Auditory, electrosensory, and mechanosensory lateral line pathways through the forebrain in channel catfishes

The forebrain auditory, electrosensory, and mechanosensory lateral line pathways in the channel catfish, Ictalurus punctatus, were examined by applying the fluorescent tracer DiI to 1) the auditory part of the torus semicircularis, 2) the electrosensory part of the torus semicircularis, 3) the lateral preglomerular nucleus, and 4) the anterior tuberal nucleus. Three distinct pathways ascend from the torus semicircularis to the telencephalon; they course through either 1) the lateral preglomerular nucleus of the posterior tuberculum, 2) the anterior tuberal nucleus of the hypothalamus, or 3) the central posterior nucleus of the dorsal thalamus. The anatomical data suggest that each of these ascending pathways carries information from more than one sensory modality. The lateral preglomerular nucleus receives an electrosensory input from nucleus electrosensorius in the diencephalon, but it also receives auditory and mechanosensory inputs directly from the torus semicircularis. The anterior tuberal and central posterior nuclei receive primarily auditory and mechanosensory, but also minor electrosensory, inputs. The efferent projections of the central posterior nucleus are presently unknown, but the lateral preglomerular and anterior tuberal nuclei project to nonoverlapping portions of the telencephalon. A cladistic analysis of these indirect torotelencephalic pathways reveals that 1) the pathway through the dorsal thalamus is probably a primitive character for gnathostomes, 2) a well-developed pathway through the posterior tuberculum is probably a derived character for actinopterygian fishes, 3) the pathway through nucleus electrosensorius is probably a derived character for catfishes and gymnotoid teleosts, and 4) auditory pathways through the hypothalamus probably evolved independently in catfishes and frogs.