Intercollicular fibers terminating in the superficial layers of the superior colliculus: a WGA-HRP study in the rat.

The intercollicular pathway in the albino rat was studied using a lectin, wheat germ agglutinin, conjugated to horseradish peroxidase (WGA-HRP) as a pathway tracer and a modified HRP histochemistry method. The results showed that a small number of intercollicular fibers were observed in the rostral half of the superficial layers and the caudal half of the intermediate layers of the superior colliculus, while the majority of the intercollicular fibers were distributed in the intermediate and deep layers in the rostral half of the colliculus. The fibers which terminated in the superficial layers of the colliculus were studied in some detail and were subdivided into 4 different morphological types. These results suggest that the intercollicular projection is more broadly distributed in the superior colliculus than has been reported previously in most of the mammalian species studied, and the optic portion of the colliculus receives direct input from different morphological types of intercollicular neurons located on the opposite side of the midbrain.