A neuromagnetic view of hippocampal memory functions.
Bursts of highly synchronized discharges of 4-7Hz sinusoidal wave activity can be recorded from the hippocampus during rapid eye movement (REM) sleep. These rhythmic discharges, the hippocampal theta activity, are generated in the dentate granule cells and the pyramidal cell layers of the CA1 field of the hippocampus. The physiological function of the hippocampal theta activity is elusive. The occurrence of this rhythm throughout the REM sleep stage suggests that it is related to some fundamental neurophysiological phenomena associated with REM sleep, particularly consolidation of memory processes. Synchronous oscillations among a population of neurons are expected to yield stronger, more coherent associated magnetic fields which, through their influence back on the electrical fields via induction, would exert an independent effect on the electrical activity of hippocampal neurons and additionally, could foster and reinforce these oscillations through self-induction. The snail-shaped structure of the hippocampal formation, which resembles a solenoid embedded in the temporal lobe, would be expected to amplify these magnetic fields. Additionally, the discovery of large ferromagnetic particles in the human hippocampus suggests that it may function as a large iron-core electromagnet. It is proposed that memory traces may be encoded or decoded magnetically and analogous to a videotape, each encoding unit (i.e., synapse, set of synapses or glial cell) could be magnetized in one direction, or the other through the flow of a strong, AC magnetic field along the hippocampal formation. The encoding of memory traces in the hippocampal formation may ultimately reflect an electromagnetic phenomenon.