Auditory radiations: Evidence for dorsal and ventral branches

The acoustic radiations connect the medial geniculate nucleus (MGN) of the thalamus with the auditory cortex in Heschl's gyrus (HG). Classic histological dissections described a trajectory that courses through the optic radiations and beneath the posterior insula en route to HG. More recent histological work, however, along with probabilistic tractography, has reported a curved pathway composed of a genu, stem, and fan that is inconsistent with the classically described topography. Thus, the precise course of the acoustic radiations is not settled, motivating renewed anatomical characterization.

In nineteen healthy adults, we used probabilistic diffusion tractography to visualize streamlines connecting the medial geniculate nucleus (MGN) with the supramarginal gyrus (SMG) bordering the ascending ramus of the lateral fissure. We compared these streamlines with the acoustic and optic radiations as defined in the XTRACT white matter atlas. We then qualitatively re-evaluated published neuroimaging figures from eight patients with severe auditory agnosia or cortical deafness caused by bilateral subcortical white matter lesions and overlaid the reported lesion locations onto the tractography-derived pathways.

In contrast to a ventral acoustic radiation (vAR) that courses through the optic radiations and beneath the insula, our tractography analysis revealed a dorsal bundle that we term the dorsal acoustic radiations (dAR). The dAR comprises two components that initially travel together: an ansa acoustica (AA) projecting from ventral MGN to Heschl's gyrus (HG), and a projection from magnocellular MGN to the supramarginal gyrus (SMG). Both components arch dorsally over the optic radiations before diverging. In all eight historical cases, bilateral lesions overlapped one or both components of the dAR while visual fields were preserved, suggesting relative sparing of the optic radiations and the vAR.

These findings support a model in which the human acoustic radiations comprise at least two neuroanatomically distinct components: an earlier-developing ventral branch (vAR) arising from ventral MGN that courses beneath the insula, and a later-developing dorsal branch (dAR) that includes the AA traveling from ventral MGN to Heschl's gyrus (HG) as well as a second pathway traveling from magnocellular MGN to the supramarginal gyrus (SMG) and neighboring auditory-responsive cortices.