A Role for Hippocampal Sharp-wave Ripples in Active Visual Search

Sharp-wave ripples (SWRs) in the hippocampus are thought to contribute to memory formation, though this effect has only been demonstrated in rodents. The SWR, a large deflection in the hippocampal LFP (local field potential), is known to occur primarily during slow wave sleep and during immobility and consummator behaviors. SWRs have widespread effects throughout the cortex, and are directly implicated in memory formation their occurrence correlates with correct performance, and their ablation impairs memory in spatial memory tasks. Though SWRs have been reported in primates, their role is poorly understood. Whether or not SWRs play a role in memory formation, as they do in rodents, has yet to be confirmed.
This work encompasses three separate studies with the goal of determining whether there is a link between SWR occurrence and memory formation in the macaque. Chapter 2 establishes the validity of the modified Change Blindness task as a memory task which is sensitive to normal hippocampal function in monkeys. Chapter 3 establishes that SWR events occur during waking (and stationary) activity, during visual search, in the macaque. Until this work, the prevalence of SWRs in macaques during waking exploration was unknown. Chapter 4 shows that gaze during SWRs was more likely to be near the target object on repeated than on novel presentations, even after accounting for overall differences in gaze location with scene repetition. The increase in ripple likelihood near remembered visual objects suggests a link between ripples and memory in primates; specifically, SWRs may reflect part of a mechanism supporting the guidance of search based on experience. The amalgamation of this work reveals several novel findings and establishes an important step towards understanding the role that SWRs play in memory formation in predominantly-visual primate brains.