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Split-brain experiments, which have been actively conducted since the twentieth century, have provided a great deal of insight into functional asymmetry and inter-hemispheric interactions. However, how communication between the left and right hemispheres directly contributes to memory formation is still poorly understood. To address this issue, we cut the rat commissural fibers prior to performing behavioral tests, which consisted of two short-term and two long-term memory tasks. The result showed that cutting the commissural fibers impairs short-term memory but not long-term memory. This suggests that the left-right hemispheric interaction through the commissural fibers contributes to the appropriate formation of short-term memory, but not that of long-term memory. Our findings would help to elucidate dynamic memory formation between the two hemispheres and contribute to the development of therapeutics for some neurological diseases which cause a reduction in the inter-hemispheric interaction.

The left–right hemispheric differences in some brain functions are well known in humans. Among them, savant syndrome has unique features, such as exceptional abilities in vision, memory, computation, and music, despite brain abnormalities. In cases of acquired savant and transient savant, brain damage or inhibition is often seen in the left hemisphere, suggesting a link between left hemispheric dysfunction and these talents. On the other hand, some functional left–right differences have been reported in rodent brains, and therefore, unilateral damage in rodents may also result in savant-like enhancements. In the present study, we examined the effects of hippocampal damage on spatial learning in rats with left, right, or bilateral hippocampal lesion. The results showed that learning performance was impaired in the bilateral lesion group, and there was no significant difference in the left lesion group, while performance was enhanced in the right lesion group. These results suggest that damage to the right hippocampus in rats may lead to savant-like enhancement in learning and memory. The construction of the savant model through these results will contribute to the neuroscientific elucidation of the paradoxical phenomenon observed in savants, that some abilities are enhanced despite their brain dysfunction.

Functional asymmetry in the left/right hemisphere is a well-known feature in human, e.g., the left-sided function of the language area and the right-sided spacial cognitive function. However, such asymmetrical function is not specific in human but also in the various animals. The left-sided planned (skilled) behavior is a one of the most well-studied lateralized function, and it has been revealed that dorsolateral striatum (DLS) plays an important role in behavioral transition from the goal-directed to the habitual. Functional inhibition of this brain area leads to inhibit the formation of the habit. These evidences would suggest the left hemispheric preference of the habitual behavior. In addition to it, the dopamine D2, not D1 receptor in DLS plays an important role in forming habits. Then, we examined how down regulation of D2R activity by microinjection of D2R-antagonist to rat DLS (left or right hemisphere) affects the habit formation. Experimental design was the place/response test with a plus-maze. As a result, the choice ratio of the response arm (habitual) remained low level through all trials in the left-injected group compared to the control group. Conversely, the habitualization of the right-injected group was significantly preceded. Thus, from present results, we concluded that the left DLS-D2 neurons have a higher contribution to the habit formation than the right one. This suggests left/right functional dissociation of the basal ganglia, at least rat striatum.