Journal Article

Cerebellar granule cells encode the expectation of reward

Wagner, Mark J., Savall, Joan, Kim, Tony Hyun, Schnitzer, Mark J., Luo, Liqun

2017

Overview

A sizable fraction of granule cells convey information about the expectation of reward, with different populations responding to reward delivery, anticipation and omission, with some responses evolving over time with learning. Reward response in granule cells Classical theories suggest that granule cells in the cerebellum carry sensory and motor signals, enabling downstream Purkinje cells to sense fine contextual changes relating to movement. Using two-photon calcium imaging in behaving mice, Liqun Luo and colleagues also show that a sizable fraction of granule cells convey information about the expectation of reward. Different populations responded to reward delivery, anticipation and omission and some responses evolved over time with learning. The discovery of reward-related signals in granule cells has implications for both models of sensorimotor learning and of cognitive processing in the cerebellum. The human brain contains approximately 60 billion cerebellar granule cells 1 , which outnumber all other brain neurons combined. Classical theories posit that a large, diverse population of granule cells allows for highly detailed representations of sensorimotor context, enabling downstream Purkinje cells to sense fine contextual changes 2 , 3 , 4 , 5 , 6 . Although evidence suggests a role for the cerebellum in cognition 7 , 8 , 9 , 10 , granule cells are known to encode only sensory 11 , 12 , 13 and motor 14 context. Here, using two-photon calcium imaging in behaving mice, we show that granule cells convey information about the expectation of reward. Mice initiated voluntary forelimb movements for delayed sugar-water reward. Some granule cells responded preferentially to reward or reward omission, whereas others selectively encoded reward anticipation. Reward responses were not restricted to forelimb movement, as a Pavlovian task evoked similar responses. Compared to predictable rewards, unexpected rewards elicited markedly different granule cell activity despite identical stimuli and licking responses. In both tasks, reward signals were widespread throughout multiple cerebellar lobules. Tracking the same granule cells over several days of learning revealed that cells with reward-anticipating responses emerged from those that responded at the start of learning to reward delivery, whereas reward-omission responses grew stronger as learning progressed. The discovery of predictive, non-sensorimotor encoding in granule cells is a major departure from the current understanding of these neurons and markedly enriches the contextual information available to postsynaptic Purkinje cells, with important implications for cognitive processing in the cerebellum.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 631/378/1788 / 631/378/3920 / Animals / Anticipation, Psychological - physiology / Behavior, Animal - physiology / Brain cells / Calcium - analysis / Calcium - metabolism / Cerebellum / Cerebellum - cytology / Cerebellum - physiology / Cognition - physiology / Conditioning, Classical - physiology / Conditioning, Operant - physiology / Female / Forelimb - physiology / Genetic aspects / Humanities and Social Sciences / Learning - physiology / letter / Male / Mice / Molecular Imaging / Movement / multidisciplinary / Natural history / Neurons / Neurons - physiology / Physiological aspects / Psychological aspects / Purkinje Cells - physiology / Reward / Reward (Psychology) / Rewards (Psychology) / Rodents / Science