A Direct Auditory Subcortical Route to the Amygdala Associated with Fear in Humans.

Rapid and efficient fear processing is essential for survival. In vision, this function is supported by a well-characterized subcortical pathway consisting of direct projections from the pulvinar of the thalamus to the amygdala in the human brain. In contrast, the existence of an analogous shortcut for fear in audition has been demonstrated in nonhuman animals but remains unconfirmed in humans. To address this question, we used probabilistic streamline tractography and fixel-based analysis on diffusion-weighted images from Human Connectome Project participants of either sex to reconstruct candidate auditory subcortical pathways and examine their associations with affective and auditory behavioral measures. Our findings revealed a robust white matter tract connecting the inferior colliculus to basolateral amygdala via the medial geniculate body (MGB) of the thalamus. Remarkably, higher fiber density in this tract was associated with better hearing ability in noise and increased self-reported fearfulness, supporting its role in auditory and affective function. Conversely, a control analysis of the core thalamocortical pathway from ventral MGB to primary auditory cortex, representing the main route for auditory processing, was associated with auditory ability but not with affective measures. These findings provide previously unreported evidence for an auditory colliculo-geniculo-amygdala "low road" in humans, aligning with evolutionarily conserved pathways for fear described in nonhuman species.