Abnormal interhemispheric connections in the visual system of Boston Siamese cats: a physiological study

In the normal cat most visual fibers in the splenium of the corpus callosum are concerned with a representation of the vertical meridian of the visual field. These fibers for the most part originate from cortical regions forming the border between adjacent architectonic areas, such as the anatomical 17-18 border. Due to a genetic mutation in Boston Siamese cats, the anatomical 17-18 border now represents a region in the ipsilateral visual field roughly 20 degrees away from the vertical meridian, and the representation of the vertical midline is displaced from the border to regions within areas 17 and 18 proper. Do visual fibers in the Boston Siamese cat corpus callosum originate, as in normal cats, from the 17-18 border even though the vertical meridian is no longer represented there? The present paper deals with this question from a physiological standpoint. Single visual fibers in the corpus callosum of 11 Boston Siamese and two normal cats were recorded using extracellular microelectrodes. Receptive fields were mapped and their positions used to infer the likely cortical site of origin for each fiber. In confirmation of earlier findings, most callosal receptive fields in the normal cat were closely apposed to the vertical meridian, as would be expected if only those visual cortical regions which represent the vertical meridian, such as the border between areas 17 and 18, contribute to the corpus callosum. In Boston Siamese cats, however, an abnormally large amount of visual field was represented in the corpus callosum, with some receptive fields located as far as 25 degrees away from the vertical meridian. The representation of the vertical meridian was nevertheless substantial. The present findings suggest, therefore, that in Boston Siamese cats callosal fibers do not originate solely from the boundary regions between adjacent cortical areas, such as the anatomical 17-18 border, but originate also from other cortical regions, especially the vertical meridian representation, regardless of their location with respect to such boundaries.