However, the timing of complex spikes during saccade adaptation (Catz, Dicke, & Thier, 2008) suggests that climbing fiber input might not be the sole teacher in the cerebellum. In this study, Nguyen-Vu and colleagues tested the hypothesis that Purkinje cells themselves can drive adaptation of the vestibulo-ocular reflex (VOR), a form of motor learning.
In this study, Nguyen-Vu and colleagues demonstrates that there exists more than one teacher for cerebellar learning and that changes in Purkinje cell activity can drive motor learning.
When the subjects were rotated sinusoidally, optogenetic stimulation of Purkinje cells of left hemisphere during rightward motion and of the right hemisphere during leftward motion (contraversive stimulation) led to an increase in VOR gain. In contrast, ipsiversive stimulation (stimulation of the right/left hemisphere during rightward/leftward motion) had no effect on VOR gain. This study provides strong evidence for role of Purkinje cell activity in motor learning
In contrast, optogenetic stimulation of climbing fibers did not induce VOR adaptation during contraversive stimulation but induced an increase in VOR gain during ipsiversive stimulation. The latter observation confirms that climbing fibers plays a role in inducing motor learning.
Surprisingly, none of the stimulation protocols induced a decrease in VOR gain.
The difference in the required timing of stimulation for the induction of motor learning between Purkinje cell (contraversive stimulation required) and climbing fiber stimulation (ipsiversive stimulation required) suggests that activation of Purkinje cells and climbing fibers modulates motor learning through different mechanisms. The existence of two different mechanisms increases the ability of the cerebellum to adapt.
Catz, N., Dicke, P. W., & Thier, P. (2008). Cerebellar-dependent motor learning is based on pruning a Purkinje cell population response. Proceedings of the National Academy of Sciences of the United States of America, 105(20), 7309–14. doi:10.1073/pnas.0706032105
Marr, D. (1969). A theory of cerebellar cortex. The Journal of physiology, 202(2), 437–70.
Najafi, F., & Medina, J. F. (2013). Beyond “all-or-nothing” climbing fibers: graded representation of teaching signals in Purkinje cells. Frontiers in neural circuits, 7(July), 115. doi:10.3389/fncir.2013.00115