Lateralized cerebellar connectivity differentiates auditory pathways in echolocating and non-echolocating whales

We report the first application of diffusion tractography to a mysticete, which was analyzed alongside three odontocete brains, allowing the first direct comparison of strength and laterality of auditory pathways in echolocating and non-echolocating whales. Brains were imaged post-mortem at high resolution with a specialized steady state free precession diffusion sequence optimized for dead tissue. We conducted probabilistic tractography to compare the qualitative features, tract strength, and lateralization of potential ascending and descending auditory paths in the mysticete versus odontocetes. Tracts were seeded in the inferior colliculi (IC), a nexus for ascending auditory information, and the cerebellum, a center for sensorimotor integration. Direct IC to temporal lobe pathways were found in all animals, replicating previous cetacean tractography and suggesting conservation of the primary auditory projection path in the cetacean clade. Additionally, odontocete IC-cerebellum pathways exhibited higher overall tract strength than in the mysticete, suggesting a role as descending acousticomotor tracts supporting the rapid sensorimotor integration demands of echolocation. Further, in the mysticete, contralateral right IC to left cerebellum pathways were 17x stronger than those between left IC and right cerebellum, while in odontocetes, the laterality was reversed, and left IC to right cerebellum pathways were 2-4x stronger than those between right IC and left cerebellum. The stronger left IC-right cerebellum connectivity observed in odontocetes corroborates the theory that odontocetes preferentially echolocate with their right phonic lips, as the right phonic lips are likely innervated by left-cortical motor efferents that integrate with left-cortical auditory afferents in right cerebellum. This interpretation is further supported by the reversed lateralization of IC-cerebellar tracts observed in the non-echolocating mysticete. We also found differences in the specific subregions of cerebellum targeted by the IC, both between the mysticete and odontocetes, and between left and right sides. This study establishes foundational knowledge on mysticete auditory connectivity and extends knowledge on the neural basis of echolocation in odontocetes.