Spatial Hearing, ANSD, and Cochlear Implants
Audiologists are familiar with the three primary acoustic cues related to spatial hearing: (1) interaural loudness differences(ILDs), (2) interaural timing differences (ITDs), and (3) concha/pinna effects. However, there is another aspect of spatial (i.e., "3D") hearing that allows us to determine how far away sounds are using reverb, loudness cues, binaural hearing, and the brain’s vast processing power.
Indeed, to understand speech in noise and to know where to "focus the brain," ("where to listen") amplification devices, hearing aids, and cochlear implants etc. must maintain the natural acoustic spatial cues such as ILDs, ITDs, etc., as those are the cues normal hearing people use to understand speech in noise (Kopco, 2012).
Culling and Akeroyd note the importance of perceiving sounds from both sides. They report that spatial hearing (processes) "assist in determining the location of the sound source, as well as detection and identification of sound in background noise." ILDs are greatest above 1,500 Hz and may be as great as 20 to 30 dB between 5000 and 10,000 Hz. ITDs are most significant below 1,500 Hz and may be as great as 600 to 800 microseconds (millionths of a second). Of note, people with normal hearing can detect 10 microseconds difference with regard to ITDs, and people with normal hearing can detect spatial angle changes as small as one degree (Akeroyd and Whitmer, 2011).
Johnstone, Yeager, and Noss (2013) refer to Zeng et al (2005), who reported three young adults with auditory neuropathy spectrum disorder (ANSD) who had normal ILDs, but could not use (i.e., lateralize) as ITD cues were manipulated. Johnstone, Yeager, and Noss report a young man with ANSD who received spatial hearing tests before and (nine months) after his second cochlear implant (CI). Following his second cochlear implant, his spatial hearing ability improved. Indeed, his sound localization accuracy (SLA) improved by some 13 degrees and his spatial release from masking (SRM) increased approximately 4 dB. The authors report that spatial hearing ability in people with ANSD is not well understood and the results of their subject with ANSD and bilateral CIs may not be typical. Nonetheless, theirs is the first study to indicate "bilateral, sequential CIs could improve both the SRM and SLA in a child with ANSD as compared to performance prior to receiving a second CI."
For More Information, References and Recommendations
Akeroyd MA, Whitmer WM. (2011) Spatial Hearing and Hearing Aids. ENT & Audiology News. 20(5):76-77.
Beck DL, Sockalingam R. (2010) Facilitating Spatial Hearing Through Advanced Hearing Aid Technology. Hearing Review April.
Beck DL, Flexer C. (2011) Listening is Where Hearing Meets Brain. Hearing Review February.
Culling JF, Akeroyd MA. (2010) "Spatial Hearing." In The Oxford Handbook of Auditory Science: Hearing. Chief Editor David R. Moore. Volume 3. Editor Christopher J. Plack. Oxford University Press.
Johnstone PM, Yeager KR, Noss E. (2013) Spatial Hearing in a Child with Auditory Neuropathy Spectrum Disorder and Bilateral Cochlear Implants. International Journal of Audiology. 52:400-408.
Zeng FG, Kong YY, Michalewski HJ, Starr A. (2005) Perceptual Consequences of Disrupted Auditory Nerve Activity. J Neurophysiol 93:3050-1063.