INTRINSIC CIRCUITRY IN THE DEEP LAYERS OF THE CAT SUPERIOR COLLICULUS

The mammalian superior colliculus is involved in the transformation of sensory signals into orienting behaviors. Sensory and motor signals a re integrated in the colliculus to produce movements of the eyes, head , and neck. While there is a considerable amount of information availa ble on the afferent and efferent connections of the colliculus, almost nothing is known about its intrinsic circuitry, particularly that of its deepest layers. It is likely that intrinsic connections in these d eeper layers of the colliculus participate in the sensory-motor transf ormations leading to orienting movements. In this study, we used the n euroanatomical tracer biocytin to label small groups of neurons in the deeper layers of the cat superior colliculus and examine the distribu tion of their axons and terminals. We found a broadly distributed netw ork of intrinsic projections throughout the deep layers of the superio r colliculus. While the majority of terminals were found in a 1-2 mm r adius around the injection site, labeled terminals were found througho ut the deep layers of the colliculus up to 5 mm from the injection sit e. In addition, these injections sometimes labeled terminals in the su perficial tectum. Extensive projections were demonstrated by the more superficial injections, but few terminals were found when injections w ere confined to the deepest layers of the colliculus. There was no evi dence of anisotropy in the distribution of terminals from injections m ade at different rostrocaudal or mediolateral locations; neurons locat ed in any one region in the colliculus could potentially influence any other region. This network of intrinsic connections in the cat superi or colliculus could provide a means for deeper-layer efferent neurons to associate, and to modulate or coordinate their output. Interneurons could also provide a substrate for mutual inhibition between neurons at the rostral pole of the colliculus that are active during fixation, and more caudally located neurons whose activity is associated with s accadic eye movements.