Regional variation in the representation of the visual field in the visual cortex of the Siamese cat.

In Siamese cats, many ganglion cell fibers from the temporal retina misproject to the contralateral hemisphere; as a result, each lateral geniculate nucleus contains an abnormally large representation of the ipsilateral visual field. The manner in which the visual cortex processes this aberrant visual information has been examined in several previous studies. In some Siamese cats, the region of the 17/18 border was found to contain an extensive, systematic map of the ipsilateral field, while in other animals no such map was found, and the 17/18 border appeared to represent the zero meridian of azimuth (as in normal cats). These results have led to the suggestion that there are two distinct types of Siamese cat ("Boston" and "Midwestern") which can be distinguished on the basis of cortical topography and the anatomical organization oif the geniculocortical pathway. In the present study, we have recorded from four Siamese cats in order to examine the visual field map in the region of the 17/18 border; in each cat we recorded at anterior coronal levels corresponding to the representation of the lower visual field, and also at more posterior levels near the horizontal meridian representation. In all of the animals we found that the anterior penetrations (corresponding to mean receptive field elevations inferior to -7 degrees) yielded 15-20 degrees of ipsilateral field representation at the 17/18 border; however, the posterior, horizontal meridian penetrations (with mean elevations from +1 degrees to -4 degrees) showed excursions of only about 5 degrees into the ipsilateral field. This large difference in the representation of azimuth was not due to rotation of the eyes during our recording sessions. The finding of appreciable differences in the amount of ipsilateral field represented at different anterior-posterior levels of the same animal might lead to the suggestion that there are not two distinct populations (or types) of Siamese cat with regard to the cortical map of the ipsilateral field. Rather, we raise the possibility that Siamese cats form one population in which there is a continuous variation in the extent of ipsilateral field represented in the cortex.