Auditory adaptation time course in the Yangtze finless porpoises, Neophocaena asiaeorientalis asiaeorientalis

Auditory adaptation manifests as temporary change in the hearing sensitivity under influence of sound stimuli, and little attention has been paid to hearing adaptation in cetaceans. In a Yangtze finless porpoise Neophocaena asiaeorientalis (Cetacea, Phocoenidae), effects of long-lasting sound stimuli (pip trains) on auditory evoked potentials (AEP) were investigated. The stimuli were rhythmic tone pips of either 90 or 128 kHz frequency. Stimulus levels varied from 80 to 120 dB re 1 μPa. During stimulation, the stimulus level was either kept constant the stimulus level changed up and down by 20 or 40 dB every 1000 ms. A stimulus level increase produced quick response amplitude increase with subsequent slower exponential decay; the exponent time constant was 59.2 ± 9.7 ms. A stimulus level decrease produced quick response amplitude suppression with subsequent exponent recovery; the exponent time constant was 153.9 ± 15.9 ms. The response variations were assessed as manifestations of adaptation processes. The finless porpoise is capable of quickly adjusting its sensitivity to the current auditory scene. This flexibility allows for quick sensitivity recovery after exposure to loud noises and is important for the effective use of auditory orientation. Abbreviations: RFR: rate-following response; AEP: auditory evoked potentials; A/D: analogue-to-digital (A/D); ERD: equivalent rectangular duration