Sound intensity processing under continuous adaptation

Abstract

Using classical respiratory conditioning, the smallest detectable changes in the intensity of a continuous sound were measured in the goldfish as functions of signal duration and overall level. Gap detection thresholds for noise were also obtained. Neural correlates were studied in single auditory (saccular) nerve units. Noise increment detection shows perfect temporal summation (10–160 ms) in that equivalent signal levels for in‐phase addition fall by 3 dB per doubling of duration. Thresholds are independent of overall level, in accordance with Weber's law. Noise decrement thresholds grow more rapidly toward a minimum gap of 35 ms. Tone increment and decrement thresholds are identical (0.1–0.2 dB), and show no duration effect. Increment thresholds decline significantly with overall level between 15 and 40 dB SL (Weber's law does not hold). Auditory nerve units vary widely in adaptation patterns and in sensitivity to intensity changes. Recovery from adaptation (a burst of spikes evoked as the sound intensity returns to the adapting level following a decrement) has a different time course for tones and noise and may account for gap detection thresholds. These results are discussed in the context of current theories on hair cell‐nerve fiber synaptic mechanisms. [Supported by the NSF and the NIH (NINCDS).]