An ipsilateral vestibulothalamic tract adjacent to the medial lemniscus in humans

We examined 14 patients with acute anteromedial pontomesencephalic infarctions for signs of vestibular and ocular motor dysfunction. In all cases, an isolated ipsilateral deviation of the subjective visual vertical (mean: 4.1, range: 2.7- 6.6) was found without any further signs of vestibular or eye movement disorders like ocular torsion or skew deviation. Distinct lesions in thin-slice brainstem MRI showed an overlap zone in the medial portion of the medial lemniscus. The finding of putative ipsilateral vestibular projections running adjacent to or within the medial lemniscus was subsequently confirmed by a reanalysis of an anterograde tracer labelling study in the primate after tracer injection in the vestibular nucleus complex. The major conclusions of this study are as follows: (i) there is evidence for an ipsilateral graviceptive pathway running from the vestibular nuclei close to and within the medial lemniscus to the posterolateral thalamus [ipsilateral vestibulothalamic tract (IVTT)], (ii) this pathway might be the human homologue of the three-neuron sensory vestibulocortical tract described in primates and (iii) unilateral lesions of this pathway cause only vestibulo-perceptive dysfunction in the roll plane in contrast to lesions of the crossed graviceptive pathways (in the medial longitudinal fascicle), which were described earlier and which manifest as a combination of tilt of the subjective visual vertical, ocular torsion and skew deviation.