Cerebellar Longterm Depression

delivered. Trace eye-blink conditioning (CS followed by an intervening time delay) is hippocampus-dependent, while delay conditioning (no intervening time delay) is hippocampus-independent, so additional circuits outside the cerebellum clearly can come into play.

Adaptation of the VOR is even more complicated (59). Shake your head back and forth while you are looking at this word. You are amazingly good at keeping your eyes pointed at exactly the right spot as your head goes back and forth. This seems simple, but remember that you are precisely moving both your left and right eyes to exactly counterbalance your head movements. (To get a feel for what it would be like to live without a VOR, hold your finger out at arm's length and stare at your fingertip. Now rotate your entire upper body back and forth—(there is quite a difference!). The VOR detects signals from your vestibular system semicircular canals (that read-out head movement) and allows triggering of the appropriate eye muscle contractions to hold the eye position constant in space. To complicate matters further, this reflex is of necessity subject to adaptation. If your eye movements are not holding the visual field constant for some reason (e.g., damage to your oculomotor system or to the eye muscles themselves, or even a new pair of eyeglasses that change your focal point), the system adapts to the change and modifies the VOR to appropriate for your new state. This depends on complicated signals from the visual cortex that provide information concerning constancy of the visual percept—obviously not a trivial matter.

Clearly synaptic plasticity at a single type of synapse cannot account for all of the elaborate behavioral changes underlying eye-blink conditioning and adaptation of the VOR. However, a wide variety of evidence from pharmacologic, genetic, lesioning, and physiologic recording studies has indicated an important role in these processes for LTD at parallel fiber-to-Purkinje cell synapses in the cerebellar cortex. This LTD in the cerebellar cortex has been extensively studied, and shortly I will highlight a few of its properties and present a simplified version of how it might participate in eye-blink conditioning.

Parallel fiber LTD in the cerebellum is a persistent, input-specific decrease in the efficacy of synaptic transmission between the parallel fibers and Purkinje cells in the cerebellar cortex (see Panel B). It is induced by low-frequency co-activation of climbing fibers and parallel fiber inputs to Purkinje neurons. Climbing fibers are highly potent inputs onto Purkinje neurons—a single climbing fiber matches to only one Purkinje neuron, and its activation is sufficient to trigger an action potential in its specific

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