A quantitative analysis of the ipsilateral retinocollicular projection in the cat: an EM degeneration and EM autoradiographic study.

The ipsilateral projection to the binocular segment of the superior colliculus in the cat was quantified by means of electron microscopic (EM) degeneration and electron microscopic autoradiography. Because of the patchy distribution of ipsilateral terminals, contiguous samples were taken across the mediolateral extent of the colliculus. The results from both EM degeneration and EM autoradiography are in substantial agreement: In the binocular segment, the ipsilateral projection represents 5.3% and 5.6%, respectively, of the total retinal input to the superior colliculus. As the contralateral projection extends over the whole colliculus but is greatest in the caudal region, while the ipsilateral projection is restricted to the middle one-third of the colliculus, it follows that the total ipsilateral retinocollicular projection is less than earlier estimates (5--10%, Kanaseki and Sprague, '74) and probably closer to 1%, as suggested by Sterling ('73) on the basis of an electron microscopic degeneration study. A significant finding is that there are many more terminals per axon in the contralateral than in the ipsilateral superficial gray. The ratio of degenerated terminals in SGS1 and SGS2 to degenerated axons in SGS3 is 3.1 contralaterally, while in the ipsilateral colliculus the figure is 1.2. This may reflect a difference in the arborization pattern of Y-type and W-type retinal ganglion cells in the colliculus.