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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