Projections of the optic tectum in two teleost species.

The efferent pathways of the optic tectum have been investigated in the percomorph Eugerres and the berycomorph Holocentrus. A portion of the dorsal-dorsolateral region of the optic tectum was unilaterally resected by suction. The animals were perfused 6-30 days thereafter, and the brains were processed according to a modification (Method 7 in Ebbesson, '70) of the Fink-Heimer ('67) technique for the selective silver impregnation of degenerating axons and terminals. Three groups of fibers emerge from the lesioned region (a) a medial group, which runs towards the midline and terminates in the ipsilateral torus longitudinalis and the contralateral tectum; (b) an ascending group, which enters the dorsocaudal region of the diencephalon and terminates in pretectal cell groups, in the dorsomedial optic thalamic nucleus, and in the nucleus rotundus or prethalamicus; and (c) a descending group, which funnels down into the midbrain tegmentum. Here abundant terminals are given to dorsolateral cell groups and to the nucleus isthmi. A recurrent fascicle leaves the mainstream and ascends to terminate in scattered diencephalic cell groups, in the nucleus geniculatus posterior pars ventralis, and in the nucleus rotundus or prethalamicus. The bulk of descending fibers then forms an ipsilateral bundle, which gives terminals to the lateral reticular formation of mesencephalon and rhombencephalon, and a contralateral (i.e., the predorsal) bundle, which terminates in the medial reticular formation of mesencephalon and rhombencephalon.