Reduction of rostral dorsal accessory olive responses during reaching.

1. Rostral dorsal accessory olive (rDAO) neurons are sensitive to light touch but have little or no discharge during active movement. We hypothesize that sensitivity of the rDAO is reduced during movement. To test this hypothesis, we evaluated sensitivity of rDAO neurons as cats reached out and retrieved a handle. On selected trials, mechanical or electrical perturbations to the forelimb were presented, and responses of rDAO neurons to the disturbances were recorded. 2. All rDAO units were highly sensitive to somatosensory stimuli during periods of stance. The cells responded to stimuli such as touch to hairs or light taps to the platform on which the cat was standing. 3. Discharges of rDAO neurons showed little or no synchronization to any aspect of the reaching task. rDAO neurons failed to fire to mechanical perturbations of the food handle during retrieval or hold phases of the task, even when their receptive fields included the surface of the paw in contact with the handle. 4. Electrical stimulation of the skin produced the greatest evoked response at all rDAO recording sites when the cats were at stance. Stimulation at any time during the reaching task, including periods of holding and licking, produced lower-amplitude evoked responses. The reduction in evoked response could be large and was restricted to the limb performing the task. 5. The data support the hypothesis that the cutaneous sensitivity of the rDAO is reduced during behavior. However, the inhibition does not appear to be tailored to specific times during the task or to neurons with specific receptive field locations on the actively moving limb. The reduction in sensitivity is as likely to be dependent on limb posture as on movement. We conclude that the rDAO discharge provides the cerebellum with information about vibration or contact during stance; it does not provide reliable information about undisturbed or disturbed movement. Climbing fiber input from rDAO might be useful in the preparation to make a movement, but it is probably not useful for correction of movement errors.