fMRI signals of pattern separation in the neocortex and hippocampus to non-meaningful objects and their spatial location

Abstract

Computational theories of memory posit that the dentate gyrus and CA3 (CA3DG) hippocampal subfields reduce mnemonic interference via a process called pattern separation. While the CA3DG is viewed as a domain-general pattern separator, the parahippocampal and perirhinal cortices may play a role in content-specific (e.g., spatial or object-related) interference reduction. Recent work highlighted the role of frontal and parietal control areas in allocating resources during mnemonic discrimination, but the interactions between the medial temporal lobe and frontoparietal regions have been rarely studied. Moreover, mnemonic discrimination tasks designed for humans almost exclusively use everyday items as stimuli, confounding retrieval processes with pattern separation. To address these challenges we acquired high-resolution structural images of the medial temporal lobe, and full-brain high-resolution functional MRI data of 39 participants while they studied non-meaningful fractals with varying degrees of interference in either their spatial or object features. We found that the parahippocampal cortex contributes to interference reduction in the spatial domain, while the perirhinal cortex contributes to interference reduction in the object domain. The dorsolateral frontal and parietal regions were recruited during the encoding of interfering stimuli in both object and location domains, and displayed strengthened within- and cross-network connectivity in response to interference. Contrary to our expectations, we did not find significantly increased activation in the CA3DG to similar trials relative to repeats, indicating a lack of sensitivity to small differences in interfering stimuli. Altogether, these results are in line with content-specific interference reduction in the medial temporal lobe, possibly orchestrated by frontoparietal regions, but challenge the view of the CA3DG as the universal pattern separator of the human brain.