Physiology

The classical definition of the cerebellar function is that of fine-tuning for muscle control. This view derived mainly from the observation that in humans and in experimental animals, cerebellar damage impairs posture and fine coordination of movements. This was supported by a variety of experimental and clinical data. Nevertheless, there is still debate regarding which physiological events support the cerebellar function as motor controller. In general, it is believed that cerebellar control is involved with motor adaptation and motor learning more for compound limb movements than for simple movements.

Figure 9 Mediolateral compartment boundaries of the vertebrate cerebellum according to a model proposed by Karl Herrup and Barbara Kuemerle. Six different compartments are located on either side of the cerebellar midline. Shaded boxes denote intensity of gene and antibody expression. Compartmental boundaries are represented by vertical solid lines and vermis-hemisphere junctions are represented by arrows and dashed vertical lines. The top half of the figure represents markers that are expressed transiently during development; the bottom half represents localization of stable markers in the adult cerebellum (from Herrup and Kuemerle, 1997. Reprinted, with permission, from the Annual Review of Neuroscience, Vol. 20. © 1997 by Annual Reviews. www.AnnualReviews.org).

Figure 9 Mediolateral compartment boundaries of the vertebrate cerebellum according to a model proposed by Karl Herrup and Barbara Kuemerle. Six different compartments are located on either side of the cerebellar midline. Shaded boxes denote intensity of gene and antibody expression. Compartmental boundaries are represented by vertical solid lines and vermis-hemisphere junctions are represented by arrows and dashed vertical lines. The top half of the figure represents markers that are expressed transiently during development; the bottom half represents localization of stable markers in the adult cerebellum (from Herrup and Kuemerle, 1997. Reprinted, with permission, from the Annual Review of Neuroscience, Vol. 20. © 1997 by Annual Reviews. www.AnnualReviews.org).

For a long time, the most widely accepted theory on how the cerebellum acts was that proposed by David Marr and James Albus and further developed by Masao Ito. According to the Marr-Albus-Ito theory, the main focus of cerebellar activity is acquisition and control of skillful movements through the interactions between climbing fiber (CF) and mossy fiber (MF) inputs in the Purkinje cells (PCs). In resting conditions, PC activity is mainly under the control of the MF-granule cell -parallel fiber (PF) system, whereas the CF system modifies the pattern of PC activation by influencing the strength of the PF/PC synapses. In particular, the convergence of PF and CF activation over the same PC is considered to be capable of inducing a long-term depression (LTD) of the efficacy of PF/PC synapses. Thus, the correct setting for a given movement is coded in the PC-deep nuclei output system by the PF fibers; this pattern is tuned by the so-called ''error signal'' conveyed through the CF afferents. Although a great deal of experimental evidence can be interpreted according to the Marr-Albus-Ito theory, an increasing body of data challenge the role of LTD in motor learning and the importance of the cerebellum as a motor memory storage site. Most of the evidence that indicates a primary role of the cerebellum in motor learning derives from experiments examining the substrate responsible for the acquisition of the nictitating membrane reflex in the rabbit or from those responsible for the vestibuloocular reflex (VOR). From these experimental approaches and from evidence derived from patients with cerebellar damage, two major hypotheses about the role of the cerebellum in motor learning have been drawn. First, the cerebellar role in learning is mainly related to participation in the execution of motor behavior (the performance hypothesis). Second, the main cerebellar role is that it acts as a storage site for the motor engrams of different motor tasks (the storage hypothesis). Despite the efforts of many different laboratories, the conflict between the storage and performance hypotheses has not been resolved and a comprehensive theory of the cerebellar role in motor control is far from established.

A different approach to the problem of the cerebellar function focuses on the importance of the cerebellum as a sensory acquisition device rather than a center for motor control and/or learning. This line of thought stems from the anatomical and physiological evidence of massive sensory information that is conveyed to the cerebellum. Recently, it has received important support from functional neuroimaging and experimental data. Within this line is the data acquisition hypothesis, suggested by James Bower and his group. It states that the cerebellum is specifically involved in monitoring and adjusting the acquisition of most of the sensory data on which the nervous system depends. According to this theoretical framework, motor deficits seen after lesion of the cerebellar circuits depend more on the disruption of the inflow of sensory information on which the motor system depends than on a lack of cerebellar control over the motor centers. This approach is in line with the organization of the fractured sensory map described previously. It has been proposed that a key for interpreting the spatial relationships among the different sensory representations is their need to cooperate for better active sensory exploration. The highly expanded and detailed representation of the whisker in the lateral hemispheres of the rat cerebellum seems to support this proposal. The massive cerebellar output toward the motor center is interpreted as the need for direct on-line control of fine movements to optimize the acquisition of sensory information by adjusting the position among the different tactile surfaces and between them and the object being explored.

Marco Molinari and collaborators have also shown the importance of the cerebellum in the acquisition of the sensory information required for implicit learning of a visuomotor task in cerebellar patients. These patients were severely impaired in the acquisition of a task that required a finger-tapping response to the presentation of visual stimuli in a random or fixed sequence. The deficit was particularly due to the inability to recognize the recurrence of the fixed sequence. As shown in Fig. 10, the declarative knowledge of the presented sequence was very poor in all groups of cerebellar patients. On the other hand, a

Figure 10 (A) Percentage of sequence items reproduced after a serial reaction time task based on an 8-digit sequence (Exp. 1), a 10-digit sequence (Exp. 2), and after visual presentation only (Exp. 3). (B) Reaction times (msec) after acquisition of a declarative knowledge of the sequence to be reproduced. Note that the presentation of random sequence (underlined blocks) induces an increase in the reaction times in all groups. CB right, group of patients with focal cerebellar lesion on the right side; CB left, group of patients with focal cerebellar lesion on the left side; controls, age- and education-matched control group. vertical bars, standard error (reproduced with permission from Molinari et al. (1997). Brain 120, 1753-1762. Reproduced with permission of Oxford University Press).

Figure 10 (A) Percentage of sequence items reproduced after a serial reaction time task based on an 8-digit sequence (Exp. 1), a 10-digit sequence (Exp. 2), and after visual presentation only (Exp. 3). (B) Reaction times (msec) after acquisition of a declarative knowledge of the sequence to be reproduced. Note that the presentation of random sequence (underlined blocks) induces an increase in the reaction times in all groups. CB right, group of patients with focal cerebellar lesion on the right side; CB left, group of patients with focal cerebellar lesion on the left side; controls, age- and education-matched control group. vertical bars, standard error (reproduced with permission from Molinari et al. (1997). Brain 120, 1753-1762. Reproduced with permission of Oxford University Press).

clear modulation of the response was observed when the subjects received instructions about the digit sequence that would be used in the test. In this condition, a clear improvement in reaction times was observed. This suggests that cerebellar patients are particularly impaired in detecting a sequence and that performance can be improved if knowledge of the sequence has been previously acquired.

The importance of the cerebellum in sensory analysis has also been reported in other forms of learning—that is, in visual perceptual learning by Lucia Vaina and coworkers or in observational learning by Maria G. Leggio and coworkers. In the former functional magnetic resonance imaging (fMRI) study, clear cerebellar activation linked to an early phase of learning a motion perception task was observed. In the latter study, it was reported that lesioning the cerebellum can impair the acquisition of spatial strategies through observation. Both cases clearly demonstrate the role of the cerebellum in processing the sensory information required by the cortical modules during learning.

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