A quantitative analysis of the spatial organization of the vestibulo-ocular reflexes in lateral- and frontal-eyed animals--II. Neuronal networks underlying vestibulo-oculomotor coordination.

The neuronal connectivity underlying the vestibulo-ocular reflexes in cat and rabbit was evaluated in the light of quantitative data of the spatial orientation on semicircular canals and extraocular muscles. Neuronal connectivity was calculated using a matrix-analysis of the sensory and motor periphery, and of the brain stem pathways connecting semicircular canals and extraocular muscles. Two cases of vestibulo-ocular reflex compensation were considered. In the first case, vestibulo-oculor reflex compensation was assumed to be isotropic, i.e. the vestibulo-ocular reflex gain is the same for all directions of rotation. In the second case, the vestibulo-oculor reflex gain was assumed to be anisotropic with the "torsional" gain smaller than the "horizontal" and "vertical" gains. The theoretical calculation predicts that besides the principal vestibulo-ocular reflex pathways (classical three-neuron-arc connectivity), several accessory connections (other than principal connections, regardless of the synapses involved) exist which are characteristic for each species. These accessory connections were compared to physiological and anatomical data. In the cat theoretical connections for an isotropic vestibulo-ocular reflex gain agree with pathways observed experimentally, of which the most characteristic are excitatory connections to the superior rectus and inhibitory connections to the inferior rectus muscle from both of the anterior canals, and a mirror image pattern of connections from the posterior canals. In the rabbit experimentally obtained data and calculated connections rarely agree. However, for an anisotropic gain we find a higher rate of coincidence between experimental and theoretical connections. Our evaluation indicates, that accessory vestibulo-ocular reflex pathways serve to compensate for the incongruence between semicircular canal and extraocular muscle planes, at least in the cat. Available experimental data suggest an important role of a special subclass of accessory pathways via axon collaterals of principal projections (three-neuron-arc nature). With certain restrictions, the presented method of calculation promises to be a useful tool for a quantitative analysis of the vestibulo-ocular reflex.