Retinotectal connections within the superficial layers of the cat's superior colliculus.

Intracellular staining of neurons located in the superficial layers of the superior colliculus (SC) of the cat was carried out through an intracellular recording micropipette with which responses to stimulation of the optic chiasm (OX) and the optic tract (OT) ipsilateral to the explored SC were recorded. :A total of 16 SC neurons were successfully stained. Retinotectal fibers which were synaptically connected to those morphologically identified neurons were classified as either W- or Y-fibers according to their conduction velocities. The conduction velocity of W-fibers ranged from 4.0 to 13.1 m/sec and that of Y-fibers from 35.5 to 42.0 m/sec. Neurons located in the upper part of the stratum griseum superficiale (SGS) were found to be monosynaptically or polysynaptically supplied by W-fibers, while some neurons located in the lower part of SGS were monosynaptically connected to Y-fibers and others monosynaptically or polysynaptically to W-fibers. Morphologically SC neurons were classified as horizontal (1), narrow-field vertical (3), wide-field vertical type II (6) and type III cells (3) with 3 unidentified neurons. W-fibers were connected to the horizontal, narrow-field vertical and wide-field vertical type-II cells while Y-fibers contacted only wide-field vertical type-III cells.