bioRxiv | 2019
The role of the pre-commissural fornix in episodic autobiographical memory and simulation
Abstract
The ability to vividly remember the past, and imagine the future, involves two key regions: the hippocampus and ventromedial prefrontal cortex (vmPFC). Despite evidence of a direct anatomical connection between these structures, it is unknown whether hippocampal-vmPFC structural connectivity supports both past and future-oriented episodic thinking. We applied diffusion-weighted magnetic resonance imaging (dMRI) and a novel tractography protocol to reconstruct distinct fornix subdivisions previously detected in axonal tracer studies, namely pre-commissural (connecting anterior hippocampus to vmPFC) and post-commissural (linking posterior hippocampus and medial diencephalon) fornix, in a group of healthy humans who undertook a past-future autobiographical interview. Inter-individual differences in pre- but not post-commissural fornix microstructure significantly correlated with the episodic richness of both past and future autobiographical narratives. These results remained significant when controlling for both non-episodic narrative content and regional volumes. Reconstructing events from one’s past, and constructing possible future events, thus involves a distinct, structurally-instantiated hippocampal-vmPFC pathway. Significance Statement A novel anatomically-guided protocol that allows the pre-commissural and post-commissural fornix fibers to be separately reconstructed in vivo (Christiansen et al., 2016) was applied to reconstruct the pre-commissural subdivision of the white matter fornix tract (anatomically linking the hippocampal formation to the vmPFC) and investigate its contribution to episodic memory and future simulation. We demonstrated that the amount of episodic details contained in past and future narratives, collected via an adapted autobiographical interview, was positively correlated with pre-, but not post-, commissural fornix microstructure. These findings highlight how inter-individual variation in the pre-commissural subdivision of the fornix underpins the construction of self-reflective, contextual events – for both the past and future.