EPISODIC MEMORY IS RELATED TO MEDIAL TEMPORAL LOBE ACTIVITY ON A FIXATION-BY-FIXATION BASIS: AN IEEG STUDY IN HUMANS

Memory formation involves encoding perceptual experiences into lasting neural traces, a process that relies on the hippocampus in humans and is mediated by visual exploration. Eye movements determine both what information is sampled and when it becomes available for encoding, structuring semantic content and its temporal order. Prior work showed that fixations to people elicit stronger memory and amygdala responses, and that early fixations, particularly the first fixation, are critical for memory formation by organizing visual sampling. At the neural level, eye movements evoke hippocampal responses, including fixation-locked event-related potentials (fERPs) and theta phase resets, suggesting a role for medial temporal lobe (MTL) circuits encoding visual experience on a fixation-by-fixation basis. However, it remains unclear how semantic content and fixation order interact to shape MTL activity during naturalistic viewing. Here, we tested whether fixation-related MTL activity is selectively modulated by what is fixated and when it is fixated, and whether these effects predict subsequent memory. We recorded intracranial EEG from the hippocampus and amygdala in six epilepsy patients performing a scene recognition task with simultaneous eye-tracking. Failing to fixate on people in people-themed scenes reduced recognition probability. Using a deconvolution approach, estimated hippocampal fERPs showed content selectivity, with larger responses for fixations to people, particularly during first fixations. Theta phase-locking in both hippocampus and amygdala was similarly enhanced for fixations to people, an effect confined to the first fixation. Together, these findings reveal that MTL activity during memory formation is structured by the content and temporal order of visual exploration.