A Test for Determining Binaural Sensitivity to Temporal Fine Structure:
The TFS-AF test
Most of the sounds that we encounter in everyday life, including speech and music, are broadband; their spectra cover a wide range of frequencies. When such sounds are analyzed in the cochlea of a normal ear, the result is a series of bandpass-filtered signals, each corresponding to one position on the basilar membrane. Each of these signals contains two forms of information; fluctuations in the envelope (E, the relatively slow variations in amplitude over time) and fluctuations in the temporal fine structure (TFS, the rapid oscillations with rate close to the center frequency of the band). Information about the TFS is conveyed in the temporal patterns of firing in the auditory nerve, and it may play an important role in pitch perception, masking, and the perception of speech in background sounds.
Recent evidence suggests that people with cochlear hearing loss, and older people with or without hearing loss, have a reduced ability to process information carried in the TFS, and this contributes to their difficulties in understanding speech in the presence of background sounds. For a review, see
Moore, B.C.J. 2008. The role of temporal fine structure processing in pitch perception, masking, and speech perception for normal-hearing and hearing-impaired people. J Assoc Res Otolaryngol, DOI: 10.1007/s10162-008-0143-x. http://dx.doi.org/10.1007/s10162-008-0143-x
The software that you can download from this website implements the TFS-AF test described in
Füllgrabe, C., Harland, A. J., Sek, A. P., and Moore, B. C. J. (2017). "Development of a method for determining binaural sensitivity to temporal fine structure," Int. J. Audiol. (in press). DOI: 10.1080/14992027.2017.1366078
The task involves discrimination of the lateral position of sinusoidal tone bursts based on an interaural phase difference (IPD). The envelopes of the tones are synchronous across the two ears, so the task can only be performed if the listener is sensitive to IPD. The task is similar to the earlier TFS-LF test described in
Hopkins, K., and Moore, B. C. J. (2010). "Development of a fast method for measuring sensitivity to temporal fine structure information at low frequencies," Int. J. Audiol. 49, 940-946.
Sek, A., and Moore, B. C. J. (2012). "Implementation of two tests for measuring sensitivity to temporal fine structure," Int. J. Audiol. 51, 58-63.
However, in the TFS-LF test the frequency was kept fixed and the IPD was varied to determine the threshold, whereas in the TFS-AF test the IPD is fixed and the frequency is varied to determine the threshold. Higher thresholds on the TFS-AF test indicate better performance.
The task structure is similar to that for the TFS-LF test. In each interval there are four successive tones. In one interval the tones are all in-phase at the two ears. For a person with symmetric hearing, these tones all appear to be at the same location within the head. In the other interval, tones 1 and 3 are in-phase at the two ears, but tones 2 and 4 have an IPD of j, giving a pattern of IPD: 0 j 0 j. If the listener is sensitive to IPD, these tones appear to move within the head. The listener is asked to pick the interval in which the tones appear to move.
A measurement for a single frequency takes about four minutes, and only a little training is required.
Use of the tests
The outcome of the TFS tests may be useful for the following purposes:
1) For those conducting research on hearing, the test may be useful for characterising the binaural abilities of individual hearing-impaired people.
2) For those involved in the clinical assessment and treatment of hearing-impaired people, the test may be useful for counselling a hearing-impaired client. A person with little or no sensitivity to TFS is likely to have difficulty in understanding speech when background sounds are present, and a hearing aid may be of limited help.
3) The tests may provide early signs of dysfunction of the auditory system, for example, associated with acoustic neuroma and mild auditory neuropathy.