Parallel ascending vestibular pathways: anatomical localization and functional specialization

Information from the vestibular nuclei ascending through the brainstem to the oculomotor and trochlear nuclei (NIII, NIV), the interstitial nucleus of Cajal (INC), the pretectum, or thalamus, is thought to be distributed in at least five different pathways. They include the medial longitudinal fasciculus (MLF), the ascending tract of Deiters (ATD), possibly the brachium conjuctivum (BC), the crossing ventral tegmental tracts (CVTT), and a recently observed ipsilateral pathway close to the medial lemniscus, possibly the equivalent of the ipsilateral vestibulo-thalamic tract (IVTT). This short review describes the location of these ascending tracts, their function with respect to ocular motor control and perception, and their clinical relevance. There is evidence that the MLF carries mainly information from the canals to NIII, NIV, INC, and possibly the thalamus, whereas otolith signals may ascend in the CVTT, along with excitatory anterior canal connections. The evidence for BC as a specific vestibulo-oculomotor pathway is weak and could be the result of the initial observations of CVTT. The ATD carries mostly ipsilateral otolithic information to the medial and inferior recti subgroups in NIII and the Edinger-Westphal nucleus. In the rostral pons an ipsilateral vestibular pathway was seen lying close to the medial lemniscus. This anatomical projection could be the equivalent of the IVTT, bypassing the ocular motor centers and projecting to the thalamus. The IVTT mediates perception of verticality and may be part of a fast three-neuron vestibulo-thalamo-cortical pathway, which provides the multisensory cortical system for spatial orientation and self-motion-perception with information about head acceleration.