Studies of the modifiability of the visual pathways in midwestern siamese cats
Authors: Guillery R. W. , Casagrande V. A. 
Published: December 2004

In Midwestern Siamese cats each cerebral hemisphere receives visual inputs from both temporal retinae, and the conflicting signals that arise from these two disparate afferent sources appear to prevent the establishment of some normal functional connections for the whole temporal retina. As a result, these cats show no behaviorally demonstrable visual orientation for the temporal retina, although they respond normally for the nasal retina. The visual capacities of Midwestern cats can be changed by modifying early visual experience, and some of the rules that govern the formation of central visual pathways can thus be defined.

The apparent blindness of the temporal retina of a Midwestern cat can be modified by monocular rearing. When such a cat is raised with one eye sutured or removed, the other eye develops a field of view that is normal in terms of visual orientation. Cortical lesions show that the vision acquired by the temporal retina depends upon areas 17 and 18 of occipital cortex. The modification of vision in the open eye can be produced by deprivation starting between the 8th and 80th day of life; later stages remain to be tested. In contrast, modifications produced in the deprived eye show a different relation to the time of the suture: if the lid suture is done at eight to ten weeks, the deprived eye develops vision for only the nasal retina and thus demonstrates the visual connections that developed while both eyes were open: if the suture is done at about the age of eye opening, the sutured eye develops no demonstrable vision at all. This is in striking contrast to the effect seen in non‐Siamese cats in which the sutured eye shows vision for the monocular crescent (Sherman, 1973).

Experiments using alternate deprivation (one eye closed on the tenth day and then opened when the other is closed or removed at the eighth to fourteenth week) show that if one eye has sole access to the geniculo‐cortical system during the first eight or more weeks of life, functional connections demonstrable by behavioral testing of visual orientation are established permanently for the temporal retina of this eye. Thereafter the other eye cannot also establish functional connections for its temporal retina, even when the “successful” eye is sutured or removed surgically. In contrast, opening a previously sutured eye at the eighth or tenth week will allow visual development for the pathways from the temporal retina of the opened eye, provided that both eyes had been kept closed initially. Thus, after the eighth week, vision for the temporal retina can still be established, but we have been unable to make both temporal retinae share the pathways available in one hemisphere.

Some segments of the geniculo‐cortical system are essentially non‐functional in Midwestern cats and other segments are made non‐functional by monocular deprivation. However, since all parts show about the same degree of retrograde reaction after cortical lesions, it appears that all parts send geniculo‐cortical axons to the visual cortex. The monocular suture itself affects geniculate cell growth in the normal segments of lamina A1 more than in the rest of the nucleus. The large monocular segments of lamina A, which are characeristic of the Siamese nucleus, do not show the sparing effect that is seen for geniculate cell growth in normal cats (Guillery and Stelzner, 1970), all parts of lamina A showing approximately the same cell size after monocular rearing.