Physiological correlates of the ‘‘mean-term’’ auditory adaptation

Abstract

In psychophysics, adaptation is defined usually as a decline of loudness of steady stimulus as a function of time. It occurs over periods ranging from seconds to minutes. Recent psychophysical data show that during a ‘‘mean-term’’ adaptation period (the first 30 seconds of the sound) the detection of amplitude changes considerably improves. Physiological correlates of ‘‘mean-term’’ adaptation in single units located in different regions (cochlear nucleus, superior olive, torus semicircularis) of the auditory system of the immobilized frog Rana temporaria are explored. Driven responses to long-duration pure tones and AM tones decreased over time periods ranging from milliseconds to minutes. The adaptation increased at the upper levels of the auditory system. The response to pure tone stimuli declined more rapidly than the response to AM tones. For AM stimuli the adaptation increased as the modulation depth decreased. The considerable enhancement of the phase locking to the sound envelope could be observed during the first 10–15 s of the AM tones with low modulation indexes. The effect of phase-locking enhancement during the mean-term adaptation matched some psychoacoustical findings. [Work supported by the RFBR.]