The Hippocampus and the Sense of Smell

Abstract

Brodal, in a 1947 review from which we boldly borrow the title of the present paper, outlined some of the critical evidence that ultimately led to the demise of the notion that the hippocampus was a part of the olfactory brain, or “rhinencephalon” as it was called according to the prevailing view of the time. Since then it has become abundantly clear that the hippocampus processes information from many input sources (cf. Deacon et al., 1983). Nevertheless, converging data from neuroanatomical, physiological, and behavioral studies indicate that the olfactory system projects heavily onto and has especially immediate access to the hippocampal system, suggesting that the olfactory-hippocampal pathway may be particularly useful for explorations of sensory-limbic interactions leading to the higher order coding of perceptual information. As will be described below, the intimate anatomical associations between the olfactory and hippocampal systems are paralleled by 1) the critical role played by the hippocampal system in odor-guided learning and memory, 2) the strong influence of olfactory processing over the physiological activity in the hippocampus both at the level of rhythmic EEG activity and at the level of neuronal firing patterns, and 3) the role these physiological processes may play in the induction of synaptic plasticity supporting memory formation. Thus, in the spirit of a “renaissance of the rhinencephalon” (Macrides, 1977), we will argue that olfaction is a particularly advantageous model system for studies of “sensory” processing by the hippocampus across behavioral, neuronal, and synaptic levels of analysis. Our data on studies at each of these levels of analysis will be discussed in turn (see also Otto and Eichenbaum, 1992b).