SUMMARY

In several forms in the mammalian brain, long-term potentiation (LTP) displays certain neurophysiologic characteristics that are commonly regarded as evidence that this mechanism for neuronal plasticity subserves the induction and/or storage of associative memories. In particular, the associative nature of LTP (induction by conjoint pre- and postsynaptic activity) and its dependence on relatively short interstimulus intervals between pre- and postsynaptic activity suggest certain qualitative similarities between LTP induction and the processes that define associative learning. Despite the popular acceptance of this cursory evidence, the processes that govern the formation and expression of associative memories are far more malleable and heterogeneous than those suggested by such mechanistic analogies. Here we describe in some detail the fundamental characteristics of the associative learning process, with an emphasis on those characteristics that are relevant to the assertion that LTP is an appropriate device for the storage of associative memories. With this as our point of reference, we are left to conclude that the mechanistic properties of LTP are in some instances irrelevant, and in some cases contraindicate, a role for LTP in the induction or storage of associative memories.

Introduction

Associative learning, an example of which is classical conditioning, is often described as a “simple” form of learning, limited in application to the modification of reflexive behaviors. This colloquial description is an unfortunate characterization of the phenomenon that misrepresents the fundamental nature and range of influence of associative learning. The insights that could be derived from a complete appreciation of the associative learning process were obvious to Ivan Pavlov, the Nobel Prize winner in 1904 for his studies on the physiology of digestion.