Adaptation effects on amplitude modulation detection: behavioral and neurophysiological assessment in the goldfish auditory system.

The ability of goldfish to detect the presence of amplitude modulations (AM) impressed on 200, 570 and 800 Hz tones was measured under stimulus conditions producing intermittent, short-term adaptation and continuous, long-term adaptation. Sensitivity to AM under intermittent conditions increased as a function of modulation rate, with thresholds of AM detection occurring between 10 and 25% modulation at 10 Hz and around 2% modulation at 100 Hz. AM sensitivity was independent of carrier frequency and did not change under randomly varying intensity changes. Under long-term adaptation, thresholds of AM detection ranged from 1.3% at 100 Hz to 2.1% at 10 Hz, showing increased sensitivity and less dependence on modulation rate. The effects of overall intensity on AM sensitivity were the same for both conditions, with sensitivity being relatively independent of overall signal level at 10 Hz modulation and dependent on level at 100 Hz. The responses of goldfish auditory neurons to modulated and unmodulated signals were measured under stimulus conditions similar to those for behavioral studies. Single saccular neurons responded to modulated signals with both an increase in average rate above that evoked by the unmodulated signal and with phase-locking to the AM envelope. Rate increments and phase-locking responses were observed in neurons showing significant short-term adaptation to the unmodulated signal, whereas neurons showing no increase in rate or synchronization to the AM envelope showed little or no adaptation to the unmodulated signal. The effects of overall intensity, modulation rate and adaptation duration on neural responses were similar to behaviorally measured effects. These results show that adaptation affects AM detection and that phase-locking to the AM envelope is the most likely basis for behavioral detection.