Wednesday, 4 December 2013

Holyhour Christian Lorenzi Friday 13 December 2013

Hearing without Hearing.
By Christian LORENZI, Ecole normale supérieure, Paris, France.

The audiogram reflects the limits of the auditory system's  ability to encode acoustic information  in a soundwave. It is measured typically as the detection threshold for a pure tone (a sinusoid) as a function of sinusoid frequency.  The audiogram provides an indication of how well variations in sound pressure are preserved by the auditory system. However, it does not inform about the fidelity of sound encoding. The question of encoding fidelity is crucial because it indicates “the extent to which variations in the strength of a given supra-threshold auditory feature can convey information about the acoustic signal. A limiting case  of encoding  fidelity is that in which sounds are well detected, but no further  information  about the changes in the sounds parameters is  preserved;  in this case, the encoding fidelity is poor” (Wakefield & Viemeister, 1990). Here, we will review psychophysical evidence that such a “limiting case of encoding fidelity” may be frequently observed for patients with sensorineural hearing loss. In other words, we will show that abnormal encoding fidelity may arise despite normal auditory sensitivity for patients with sensorineural hearing loss. This will be demonstrated for several speech identification tasks performed in the low- or mid-frequency regions where patients with high-frequency sensorineural hearing loss show normal or near-normal auditory sensitivity (Léger et al., 2012a,b,c; Bruce et al., 2013; Stasiak et al., 2013; Goodman et al., 2013). Taken together, these results: (i) confirm that normal auditory sensitivity (as measured clinically by the audiogram) is not sufficient to guarantee robust encoding of certain spectral and temporal suprathreshold auditory features critical in daily listening situations, and (ii) raise the need for using discrimination/identification tasks in auditory screening.

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