Manual bias, behavior, and cognition in common marmosets and other primates.

This chapter examines the importance of studying hand preference together with different expressions of behavior. Cognitive differences between left- and right-handed primates are discussed. As shown in several species of primate, eye preference, but not hand preference, is biased at the level of the population and reflects hemispheric asymmetry of processing. Hand preference, determined from simple grasping of pieces of food and taking them to the mouth, is consistent for individuals but it is not population biased. It is a measure of an individual's preference to use a particular hemisphere, and hence which cognitive processes are characteristic of the individual. Compared to left-handed subjects, right-handed subjects are more active in exploring novel objects, show more social facilitation of behavior, have a positive cognitive bias, and express lower levels of fear and stress responses. In marmosets, learning of food searching tasks is not associated with hand preference. Strength of hand preference, rather than its direction, is linked to the ability to perform two tasks at once, viz., detection of a predator while searching for food. Marmosets with strong hand preferences are able to perform these two tasks at once but those with weak or no hand preference are unable to do so.