CEREBELLUM the Little Learning Brain

RESEARCH HIGHLIGHTS Nature Reviews Neuroscience | Published online 6 Apr 2017; doi:10.1038/nrn.2017.47 CEREBELLUM The little learning brain Cerebellar granule cells receive Trial-by-trial analyses revealed tone with a normal or large sugar inputs to the cerebellum and provide that most granule cells responded to water reward showed similar reward- many granule Purkinje cells with sensorimotor the US and the CS, and the activity related signals; moreover, some of cells showed contextual information. Two recent of many of the granule cells was the other granule cells in these mice studies used calcium imaging to associated with different movements represented reward expectation or reward-related investigate changes in granule cell (for example, snout movement or reward magnitude. These findings responses activity during two different types of locomotion), which was consistent suggest that cerebellar granule learning, providing insights into the with previous research indicating cells represent different types of function of these cells. that these cells represent sensorimo- reward-related information. Giovannucci et al. measured cal- tor information. Importantly, almost The authors next investigated cium responses in granule cells in the two-thirds of the studied granule how granule cell responses changed cerebellar lobule VI in head-fixed mice cells produced CR-related calcium with training on the operant task. over the course of classical eyeblink responses; that is, responses that Over the course of several days of conditioning. In each trial, an aversive followed the CS and preceded the CR training, fewer neurons responded periorbital airpuff (the unconditioned and that, in trials in which mice pro- to the reward, whereas more stimulus (US)) was delivered to mice duced a CR (but not in trials in which neurons became responsive to shortly after a flash of light or a mild they did not produce a CR), grew in reward anticipation. In addition, puff of air was targeted to the ipsilat- amplitude with training. Thus, learn- the responses of granule cells that eral whisker area (the conditioned ing was accompanied by a widespread were most tuned to reward omission stimulus (CS)). With repeated trials and dense granule cell response. or reward anticipation at day 6 of over several days, the mice learned to In the other study, Wagner et al. training had become progressively blink (a conditioned response (CR)) in recorded calcium responses in stronger with each day of training, anticipation of the airpuff. granule cells in cerebellar lobules V whereas granule cells that showed and VI in head-fixed mice perform- strong responses during forelimb ing an operant task in which they movement showed little change moved a manipulandum with their in their responses with training. forelimb to obtain a delayed sugar Therefore, reward-related granule water reward. Again, many granule cell responses are particularly cells showed activity during different dynamic during learning. movements, such as forelimb move- Together, these two studies ment or licking. Strikingly, many describe changes in granule cell activ- granule cells showed reward-related ity that occur with different types of responses. Specifically, some learning, and imply that granule cells granule cells showed activity during may provide much more than sen- reward anticipation, whereas others sorimotor contextual information to preferentially responded during Purkinje cells. Moreover, these stud- reward consumption (but not during ies provide support for the hypothesis licking when the reward was omit- that the cerebellum may be involved ted). Furthermore, many neurons in various cognitive processes. were sensitive to reward omission: Natasha Bray some were active in anticipation of reward but became even more ORIGINAL ARTICLES Giovannucci, A. et al. active if the reward was omitted, Cerebellar granule cells acquire a widespread predictive feedback signal during motor learning. and others showed an increase in Nat. Neurosci. http://dx.doi.org/10.1038/nn.4531 calcium signal only after the omitted (2017) | Wagner, M. J. et al. Cerebellar granule cells reward. Granule cells in mice that encode the expectation of reward. Nature http:// dx.doi.org/10.1038/nature21726 (2017) decade3d/Alamy Stock Photo Stock decade3d/Alamy were reconditioned to associate a NATURE REVIEWS | NEUROSCIENCE www.nature.com/nrn ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. .

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