Development of a new test for determining binaural sensitivity to temporal fine structure
Speech identification in noise has been shown to be associated with a listener’s sensitivity to the temporal-fine-structure (TFS) of sounds. Both hearing loss and age (Hopkins & Moore, 2011; Füllgrabe, 2013) seem to adversely affect the ability to process TFS information. Hence, there has been keen interest in the development of tests that could be used in the clinic or in large-scale research studies to assess sensitivity to TFS.
Moore and colleagues designed two such tests: the monaural TFS1 test that requires listeners to discriminate complex harmonic tones from the same tones with all components shifted upwards by the same amount in Hertz (Moore & Sek, 2009); and the binaural TFS-LF test that determines a listener’s detection threshold for an interaural phase difference (IPD) for pure tones of a given low frequency (Hopkins & Moore, 2010). However, several studies have reported that some listeners were unable to reliably perform the tests, so a graded measure of sensitivity to TFS could not be obtained for all listeners.
To address this limitation, a new binaural TFS test was developed and validated, the so-called TFS-AF test (with AF standing for ‘adaptive frequency’). In this test, listeners are required to detect an IPD in pure tones but, in contrast to the TFS-LF test, the IPD is fixed while the frequency of the pure tones is adaptively varied to determine the threshold.
Normative data for a range of IPDs were obtained for young (19-25 years) listeners with audiometrically normal hearing (≤20 dB HL). Results revealed thresholds between 950 and 1600 Hz for IPDs of at least 90º, and no evidence for improved sensitivity following protracted practice (Füllgrabe et al., 2015). In a follow-up study, IPD sensitivity was assessed in older (>60 years) adults with normal hearing to investigate if: (i) listeners can perform the TFS-AF test even when performance on the TFS-LF test is not possible, and (ii) familiarity with the test procedure and stimuli plays a greater role for listeners from an age range that is more representative of audiological patients. Results indicated that all older listeners could perform the TFS-AF test and there were no or only small practice effects.
The results suggest that IPD sensitivity is fairly stable across frequencies (on average, ≤1300 and 1000 Hz for the young and older normal-hearing listeners, respectively) and that the TFS-AF test is suitable for the rapid assessment of TFS sensitivity in untrained listeners.