Hippocampal Subfield Contributions to Discrimination in Perception and Memory: Insights from Human Case Studies and Rats

The hippocampus (HPC) is known to be necessary for memory, but it is not a unitary structure and its individual subregions exhibit preferential functions. Specifically, area CA1 has shown involvement in memory integration/retention/retrieval. Conversely, the dentate gyrus (DG) has been implicated in information encoding and pattern separation, or the orthogonal representation of similar items in the service of distinct memory formation and subsequent discrimination. As the DG is proposed to engage in pattern separation during encoding, it is possible that it engages in discriminatory processes within perception as well as memory. Here, I investigated the involvement of the DG and CA1 in perceptual discrimination across various stimuli, modalities, and levels of difficulty. Chapter 2 describes a study assessing performance in the unique DG-lesioned case study BL on an object oddity task. In the context of intact general perception of the objects, BL was significantly worse at determining the odd object amongst three identical distractors, compared to matched controls. Moreover, his atypical eye fixation pattern was possibly suggestive of an increased effort to compare objects. Chapter 3 presents an in-depth neuropsychological evaluation of BL and a CA1-lesioned case BR. Here, several measures were co-opted to assess perceptual discrimination. BL exhibited consistent weakness on difficult perceptual discrimination tasks across stimulus types, whereas BR exhibited intact performance. BL additionally demonstrated poor encoding but intact retention, while BR exhibited a generally opposite pattern; these patterns of performance were also evident on a mnemonic discrimination task. Chapter 4 details novel visual-, spatial-, and tactile- oddity tasks adapted for rats, with three degrees of difficulty. DG-compromised rats were significantly worse than controls at intermediate levels of difficulty across all tasks, with spared performance on the easiest versions. Contrary to expectations, CA1-compromised rats exhibited enhanced performance on the most difficult versions of all three tasks. Collectively, these results provide the first demonstration that the DG is necessary for fine-grained perceptual discrimination in a domain-general manner. Furthermore, the DG and CA1 appear specialized for encoding and retention/retrieval, respectively, and might even have an inhibitory relationship under some circumstances.