A role for pre-stimulus theta activity in the generalization of salient stimuli
- Author(s): Harriger, Logan D;
- Advisor(s): Yassa, Michael A;
- et al.
The hippocampus’s role in memory was first recognized in 1957 after the bilateral resection of patient HM’s medial temporal lobes left him with profound amnesia; HM’s case study supported hypotheses that the brain possesses multiple memory systems, and suggested the hippocampus is critical for memories of experience. Subsequent lesion cases bolstered additional support, but initially, experimental progress was limited by a lack of technology that could ethically probe hippocampal function in humans, and in animals, was confounded by flawed experimental design and sidetracked by a competing theory about the hippocampus’s role in spatial navigation. Nevertheless, research on navigation became complementary, offering new experimental paradigms also suitable for probing hippocampal memory function and revealing many noteworthy features of hippocampal function like the theta rhythm. Today these research trajectories have begun to coalesce, and now prominent theories presume a more general role of the hippocampus as an index linking arbitrary cortical activation patterns; furthermore, these index representations are thought to relate to the ability of generalizing over or discriminating between similar stimuli via pattern separation/completion processes.Brain electrophysiology is composed of a geometric progression of distinct rhythms, and the theta rhythm (3-8 Hz) has been elegantly linked to navigation processes in the hippocampus of various mammalian species. Additionally, this activity has been associated with memory in mammals, including humans, albeit, primarily with non-invasive methods incapable of attributing this activity to the hippocampus; notably, several studies have also demonstrated that pre-stimulus theta activity is correlated with recognition memory strength. Nevertheless, in the last couple of decades, direct recordings of human hippocampi have been conducted in patients implanted with intracranial EEG (iEEG) prior to brain surgery, but these studies related to memory have provided conflicting results, casting some doubt on the function of hippocampal theta. In this dissertation, I analyze iEEG data collected from human epilepsy patients while engaged in a recollection memory task. I demonstrate that post-stimulus theta during encoding is indeed associated with successful recollection memory. While the first result runs counter to the several iEEG studies on memory, it is consistent with long-standing theories about theta’s role in hippocampal memory processes. Additionally, I provide evidence that pre-stimulus theta activity at encoding is associated with poor recollection memory in the presence of mnemonic interference. Although this result also seems to be in contradiction with all prior accounts of pre-stimulus theta’s effect on memory, it might be reconciled by interpreting the role of pre-stimulus theta as facilitating generalization of memory, rather than memory in general.