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Evaluation of Sites Storing Novel Memories of Adapted VOR Gains

Stephen M. Highstein

Washington University School of Medicine, St. Louis MO

Learning experiments can be divided into those evaluating the acquisition of new memories, and those designed to evaluate the storage of these memories. The cerebellar cortex and its target neurons in the deep cerebellar nuclei and brainstem (FTN) are candidates for this storage aspect. A model system for motor learning is the vestibulo-ocular reflex (VOR) that generates compensatory eye movements during head turns. To evaluate the storage of motor learning, we recorded the activity of Y neurons, a type of FTN that participates in vertical eye movements, before and after motor learning. Y neuron discharge was compared with that of Purkinje cells in the cerebellar flocculus. Comparison illustrates that the changes observed in the eye velocity sensitivity of Purkinje cells following motor learning are not directly transferred to Y neurons, suggesting an additional plastic site, possibly the synapse between the Purkinje cell and Y neuron. We quantified changes in the head velocity pathway to Y neurons using multiple regression models. This quantification was later employed to build model simulations that account for changes in parallel pathways to Y neurons and to explain the role of the cerebellum in VOR memory. Our results suggest that low gain adaptation results in more drastic changes than high gain adaptation. Low gain adaptation causes increases in the head velocity sensitivity in parallel pathways. Model simulations suggest that cerebellar and brainstem plasticity are necessary for chronic memory storage. Further, we posit that results obtained following cerebellar flocculus lesion are the product of different mechanisms than those operating during memory storage in the intact animal.