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Cerebellum CEREBELLAR CORTICAL NEURONS EXHIBIT BIMODALITY IN FREELY MOVING ANIMALS Iddo Lev1, Gilad Jacobson2,3, Yosef Yarom2,3, Dana Cohen1 1 The Gonda Multidisciplinary Brain Research Center, Bar-Ilan University, Ramat-Gan 52900, Israel. 2 Department of Neurobiology, Life Science Institute and 3The Interdisciplinary Center for Neural Computation, Edmond Safra Campus, Jerusalem 91904, Israel. The information content transferred by neurons is encoded in their time dependent firing patterns. Some neurons exhibit bimodal firing patterns in which neurons alternate between periods of high firing rates (up state) to periods of quiescence (down state). Intracellular recordings from Purkinje cells in cerebellar slices showed that the bimodal firing pattern reflects a bistable membrane potential. Recently it has been shown that the membrane potential of Purkinje cells in intact, anesthetized brain is also bistable. This finding was challenged by a report claiming that Purkinje cells in awake animals are continuously in their up state and quiescent periods of Purkinje cells could not be detected in awake animals. We reexamined this issue by implanting microwire arrays into the cerebellar cortex and recording activity of cerebellar cortical neurons while animals walked freely in their home cages. An array of 32 isonel coated tungsten microwires (35 microns in diameter) was implanted into the posterior part of the cerebellar vermis. After about 10 days of recovery, we observed high levels of spontaneous activity. The majority of the clearly sorted single units, displayed epochs of high firing rates followed by prolonged quiescent periods. For the identification of Purkinje cells we compared these recordings with recordings under isoflorane anesthesia of the same animals. Although, the transition from anesthetized to awake state was accompanied by an increase in firing rate the bimodal pattern was clearly observed. These results show beyond a doubt that cerebellar cortical neurons recorded chronically in freely moving animals, exhibit bimodal firing patterns.