Are representations in the hippocampus organized by the emotional content of stimuli? A multivariate analysis of intracranial electrode recordings

Poster Session C - Sunday, April 14, 2024, 5:00 – 7:00 pm EDT, Sheraton Hall ABC

Alexander Lawriw¹ (alawri1@lsu.edu), Christopher R. Cox¹; ¹Louisiana State University

Although long understood to be sparse, episodic, and not organized around the similarity of stimuli or events, recent work suggests that representations in the hippocampus may have important structure. Within-category stimulus correlations among patterns of activity measured with fMRI from subfield CA1 can be significantly larger than between-category correlations (Schapiro et al., 2018; Nature Commun.) and some individual variability in rapid category learning can be explained by white matter integrity in the trisynaptic and monosynaptic hippocampal pathways (Schlichting et al., 2021; Hippocampus). We examined spike rates and local field potentials (LFPs) recorded by intracranial electrodes implanted in the hippocampus, amygdala, anterior cingulate cortex, and prefrontal cortex of 14 epileptic patients while they viewed and evaluated faces expressing positive, negative, or neutral affect. Using regularized logistic regression (elastic net), we attempted to discriminate patterns of activity associated with positive and negative faces within the hippocampus and spanning the four regions with electrodes. After exhaustive computational experimentation, modeling trial-level spike rates as well as the time-varying spectral power of the LFPs, it was not possible to discriminate positive from negative faces in this dataset. However, these null results do not provide tacit support for the conventional view. Instead, we situate these important null findings in a hypothesis space that considers the importance of time for how category structure may influence the hippocampus. In doing so, we discuss the limitations of intracranial recordings (our dataset, and the modality in general) for addressing questions of neurocognitive representation in the brain.

Topic Area: LONG-TERM MEMORY: Semantic