A cell assembly theory of hippocampal amnesia.

Recent memories are more susceptible to amnesic loss than older memories, the time scale being much longer than can reasonably be explained by a passive chemical or morphological change. A possible explanation is that memories are initially sustained by "soft", easily produced but ephemoral, synaptic changes to which are later added "hard" changes that are more durable but require repeated synaptic activity over a long period to become established. "Soft" synapses are assumed to be concentrated in parts of the limbic system, "hard" synapses in the neocortex. The theory can also explain why objects encountered by patients with anterograde amnesia never become familiar to them.