Postdoctoral Fellow, University of California, San Francisco
Abstract: The brain is capable of modifying its structures and functions to adapt to existing conditions, a form of plasticity vital for animal’s survival in changing environments. Mammalian nerve systems have superior adaptability compared to the primitive ones of amphibians and reptiles and their superiority in adaptability largely results from the emergence of the cerebral cortex, a phylogenetically modern structure. Mounting evidence indicates that the cortex can adaptively modulate innate reflexive behaviors that are mediated by the brainstem, an ancient subcortical structure, depending on prevailing condition and past experience. This modulation becomes particularly pronounced in organisms with high levels of encephalization, humans being the supreme example. The mechanisms underlying the cortical adaptive control of innate behaviors, however, remain unknown. Here I will present my recent study on the impact of visual cortex on the optokinetic reflex (OKR), an involuntary eye movement that stabilizes images on the retina while the animal moves. Although innate, the OKR is highly plastic. This plasticity is essential to maintain image stability, since the amplitude of the OKR needs to be continuously readjusted as the animal matures, grows, ages or suffers from impairments of other ocular motor reflexes. Previously, OKR plasticity was thought to be entirely mediated by the brainstem and cerebellum. However, I will show that the visual cortex plays a prominent role of in this plasticity. In particular, I will show that a plastic increase in the amplitude of the OKR, induced by impairing another image stabilization mechanism, can be reversed upon silencing visual cortex. Furthermore, selectively ablating visual cortical neurons projecting to the brainstem nuclei responsible for the OKR strongly reduces OKR plasticity. Finally, I will show that OKR plasticity is generated by an enhanced drive exerted by visual cortex onto these brainstem nuclei. These results demonstrate how the innervation of a phylogenetically older structure by the neocortex can modulate the performance of reflexive behaviors in an experience dependent manner. This ability of the cortex to modulate subcortically mediated innate behaviors represents a fundamental evolutionary adaptation as it expands the behavioral repertoire of mammals.