A biologically based model of functional properties of the hippocampus

Abstract

The hippocampus is a brain structure essential for learning and memory processes, although its precise role has yet to be determined despite intensive experimental study. A combined experimental/theoretical approach is outlined for realizing a biologically based representation of the hippocampal formation. The approach involves developing two models, one a “nonparametric” model in which the subsystems, principal neurons, and subcellular processes of the principal neurons are characterized experimentally using random impulse train stimulation. Nonlinearities in the input/output relation are represented as the kernels of a functional power series. Using multidimensional z-transforms, a procedure is demonstrated for deriving kernel functions for interneurons that are not directly observable. A scheme is proposed for developing an “external” model of the hippocampus, in which the system is represented as the composite of the input/output functions of its intrinsic elements. The second model is an “internal” model, derived from an n-level field theory, in which specific cellular and subcellular processes are included as the parameters of coupled field equations describing the dynamics at a different hierarchical levels of nervous system function. The current model consists of two field equations for each of the synaptic and neuronal levels, respectively; included in each are geometrical relations to incorporate anatomical characteristics (e.g., connectivity patterns, synaptic, and cell densities) of the system. It is proposed that the two models be used in a complementary manner to achieve an understanding of the neurobiological basis of the system dynamics, and thus the mnemonic function, of the hippocampus.