Intact CA3 in the hippocampus is only sufficient for contextual behavior based on well-learned and unaltered visual background.
Computational models suggest that the dentate gyrus and CA3 subfields of the hippocampus are responsible for discrete memory representations using pattern separation and pattern completion when a modified external stimulus is recognized as an old memory or encoded as a new memory. Experimental evidence of such computational processes in the hippocampus has been obtained mostly from spatial navigational tasks, and little is known about the proposed computational functions of the hippocampal subfields in "nonspatial" memory tasks. We tested whether rats with major damage in the dentate gyrus induced by colchicine lesions could remember patterned visual scene stimuli presented on LCD screens in the background. Rats responded using a touchscreen to indicate the identity of the visual scene. Performance of the lesion group was normal when tested with familiar visual scenes that had been learned prior to surgery. Lesioned rats exhibited severe deficits in learning novel visual scenes, but eventually reached the same level of performance as controls. However, unlike in controls, novel scene-associated memories formed in the lesion group were unstable and easily disrupted when ambiguous versions of the novel scenes were presented intermixed with the original stimuli. Our findings confirm that the prior computational models can also be applied to the nonspatial memory domain and suggest that the dentate gyrus is not necessary for the retrieval of learned visual scene-associated behavioral responses but plays a crucial role in forming novel visual scene-dependent memory and recognizing altered or ambiguous visual scenes in the background.