1Climbing fiber input to Purkinje cells has been thought to instruct learning related changes 2 in simple spikes and cause behavioral changes through an error-based learning mechanism. 3 Although, this framework explains simple motor learning, it cannot be extended to learning higher-4 order skills. Recently the cerebellum has been implicated in a variety of cognitive tasks and 5 reward-based learning. Here we show that when a monkey learns a new visuomotor association, 6 complex spikes predict the time of the beginning of the trial in a learning independent manner as 7 well as encode a learning contingent reward expectation signal after the stimulus onset and reward 8 delivery. These complex spike signals are unrelated to and were unlikely to instruct the reward 9 based signal found in the simple spikes. Our results provide a more general role of complex spikes 10 in learning and higher-order processing while gathering evidence for their participation in reward 11 based learning. 12 13
Introduction: 14The cerebellum has been classically considered to be one of the centers for supervised 15 learning in the brain, where the predicted results of movement are compared with the animal's 16 actual performance in relation to their sensory experience, in order to correct the errors in the 17 action that led to the mismatch. The cerebellar cortex has been posited to achieve this via its two 18 distinct types of inputs to its principle output cells, the Purkinje cells (P-cells). First, an efference 19 copy of the ongoing plan generated by other areas in the brain, communicated to the P-cells 20 through mossy fibers, and read out as high frequency simple spikes (SS); and second, a putative 21 instruction signal, motioning unexpected events, communicated through the projections from the 22 inferior olive, the climbing fibers, which evoke low frequency complex spikes (CS). The precisely 23 timed relationship between the coincidence of complex spikes and simple spikes has been to shown 24 to cause a long-term depression at the granule cell->P-cell synapse thereby supervising the 25 information being learned at the level of P-cells 1 . 26The CS have several characteristics: First, they fire with extremely low firing rates 27 (baseline: 0.5-1 Hz, to ~10 Hz), second, they encode sparsely by only firing in about 20-30% of 28 the trials 2,3 , third: some are predominantly evoked by mismatches in sensory predictions 4 , fourth, 29 they can encode information in their rate of firing 2 , timing of firing 3 or the duration of spikes 5 , 30fifth, presence of a single complex on a trial could potentially drive learning and might cause about 31 1 -10 sp/s change in the magnitude of simple spike firing rate in the next trial 3,5 . However, due to 32 the combined effect of low frequency, sparseness and low amount changes that it causes to simple 33 spike, significant changes in behavior, as a consequence of learning, only happens over several 34 tens of trials. This flow of information and circuitry explains many simple motor learning 35 b...