  Cerebellar climbing fiber activity encodes performance errors during many
motor learning tasks, but the role of these error signals in learning has been
controversial. We compared two motor learning paradigms that elicited equally
robust putative error signals in the same climbing fibers: learned increases
and decreases in the gain of the vestibulo-ocular reflex (VOR). During
VOR-increase training, climbing fiber activity on one trial predicted changes
in cerebellar output on the next trial, and optogenetic activation of climbing
fibers to mimic their encoding of performance errors was sufficient to implant
a motor memory. In contrast, during VOR-decrease training, there was no
trial-by-trial correlation between climbing fiber activity and changes in
cerebellar output, and climbing fiber activation did not induce VOR-decrease
learning. Comparison of the two training paradigms suggests that the ability of
climbing fibers to induce plasticity can be dynamically gated in vivo by the
state of the cerebellar circuit, even under conditions where the climbing
fibers are robustly activated by performance errors.
