Eyelid conditioning to a target amplitude: adding how much to whether and when.

Conceptual and practical advantages of pavlovian eyelid conditioning facilitate analysis of cerebellar computation and learning. Even so, eyelid conditioning procedures are unrealistic in an important way. The error signal to the olivocerebellar system does not decrease as learning adapts response amplitude or gain. This inherently limits the utility of eyelid conditioning for studies investigating how cerebellar learning mechanisms acquire and store an adaptive response amplitude. We report the development and characterization of a training procedure in which conditioned response amplitude is brought under experimental control with contingencies that more closely parallel natural conditions. In this procedure, the delivery of the unconditioned stimulus (US) is made contingent on conditioned response amplitude: the US is delivered for responses that fail to reach a specified target amplitude and is omitted for responses that meet or exceed the target. We find that rabbits trained with either a tone or with mossy fiber stimulation as the conditioned stimulus learn responses that approach target amplitudes ranging from 2 to 5 mm. Inactivating the interpositus nucleus with muscimol infusions abolished these conditioned responses, indicating that cerebellar involvement in eyelid conditioning is not tied explicitly to the use of pavlovian procedures. Together with previous studies, these data suggest that response amplitude is learned and encoded in the cerebellum during eyelid conditioning. As such, these results provide a foundation for systematic and controlled investigations of the cerebellar mechanisms that learn and encode the proper amplitude of adaptive movements.