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the retinal slip. Adaptation of VOR or OKR is impaired by surgical lesion, pharmacological
inactivation or genetic disruption of a part of the cerebellum, suggesting that the cerebellum is
required for memory formation [60,114,115]. In a lurcher mouse, which lacks outputs from
the cerebellar cortex due to the developmental loss of Purkinje cells, neither VOR nor OKR
adaptation occurs [60].
The main neural pathway controlling VOR consists of vestibular organs, vestibular nuclei
and oculomotor nuclei. In addition, there is a regulatory side pathway including the flocculus
and the ventral paraflocculus of cerebellum. These regions receive inputs from the vestibular
organ through mossy fibers and project to the vestibular nucleus through Purkinje cell axons.
Where is the motor memory for VOR adaptation stored in these neural circuits? Two
candidate sites have been considered. One is the synapses between parallel fibers and
Purkinje cells in the cerebellar cortex, and the other is the synapses between vestibular
afferents and neurons in the vestibular nuclei. Ito proposed that the cerebellar LTD at parallel
fiber - Purkinje cell synapse is responsible for VOR adaptation [111]. The correlation of
retinal slip (the visual image motion on a retina) and climbing fiber activity was reported
[116]. On the other hand, Lisberger and colleagues considered that the synaptic plasticity in
the vestibular nucleus plays an essential role in VOR adaptation [117]. Later, implication of
both forms of synaptic plasticity in the VOR adaptation has been suggested [60,113,118,119].
E YEBLINK C ONDITIONING
Eyeblink conditioning is another model task for motor learning which depends on the
cerebellum, and has been studied extensively [120,121] (Figure 4).
Eyeblinking is elicited by applying aversive stimulus such as an air puff to an eye or the
electrical stimulation. Such stimulus is called unconditioned stimulus (US), and the reflexive
eyeblink response is called unconditioned response (UR). In eyeblink conditioning, US is
preceded by a neutral stimulus that by itself does not elicit UR such as tone (conditioned
stimulus, CS). Repeated CS-US pairings make an animal elicit a response similar to UR upon
CS (conditioned response, CR). Two training procedures have been used. One is the delayed
procedure, in which CS and US overlap in time. The other is the trace procedure with a time
interval between the CS offset and the US onset. In both procedures the cerebellum is
implicated. Additionally, the hippocampus plays an essential role in the trace training [120].
Lesion experiments, electrophysiological recordings and electrical stimulation of the inferior
olivary nuclei have shown that US information is transmitted to the cerebellar cortex and also
to the interpositus nucleus of DCN through climbing fibers. Similarly, lesion and stimulation
of pontine nuclei revealed that CS information is transmitted to the cerebellar cortex and the
interpositus nucleus through the mossy fiber pathway. Therefore, CS and US signals are
integrated at both Purkinje cells and neurons in the interpositus nucleus .
The interpositus nucleus is involved in both UR and CR [120]. Some neurons in the
interpositus nucleus discharge spikes preceding the onset of CR and in a precise temporal
pattern related to the onset of CS. Lesion or reversible inactivation of the interpositus nucleus
abolishes acquired CR, but not UR, in well-trained animals, and also prevents acquisition of
new CR. On the other hand, inactivation of the red nucleus, which is located downstream of
the interpositus nucleus, prevents expression of acquired CR without preventing CR
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