Gain control in the electrosensory system mediated by descending inputs to the electrosensory lateral line lobe

The electrosensory lateral line lobe (ELLL) of weakly electric fish, the primary electrosensory processing station, receives a large descending input from the midbrain in addition to the input from the electroreceptor afferents. The role of a major component of this descending input in determining the properties of ELLL output neurons was investigated. The descending input was reduced or eliminated by microinjections of the local anesthetic lidocaine or by small lesions. This treatment increased the responses of the ELLL output neurons to suprathreshold stimuli by about 300% and also increased the size of the neurons' receptive fields for moving electrolocation targets and the resolution with which they encode target distance. The neurons' threshold sensitivity and tuning to amplitude modulation frequency were unchanged by removal of the descending input. The results of this study show that this portion of the descending input to the ELLL normally mediates an inhibition that controls the responsiveness of ELLL output neurons. This descending input could function as a gain control mechanism, allowing the animal to modulate the sensitivity of the electrosensory system in response to changing environmental conditions.