Charles R. Hamilton

Lateralization for orientation in split-brain monkeys

Split-brain monkeys learned with each cerebral hemisphere to discriminate lines differing in slope by 15 degrees. This type of spatial discrimination is usually performed better by the right hemisphere of humans. The left hemisphere of 8 monkeys learned this type of problem much more readily than did the right hemisphere. Learning to discriminate simple patterns in the same apparatus was done equally well by either hemisphere, demonstrating that the lateralized ability is specific to the stimuli employed.

Discrimination of monkey faces by split-brain monkeys

Eighteen split-brain rhesus monkeys were tested with each hemisphere for the ability to learn to discriminate photographs of the faces of other monkeys. Seven subjects also ran tests of generalization to new photographs of the discriminated monkeys; these tests confirmed that facial features pertaining to individual monkeys were learned. Equal numbers of male and female monkeys and nearly equal numbers of right and left handed monkeys were tested. Over all the monkeys there was no significant advantage in learning with either the left or right hemisphere or with the hemisphere contralateral or ipsilateral to the preferred hand. The group of 9 female monkeys, however, did shown a significant advantage in learning with the left hemisphere. Furthermore, there was a tendency for monkeys older at the time of surgery to show greater hemispheric specialization.

Hemispheric differences in split-brain monkeys learning sequential comparisons

Twelve split-brain rhesus monkeys were tested for differences in the abilities of their two cerebral hemispheres to learn discriminations based on the comparison of sequentially-presented visual stimuli. Overall there was no generalized advantage for either the left or right hemisphere. There was, however, a significant correlation between each monkeys handedness and the hemisphere that learned more readily; the more proficient hemisphere tended to be contralateral to the preoperatively preferred hand. This result raises the possibility that handedness in monkeys may be more closely related to cognitive processing than is usually believed.

An assessment of hemispheric specialization in monkeys.

Split-brain monkeys learned several sets of visual discriminations with each hemisphere. Some stimuli, such as photographs of monkeys faces, were intended to favor mechanisms similar to those of mans 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.

Hemispheric differences in split-brain monkeys viewing and responding to videotape recordings.

Eight split-brain monkeys were tested for hemispheric differences in their viewing of and responses to colored videotape recordings of monkeys, people, animals, and scenery. The number of facial expressions elicited from the right hemisphere was significantly greater than the number made when using the left hemisphere. Monkeys also tended to look longer when viewing with their right hemispheres than when viewing with the left.

Right-hemispheric superiority in split-brain monkeys for learning and remembering facial discriminations.

Twenty-six split-brain rhesus monkeys learned and remembered 8 go/no-go discriminations of monkey faces significantly better with the right hemisphere than with the left. Four discriminations required differentiating individual identity with expression held constant, and 4 required discriminating facial expression with identity held constant. There was no significant difference in the degree of laterality shown for these 2 types of problems. Female monkeys were more lateralized for learning to discriminate faces than were males. This sex difference in laterality was significant for learning but not for memory. Laterality for the facial discriminations was not significantly related to handedness of the monkeys. Overall, rhesus monkeys, like humans, show a right-hemispheric superiority for facial processing.

Partial interocular transfer of brightness and movement discrimination by split-brain cats

Interhemispheric transfer of discriminations of brightness and direction of movement, two types of stimuli which may be processed by midbrain visual areas, was compared to transfer of control patterns in cats with sections of the optic chiasm and forebrain commissures. Transfer in these split-brain cats was further compared to transfer in partially split-brain cats with sparing of either the splenium of the corpus callosum or the anterior commissure. An intact splenium was necessary for high levels of interhemispheric transfer of all of the discriminations, which implies a prominant role for cortical mechanisms in such tasks. However, animals with complete section of the corpus callosum often showed partial interocular transfer of both brightness and movement discriminations, which may reflect a limited but selective participation of subcortical areas in visual perception. Cats in which the anterior commissure was spared did not show greater transfer on any of these tasks than did cats with section of both callosum and anterior commissure.

Effects of facial identity, facial expression, and subject's sex on laterality in monkeys.

Previously we showed that rhesus monkeys processed discriminations of monkey faces significantly better with the right hemisphere of the brain than with the left. The overall effects of the type of discrimination, i.e. facial identity or expression, and the sex of the subject on laterality are examined here for seven phases of this series of experiments. Both types of discrimination produced a right hemispheric advantage, with slightly greater laterality for expression, but generally the laterality did not differ significantly for the two types. Female monkeys demonstrated more consistent and significant right hemispheric laterality than did males. Furthermore, female monkeys tended to be more lateralised for discriminations of expression, whereas males were about equally lateralised for both types. Thus, in these experiments the overall right hemispheric advantage for facial discriminations in monkeys reflects the contribution of the female subjects, especially when discriminating expression, more than that of the males.