Differences in binocular interactions between cortical areas 17 and 18 and superior colliculus of Siamese cats

In Siamese cats most retinal projections are crossed and almost all neurons in cortical areas 17 and 18 respond exclusively to the contralateral eye. However, neurons in visual areas in the suprasylvian sulci (LSA) can be activated from both eyes, since the input from the ipsilateral eye is relayed to them by the corpus callosum (Marzi et al., '80). This study shows that the superior colliculus (SC) is also the site of binocular interactions in Siamese cats with a predominantly monocular organization of areas 17 and 18. In 8 unanesthetized, brainstem-sectioned Siamese cats, identified as "Boston" on the basis of the pattern of visual field representation in areas 17 and 18 (Hubel and Wiesel, '71; Shatz, '77a), only 25 out of 140 neurons in these areas could be driven from both eyes, whereas as many as 124 out of 143 SC neurons showed a clear binocular input. The input from each eye to the ipsilateral SC and the resulting binocular interactions in our Siamese cats might have depended on corticotectal projections from LSA. If so, since a posterior callosal section abolishes the input from each eye to ipsilateral LSA, the SC should similarly be affected by callosotomy. The posterior half of the corpus callosum was sectioned acutely in three Siamese cats after recording from SC; the input from the ipsilateral eye to SC was suppressed by this operation in one cat (which, however, also had an unintended interruption of the posterior, habenular, and anterior intertectal commissures), and drastically reduced in the other two. In a fourth cat, in which callosotomy was performed three weeks before recording from SC, the number of SC neurons responsive to the ipsilateral eye was significantly inferior to that of Siamese cats with an intact corpus callosum. These findings indicate the importance of the corpus callosum for ipsilateral eye-SC relations, as well as for SC binocularity, in Siamese cats. At the same time they imply that noncallosal routes can also transmit information from each eye to the ipsilateral SC in callosotomized Siamese cats.