Assessing Temporal Fine-Structure processing: "Derived Measures"
Wallaert, Nicolas; Grousseau, Amandine; Gnansia, Dan; Lorenzi, Christian
Ecole normale supérieure, Paris
Background: Several studies suggest that ageing and cochlear damage degrade the ability to use temporal fine-structure (TFS) cues. However, these deficits may partly result from reduced "processing efficiency". The latter reflects high-level (i.e. cognitive) factors such as the ability to concentrate, the fidelity of the short-tem memory, and the ability to make an optimal decision. Here, we assessed the effect of changing (center) audio-frequency in several auditory tasks to separate sensitivity to TFS and processing efficiency for normal-hearing (NH) and hearing-impaired (HI) listeners. If processing efficiency is the same for extraction of TFS information at two (low) frequencies, then the effect of efficiency should cancel out when subtracting perceptual scores measured at each frequency, leaving only the temporal factor. It was reasoned that derived scores (estimated for each task) should be affected by cochlear damage and/or ageing if the latter affects the roll-off of phase locking to TFS cues in auditory-nerve fibers.
Methods: Psychoacoustic measures of sensitivity to TFS were obtained at two low centre frequencies of 0.5 and 1 kHz. We measured: i) thresholds for detecting a sine frequency modulation (FM) of 5 Hz (a masking sine AM (5 Hz) was added to the FM tone to disrupt excitation-pattern cues), (ii) thresholds for detecting a change in interaural phase (IPD), and iii) the “binaural masking level difference” effect (BMLD) which involves detection of a pure tone in noise, in diotic and dichotic conditions. Psychoacoustic measures of sensitivity to temporal-envelope cues were also obtained at the same centre frequencies by measuring thresholds for detecting a 5-Hz, sine amplitude modulation (AM). In each task, the derived score was computed by subtracting perceptual thresholds at the two center frequencies. Two groups of young and elderly NH and HI listeners participated in this study.
Results: Preliminary results showed that only the derived measures assumed to rely on the fidelity of TFS encoding (FM, IPD, BMLD tasks) were affected by cochlear damage and ageing. These pilot data are consistent with the notion that cochlear damage and ageing degrade the accuracy of neural phase locking to TFS cues.
Funding: This work was supported by NEURELEC/Oticon-Medical and ANR HEARFIN.