Balance before reason in rats and humans.

Considerable clinical and experimental evidence indicates that loss of vestibular function results in cognitive deficits, especially deficits in spatial memory. These studies demonstrate the importance of balance for the most fundamental of cognitive processes and suggest that information about head acceleration and orientation must have been critical to the evolution of brain structures such as the hippocampus. Studies of animals with bilateral vestibular lesions have shown that theta rhythm and the activity of hippocampal place cells are severely disrupted; recent human studies show that bilateral vestibular loss is even associated with hippocampal atrophy. While it is conceivable that the effects of vestibular lesions on the hippocampus are due to chronic stress and increased glucocorticoid levels, at present there is little evidence to support this hypothesis. It is also possible that the hippocampal changes are due to a reduction in exploration and active behavior; however, in rats, at least, bilateral vestibular lesions cause hyperactivity rather than hypoactivity. Alternatively, the hippocampus may have developed a special dependence upon the vestibular system during evolution, since it was the first sensory system to reliably indicate gravitational vertical.