Neuronal circuitry for comparison of timing in the electrosensory lateral line lobe of the African wave-type electric fish Gymnarchus niloticus.

An African wave-type electric fish, Gymnarchus, compares timing on the order of microseconds of sensory feedback from from its high-frequency (approximately 400 Hz) electric organ discharges (EODs) received at different parts of its body surfaces. This capability is essential for and demonstrated by the jamming avoidance response (JAR). The organization of the timing comparison mechanisms was identified in the electrosensory lateral line lobe (ELL) in the hindbrain by field potential, extra- and intracellular recordings, and intracellular labeling with biotinylated agents. Timing of phase of the EOD feedback is carried by action potentials of S-type primary afferent fibers that project to the inner cellular layer (ICL) of the medial zone of the ELL and to the giant neurons in the ELL. The giant neurons bilaterally project to the ICL, where neurons sensitive to phase differences between different parts of the body occur. Although sensitive to dynamic phase changes of several microseconds, these differential-phase-sensitive neurons showed adaptation to steady-state changes of phase difference over a wide range (greater than +/- 100 microseconds) and continued to respond to small modulations after the mean difference was shifted. Gymnarchus and an independently evolved South American electric fish, Eigenmannia, exhibit nearly identical JARs and share a rather complex but identical set of computational algorithms for JAR. This study showed that one of the computational steps, the timing comparison between body surfaces, occurs in the hindbrain in Gymnarchus, in contrast to the midbrain in Eigenmannia. Thus, similar systems with a similar overall function may have evolved differently in different genera by assigning a subfunction to different substructures within the brain.