Hippocampally-dependent learning and spatial representation in the subiculum

The hippocampal formation is a part of the brain that is essential for everyday learning and memory, including the ability to remember places and scenes. The mnemonic function of the hippocampal formation can be experimentally investigated in rats using behavioral paradigms that engage spatial learning and memory. Such behavioral paradigms have compelling neural correlates, as neurons in the hippocampal formation fire in response to spatial locations and environmental contexts. Here I present two significant original contributions to our understanding of the cognitive function of the hippocampal formation and the neurophysiology that underlies this function.The first part of this dissertation is a behavioral study of the effects of hippocampal lesions on learning of the W-maze continuous spatial alternation task. The W-maze task is a test of spatial working memory and rule-learning. Neurons in the hippocampal formation exhibit task-relevant activity during performance of this task. However, previous to this study, it was not known whether learning of the W-maze task really depends on the hippocampal formation. I found that rats with excitotoxic lesions of the hippocampal formation made unusual perseverative errors and were significantly slower to learn the W-maze task than sham-operated controls. This finding suggests that the hippocampal formation contributes to rapid learning of spatial trajectories that lead to reward. The second part of this dissertation is a single-unit recording study of the subiculum. The subiculum is a region within the hippocampal formation that has received little previous investigation, even though it is a major output structure through which information from the hippocampal formation reaches the rest of the brain. I recorded spikes and local field potentials in the subiculum while rats ran in two environments. I found that neurons in the subiculum provide a highly informative representation of the animal's spatial location and environmental context, and that the sparseness of this spatial representation exhibits a gradient along the proximal-distal anatomical axis. Additionally, I discovered that neurons in the subiculum exhibit theta phase precession, an oscillatory phase coding phenomenon that is thought to be important for coordinating information transfer and spike timing-dependent plasticity