Anatomy of interhemispheric connections in the visual system of Boston Siamese and ordinary cats.

In Siamese cats, previous studies have shown that a genetic mutation causes retinogeniculate fibers in each eye which arise from the temporal retina representing the 20 degrees of ipsilateral visual field adjacent to the vertical meridian to cross aberrantly in the optic chiasm, thereby terminating in the wrong lateral geniculate nucleus. The abnormality is expressed subsequently at the level of the visual cortex. This paper presents anatomical evidence that the pattern of commissural visual connections in the "Boston" variety of Siamese cat also is highly abnormal in comparison to that of ordinary cats. The topographical distribution of neurons supplying visual fibers to the splenium of the corpus callosum was studied in Boston Siamese and ordinary cats using the method of retrograde transport of horseradish peroxidase (HRP) following localized cortical injections made through a recording micropipette. In ordinary cats, after an HRP injection at the border between cortical areas 17 and 18, which represents the vertical meridian of the visual field, HRP-labeled cells in areas 17 and 18 of the opposite hemisphere were found only immediately adjacent to the 17-18 border, thus confirming the results of previous investigations. In Boston Siamese cats, the border represents a region in the ipsilateral visual field roughly 20 degrees away from the vertical meridian, and the vertical meridian representation is displaced to sites within areas 17 and 18 proper. When HRP was injected at the 17-18 border, labeled cells in the opposite hemisphere were located well within area 17 near the suprasplenial sulcus, and also well within area 18; few labeled cells were found at the 17-18 border. When an HRP injection was placed at the vertical meridian representation, again few HRP-labeled cells were found at the opposite 17-18 border, but instead most were found in area 17 slightly medial to the border, and in area 18 slightly lateral to it. Thee findings were complemented in an autoradiographic study in which orthograde transport of tritiated proline after a localized cortical injection was used to demonstrate the distribution of callosal terminals. Thus the pattern of callosal connections revealed in Boston Siamese cats, although anatomically different from that of ordinary cats, was nevertheless consistent with the proposal that cortical sites representing similar visual field coordinates in each hemisphere are appropriately interconnected via the corpus callosum. The laminar distribution of callosal connections was examined briefly. Layer III pyramidal cells of areas 17 and 18 supplied the majority of terminals to the opposite 17-18 border. Pyramidal cells of Layers II and VI, and Layer IVa in area 18, made a smaller contribution. In areas 17 and 18, the same cortical layers (II, III, and VI; and IVa in 18) were again the major sites of callosal termination. A clear projection to the base of layer I was also noted. The laminar distribution of callosal connections in ordinary and Boston Siamese cats were not substantially different.