Trends in Cognitive Sciences
Cerebellar learning in the vestibulo–ocular reflex
Section snippets
The neuronal circuit for the VOR–flocculus system
Of the many component pathways of the VOR arc operating under the influences of the flocculus[7], that serving the horizontal VOR is illustrated in Fig. 1. The horizontal semicircular canals are stimulated by ipsilateral head rotations (as indicated by broken arrows in Fig. 1 for the left horizontal canal). The stimulated horizontal semicircular canal on either side of the head sends neural signals via the primary vestibular nerve fibers to the relay cells of the VOR located in the vestibular
The VOR and its adaptation
In laboratory experiments, the horizontal VOR is induced by sinusoidal or velocity-step head rotation, and the VOR gain is measured as the ratio of the attained eye velocity to the applied head velocity. Using sinusoidal rotation is convenient for measuring the gain and phase of the VOR separately, while velocity steps enable us to separate components of VOR responses, which arise with different latencies. The measurement of the VOR is performed in the dark or with the eyes closed, in order to
Evidence for the flocculus hypothesis
The first lines of evidence in support of the flocculus hypothesis were derived by lesioning the flocculus or impairing its function. VOR adaptation no longer occurs after surgical ablation of, or the injection of toxic amino acids into, the flocculus in cats[28], rabbits29, 30, monkeys[31]and goldfish32, 33. Microdialysis of lidocaine into the goldfish cerebellum blocked both adaptive increase and decrease of VOR gain[34]. Interruption of the climbing-fiber input to the flocculus reproduced
Debate 1: Memory site for the VOR adaptation
Although the aforementioned three lines of evidence consistently indicate that the flocculus plays a crucial role in the induction of the VOR adaptation, there has been dichotomy of opinion about the role of this structure in its retention. To determine whether the VOR adaptation is retained in the flocculus or not, the effects of the (surgical or functional) removal of flocculus functions after the VOR adaptation had developed was studied. In goldfish, no less than 30% of the altered VOR gain
Debate 2: Purkinje-cell behavior associated with VOR adaptation
Another debate concerns the behavior of flocculus Purkinje cells. Even though this behavior conforms to the flocculus hypothesis in the rabbit H zone (Appendix Aand Appendix D), inconsistent results have been reported in monkey and goldfish experiments. However, these experiments involve serious technical problems: the earlier data on monkeys were obtained without distinguishing between the ventral paraflocculus and the flocculus[8]. It is also questionable whether the vestibular responses of
Debate 3: implications of oculomotor signals
In the aforementioned visual-suppression method, the difference between the responses during VOR and those during visual suppression was considered to represent oculomotor signals encoding eye velocity or eye position. As the oculomotor signals so derived were large, it has been hypothesized that oculomotor signals provide positive feedback from the oculomotor system to the VOR-relay neurons via the flocculus[53](Fig. 3B). If this were the case, changes in Purkinje-cell behavior observed during
Conclusion
The VOR–flocculus system conforms to the general idea of a cerebellar corticonuclear microcomplex as a module capable of error-driven learning to modify its input–output relationships59, 60. Controversies about the neuronal mechanisms in a microcomplex need to be resolved in future but, as the microcomplex includes both a cerebellar cortical zone and a nuclear cell group (Fig. 4), the debate on the memory site for VOR adaptation is not critical for the microcomplex concept. Such microcomplexes
Outstanding questions
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Does LTD account for long-term or even permanent memory? This has not been answered because of technical difficulties that currently do not allow us to follow the time course of LTD for more than three hours. This question is central to the cerebellar-learning theories and, to answer it, new technologies for marking LTD are required.
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Is the memory in VOR adaptation controlled by the flocculus alone or by brainstem pathways, or by both of them? Although the available evidence consistently
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2021, Neural NetworksCitation Excerpt :The goal of cerebellar motor learning is to perform precise gain and temporal control for movements. The cerebellar mechanisms for gain and timing control for eye movements have been studied in the two types of experimental paradigms; (1) gain control for the optokinetic response (OKR) and the vestibulo-ocular reflex (Ito, 1984, 1998) and (2) timing control for the eyeblink conditioning (Christian & Thompson, 2003; Mauk & Donegan, 1997). Here, we are concerned about gain adaptation of OKR.
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2019, Neurobiology of Learning and MemoryCitation Excerpt :Finally, the flocculus and paraflocculus are critical for eye-related movements. PCs in the flocculus control the vestibulo-ocular reflex as well as balance control which is a determinant of optimal gait characteristics (Ito, 1998). Human patients with cerebellar disease quite often show gait ataxia, however, since the cerebellar cortex specializes in error-based conditional, adaptive learning, it is imperative to address the role of adaptive learning within a locomotor context, and how this is affected in cases of injury.