Cochlear compression delay implies automatic gain control circuitry

Evidence

Our observation of a distinct delay to the appearance of compression (or gain) in the response envelopes is new, however. We currently believe that it is impossible to produce such a delay without resorting to a non-instantaneous form of gain control, such as an AGC. Other (non-AGC) types of system which produce level-dependent group delays can easily mimic the envelope asymmetry (hysteresis) effects that we observe, but (to the best of our knowledge) they cannot produce an actual delay in the “effect” of the nonlinearity. — Dijk, Pim van, Physiology, Psychoacoustics and…, 3659

Basilar-membrane (BM) vibrations in healthy cochleae grow compressively with the intensity of stationary sounds such as tones and broad band noise (de Boer and Nuttall 1997 ; Rhode 1971 ; Robles and Ruggero 2001 ). Similarly compressive nonlinearities also affect mechanical responses to pulsatile stimuli such as clicks (e.g. Robles et al. 1976 ), and in two-tone experiments (Robles et al. 1997 ), where they reputedly act on a cycle-by-cycle basis to generate distinctive patterns of intermodulation distortion products. The extent to which a compressive input-output relation holds when the intensity of a stimulus fluctuates in time has not been studied systematically, however, and the possibility that the cochlea operates more like an automatic gain control (AGC) system than an instantaneously nonlinear system remains feasible (van der Heijden 2005 ). — Dijk, Pim van, Physiology, Psychoacoustics and…, 3629

Our observation of a distinct delay to the appearance of compression (or gain) in the response envelopes is new, however. We currently believe that it is impossible to produce such a delay without resorting to a non-instantaneous form of gain control, such as an AGC. Other (non-AGC) types of system which produce level-dependent group delays can easily mimic the envelope asymmetry (hysteresis) effects that we observe, but (to the best of our knowledge) they cannot produce an actual delay in the “effect” of the nonlinearity. If this delay really does signify the presence of an AGC in the cochlea, it may well prove significant for the coding of a wide range of stimuli, and have consequences for numerous psychophysical phenomena. — Dijk, Pim van, Physiology, Psychoacoustics and…, 3657

Categorization

• QUESTION: Does the cochlea genuinely implement an AGC-like mechanism, and what psychophysical phenomena does this delay explain?
• XREF: Connects to the reading purpose on physiology and psychoacoustics, and to how compression underlies loudness perception and OAE-based clinical tests. — connects to: The cochlea compresses a 100 dB input range into about 57 dB of response.; Ideal audio reproduction tolerates delay and gain because systems are causal; Cochlear compression operates via a gain control with finite attack and release times. — Dijk, Pim van, Physiology, Psychoacoustics and…, 3625 (+4 more)
• DEEPTHINKING: The argument that only a true non-instantaneous AGC (not mere level-dependent delay systems) can produce a delayed onset of compression is a subtle inference distinguishing mechanisms — could repay long thought.
• corroborated: 4 independent passages support this claim — Dijk, Pim van, Physiology, Psychoacoustics and…, 3659; Dijk, Pim van, Physiology, Psychoacoustics and…, 3629; Dijk, Pim van, Physiology, Psychoacoustics and…, 3657

Broader Connections

• Cochlear gain control lags stimulus envelope changes by a fixed ~50 μs delay #corroborates (0.85)
• The cochlea may regulate gain slowly rather than compress sound instantly #expands (0.82) — Dijk, Pim van, Physiology, Psychoacoustics and…, 3628
• Faster-fluctuating sounds trigger less cochlear compression, trading sensitivity for better dynamic encoding #expands (0.73) — Dijk, Pim van, Physiology, Psychoacoustics and…, 3637
• Cochlear compression shows hysteresis with faster attack than decay at high modulation rates. #complicates (0.72)