Narrow Results

Networks of neurons in the nervous system can produce a variety of temporal patterns of different frequency; the same network can produce different rhythms at different times, or several rhythms at one time. We focus here on the gamma (30–90 Hz) and theta (4–12 Hz) rhythms produced in the hippocampus, a part of the nervous system critical for learning and recall. We discuss experiments and models that suggest that there are separate subnetworks that produce the different rhythms; the sharing of components of the networks induces competition between the rhythms, which can lead to suppression of one of the rhythms, or nesting of the rhythms. We show how low-dimensional maps can help to understand the properties of the cells and networks to allow this to happen.

Our best conception for the representation of learned information is the cell assembly, a self-exciting configuration of nerve cells selected from the larger network of the cerebral cortex. Despite its advantages, there is a difficulty that such assemblies may not be able to operate adequately because of insufficient connections. In the first part of this chapter, an attempt is made to define this limitation on cell assemblies, using probability calculations. The difficulty is most severe for large scale integration of cortical cell assemblies, and at the stage of formation of cell assemblies, rather than in local integration, and for the routine operation of well-established assemblies. It has previously been suggested that circuits of neural activity between cortex and hippocampus, resonating at the frequency of the theta rhythm, are important in reducing these difficulties in cell assembly function. This theory is briefly summarized. The second half of the chapter links these ideas about cell assemblies to the various concepts of the psychological impairment which follows damage to the hippocampus and related structures. These concepts include place representation, context representation, working memory, cross-temporal associations, configural representation, conditional responding and declarative memory. Despite the wide range of these various concepts, they can all be regarded as instances when the integration of activity in wide areas of cerebral cortex, and/or its standardization and stabilization over time are required. The prediction is made that psychological functions vulnerable to hippocampal damage should also be vulnerable to cortical damage in many cortical fields rather than single areas.