ReviewLTP and LTD : An Embarrassment of Riches

control their firing properties, neurons can vary in terms of the specific forms of LTP and LTD they express. Palo Alto, California 94304 2 Howard Hughes Medical Institute and the We would also argue that it is no longer particularly productive to debate the generic question of whether Picower Center for Learning and Memory Massachusetts Institute of Technology LTP and LTD are cellular/synaptic mechanisms for memory. LTP and LTD are experimental phenomena, Cambridge, Massachusetts 02139 which can be used to demonstrate the repertoire of long-lasting modifications of which individual synapses are capable. It is a daunting task to demonstrate that LTP and LTD, the long-term potentiation and depression of excitatory synaptic transmission, are wide-identical synaptic modifications due to the same mechanisms underlying some form of LTP or LTD occur in vivo spread phenomena expressed at possibly every excit-atory synapse in the mammalian brain. It is now clear in response to experience. It is even more difficult to prove that these LTP or LTD-like modifications subserve that " LTP " and " LTD " are not unitary phenomena. Their mechanisms vary depending on the synapses and cir-essential functional roles. Nevertheless, given the ubiq-uity of various forms of LTP and LTD at excitatory syn-cuits in which they operate. Here we review those forms of LTP and LTD for which mechanisms have apses throughout the brain and the clear computational advantages they afford, it seems virtually certain that been most firmly established. Examples are provided that show how these mechanisms can contribute to the brain takes advantage of the neuronal capability to express long-lasting activity-dependent synaptic modi-experience-dependent modifications of brain function. fications as at least one of the key mechanisms by which experiences modify neural circuit behavior. Exactly a decade ago, we wrote a review on NMDA Thus, questions about the functional roles of " LTP " receptor (NMDAR)-dependent LTP and LTD in the hip-or " LTD " must be placed into very specific contexts. It pocampus (Bear and Malenka, 1994)—a review that up-first must be determined which synapses and circuits dated the reader on the latest findings on the mecha-mediate a specific type of experience-dependent plas-nisms of these two forms of synaptic plasticity. At the ticity. Then the types of LTP and LTD that can occur at time, controversy was swirling around the issue of those synapses must be defined. Finally, largely through whether LTP was primarily due to pre-or postsynaptic correlational …