Horseradish peroxidase labeling of cat tectotectal cells

By means of unilateral intracollicular injections of horseradish peroxidase (HRP), the cells of origin of the tectotectal projection in cats were detected in the opposite colliculus. The 9 injected animals were divided into two groups in accordance to the location of their injections along the collicular rostrocaudal axis. The majority of the labeled cells were obtained from two injections performed at the level of the commissure of the superior colliculus (231 cells) whereas 5 injections performed at lower levels led to labeling of 49 cells. In two animals, the injections did not cause labeling of cells, a fact that was attributed to the superficiality of the injections. Concerning the distribution of the labeled cells over the collicular layers and along the collicular rostrocaudal axis, there was no difference between the two groups of animals. Among the 280 labeled cells, 1 cell (0.35%) was found in stratum griseum superficiale, 83 cells (29.64%) in stratum opticum, 182 cells (65.00%) in stratum griseum intermediale and 14 cells (5.00%) in stratum griseum profundum. Ninety-two per cent of the cells were located in the rostral portion of the colliculus, in the region between the collicular rostral tip and the collicular rostral plane that corresponds to the vertical meridian representation. Eight per cent of the cells were concentrated in a zone comprising the lower half of stratum opticum and the upper half of stratum griseum intermediale, a location that corresponds to the transition between the visual-sensory and the visual-motor colliculus. On the basis of laminar segregation of collicular functions, the possibility that the tectotectal projection could be related to visual orienting, following and attentive collicular functions through the suppression of activity (a non-enhancement effect?) of homologous cells in the opposite colliculus is discussed.