Eigenmannia's frequency and intensity discrimination thresholds were determined for a range of seven frequencies (50–1700Hz), centered on a fish's individual discharge frequency, using a conditioned go/no-go paradigm. The threshold criterion was ‘50% correct choices’ (of the rewarded stimulus, S+, over the unrewarded stimulus, S-); this was validated by testing for statistically significantly shorter response latencies for the S+ compared with the S- stimulus.
The stimuli consisted of sine wave bursts presented for up to 20 s, repeated at 2 s−1 (rise and fall times of an individual burst, 50 ms; holding time, 250 ms; silence, 150ms). When testing for frequency discrimination, the sine wave bursts alternated in frequency (λf); for intensity discrimination, every other sine wave burst was of increased intensity (λI). The reference stimulus intensity was 30 dB with reference to a fish's individual absolute threshold for a continuous sine wave at that frequency, previously determined using a conditioned go response.
Sensory discrimination was best close to a fish's individual discharge frequency. At 30 dB sensation level, fish discriminated frequency differences as small as 0.52 Hz (0.60 and 0.79 Hz in two other individuals) and intensity differences as small as 0.56dB (1 dB in two other fish).
At stimulus frequencies different from a fish's discharge frequency, Eigenmannia's frequency discrimination declined at lower frequencies at a rate of up to 1 Hz octave−1, and at higher frequencies at a rate of up to 3 Hz octave−1. For Eigenmannia's intensity discrimination a similar loss was observed: at frequencies lower than a fish's discharge frequency, intensity discrimination thresholds rose at a rate of less than 1 dB octave−1, while the rate was below 2 dB octave−1 for higher frequencies.
Compared with other acoustico-lateral senses in lower vertebrates, Eigenmannia's electrosensory frequency and intensity discrimination is unusually high, in the range of that known for audition in the most sensitive higher vertebrates with a cochlea (for example, human). This emphasizes Eigenmannia's specialized ‘active’ electrosensory system, which detects the presence of a stimulus field as the modulation of a fish's own ‘carrier’ signal in amplitude and phase (beat analysis), as opposed to ‘passive’ sensory systems, which must deal with unpredictable signals from the environment as they occur.