Spatial Cues and Sensorineural Hearing Loss
Spatial cues such as inter-aural timing differences (ITDs) and inter-aural loudness differences (ILDs) allow listeners to determine the location of sounds in space (i.e., knowing where sounds originate) thus allowing the listener to know where to focus their attention. Further, inter-aural cues have been shown to allow people with normal hearing to better understand speech in degraded signal-to-noise ratios (SNRs). This brain function is referred to as spatial processing and is also referred to as spatial release from masking and is generally compromised in people with sensorineural hearing loss (SNHL).
Understanding speech-in-noise depends on many factors including SNR, the type and degree of hearing loss, the individual’s cognitive processing speed, attention, working memory, and more. However, to understand speech-in-noise also includes the ability to segregate the primary speech signal from background noise.
In this case, the background noise might be referred to as the secondary speech signal. With regard to primary and secondary speech sounds, one well-known dilemma is the fact that human voices (the primary and secondary) overlap with regard to loudness, spectral content and timing cues! That is, we label the speech signal of interest the primary speech signal, and the other voices are generally labelled noise. Nonetheless, the ability to attend to the primary signal despite (multiple) secondary signals is referred to as auditory stream segregation.
Spatial processing is extraordinarily important. People with normal hearing and normal cognitive ability can realize a 15 dB advantage (for the primary speech signal) as the speech source and/or noise source move around the listener. Unfortunately, people with SNHL acquire less useful information from spatial cues and people with SNHL require a better SNR than normal hearing people to understand speech in noise equally well.
Glyde et al (2014) sought to determine whether the LiSN & Learn auditory training program would be beneficial with regard to improving the spatial processing ability of hearing-impaired adults and children. The LiSN & Learn program was designed for normal hearing children and has been successfully applied to that same population. LiSN & Learn has five computer-based games in which target sentences are processed with head-related transfer functions and are perceived as coming from 0 azimuth while simultaneous distracting speech is delivered from other locations. The test is adaptive and the SNR changes based on the correct or incorrect response from the subject.
The authors report training with “LiSN & Learn did not significantly improve spatial processing deficits in adults or children with SNHL....”
Glyde H, Cameron S, Dillon H, Hickson L. (2014) Remediation of Spatial Processing Deficits in Hearing-Impaired Children and Adults. Journal of the American Academy of Audiology 25(6):549-561.