A Test for
Determining Sensitivity to Temporal Fine Structure
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 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
TFS1 is a computer program that can be used to determine the sensitivity of an individual to the temporal fine structure (TFS) of a sound. It was developed by Aleksander Sek and Brian Moore. TFS1 can be installed on a PC with a standard (good-quality) sound card. It is intended that sounds are delivered via headphones. Each ear is tested separately (the ear to be tested is selected in the software). If required, an external amplifier can be used between the sound card and the headphones.
The method is based on that described in:
Hopkins, K., and Moore, B. C. J. (2007). "Moderate cochlear hearing loss leads to a reduced ability to use temporal fine structure information," J. Acoust. Soc. Am. 122, 1055-1068.
but modified as described in:
Moore, B. C. J., and Sek, A. (2009). "Development of a fast method for determining sensitivity to temporal fine structure," Int. J. Audiol. 48, 161-171.
The task is to discriminate a harmonic complex tone (called H), with a fundamental frequency F0, from a tone in which all frequency components are shifted upwards by the same amount in Hz, D, so as to create an inharmonic tone (called I). Both tones are passed through a fixed bandpass filter which is centred on the upper (unresolved) components. The value of F0 and the centre frequency of the filter can be set by entering the desired values in the boxes which are displayed on the screen.
Some modifications have been made to the procedure described by Moore and Sek (2009):
(1) The starting frequency shift, D, is forced to be 0.5F0. This makes it less likely that the listener will complete the task by chance, when they cannot hear a difference between the H and I tones.
(2) The adaptive procedure used to measure the “threshold” value of D initially uses large steps in D, but switches to smaller steps in D after a while. If the maximum possible shift (0.5F0) is reached at any time following the switch to small steps in D, the value of D is fixed at 0.5F0 and 40 more trials are presented.
A measurement for a single F0 and centre frequency takes about 2 minutes, and only a little training is required.
The software includes an extensive help file with instructions. The software is installed from a CD.
The outcome of the TFS1 test may be useful for the following purposes:
1) For those conducting research on hearing, the test may be useful for characterising the auditory abilities of individual hearing-impaired people, especially when a battery of tests is being administered.
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 not help much with this problem.