Twenty-five split-brain monkeys were taught to discriminate two types of visual stimuli that engage lateralized cerebral processing in human subjects. Differential lateralization for the two kinds of discriminations was found; the left hemisphere was better at distinguishing between tilted lines and the right hemisphere was better at discriminating faces. These results indicate that lateralization of cognitive processing appeared in primates independently of language or handedness. In addition, cerebral lateralization in monkeys may provide an appropriate model for studying the biological basis of hemispheric specialization.
Split-brain monkeys learned several sets of visual discriminations with each hemisphere. Some stimuli, such as photographs of monkey's faces, were intended to favor mechanisms similar to those of man's nondominant hemisphere, while other tasks, requiring sequential comparison of visual stimuli, should favor mechanisms similar to ones in the dominant hemisphere of man. The tests uniformly demonstrated hemispheric equivalence for solving all types of problems, regardless of handedness, sex, or side of surgical retraction. A review of the literature also offers little support for the concept of hemispheric specialization in infra-human mammals although a few leads still need to be explored before abandoning the hope of finding the roots of human cerebral dominance in monkeys.
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