The superior colliculus (SC) is believed to play an important role in sensorimotor integration and orienting behavior. It is classically divided into superficial layers predominantly containing visual neurons and deep layers containing multisensory and premotor neurons. Investigations of intrinsic connectivity within the SC in non-human species initially led to controversy regarding the existence of interlaminar connections between superficial and deep layers. It now seems more likely that such connections exist in a number of species, including non-human primates. In the latter, anatomical data concerning intrinsic SC connectivity are restricted to a limited number of intracellularly labeled neurons. No studies have been conducted to investigate the existence of intrinsic connections of human SC. In the present study, DiI (1,1'-dioctadecyl-3,3,3',3'- tetramethylindocarbocyanine perchlorate) and BDA (biotinylated dextran amine) were two tracers used in post-mortem human brains to examine intrinsic SC connections. Injections into the superficial layers revealed tangential connections within superficial layers and radial superficial-layer to deep-layer connections. Within superficial layers, horizontal connections were found over the entire rostro-caudal axis and were mostly directed laterally, i.e. toward the brachium of the inferior colliculus. Superficial-layer to deep-layer connections were more prominent in sections containing the injection site or located close to it. In these sections, an axon bundle having roughly the same diameter as the injection site crossed all deep layers, and individual axons displayed en passant or terminal boutons. The present results suggest that intrinsic connections within superficial layers and radial superficial-layers to deep-layers exist in human SC. The putative roles of these connections are discussed with regard to visual receptive field organization, as well as visuomotor and multisensory integration
