Recollection is impaired, but familiarity remains intact in rats with lesions of the fornix

Abstract

It has been argued that a neural system including the hippocampus, fornix, mamillary bodies, and anterior thalamus is specifically involved in recollection, but not in familiarity based memory processes. Here we test this hypothesis using a task of episodic-like memory within an E-shaped maze. Animals seek out a preferred object (what) in a particular location (where) that is unique to a particular context (which occasion). As objects are hidden from view at the point of decision making, the animals can only base their decision on recall of their previous episode in the E-shaped maze. In contrast, once a decision has been made animals are free to explore both objects and display an object preference when objects are visible to the animal and decisions can be made on the basis of familiarity. Animals with fornix lesions are impaired at recalling a past event. However, the same animals on the same trials show no such impairment in a judgement of familiarity. We therefore demonstrate that recall is dependent upon the fornix, while familiarity based memory is not.