Responses of electrosensory neurons in the torus semicircularis of Eigenmannia to complex beat stimuli: testing hypotheses of temporal filtering

The weakly electric fish, Eigenmannia, changes its frequency of electric organ discharges (EODs) to increase the frequency difference between its EODs and those of a jamming neighbor. This jamming avoidance response is greatest for frequency differences (i.e., beat rates) of approximately 4 Hz and barely detectable at beat rates of 20 Hz. A neural correlate of this behavior is found in the torus semicircularis, where most neurons act as low-pass or band-pass filters over this range of beat rates.This study examines two mechanisms that could possibly underlie low-pass temporal filtering: 1) Inhibition by a high-pass interneuron. 2) Voltage and time-dependent conductances associated with ligand-gated channels. These mechanisms were tested by recording intracellularly while employing stimuli consisting of simultaneous low and high beat rates. A neuron's response to the low beat rate was not diminished by the addition of the higher frequency jamming signal (thereby superimposing a high rate of amplitude and phase modulation onto the lower rate), and the inhibitory interneuron hypothesis is, therefore, not supported. Also, the responses to the high beat rate were not facilitated during maintained depolarization in response to the low beat rate.In some cases, particularly band-pass neurons, accommodation processes appeared to contribute to the decline in the amplitude of psps at high beat rates.