The electric sense of weakly electric fish

Recent studies of electroreception have been particularly successful in three different areas: Electroreceptors are tuned to the dominant frequency of the animal's EOD, and their tuning follows natural and experimentally induced shifts in EOD frequency. Steroid hormones influence the electric organ pacemaker frequency in the genus Sternopygus, and the tuning of electroreceptors will follow hormone-induced frequency shifts only if the receptors experience the animal's EOD. The frequency tuning of electroreceptors reveals properties similar to those of cochlear hair cells, and electroreceptors may be suitable model systems for in vitro studies of cellular and molecular aspects of electrical filter mechanisms in hair cells. In contrast to the South American or gymnotoid electric fish, the African or mormyrid electric fish evaluate electroreceptive information with the help of corollary discharges of their electric-organ pacemaker. The corollary discharge inhibits input from knollenorgan receptors so that, in the context of social communication, the animal only perceives EODs of neighbors but not its own. The corollary discharge at the same time enhances input from most mormyromasts so that the animal, in the context of electrolocation, selectively receives feedback from its own EODs. Finally, responses of ampullary electroreceptors to the animal's own EODs are centrally nulled by an elaborate and modifiable efference copy so that the animal is only informed about "nontrivial", low-frequency events in its environment. Laminated and topographically organized structures in the hindbrain and midbrain of gymnotoid fish are being studied with regard to neuroanatomical fine structure and functional organization. Different laminae and cell types in the hindbrain are specialized for the extraction of specific stimulus features, such as modulations of phase or amplitude in a sinusoidal stimulus regime. This information is passed on to the midbrain for the computation of more complex stimulus variables, such as the difference in phase modulations reported from different parts of the body surface. The torus semicircularis of the midbrain is designed for parallel processing of information from different parts of the body surface and for parallel computation of different stimulus variables for the control of behavioral responses. Electrical and visual information converge in the tectum opticum, which harbors a multimodal representation of sensory space.