Lateralized preferences and right-left discrimination in the monkey: choice of mirror-image shape, position of recovered food and hand usage.

Rhesus monkeys were tested for lateral preferences during mirror-image shape discrimination and food recovery from a row of pellets. Position of recovered food was unrelated to hand preference or mirror-image preference. There was no significant difference in the number of individuals preferring right and left lateral mirror-image shapes. Individuals tended to retain the same mirror-image preference to the same degree upon repeated testing. Animals of greater weight had stronger preferences. In all these respects, the features of mirror-image preference resembled those previously described for hand preference during a simple reaching task. The strength of different lateral preferences expressed by each animal were not significantly related thus failing to support the concept that asymmetric bodily responses can serve as cues for right-left discrimination in the environment. However, when the direction as well as strength of preference was considered, there appeared to be a tendency for animals to prefer the mirror-image shape whose reward bias was opposite to that of the preferred hand. This finding might have been due to a positional bias though there was evidence against this possibility.