Some Neural Connections Subserving Binocular Vision in Ungulates; pp. 65–79

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Abstract
Using a combination of anatomical and physiolgical techniques some of the neural connections subserving binocular vision in 2 species of artiodactyl ungulates (the sheep, Ovis sp., and the goat, Capra hircus) were studied. After monocular injections of [3H] proline, transsynaptic transport was observed bilaterally in layers 4 and 6 of visual cortical areas V1 and V2, but there were no sharply defined ocular dominance columns of the kind seen in cats and rhesus monkeys. In coronal sections there was a discontinuity in density of labeling between areas V1 and V2 corresponding to a point in the visuotopic map about azimuth -15.degree. in the ipsilateral visual field. This discontinuity was most pronounced in the hemisphere ipsilateral to the injected eye. While the cortical representation of ipsilateral visual space can be explained by the retino-geniculo-cortical input pathway from the contralateral eye, the physiologically demonstrated cortical contribution to ipsilateral visual space from the ipsilateral eye cannot be explained in this way. This conclusion was reinforced by experiments using retrograde transport of horseradish peroxidase [HRP] from the lateral geniculate nucleus (LGN) and medial interlaminar nucleus (MIN) to retinal ganglion cells in flattened whole mounts. These revealed a sharp nasotemporal decussation in the ipsilateral retina, which could not thereby subserve any significant representation of the ipsilateral visual field. In contrast the contralateral nasotemporal decussation was smeared, with many labeled ganglion cells in the temporal retina which could subserve visual input from the ipsilateral hemifield. The projection of the nasotemporal decussation line into visual space, was estimated to be tilted from vertical by .apprx. 5.degree. in each eye, in a similar way to that already reported in the cat. Neurophysiological recordings from binocular neurons in area V1 with different vertical eccentricities also showed that the vertical horopter (the midsagittal reference plane for binocular vision) would be tilted in life when the cyclotorsional position of the eyes was taken into account. Both anatomical and physiological methods concur in the prediction that ungulates have a tilted vertical horopter like that described for 2 other terrestrial species, the burrowing owl and the cat. Anatomical experiments reveal other similarities between the organisation of the ungulate''s visual pathways and that of the cat. After [3H] proline injections in V1, visuotopic labeling was found in the claustrum, dorsal LGN, cortical area V2 and the superior colliculus. HRP injections in V2 also revealed a direct input to that area from the MIN and dorsal LGN and also from layers, 2, 3 and 5 of the splenial area.

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