Visuotopic organization of the superior colliculus in normal and Siamese cats

Abstract

The visuotopic organization of the superior colliculus of normally pigmented and Siamese cats was investigated with microelectrodes. In normal cats, the representation of the ipsilateral hemifield in the rostral tectum was found to be binocular. In addition, this representation included more of the ipsilateral hemifield than has been previously reported. Both normally pigmented and Siamese cats were found to have about 40° of the ipsilateral hemifield represented in the rostral tectum and normally pigmented and Siamese cats do not appear to differ in this regard (see ref. 2). However, the superior colliculus of Siamese cats is abnormal in other ways. The representation of the ipsilateral hemifield in the rostral tectum of Siamese cats was activated only by the contralateral eye. Furthermore, in the larger caudal representation of the contralateral hemifield, very few neurons were activated by the ipsilateral eye. The reduction in such activation is not simply the result of misrouting of ipsilateral retinotectal fibers to the contralateral tectum in a manner similar to the misrouting of retinogeniculate fibers11,12, since there were also very few neurons with abnormally placed receptive fields. Instead, the recordings from the tectum are similar to those obtained from striate cortex of some Siamese cats13 where few neurons were activated by input relayed from the normal ipsilateral or the abnormal contralateral retinogeniculate projections. As in the geniculostriate system11–13, those neurons with receptive fields abnormally placed in ipsilateral hemifield were in part of the tectal representation of the first 20° of the contralateral hemifield; neurons with normally placed receptive fields related to the ipsilateral eye were 20°–40° into the representation of the contralateral hemifield.