In hereditary neurodegenerative Huntington disease (HD), early cognitive impairments before motor deficits have been hypothesized to result from dysfunction in the striatum and cortex before degeneration. To test this hypothesis, we examined the firing properties of single cells and local field activity in the striatum and cortex of pre-motor-symptomatic R6/1 transgenic mice while they were engaged in a procedural learning task, the performance on which typically depends on the integrity of striatum and basal ganglia. Here, we report that a dramatically diminished recruitment of the vulnerable striatal projection cells, but not local interneurons, of R6/1 mice in coding for the task, compared with WT littermates, is associated with severe deficits in procedural learning. In addition, both the striatum and cortex in these mice showed a unique oscillation at high γ-frequency. These data provide crucial information on the in vivo cellular processes in the corticostriatal pathway through which the HD mutation exerts its effects on cognitive abilities in early HD.single unit | local field potential | high γ-oscillation | operant learning | pre-motor-symptomatic R6/1 mice H untington disease (HD) is a progressive and inherited neurodegenerative disorder associated with selective degeneration of medium-sized spiny projection neurons in the striatum (1). Although the disease is well characterized by symptoms such as involuntary choreiform movements, dystonias, and rigidity, these motor symptoms have been found to be preceded by personality, mood, and cognitive disturbances (2). Some postmortem studies have also shown only limited signs of pathologic processes or cell loss in the brain despite substantial clinical evidence of HD (3), suggesting that neuronal-and, more precisely, synapticdysfunction, rather than cell death, may predominantly underlie early behavioral manifestations of the HD mutation.Studies of the effects of the mutant HD gene in several transgenic mouse models, including the R6 lines, have indeed shown changes in the membrane properties, biochemistry, and morphology of fragile striatal cells (4, 5), suggesting compromised functional integrity. They have also shown early and progressive alterations in synaptic plasticity in the corticostriatal pathway (6). These changes have been hypothesized to underlie early cognitive and behavioral deficits in transgenic mice (and patients with HD) by altering striatal information processing and transfer within basal ganglia loops.To verify this hypothesis, we recorded single-unit activity and local field potentials (LFPs) in the dorsal striatum and cortex, whereas behaviorally naive pre-motor-symptomatic R6/1 mice (7) (14-18 wk old) and age-matched WT littermates acquired an association between a nose-poke (NP) action and reward through operant learning. This procedural learning of an action-reward association is known to critically involve the striatum, a crucial site for the integration of the sensorimotor, limbic, and cognitive information required for the selecti...