WAVE-FORM TUNING OF ELECTRORECEPTOR CELLS IN THE WEAKLY ELECTRIC FISH, GNATHONEMUS-PETERSII

Electroreceptive afferents from A- and B-electroreceptor cells of morm yromasts and Knollenorgans were tested for their sensitivity to differ ent stimulus waveforms in the weakly electric fish Gnathonemus petersi i . Both A- and B-mormyromast cells had their lowest sensitivity to a waveform similar to the self-generated electric organ discharge (EOD) (around 0 degrees phase-shift). Highest sensitivities, i.e. lowest res ponse thresholds, in both A- and B-cells were measured at phase shifts of + 135 degrees. Thus, both cell types were inversely waveform tuned . The sensitivity of B-cells increased sharply with increasing wavefor m distortions. Their tuning curves had a sharp minimum of sensitivity at + 7 degrees phase shift. A-cells had a much broader waveform tuning with a plateau level of low sensitivity from + 24 degrees to -15 degr ees. Across a 360 degrees cycle of phase-shifts, the range of threshol ds was 16 dB for individual B-cells and 4.5 dB for individual A-cells. Knollenorgan afferents were tuned to 0 degrees phase-shifted EODs and had a dynamic range of 12 dB. Lowest sensitivities were measured at a phase shift of + 165 degrees. Experiments with computer-generated sti muli revealed that the strong sensitivity of mormyromast B-cells of EO D waveform distortions cannot be attributed to any of the seven wavefo rm parameters tested. In addition, EOD stimuli must have the correct d uration for B-cells to respond to waveform distortions. Thus, waveform tuning appears to be based on the specific combination of several wav eform parameters that occur only with natural EODs.