Biomedical Engineering Reference
In-Depth Information
speculative ideas about its function. From the beginning of the 1960s theoretical
approaches, hypotheses, and models of the cerebellum have been created. Some
models were derived from the strategic position of the cerebellum within closed
or open loops of structures assumed to be involved in active movements (e.g.
(29,3,59)). Others were based on the unique anatomical microarchitecture of the
cerebellum (e.g. (12,48,2)) or on the requirement to transform motor planning
into motor execution coordinates (56). More recent models assume the cerebel-
lum to be an adaptive controller, or, more exactly, that the cerebellum could be
modeled as an adaptive controller, as pointed out by Barlow (6). In his recently
published topic, he reviews different types of models as well as the experimental
sources these models were derived from (6).
In the subsequent paragraphs and based on experimental approaches, we
will first review the evidence in human lesion and neuroimaging studies about
involvement of the human cerebellum in classical conditioning of the eyeblink
response. In addition, based on the recent discussion of the cerebellum's in-
volvement in non-motor functions, we will review evidence that the cerebellum
may be involved in visuomotor associative learning as well as in habituation
processes (69). Different hypotheses and models of cerebellar function in learn-
ing will be discussed in the theoretical part of this chapter.
1.2. Anatomy of the Cerebellum
The cerebellum is located behind and below the cerebral hemispheres, over-
lying the brain stem (for a review, see (14,77)). The cerebellum consists of two
large hemispheres and has, in contrast to most other parts of the brain, a midline
structure, the vermis. In the anterior-posterior direction, the cerebellum is subdi-
vided into the anterior, posterior, and flocculonodular lobes. The three lobes are
subdivided into several lobules. Larsell has introduced a numbering system,
based on comparative studies on phylogenetic similarities (44), which consists
of Roman numerals in the vermis and the prefix H in the hemispheres.
Each half of the cerebellum contains four distinct nuclei. The fastigial nu-
cleus is located most medially, followed by the globose and emboliform nucleus
and, most laterally, the dentate nucleus. The emboliform and globose nuclei in
humans most likely resemble the anterior and posterior interposed nuclei in
animals.
On the basis of efferent projections from the cerebellar cortex to the cere-
bellar nuclei, Jansen and Brodal (32), and later Chambers and Sprague (16),
suggested a subdivision into three longitudinal (sagittal) zones: a medial zone
(vermis) projecting to the fastigial nucleus, an intermediate (paravermal part of
the cerebellar hemisphere) zone projecting to the interposed nuclei and a lateral
(lateral part of the cerebellar hemisphere) zone projecting to the dentate nucleus.
