Vestibulo-thalamic projections and head posture in the cat.

The vestibulo-thalamic projections were studied using the retrograde axonal transport technique by injection of horseradish peroxidase (HRP) into the unilateral ventrolateral (VL) thalamic nucleus in 12 cats. The results of experiments in which HRP was injected into the VL-thalamus, including the dorsomedial and central lateral thalamic nuclei, of two cats suggest that major inputs to the thalamus arise predominantly from the contralateral superior nucleus and ipsilateral medial and lateral vestibular nuclei.The functional significance of the connections was studied in 15 cats by eliciting head postures such as turning, tilting or a combined form following electrical stimulation of vestibular nuclei. Horizontal turning toward the ipsilateral side was evoked by stimulation of unilateral medial and lateral vestibular nuclei, and it was abolished by bilateral thalamotomy. The turning posture was also eliminated by producing a lesion in the VL-thalamus, the field of Forel H(1) and zona incerta in the contralateral side to the vestibular nuclear stimulation. However, a lesion in the same structure on the ipsilateral side made the posture worse. Therefore, it is concluded that horizontal turning of the head toward the ipsilateral side is caused by the activation of both ipsilateral vestibulo-reticulo-thalamic projections and contralateral vestibulo-thalamo-striate projections.Lateral tilting of the head toward the contralateral side was evoked by stimulation of the superior vestibular nucleus or by producing a lesion in the mesencephalic ventromedial tegmentum including the interstitial nucleus of Cajal. The effect of thalamotomy on the tilting was slight, and the posture was maintained after bilateral thalamotomy. These results suggest that lateral tilting toward the contralateral side of vestibular nuclear stimulation is caused to some extent by the activation of contralateral reticulo-thalamic projections, but that it is primarily related to descending projections from the striatum and mesencephalic ventromedial tegmentum