Subject: Re: Cochlear AGC attack/releases times? From: David Mountain <dcm(at)BU.EDU> Date: Mon, 23 May 2005 14:56:41 -0400I'll start my answer with my engineer's hat on. I usually usually refer to nonlinearities that are essentially instantaneous as "compression" and nonlinearities that respond to some time average of the signal as "automatic gain control" or AGC. The auditory system includes a lot more than the cochlea so the answer to your question is not straight forward. If you are asking about the nonlinearity in cochlear mechanics, I would say that the answer is that it is essentially instantaneous (comparable to the rise time of the cochlear filters) and hence refer to it as compression. Additional compression takes place in the hair cells due to the nonlinear nature of the transduction process. The synapse between the inner hair cell and the auditory nerve fibers is best described as an AGC with two or more time constants. There is a fast component to the gain change (adaptation in the physiological jargon) that is the major component and has a time constant in the 2-4 ms range. There is a slower component with a time constant in the 10's of ms and there are other components that operate on time scales over over 1 s. Additional adaptation takes place in the central nervous system. The cochlear efferent system also can alter the gain. The primary efferent effect build up with a time constant in the 50-100 ms range but here again there are longer term effects as well. -------------------------------------------------------------------- David C. Mountain, Ph.D. Professor of Biomedical Engineering Boston University 44 Cummington St. Boston, MA 02215 Email: dcm(at)bu.edu Website: http://earlab.bu.edu/external/dcm/ Phone: (617) 353-4343 FAX: (617) 353-6766 Office: ERB 413 On Mon, 23 May 2005, Richard H. wrote: > Hi, > > Does anyone know how the cochlear AGC can be modelled? > > Suppose a short tone burst hits the cochlear, how long does it take for the "gain" to be reduced? And then to recover? > > Are we talking 1 millisec or 1000 millisecs? [Or 0 millisecs i.e. the transfer function is more like an amplitude compander, without > any time aspect] > > And is this AGC very frequency specific - narrow nand, or perhaps critical band? > > Or does the cochear not really work like an electronic AGC [or compander] at all? > > kind regards, > > Richard > >