference for mounting and dismounting a chain link fence. However, strongly right handed monkeys showed weakest left hindlimb biases and strongly left handed monkeys showed strongest left hindlimb biases.