The hippocampus, an area in the temporal lobe of the mammalian brain, participates in the storage of personal memories and life events, including traumatic memories and the consequent symptoms of post-traumatic stress, giving importance to the study of the machinery of hippocampal memory storage and retrieval. The circuit is known to be controlled by the neuromodulator Acetylcholine, which switches the circuit between the memory storage state and the memory retrieval state.

We built a computational model of the hippocampus with the ability to perform both memory storage and retrieval functions, controlled by the level of Acetylcholine. This functional separation decrease interference between the two circuit functions while sharing the same physical implementation of a network of spiking neurons.

We discovered three important differences between the storage and retrieval circuits. First, they had difference in how they produced runaway excitation, an aberrant spread of brain activity leading to seizures. Second, the two circuits had distinct mechanisms to maintain control over runaway excitation spread. These two findings provided the first classification of seizures based on the functional state of the brain, and suggested the need for specific treatments for each type.

Third, we found the two circuits also had unique ways of generating theta rhythmic activity, which is theorized to have a fundamental role in memory storage and retrieval. Our model uncovered an unexpected complexity in theta rhythm generation across functional states of the circuit. These findings can allow for deciphering the computations carried out by the circuit, based on the engaged mechanisms of rhythm generation.