Imaging the three human primary amygdala output tracts with age and sex characterisation across the lifespan

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Abstract

Background - The amygdala is involved memory and emotion processing and communicates with the rest of the brain through three efferent tracts: the stria terminalis (ST), ventral amygdalofugal pathway (VAP), and anterior commissure (AC). This is the first study to isolate all three major amygdalar output pathways using novel diffusion tractography protocols with exploration of the diffusion, age, and sex characteristics of each tract. Methods − 64 healthy individuals aged 15–64 underwent high-resolution T1, T2, and diffusion MR brain imaging with constrained spherical deconvolution tractography. Individually generated amygdalae aided the virtual dissection of the ST, VAP and AC using novel bespoke protocols for each tract based on anatomical principles. Age and sex diffusion characteristics were explored. Results - The ST showed age-related decreases in fractional anisotropy (left: p = 0.00018; right: p = 0.00032), mean diffusivity (left: p = 0.0017; right: p = 0.00058), and radial diffusivity (left: p = 0.00015; right: p = 3.44E-05). The AC showed decreases in mean diffusivity (p = 0.0022) and axial diffusivity (p = 0.00015). Sex had no significant effect on the diffusion metrics apart from the right ST, showing higher fractional anisotropy in males than in females (p = 0.001). Conclusion - This is the first study to virtually dissect the three main output tracts of the amygdala from neuroimaging. We also show age related changes in markers of neuronal integrity with age. No sex differences were found apart from potentially more robust integrity in the right ST in males. The novel anatomically-driven and reproducible protocols for ST and VAP isolation presented may guide future investigation of the connectivity and efferent circuitry of the amygdala.

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