Modulation of Fixation-Locked Hippocampal iEEG Activity by Visual Content

Poster Session C - Sunday, March 30, 2025, 5:00 – 7:00 pm EDT, Back Bay Ballroom/Republic Ballroom

Arantzazu San Agustín¹ (asanagustin@uchicago.edu), James E. Kragel¹, Joel L. Voss¹; ¹University of Chicago

Saccadic eye movements sample visual content during memory encoding. Hippocampal activity is sensitive to content, with activity modulations noted for different categories (e.g., human faces versus various object categories), potentially providing a bridge between semantic and episodic memory processing within hippocampus. However, previous studies have not determined whether content-specific effects occur in immediate response to eye-fixations versus slower emergence across the viewing episode. To investigate this, we used eye-movement tracking during encoding of naturalistic scenes comprising objects of distinct categories while hippocampal activity was recorded using intracranial EEG (iEEG) in 6 neurosurgical patients with refractory epilepsy. In prior work, we described analyses of phase locking of theta oscillations to fixations, which showed greater phase locking immediately before and after fixations to “people” versus other visual categories. Here, we characterized ERPs to examine differences among visual categories in the time course of fixation-evoked activity. ERP amplitude varied for “people” (n=544 eye-fixations) versus “food” (n=824) and “animals” (n=789) categories from approximately 300-450 ms after fixation (maximum differentiation at 363 ms, F(2,10)=8.855, p=0.006). These findings build on our prior work by showing that visual sampling of specific content evokes rapid changes in hippocampal activity and therefore, that the processing of visual concepts could be an immediate response to saccadic eye movements. These processes may thus underlie the relational processing of semantic and episodic content during scene encoding. Future research aims to explore further evidence of rapid memory processes on the fixation-locked hippocampal activity.

Topic Area: LONG-TERM MEMORY: Episodic