Optic Tectum

In nonmammalian vertebrates the optic tectum is the largest and most conspicuous visual center: its twin lobes are literally the roof of the midbrain, as implied by its Latin root. Each tectal lobe receives the majority of retinal fibers from the contralateral eye, and these are distributed in a precise retinotopic order such that contralateral visual Space is mapped out on the surface of the tectum. The great convenience of defining this mapping order by means of microelectrode recordings or by various anatomical tracer methods has made the tecta of amphibians and fish popular structures for developmental and regeneration studies aimed at elucidating the underlying mechanisms for guidance of retinal axons toward their cellular targets in the brain. The tectal motor map is activated by retinal input and descends via efferent pathways to brain stem and spinal motor centers. This has been demonstrated in a few species (trout, cod, toad, and alligator) by electrically stimulating the various regions of optic tectum in a freely behaving animal. Activation of rostral tectum elicits short turns of head or body toward the frontal field; Stimulation of caudal tectum elicits large orientations designed to fixate Stimuli appearing contralaterally in the rear field. Although the optic tectum resembles its mammalian homolog, the superior colliculus, it differs in that most of its descending efferent neurons are in direct contact with incoming retinal fibers. Three key questions under current investigation are these: (1) What kinds of afferent input trigger orienting behavior? (2) Which efferent pathways mediate specific response components? (3) How is the input-output transformation facilitated or inhibited by other nonretinal inputs to tectum? These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.