Subject: Re: AW: Cochlear nonlinearity & TTS From: Erik Larsen <elarsen@xxxxxxxx> Date: Thu, 18 Jan 2007 16:04:52 -0500 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>From that point of view, the cochlea might be better described as an acoustic feature extractor rather than a compression system. The latter would imply that at some point decompression occurs, yielding a (approximate) replica of the acoustic signal somewhere in the CNS; I would guess this does not happen (what would be the point?). If we agree that the brain is only interested in specific features of the acoustic signal (those that we perceive as pitch, loudness, location, timbre, etc), there is no need for a compression/decompression system. It would be more efficient to extract the features as accurately as possible without considerations as to whether the operations are invertible. Whether it is possible in principle to reconstruct the acoustic signal from the nerve output is a different question. You would definitely need the spikes from all nerve fibers (at least the more, the better), and you won't be able to get it exactly right (if you don't have a priori knowledge about the signal). Erik -- Erik Larsen, Ph.D. candidate Speech and Hearing Bioscience and Technology Harvard-MIT Division of Health Sciences and Technology Cambridge MA 02139 http://web.mit.edu/shbt David Mountain wrote: > believe that the available data suggest that the cochlear amplifier only > has a significant effect over a small region at and basal to the peak of > the traveling wave. See for example: > > Cody AR (1992) Acoustic lesions in the mammalian cochlea: implications for > the spatial distribution of the 'active process'. Hear Res. 1992 > Oct;62(2):166-72. PMID: 1429258 > > I would rephrase Ramda's question to be the question of whether the > transformation between sound and auditory nerve firing pattern is > invertable. I think the answer in the exact sense is that it is not > invertable. It is my belief that we are dealing with a compression system > with some loss but one that preserves the features of biological > relevance. Modern audio compression schemes take advantage of this fact > and throw away acoustic information that is not perceived or only barely > detectable by the listener. > > > -------------------------------------------------------------------- > > David C. Mountain, Ph.D. > Professor of Biomedical Engineering > > Boston University > 44 Cummington St. > Boston, MA 02215 > > Email: dcm@xxxxxxxx > Website: http://www.bu.edu/dbin/bme/faculty/?prof=dcm > Phone: (617) 353-4343 > FAX: (617) 353-6766 > Office: ERB 413 > On Thu, 18 Jan 2007, Ramdas Kumaresan wrote: > >> Navid, Richard and the listees, >> >> I have heard a lot of speculation about the cochlear amplifier for many >> years. One of the questions that I have wondered about >> as a signal processing engineer for many years, is with all the >> sophisticated nonlinearities, delays, amplifiers, filters >> etc that are present in the auditory periphery, how does it "represent" >> an acoustic signal in the neural spike patterns >> that emanate from the auditory periphery? (I guess everyone wonders >> about it.) >> Is it possible to reconstruct the acoustic signal if you were able to >> measure/monitor the >> spike patterns that are put out by all the auditory nerve fibers? What >> is the reconstruction 'algorithm"? >> (I know about Egbert deBoer's reconstruction method for a single >> nerve fiber.) Is'n't the information about the signal >> distributed across many, many nerve fibers? Should'nt the >> reconstruction take information from >> all nerve fibers and fuse them to reconstruct the signal? Just wondering >> aloud. RK >> >> >> >> >> >> Richard F. Lyon wrote: >> >>> At 9:17 AM -0800 1/16/07, Navid Shahnaz wrote: >>> >>>> Thank you Reinhart for your clarification. Does the cochlear >>>> amplifier works on both sides of the excitation pattern peak on the >>>> BM? or the amplifier operates wore efficiently at a place that is >>>> just above or toward the apex from the point of disturbance created >>>> by travelling wave? Operationally this point may be an ideal point as >>>> it is less likely saturates the amplifier due to sharp slope of the >>>> travelling wave on the apical side. >>>> Cheers >>>> Navid >>> >>> Navid, >>> >>> Both Monita and Reinhart have given good explanations, but let me add >>> a bit. >>> >>> The way I think of it, the active amplification is active everywhere, >>> but it competes with the passive loss mechanisms, and is only >>> significant at low enough levels. The active loss mechanism (damping) >>> increases rapidly apically when a sine wave travels past a >>> characteristic place. Because of the active gain, the response to a >>> sine wave can travel further before it damps out; from the "passive >>> peak" that Reinhart mentions, the peak response location can be >>> further apical, up to about a half octave worth of place further, when >>> the active amplification is significant, to the "active peak". The >>> "net" amplification is positive (in dB per mm or whatever) before the >>> response peak, and negative after the response peak, pretty much by >>> definition of peak. That net includes the active gain, which >>> saturates, and the passive loss, which doesn't, so it's level dependent. >>> >>> In addition to the saturation that reduces the active gain at high >>> level, there is also efferent control that turns down the gain in >>> response to afferent response level and possibly other central control >>> signals. This effect of efferent control of mechanical gain has been >>> directly demonstrated, but I don't recall exactly who/when/where to >>> cite right now. >>> >>> Dick >>> -- Erik Larsen PhD candidate Speech and Hearing Bioscience and Technology http://web.mit.edu/shbt 'The scientist does not study Nature because it is useful; he studies it because he delights in it, and he delights in it because it is beautiful. If Nature were not beautiful, it would not be worth knowing, and if Nature were not worth knowing, life would not be worth living.' -Henri Poincare