Biology subjects

Groppe, D.

Publications and source records attributed to Groppe, D..

3 recordsLinked to original sources

bioRxiv2024

Precise Timing Matters: Modulating Human Memory by Synchronizing Hippocampal Stimulation to Saccadic Event Related Potentials

Episodic memory, the ability to record and relive experiences, is intricately connected to visual exploration in most humans. This study explores the possibility that eye movements create physiological states relevant for memory, analogous to those associated with hippocampal theta. Previous work has demonstrated that saccadic eye movements, which occur roughly at theta frequency, elicit hippocampal event-related potentials (ERPs). Building on the Separate Phases of Encoding and Retrieval (SPEAR) model, we asked if the peaks and troughs of this saccadic ERP are differentially important for memory formation. Specifically, we applied saccade-contingent hippocampal electrical stimulation at estimated ERP peaks and troughs while individuals with epilepsy visually explored natural scenes across 59 sessions. We subsequently assessed their recognition memory for scenes and their recall of associated targets. Results indicate that memory is robust when stimulation precisely targets the peak or trough, contrasting with impairments observed with random stimulation. Moreover, memory impairment is prominent when stimulation is applied within 100 ms of saccade initiation, a time that reflects high medial temporal lobe inhibition. Our findings suggest that the hippocampus rapidly evolves through memory-relevant states following each eye movement, while also challenging the assumption that human saccadic ERP peaks and troughs mirror the encoding and retrieval phases of theta rhythms studied in rodents. The study sheds light on the dynamic interplay between eye movements, hippocampal activity, and memory formation, offering theoretical insights with potential applications for memory modulation in neurological disorders. Significance StatementWhy do eye-movements enhance memory formation? Here, we causally tested if eye-movements initiate short-lived states critical for memory formation within the hippocampus, a brain region known to support memory. We built a system that could precisely apply hippocampal electrical stimulation at key moments after eye-movements to test how the timing of this stimulation influenced peoples ability to form memories. We found that stimulation was particularly disruptive to memory formation when applied within 100 ms of initiating an eye movement. By contrast, memory was robust to stimulation precisely timed to the peak and trough of hippocampal eye-movement responses. We interpret this temporal evolution of memory-relevant states within a prominent model of how theta phases relate to rodent memory.

neuroscience↗

bioRxiv2018

Differential generation of saccade, fixation and image onset event-related potentials in the human mesial temporal lobe

The electrophysiological signatures of encoding and retrieval recorded from mesial temporal lobe (MTL) structures are observed as event related potentials (ERPs) during visual memory tasks. The waveforms of the ERPs associated with the onset of visual stimuli (image-onset) and eye movements (saccades and fixations) provide insights into the mechanisms of their generation. We hypothesized that since eye movements and image-onset (common methods of stimulus presentation when testing memory) provide MTL structures with salient visual information, that perhaps they both engage similar neural mechanisms. To explore this question, we used intracranial electroencephalographic (iEEG) data from the MTLs of 11 patients with medically refractory epilepsy who participated in a visual search task. We sought to characterize electrophysiological responses of MTL structures to saccades, fixations and image onset. We demonstrate that the image-onset response is an evoked/additive response with a low-frequency power increase and post-stimulus phase clustering. In contrast, ERPs following eye movements appeared to arise from phase resetting of higher frequencies than the image onset ERP. Intriguingly, this reset was associated with saccade onset and not saccade termination (fixation), suggesting it is likely the MTL response to a corollary discharge, rather than a response to visual stimulation - in stark contrast to the image onset response. The distinct mechanistic underpinnings of these two ERP may help guide future development of visual memory tasks.

neuroscience↗

bioRxiv2017

Repeated performance in problem-solving tasks attenuates human cortical responses

A ubiquitous characteristic of human cortical networks is their tendency to rapidly change their response properties upon repetition. While this phenomenon has been amply documented using simple sensory-motor tasks, it is still unclear to what extent brain activations change on a short time scale when we are engaged in high level, complex tasks. Here, we examined this question using three types of high-level visual problems. We analyzed data from intracranial recordings performed in eight patients, focusing on the location and type of changes and on their relationship to overt behavior. Our results show significant repetition effects, manifested as signal decrease with repetition, in three different groups of electrodes: Visual sites, which increased their activity during stimuli presentation; Processing Positive sites, which demonstrated increased activity throughout the experimental trial; and Processing Negative sites, which demonstrated suppression of activity during the trial as compared to baseline. Interestingly, despite these significant repetition effects, response time remained unchanged with repetition. These findings bear directly upon our ability to interpret results aggregated across multiple repetitions of the same complex task.

neuroscience↗