The emerging concept of down-bound and up-bound modules in the learning cerebellum

The olivo-cerebellar system is organized into so-called three-element modules. These modules are characterized by sagittal zones of Purkinje cells that project onto specific parts of the cerebellar and/or vestibular nuclei, which in turn inhibit the subnucleus of the inferior olive that provides the climbing fibers to the connected sagittal zone involved. Yet, despite the generic character of this topographical neuro-anatomical organization, recent studies indicate that the modules can be functionally segregated into two main groups; those with Purkinje cells that fire at a relatively low baseline firing frequency of their simple spikes and tend to increase this frequency during learning (referred to as up-bound modules), and those with Purkinje cells that fire at a relatively high baseline frequency and tend to decrease this frequency during learning (referred to as down-bound modules). To date, this concept has now been shown for learning paradigms like adaptation of the vestibulo-ocular reflex, modification of whisker reflexes, and eyeblink conditioning. In addition, evidence is emerging that these rules can be extrapolated to acquiring cognitive functions like decision making and navigation control. In my discussion I will highlight that for each of these forms of learning one can predict whether an increase or decrease of simple spike activity is required based on the downstream connectivity.

 
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