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The hippocampus is involved in encoding and integrating contextual information. Recently, it has been suggested that the dorsal dentate gyrus (dDG) hippocampal subregion may mediate the formation of contextual representations of the spatial environment through a conjunctive encoding process whereby incoming multimodal information is integrated into a single higher-order representation. Despite anatomical evidence in support of this claim, behavioral evidence is limited. Therefore, a contextual associative learning paradigm was used to determine whether the dDG supports the formation of integrated contextual representations. Male Long-Evans rats were randomly assigned as controls or to receive bilateral intracranial infusions of colchicine into the dDG. Following recovery from surgery, each rat was tested on an appetitive task that required animals to form an association between a cue (odor) and a context to receive a food reward. Each rat received 10 trials per day and was tested for 10 consecutive days. Upon completion of testing, animals were tested on an additional two-choice olfactory and contextual discrimination task. The testing order was counterbalanced across animals. Results showed that control animals successfully acquired the contextual associative learning task for olfactory stimuli as indicated by improved performance across the 10 testing days. In contrast, animals with dDG lesions were impaired in the ability to acquire the odor-context associations. Results from follow-up odor and context discrimination tests indicated that both groups acquired the discriminations at similar rates. Therefore, it is unlikely that deficits in performance on the contextual associative learning task were due to an inability to discriminate between odors or contexts. The present findings provide further support for DG involvement in the formation of conjunctive contextual representations.

Dorsoventral lesion studies of the hippocampus have indicated that the dorsal axis of the hippocampus is important for spatial processing and the ventral axis of the hippocampus is important for olfactory learning and memory and anxiety. There is some evidence to suggest that the ventral CA3 and ventral CA1 conduct parallel processes for pattern completion and temporal processing, respectively. Studies have indicated that the dorsal dentate gyrus (DG) is importantly involved in processes reflecting underlying pattern separation activity for spatial information. However, the ventral DG is less understood. The current study investigated the less-understood role of the ventral DG in olfactory pattern separation. A series of odor stimuli that varied on only one level, number of carbon chains (methyl groups), was used in a matching-to-sample paradigm in order to investigate ventral DG involvement in working memory for similar and less similar odors. Rats with ventral DG lesions were impaired at delays of 60 sec, but not at delays of 15 sec. A memory-based pattern separation effect was observed performance was poorest with only one carbon chain separation between trial odors and was highest for trials with four separations. The present study indicates that the ventral DG plays an important role in olfactory learning and memory processes for highly similar odors. The results also indicate a role for the ventral DG in pattern separation for odor information, which may have further implications for parallel processing across the dorsoventral axis for the DG in spatial (dorsal) and olfactory (ventral) pattern separation.

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