The Primary Hippocampal Circuit: New Lessons about an Old Network

The hippocampus, a brain region intimately associated with memory, has been the subject of intense research interest for more than half a century. These efforts have produced a number of theories that postulate links between hippocampal components and specific aspects of memory processing. However, progress in the field has been hindered by a surprising lack of information about the propagation of signals from the cortex across the full-length hippocampal circuit. Recent experimental work addressed this problem and found that activation of an input from the cortex elicits an entirely unexpected response by the CA1 output station. Subsequent work identified the pathways and local operations responsible for the pertinent signal transformations. Stimulation of the input with behaviorally relevant frequencies uncovered filtering, amplification, and cycling operations executed by the various links and nodes of the circuit. Collectively, the results indicate that the system switches among different operating modes-as such, we propose that these modes are associated with distinctly different spiking patterns by CA3 pyramidal cells. In any case, circuit analysis suggests that the hippocampus makes multiple, diverse contributions to physiology and behavior via dynamic states initiated by ever-changing patterns of input from the cortex and lower brain.