Cochlear Modelling

In this section you can find two types of information : (1) Brief descriptionsof recent developments in cochlear modeling, related more to the OAE function than to the physiological structure of the cochlea; (2) Brief comments on issues related to cochlear functionality.

Recent Developments

        From the May -June 2002 Editorial

The traditional categorization of OAEs often divides them simply into two categories based upon the stimulus parameters needed to evoke the specific classes of OAEs (Probst et al 1991). For example, spontaneous OAEs (SOAEs) are in a class of their own in that no stimulus is required to evoke these emissions. Transient-evoked OAEs (TEOAEs), stimulus-frequency OAEs (SFOAEs), and DPOAEs are placed in the other category referred to as the stimulus-evoked emissions in that all these OAEs are elicited by applying deliberate acoustic stimulation to the ear. A major limitation of this simple classification scheme is that little information is provided about the mechanisms of generation for the unique subtypes of OAEs. Generally, under this schema, all OAEs are assumed to arise from the same nonlinear mechanical workings that underlie cochlear processing (eg, Kemp 1978; Kemp & Brown 1983). Recently, Shera and Guinan (1999) presented a taxonomy for mammalian OAEs that can be experimentally verified (Kalluri & Shera 2001). In this conceptualization, Shera and Guinan (1999) proposed that OAEs arise from two fundamentally different mechanisms. Thus, there are OAEs that arise by linear reflection and those that are generated by nonlinear distortion. This distinction forms a 'family tree' of OAEs in which TEOAEs, SFOAEs, and SOAEs are based upon linear reflections, whereas DPOAEs are produced mainly from nonlinearities acting as emission sources. This classification system is extremely useful in that OAEs can be categorized based upon their mechanisms of generation. Thus, the familiar click-evoked TEOAEs come from reflection off of pre-existing micromechanical impedance perturbations, distributed along the organ of Corti, which might include such conditions as disorganized outer hair cell (OHC) arrays (eg, Lonsbury-Martin et al 1988), that are unique to each cochlea. On the other hand, DPOAEs arise primarily from nonlinear elements in the cochlea that are stimulated by the in-coming traveling waves. What is most important to realize is that OAEs recorded in the ear canal, especially in humans, are rarely due purely to one form or the other, but represent a mixture of the two emission sources.


Recent Comments



  •  We have recommended numerous times to our visitors, who are interested in cochlear biophysics to visit the site, in order to get the latest information on the status of cochlear models and OAEs. One of the most recent postings is quite interesting, as it refers to new studies suggesting that the backward traveling wave (which in theory generates what we record as TEOAEs and DPOAEs) actually does not exist.
                      "On Tues, 30 Mar 2004, This email address is being protected from spambots. You need JavaScript enabled to view it. wrote: List members will no doubt be interested to note a recent paper by Tianying Ren [Nature Neuroscience, 21 March 2004] which supplies a convincing demonstration that the reverse traveling wave in the cochlea does not exist. Instead, Ren could find only a fast backward compressional (pressure) wave. The reverse traveling wave is a key entity required by conventional cochlear mechanics to explain acoustic emissions of all kinds, including, in the case under observation, distortion product emissions (DPOAEs). The 2f1-f2 distortion product, for example, originates at a location on the partition through non-linear interaction of two primary tones (e.g., 17000 Hz and 15455 kHz in one gerbil experiment reported) giving a strong vibration at 13910 Hz; this tone is presumed to travel backwards, via a traveling wave of displacement, to the stapes and the ear canal, where it is detected. To Ren's surprise, he found with his scanning laser interferometer that he could detect no displacement of the basilar membrane at 13910 Hz until after_ the stapes had started vibrating at this frequency. That is, the tapes vibrated some 50 us before the basilar membrane did. There must have been a fast (nearly instantaneous) compressional wave at 13910 Hz which originated at some distance along the partition and excited movement of the stapes; only later did the basilar membrane join in.
                       This paper will cause us to reexamine the basics of how the cochlea works. A couple of years back discussion on this list centred around the question of whether the (forward-) traveling wave is itself an epiphenomenon, arising only as a consequence of outer hair cells reacting to a fast pressure wave entering the cochlea. This latest work gives strong support to this notion. If outer hair cells can produce a fast pressure wave, it seems natural to suppose that, by reciprocity, the fast pressure wave is the key stimulus in the cochlea."