ON CLIMBING FIBER SIGNALS AND THEIR CONSEQUENCE(S)

The persistence of many contrasting notions of climbing fiber function after years of investigation testifies that the issue of climbing fib er contributions to cerebellar transactions is still unresolved. The p roposed capabilities of the climbing fibers cover an impressive spectr um. For many researchers, the climbing fibers signal errors in motor p erformance, either in the conventional manner of frequency modulation or as a single announcement of an ''unexpected event.'' More controver sial is the effect of these signals on the simple spike modulation of Purkinje cells. In some hands, they lead to a long-term depression of the strength of parallel fiber synapses, while, in other hands, they l ead to a short-lasting enhancement of the responsiveness of Purkinje c ells to mossy fiber inputs or contribute to the often-seen reciprocal relation between complex and simple spike modulation. For still other investigators, the climbing fibers serve internal timing functions thr ough their capacity for synchronous and rhythmic firing. The above vie wpoints are presented in the spirit of trying to reach some consensus about climbing fiber function. Each point of view is introduced by sum marizing first the key observations made by the respective proponents; then the issues of short-lasting enhancement, reciprocity between com plex and simple spikes, and synchrony and rhythmicity are addressed in the context of the visual climbing fiber system of the vestibulocereb ellum.