A BIOLOGICALLY-BASED MODEL OF FUNCTIONAL-PROPERTIES OF THE HIPPOCAMPUS

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 app roach is outlined for realizing a biologically based representation of the hippocampal formation. The approach involves developing two model s, one a ''nonparametric'' model in which the subsystems, principal ne urons, and subcellular processes of the principal neurons are characte rized experimentally using random impulse train stimulation. Non-linea rities in the input/output relation are represented as the kernels of a functional power series. Using multidimensional z-transforms, a proc edure is demonstrated for deriving kernel functions for interneurons t hat are not directly observable. A scheme is proposed for developing a n ''external'' model of the hippocampus, in which the system is repres ented 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 proces ses are included as the parameters of coupled field equations describi ng the dynamics at a different hierarchical levels of nervous system f unction. 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 (eg., connectivity patterns, synaptic, and cell densities) of the system. I t is proposed that the two models be used in a complementary manner to achieve an understanding of the neurobiological basis of the system d ynamics, and thus the mnemonic function, of the hippocampus.