Functional MRI of the superior colliculus in dissociated vertical deviation.

Dissociated vertical deviation (DVD), first described in 1894, is commonly encountered in the infantile strabismus syndrome. The vertical position of the eyes in DVD depends on fixation effort and luminance differences. The neural mechanism behind DVD is still not clear. Visual and oculomotor pathways in the brainstem are phylogenetically old pathways which in origin are basically monocular: the eye projects to the contralateral brainstem. Cortical visual and oculomotor pathways on the other hand are only found in birds and mammals and are basically binocular: the visual field projects to the contralateral cortex. In primates, stereopsis driven pathways were found to dominate gravity driven brainstem pathways. In early onset strabismus stereopsis fails to develop. The cause of latent nystagmus which also occurs in infantile strabismus syndrome has been tied to a changed hierarchy between cortical and brainstem velocity tracking systems. The same may hold true for DVD. With scleral search coil recordings in individuals with DVD, we showed that motor characteristics in DVD are similar to disparity induced vertical vergence. Vergence is a phylogenetically new system that facilitates binocular vision and is even weaker in cats than in primates. Vergence pathways from the cortex to the superior colliculus are strictly unilateral. The rostral pole of monkey SC projects to omnipause neurons. Both stimulation of the rostral pole of the superior colliculus and stimulation of omnipause neurons result in changes in vertical vergence. At least for torsional eye movements, it is shown that by equally activating populations of neurons in the right and left interstitial nucleus of Cajal, the head is kept in position. It is likely that disparity induced vertical vergence is realized by weighted activation of right and left omnipause neurons. The nasal bias in ocular dominance columns and interocular suppression found in infantile strabismus may give rise to vertical vergence known as Dissociated Vertical Deviation.