Pretectum and accessory optic system in the filefish Navodon modestus (Balistidae, Teleostei) with special reference to visual projections to the cerebellum and oculomotor nuclei.

The fiber connections of the accessory optic system (AOS) were investigated in a balistid fish, Navodon modestus (filefish), by means of horseradish peroxidase (HRP) and degeneration methods. Following injections of HRP into the corpus cerebelli, neurons in two retinal recipient nuclei, the area pretectalis pars dorsalis (APd) and area pretectalis pars ventralis (APv), were labeled retrogradely. In addition, a few neurons near the nucleus of the accessory optic tract (nAOT) were labeled. These neurons have dendrites extending into nAOT. Neurons in APv were also labeled by HRP injections into the oculomotor complex (nIII). However, no neurons were labeled in APd or nAOT. A few neurons in the lateral part of APv were labeled by HRP injections into the abducens nucleus (nVI). Three nuclei of the AOS, APd, APv and nAOT, were shown to receive tectal projections by the Fink-Heimer method. Thus, APv receives retinal and tectal projections, and in turn projects to corpus cerebelli, nIII and nVI. Specific efferent connections of the AOS in teleosts are discussed from phylogenetic aspects.