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Abstract

Imaging brain activity evoked by intracortical electrical stimulation with fMRI is proving to be a useful tool to study functional characteristics of the brains connectivity in vivo. Here we stimulated the cerebellar cortex with microelectrodes in the anaesthetized rhesus monkey. BOLD responses in the cerebellar cortex were easily evoked with currents of 250 µA (pulse duration: 200µs; frequency: 100Hz). The spatial spread of the BOLD response after stimulation in the anterior lobe (intermediate zone of Lobule IV and V) was large and extended well to the contralateral cerebellar side. Based on the well known connectivity of the cerebellum the spread of such a bilateral activation can be explained through an antidromic excitation of mossy fibres, since these are the only excitatory fibres that can extend bilaterally in the cerebellum. Mossy fibres originating from the lateral reticular nucleus (LRN), for instance have a substantial bilateral contribution (Pijpers et al., 2006) and project heavily to the vermal and intermediate zone of the lobus anterior. So far our stimulation of the posterior lobe (Crus II) on the other hand yielded largely ipsilateral cerebellar activation. This cerebellar region receives its mossy fibres mainly from the pontine nuclei which have a strong contralateral and a weak ipsilateral contribution, indicating a lesser degree of bilaterality.
Cerebellar stimulation yielded a relatively larger spatial spread of BOLD responses than what we have previously observed after cerebral cortical extrastriate and striate stimulation. This is in contradiction to predictions based on the facts that cerebral cortex intraconnectivity is much more extended than the cerebellar short-range intracortical connections. Our observations indicate that this is either due to a larger bifurcation pattern of the cerebellar mossy fibres, or/and to lower thresholds for the activation of mossy fibres and for triggering metabolic changes. In summary, microstimulation-evoked BOLD responses of the cerebellar cortex reveals different patterns of connectivity within the cerebellum and points to some important functional characteristics of these connections.