A New Functional Role for Cerebellar Long Term Depression

the brain (Artola and Singer, 1993; Linden, 1994). In most cases, the opposite process, i.e. long-term potentiation (LTP) (Bliss and Collingridge, 1993), is present at the same synapses. Most theories on the functional role of LTD in such neurons assume that LTD acts by resetting synapses that have undergone LTP (Tsumoto, 1993; Linden, 1994). In the cerebellum, however, LTD of the parallel fiber to Purkinje cell synapse (Ito, 1989) is distinguished by the absence of LTP at this synapse (Artola and Singer, 1993). This form of LTD has been extensively investigated in the context of a specific theory of the function of the cerebellum, first proposed by Marr (Marr, 1969) and subsequently elaborated by Albus (Albus, 1971) and Ito (Ito, 1984). However, the conditions under which cerebellar LTD can be induced do not seem to be compatible with some predictions of the Marr-Albus-Ito theories (Schreurs and Alkon, 1993). I will first review and criticize the Marr-Albus-Ito theories and then describe the properties of Purkinje cells and of LTD induction that are relevant to a new hypothesis on the function of cerebellar LTD that I have proposed recently (De Schutter, 1995a).