Thus far, we have concentrated on collective neuronal behaviors in which the only interactions have been mediated by chemical synapses. Another type of neuronal interaction is clearly of importance during hyperexcitable states and may exert at least subtle influences even under normal conditions. Influences between neurons may be mediated by the flow of transmembrane current through the extracellular medium: field effects (Dudek and Traub, 1989). Field effects provide a means for the synchronization of action potentials in different neurons on a time scale of about 1 ms. Slow or DC extracellular fields can also bias a large population of neurons, rendering them all more excitable. The general subject of electrical field effects in the brain has been reviewed by Faber and Korn (1989). We shall restrict our attention here to the hippocampus.

There are several structural features and electrophysiological observations suggesting that field effects might be important in the hippocampus. First, there is the large amplitude of extracellular population spikes that can be obtained after synchronized stimulation of an afferent pathway. Amplitudes of 5–10 mV or more are not unusual in the stratum pyramidale during epileptiform activity in vitro, and population spikes of 20 mV and more in vivo are not unusual (Somjen et al., 1985). These extracellular negative potentials would be expected to produce transmembrane positivities.