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Abstract:
Electrical stimulation, combined with functional magnetic resonance imaging (es-fMRI), is proving to be
an important tool to study the functional properties of spatially distributed neuronal networks of the brain.
Here, we want to understand how information is propagated between the two major cortices of the primate
brain, the neocortex and the cerebellar cortex. We therefore electrically stimulated the deep cerebellar
nuclei of rhesus monkeys. So far we have electrically stimulated 19 different sites in different parts of the
deep cerebellar nuclei. Electrical stimulation of the DCN leads to reliable transsynaptic responses in the
neocortex. Surprisingly, the BOLD responses can be observed in multiple neocortical sites extending
beyond classical cerebellar targets (such as primary motor cortex) and also extending to the hemisphere
ipsilateral to the stimulation site. An analysis of the BOLD amplitude in cortical and subcortical structures
indicated that the bilateral spread of activity is already present at subcortical levels, i.e. the thalamus.
Currently we cannot exclude the possibility that we stimulated fibres of passage that then activated the
contralateral DCN and hence contributed to the bilateral neocortical activation patterns. However, the
observation of wide-spread BOLD responses in thalamic regions outside the known thalamic termination
sites of the DCN indicates that the DCN are able to drive brainstem circuits effectively that then reach
neocortex through several thalamic nuclei. These results indicate that, apart from the direct DCN -thalamic
projection, indirect routes exist by which the cerebellum can mediate information to the neocortex that may
be equally important and effective despite requiring the additional passage through synapses in met- and
mesencephalic structures.
