The handedness of rhesus monkeys. III. Consistency within and across activities.

The results of this study of the hand preferences of rhesus monkeys on three different tasks are threefold: (1) When retested on the same task at intervals exceeding one month virtually all individuals prefer the same hand as they did during the original test, (2) When retested on the same task, the strength of hand preference displayed by each individual is increased. (3) When tested on differing tasks, monkeys display little consistency in the laterality of hand preference or the strength of handedness expressed during different tasks. Many authors have concluded that the lack of obvious intertask consistency in the laterality of hand preference expressed by lower primates constitutes evidence for a corresponding lack of consistent laterality in the cerebral control of this behavior. This has led to them to conclude that cerebral dominance probably does not exist in these animals (Deuel, 1975; Warren, 1977). However, where data is available from the literature, including the present study, all reports show monkeys to more frequently prefer the same hand on all of the unimanual tasks they were given than would be expected by chance alone. This finding suggests that there is a weak tendency for consistent lateralization of hand usage in the monkey. Presumably, there is a corresponding predominance of the cerebral hemisphere contralateral to the preferred hand over its mate. Other studies consistent with the concept of cerebral predominance in the monkey were reviewed. These findings do not constitute evidence for cerebral dominance in the monkey akin to that found in man. They do suggest that when performing certain activities, monkeys may have one hemisphere predominant over the other even though the degree and laterality of predominance may vary greatly from one individual and task to another.