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 electr ic organ discharges (EODs) to increase the frequency difference betwee n its EODs and those of a jamming neighbor. This jamming avoidance res ponse is greatest for frequency differences (i.e., beat rates) of appr oximately 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 und erlie low-pass temporal filtering: 1) Inhibition by a 'high-pass' inte rneuron. 2) Voltage and time-dependent conductances associated with li gand-gated channels. These mechanisms were tested by recording intrace llularly while employing stimuli consisting of simultaneous low and hi gh beat rates. A neuron's response to the low beat rate was not dimini shed by the addition of the higher frequency jamming signal (thereby s uperimposing a high rate of amplitude and phase modulation onto the lo wer rate), and the 'inhibitory interneuron' hypothesis is, therefore, not supported. Also, the responses to the high beat rate were not faci litated during maintained depolarization in response to the low beat r ate. In some cases, particularly band-pass neurons, accommodation proc esses appeared to contribute to the decline in the amplitude of psps a t high beat rates.