Collothalamic projections to the human amygdala: hemispheric asymmetry modulates trait anxiety

In 19 people, probabilistic DTI tractography was used to visualize the topographic relationships between three white matter components of a fascicle, the supraventricular temporal bundle, that traverses above the temporal horn of the lateral ventricle: collothalamic auditory and visual projections to the amygdala via the posterior thalamus, and the amygdalofugal stria terminalis. This bundle constitutes a subcortical, "low road" pathway that transmits threat signals to the amygdala, and that projects signals that bias orienting toward visual threat to the bed nucleus of the stria terminalis. The course of the visual streamline passes below the brachium of the superior colliculus through the position of two thalamic nuclei that have been shown to both receive afferents from the superficial layers of the superior colliculus and to also project to the amygdala: the suprageniculate nucleus and the inferior pulvinar. The visual streamline passes laterally dorsal to the auditory streamline and both collothalamic streamlines then traverse together above the temporal horn of the lateral ventricle, dorsal to the stria terminalis, with the auditory streamline dorsal to the visual streamline, and entering the lateral amygdala dorsal and medial to it. Individual differences in the degree of hemispheric asymmetry of the fractional anisotropy of the visual streamline, but not the auditory streamline, predicted trait anxiety: weaker left hemisphere connectivity relative to those in the right hemisphere was associated with higher trait anxiety. There was no correlation between individual differences in the microstructure of either the stria terminalis or the ventral amygdalofugal pathway and trait anxiety.NEW & NOTEWORTHY Three components of a white matter bundle, auditory and visual collothalamic projections to the amygdala and the stria terminalis, traverse above the temporal horn of the lateral ventricle. This bundle constitutes a "low road" pathway that transmits threat signals to the amygdala, via the posterior thalamus, and that biases spatial orienting toward visual threat. Hemispheric asymmetry of the microstructure of the visual pathway predicts individual differences in trait anxiety.