The cerebellar cortex is an important part of our motor system. The sole output cells of the cerebellar cortex are Purkinje cells, receiving input from climbing fibers arriving from the brain stem. The climbing fiber to Purkinje cell synapse is the strongest synapse in the central nervous system and when it is active, it generates a unique event termed a complex spike. Thus, Purkinje cells display two types of action potentials: complex and simple spikes. Multiple studies have shown that complex spikes encode error signals that instruct motor learning, but recent evidence suggest that the cerebellum is also involved in the processing of reward. To study how rewarding events are encoded, we recorded complex spikes when monkeys were engaged in an eye movement task, which were cued to the size of the expected reward. We found that complex spike rate increased when the monkeys were presented with a cue indicating a large reward size. Reward size did not modulate activity at reward delivery or during eye movements. Comparison between complex spikes and simple spike activity indicated different interactions for coding of movement and reward. These results indicate that complex spikes encode the expected reward size and suggest a general role of the cerebellum in associative learning beyond error correction.
Paper of the month
Joshua's Lab: Cerebellar climbing fibers encode expected reward size
eLife 2019;8:e46870 (2019)