Neuronal plasticity of interrelated cerebellar and cortical networks
Section snippets
Anatomical organization of the corticocerebello-cortical loop
The organization of the cerebellocortical loop has been widely investigated in different animal models and the topography of the corticoponto cerebellar system (Schmahmann and Pandya, 1997) as well as that of the cerebellothalamic one (Asanuma et al., 1983) have been studied in detail in the monkey. However, only recently have direct bi-synaptic cerebellar connections been shown outside the primary motor cortex, namely over prefrontal and posterior parietal areas (Middleton and Strick, 1997).
Neurophysiology of cerebellocortical interactions
The introduction of electrical and magnetic stimulation of brain structures through the scalp has made the neurophysiological study of neuronal circuitries in humans possible on a large scale. This technique has been successfully applied in the study of the cerebellothalamo cortical pathway in normal subjects (Ugawa et al., 1991) and in patients with unilateral focal cerebellar lesions (Di Lazzaro et al., 1994a, Di Lazzaro et al., 1994b). It allowed analyzing for the first time in humans the
Cerebellar-induced physiological changes in the primary motor cortex
Physiologically, stimulation of the intact cerebellum produces a clear reduction of the excitability of the contralateral motor cortex. The deep cerebellar nuclei have facilitatory effects on the contralateral cerebral cortex while the cerebellar cortex inhibits the activity of the nuclei. Considering this biological substrate, the reported neurophysiological evidence indicates that transcranial activation is more effective on the cerebellar cortex than on the deep nuclei, thus inducing the
Cortical map plasticity and cerebellar input
It has been suggested that the organization of the motor cortex is greatly dependent on the balance between excitatory and inhibitory influences over the network of cortical connections. In particular, the extent of the horizontal connections has been considered as a possible substrate for the rapid somatotopical changes observed after peripheral manipulations or exercise (Huntley, 1997). Among the different systems that might be able to control cortical excitability, the
Corticocerebellar interaction for cortical plasticity during implicit learning
Correlations between cerebellar input and plastic changes in the motor cortex can be demonstrated during the acquisition of a visuomotor skill (Doyon et al., 1996). Through the use of PET, different groups have demonstrated that cerebellar and motor cortex activations are coincident in the early phases of learning and that both present a decrement in later stages of learning. This is particularly evident when the learning is implicit. When a skill is acquired through declarative mechanisms, the
Cerebellar influences on cortical somatosensory processing
Another interesting aspect of the interrelations between cerebellum and cerebral cortex in serial learning is the importance of their interplay for sequence detection. Two focal aspects of this function, namely sensory processing and timing, have been considered to be carried out by the cerebellar circuits (Ivry, 1997, Bower, 1997a). To recognize that stimuli are presented in a given order, the sensory information pertaining to one stimulus must be kept active, so to speak in a ‘working
Somatosensory-evoked potential (SEP) recordings in the cerebellum
The cerebellum receives inputs virtually from every sensory system. However, its lesion does not cause clinically evident sensory deficits. Starting from this evidence, the sensory input to the cerebellum was considered as being strictly related to motor functions and not involved in the cortical processing of sensory data. However, functional neuroimaging data have challenged this interpretation. Cerebellar activation has been observed in conjunction with somatosensory cortical activation in
Cerebellar influences on cortical processing of sensory stimuli
The growing evidence pointing to the importance of the cerebellum in cortical processing has been widely corroborated by neurophysiological studies indicating cerebellar influence on the cortical motor area (previous paragraph). However, until recently no evidence was available on cerebellar-induced neurophysiological changes in the somatosensory cortex. We studied this aspect using SEPs with the brain electrical source analysis technique in subjects with unilateral lesions of a cerebellar
Physiological properties of cerebellar inputs to MI and to SI
It is interesting to note that unilateral cerebellar lesions affect both MI and SI contralateral cortices but in a different manner. They primarily affect the excitatory properties of pyramidal neurons in MI and act on inhibitory cortical circuitries in SI. One explanation may be based on the anatomical differences between cerebello-MI and cerebello-SI loops. Cerebellar information is funnelled to MI through the VL thalamic nuclear group that projects mainly to layers IV and V and, in this
Conclusion
Recent data are rapidly changing our understanding of cerebellocortical interrelations. The new neurophysiological and neuroimaging techniques together with more classical lesion studies are filling the gap providing exciting new evidence about the importance of the cerebellar loop for cortical processing. In this field, the study of implicit learning and sensory cerebellar processing is providing new elements that indicate how the cerebellum intervenes in controlling cortical plastic changes.
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