Editing Cerebellum (section)

===Learning===
There is considerable evidence that the cerebellum plays an essential role in some types of motor learning. The tasks where the cerebellum most clearly comes into play are those in which it is necessary to make fine adjustments to the way an action is performed. There has, however, been much dispute about whether learning takes place within the cerebellum itself, or whether it merely serves to provide signals that promote learning in other brain structures.<ref name="Boyden"/> Most theories that assign learning to the circuitry of the cerebellum are derived from the ideas of [[David Marr (neuroscientist)|David Marr]]<ref name=Marr>{{cite journal | vauthors = Marr D | title = A theory of cerebellar cortex | journal = Journal of Physiology | volume = 202 | issue = 2 | pages = 437–70 | date = June 1969 | pmid = 5784296 | pmc = 1351491 | doi = 10.1113/jphysiol.1969.sp008820 | author-link = David Marr (neuroscientist) }}</ref> and [[James S. Albus|James Albus]],<ref name=Albus>{{cite journal |title=A theory of cerebellar function | vauthors = Albus JS |journal=Math. Biosciences |year=1971 |volume=10 |issue=1–2 |pages=25–61 |doi=10.1016/0025-5564(71)90051-4| citeseerx = 10.1.1.14.7524 }}</ref> who postulated that [[climbing fiber]]s provide a teaching signal that induces synaptic modification in [[parallel fiber]]–[[Purkinje cell]] synapses.<ref name=Houk1996/> Marr assumed that climbing fiber input would cause synchronously activated parallel fiber inputs to be strengthened. Most subsequent cerebellar-learning models, however, have followed Albus in assuming that climbing fiber activity would be an error signal, and would cause synchronously activated parallel fiber inputs to be weakened. Some of these later models, such as the ''Adaptive Filter'' model of Fujita<ref>{{cite journal | vauthors = Fujita M | title = Adaptive filter model of the cerebellum | journal = Biological Cybernetics | volume = 45 | issue = 3 | pages = 195–206 | year = 1982 | pmid = 7171642 | doi = 10.1007/BF00336192 | s2cid = 3695770 }}</ref> made attempts to understand cerebellar function in terms of [[optimal control]] theory.

The idea that climbing fiber activity functions as an error signal has been examined in many experimental studies, with some supporting it but others casting doubt.<ref name=Simpson/> In a pioneering study by Gilbert and Thach from 1977, Purkinje cells from monkeys learning a reaching task showed increased complex spike activity—which is known to reliably indicate activity of the cell's climbing fiber input—during periods when performance was poor.<ref>{{cite journal | vauthors = Gilbert PF, Thach WT | title = Purkinje cell activity during motor learning | journal = Brain Research | volume = 128 | issue = 2 | pages = 309–28 | date = June 1977 | pmid = 194656 | doi = 10.1016/0006-8993(77)90997-0 | s2cid = 40799652 }}</ref> Several studies of motor learning in cats observed complex spike activity when there was a mismatch between an intended movement and the movement that was actually executed. Studies of the [[vestibulo-ocular reflex]] (which stabilizes the visual image on the retina when the head turns) found that climbing fiber activity indicated "retinal slip", although not in a very straightforward way.<ref name=Simpson/>

One of the most extensively studied cerebellar learning tasks is the [[eyeblink conditioning]] paradigm, in which a neutral conditioned stimulus (CS) such as a tone or a light is repeatedly paired with an unconditioned stimulus (US), such as an air puff, that elicits a blink response. After such repeated presentations of the CS and US, the CS will eventually elicit a blink before the US, a conditioned response or CR. Experiments showed that lesions localized either to a specific part of the interposed nucleus (one of the deep cerebellar nuclei) or to a few specific points in the cerebellar cortex would abolish learning of a conditionally timed blink response. If cerebellar outputs are pharmacologically inactivated while leaving the inputs and intracellular circuits intact, learning takes place even while the animal fails to show any response, whereas, if intracerebellar circuits are disrupted, no learning takes place—these facts taken together make a strong case that the learning, indeed, occurs inside the cerebellum.<ref>{{cite journal | vauthors = Christian KM, Thompson RF | title = Neural substrates of eyeblink conditioning: acquisition and retention | journal = Learning & Memory | volume = 10 | issue = 6 | pages = 427–55 | year = 2003 | pmid = 14657256 | doi = 10.1101/lm.59603 | doi-access = free }}</ref>