Functional Significance of the Basic Cerebellar Circuit in Motor Coordination

Topographic Cortico-cerebellar Networks Revealed by Visual Attention and Working Memory

Substantial portions of the cerebellum appear to support non-motor functions; however, previous investigations of cerebellar involvement in cognition have revealed only a coarse degree of specificity. Although somatotopic maps have been observed within cerebellum, similar precision within cortico-cerebellar networks supporting non-motor functions has not previously been reported. Here, we find that human cerebellar lobule VIIb/VIIIa differentially codes key aspects of visuospatial cognition. Ipsilateral visuospatial representations were observed during both a visual working memory and an attentionally demanding visual receptive field-mapping fMRI task paradigm. Moreover, within lobule VIIb/VIIIa, we observed a functional dissociation between spatial coding and visual working memory processing. Visuospatial representations were found in the dorsomedial portion of lobule VIIb/VIIIa, and load-dependent visual working memory processing was shifted ventrolaterally. A similar functional gradient for spatial versus load processing was found in posterior parietal cortex. This cerebral cortical organization was well predicted by functional connectivity with spatial and load regions of cerebellar lobule VIIb/VIIIa. Collectively, our findings indicate that recruitment by visuospatial attentional functions within cerebellar lobule VIIb/VIIIa is highly specific. Furthermore, the topographic arrangement of these functions is mirrored in frontal and parietal cortex. These findings motivate a closer examination of cortico-cerebellar functional specialization across a broad range of cognitive domains.

The organization of cerebellar cortical circuitry revisited: implications for function

For more than 35 years there has been experimental evidence that parallel fiber activity does not generate the beams of activated Purkinje cells hypothesized on the basis of cortical anatomy and assumed by most theories of cerebellar cortical function. This paper first reviews the evidence for and against the parallel fiber beam hypothesis, and then discusses the findings of our recent experimental and model-based investigations intended to better understand parallel fiber effects on Purkinje cells. A principal conclusion of these studies is that the excitatory effects of parallel fibers on Purkinje cell dendrites are modulating and must be considered in the context of a balancing inhibitory influence provided by molecular layer interneurons to these same dendrites. It is proposed that this association of excitation and inhibition can account for the lack of beam-like effects on Purkinje cells. The paper concludes by considering the consequences of this new interpretation of cerebellar cortical circuitry for current theories of cerebellar function.

Quantitative analysis of converging spinal and cuneate mossy fibre afferent projections to the rat cerebellar anterior lobe

The convergence/divergence of mossy fibre afferent projections to the cerebellar anterior lobe from a single lumbar segment, from adjacent or widely separated lower thoracic and lumbar segments, and finally from the lower thoracic-upper lumbar spinal cord and the brainstem cuneate nuclei was quantitatively analysed in adult rats. Spinal and cuneate mossy fibre terminals were differentially labelled with biotinylated dextran amine and cholera toxin subunit B, immunohistochemically identified in the same histological sections, and their spatial distributions quantitatively plotted in computer reconstructions of the unfolded anterior lobe cortex. Afferent convergence was quantified by calculating the number of biotinylated dextran amine-labelled terminals that radially overlapped with cholera toxin-labelled terminals at points on the unfolded cortical map that represented theoretical Purkinje cells. Spino- and cuneocerebellar mossy fibre terminals are organized in patches that are oriented in parasagittally-oriented stripes or transversely oriented bands. Afferent convergence was greatest following biotinylated dextran amine and cholera toxin injections in the same or adjacent spinal lumbar segments (60 and 52%, respectively). When biotinylated dextran amine and cholera toxin were injected in a single segment differentially labelled terminals appeared randomly intermingled in common patches. There was a trend for terminals labelled from adjacent lumbar segments to be more segregated in the patches. Segmentally separated biotinylated dextran amine and cholera toxin spinal cord injections (four lumbar segments) resulted in clearly segregated (80%) biotinylated dextran amine from cholera toxin-labelled terminal patches or patches with distinct divergence of the differentially labelled terminals in the patch. Cuneocerebellar terminals labelled with biotinylated dextran amine were located in patches, stripes, and bands spatially segregated from terminal patches, stripes, and bands of cholera toxin-labelled spinal afferents except at their immediate borders where some radial overlap occurred (9-22%). These anatomical findings for a fractured somatotopy of spinal and cuneate inputs to the cerebellar anterior lobe complement neurophysiological findings for a very similar pattern of organization of cutaneous inputs to the posterior lobe, and are discussed in light of potential mechanisms for anterior lobe processing of somatosensory information.

Lateral cerebellar hemispheres actively support sensory acquisition and discrimination rather than motor control

This study examined a new hypothesis proposing that the lateral cerebellum is not activated by motor control per se, as widely assumed, but is engaged during the acquisition and discrimination of tactile sensory information. This proposal derives from neurobiological studies of these regions of the rat cerebellum. Magnetic resonance imaging of the lateral cerebellar output nucleus (dentate) of humans during passive and active sensory tasks confirmed four a priori implications of this hypothesis. Dentate nuclei responded to cutaneous stimuli, even when there were no accompanying overt finger movements. Finger movements not associated with tactile sensory discrimination produced no dentate activation. Sensory discrimination with the fingers induced an increase in dentate activation, with or without finger movements. Finally, dentate activity was greatest when there was the most opportunity to modulate the acquisition of the sensory tactile data: when the discrimination involved the active repositioning of tactile sensory surface of the fingers. Furthermore, activity in cerebellar cortex was strongly correlated with observed dentate activity. This distinct four-way pattern of effects strongly challenges other cerebellar theories. However, contrary to appearances, neither our hypothesis nor findings conflict with behavioral effects of cerebellar damage, neurophysiological data on animals performing motor tasks, or cerebellar contribution to nonmotor, perceptual, and cognitive tasks.