The cerebellum is partitioned into three functional units: the anterior lobe (lobules I to V) plus vermal lobule VI, the posterior lobe (lobules VIII to X), and the mediolateral lobe (lobules VII, Crus I and II) (Fig. 1, Tehovnik, Patel, Tolias et al. 2021): the anterior lobe plus vermal lobule VI mediates oculo- and skeletomotor functions (particularly saccadic eye movements), the posterior lobe mediates spatial functions associated with proprioception and balance including pursuit eye movements (which are utilized for the vestibulo-ocular reflex and optokinetic nystagmus), and the mediolateral lobe mediates object perceptions (visual, auditory, tactile, gustatory, olfactory) and language. These three functional units, respectively, innervate the output nuclei of the cerebellum: the fastigial nucleus (an oculo-skeletomotor nucleus), the interstitial nucleus (a proprioceptive-vestibular-allocentric nucleus), and the dentate nucleus (an object- language nucleus) (Fig. 2). Given that the object and language areas of neocortex are highly non-topographic (Ojemann 1991; Schiller and Tehovnik 2015; Wilson and Rennaker 2010), a similar non-topography would be expected for the mediolateral lobe of the cerebellum; indeed, no clear somatotopy has been described for the mediolateral lobe. The mossy-fibre input to the cerebellum is such that M1, S1, and the frontal eye fields send signals to the anterior lobe plus lobule VI, the retrosplenial, medial temporal, medial superior temporal, intraparietal, supplementary motor area (including the supplementary eye fields) send signals to the posterior lobe, and the temporal and orbital cortices send signals to the mediolateral lobe (Tehovnik, Patel, Tolias et al. 2021).
So, how are the three functional units of the cerebellum linked, as automated behavioral routines are executed, e.g., as commanded by a brief glance of the keypad before automatically punching in the number code? Thach et al. (1992) have suggested that as signals loop from the neocortex (the conscious organ) through the pons to the cerebellar cortex and return to the neocortex via the thalamus, the signals from the three units converge in the thalamus and area M1 before being re-distributed (to refresh the signal) throughout the neocortex (see Fig. 2). Accordingly, the thalamus-M1 links the three representations, the oculo-skeletomotor, the spatial, and the object information. Varela (1999ab) has suggested, based on binocular rivalry experiments, that the temporal duration of consciousness is about 500 ms. As a stream of consciousness is being looped through the neocortex and cerebellum, 500 ms would be sufficient to concatenate all the essential elements (oculo-skeletomotor, spatial, and object) to transmit a continuous flow of information, which in the case of language (or playing a musical instrument) is transmitted at a rate of 40 bits per seconds (Tehovnik and Chen 2015) (see Fig. 3).
Figure 1: In humans, the cerebellum is divided into three regions: the anterior lobe, the posterior lobe, and the mediolateral lobe. From figure 8 of Tehovnik, Patel, Tolias et al. (2021).
Figure 2: Projections of the cerebellar nuclei (fastigial, interstitial, and dentate) converge in the thalamus and area M1. From figure 1 of Thach et al. (1992).
Figure 3: Neocortical-cerebellar loops over time. A command is issued from the neocortex (cortex) which has access to the cerebellar cortex at the Purkinje cells (Purkinje) via the pons (Pons). The return portion of the loop passes through the cerebellar nuclei (Nuclei) and thalamus (Thal) en route to the neocortex. Based on the work of Varela (1999ab), the loop duration for the mediation of a fragment of consciousness is about 500 ms, but the information flow for watching a film, reading a book, delivering a speech, or playing a musical instrument is continuous. The loop configuration is based on the anatomical, unit recording, and optogenetic experiments of Hasanbegović (2024), all performed on the mouse and generalized to the primate (Thach et al. 1992).
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