Neuronal sensitivity to microsecond time disparities in the electrosensory system of Gymnarchus niloticus

To perform the jamming avoidance response (JAR), the weakly electric fish Gymnarchus detects time disparities on the order of microseconds between electrosensory signals received by electroreceptors in different parts of the body surface. This paper describes time-disparity thresholds of output neurons of the electrosensory lateral line lobe (ELL), where the representation of timing information is converted from a time code to a firing-rate code. We recorded extracellular single-unit responses from pyramidal cells in the ELL to sinusoidally modulated time disparity with various depths (0-200 micros). Threshold sensitivity to time disparities measured in 123 units ranged from 0.5 to 100 micros and was < or =5 micros in 60% of the units. The units from pyramidal cells in the inner and outer cell layers of the ELL responded equally well to small time disparities. The neuronal thresholds to time disparities found in the ELL are comparable with those demonstrated in behavioral performance of the JAR. The sensitivity of ELL units to small time disparities was unaffected when the center of the cyclic time-disparity modulation was shifted over a wide range (up to 250 micros), indicating an adaptation mechanism for steady-state time disparities that preserves the sensitivity to small dynamic changes in time disparities. Phase-locked input neurons, which provide time information to the ELL by phase-locked firing of action potentials, did not adapt to steady-state time shifts of sensory signals. This suggests that the adaptation emerges within the ELL.