Dissociations in perceptual discrimination following selective damage to the dentate gyrus versus CA1 subfield of the hippocampus.

The hippocampus (HPC) is well-known for its involvement in declarative (consciously accessible) memory, but there is evidence that it may also play a role in complex perceptual discrimination. Separate research has demonstrated separable contributions of HPC subregions to component memory processes, with the dentate gyrus (DG) required for mnemonic discrimination of similar inputs and the CA1 subfield required for retention and retrieval, but contributions of these subregions to perceptual processes is understudied. The current study examined the nature and extent of a double dissociation between the dentate gyrus (DG) to discrimination processes and CA1 subfield to retention/retrieval by testing two unique individuals with bilateral damage to the DG (case BL) and CA1 (case BR). We tested BL and BR on a wide range of standardized neuropsychological tests to assess information encoding and retention/retrieval and co-opted many measures to assess perceptual discrimination. Compared to normative data, BL exhibited performance below expectations on most measures requiring perceptual discrimination and on measures of encoding but demonstrated intact retention. Conversely, BR showed no difficulties with perceptual discrimination or verbal encoding but exhibited poor verbal retention, as well as poor encoding and retention of spatial/integrative tasks (e.g., object in a location). These results indicate that, despite its prominent role in memory, the DG is necessary for perceptual discrimination and encoding, whereas CA1 is necessary for retention/retrieval and encoding of spatial information. The pattern of results highlights the critical nature of individual case studies in the nuanced understanding of HPC subfield contributions to different memory processes, as well as the utility of repurposing neuropsychological measures to capture individual differences.