A possible neural basis for the phenomenon of savings in the cerebellum. A, Saccadic adaptation. The monkey's rightward saccades are perturbed via intrasaccadic movement of the target. Positive perturbations produce rightward errors, resulting in an increase in saccadic gain. Negative perturbations produce leftward errors, resulting in a decrease in saccadic gain. After gain-up, gain-down training, the animal exhibits savings in the gain-up period, demonstrating that reversal of errors did not erase the memory. The lines and the numbers indicate the slope of the behavioral data. Data from Kojima et al. (2004). B, Hypothesized model of the oculomotor vermis region of the cerebellum. Rightward errors during gain-up training engage the right inferior olive, producing complex spikes in the left cerebellum. Gain-down training reverses the direction of error, producing complex spikes in the right cerebellum. Perturbation reversal changes the direction of error. If P-cells are organized based on their preference for error, reversal of error engages a new micro-cluster of P-cells, laying down a competing memory. As a result, training followed by “washout” produces two anatomically distinct memories: one for the errors experienced during training and one for the errors experienced during washout. Unfilled triangles represent excitatory synapses. Filled triangles represent inhibitory synapses.