Spatial Hearing Disorders: Update 2014
Harvey Dillon (2012) defined spatial processing disorders (SPD) as a specific form of auditory processing disorders (APD). He noted that people with SPDs demonstrate a reduced ability to separate speech from noise using spatial cues, which generally manifests in particularly noisy situations. Dillon reported one of the major reasons people with sensorineural hearing loss fail to do well in noise, even with well-fitted hearing aids, is they are not able to benefit from acoustic spatial cues (which people with normal hearing use) to know where to focus their attention. Dillon (2013) added "…some children…are not able to use acoustic spatial cues like most people can…. These children generally have normal hearing thresholds…normal intelligibility for words in quiet, and very likely they have normal intelligibility in noise, too. However, if you move the speech and the noise around the child, such that the speech signal is coming from straight ahead, and let's presume the noises are on the left and the right, these children are not able to focus on the frontal sound and suppress the other sounds as well as most people can. We refer to these children as having a "spatial hearing disorder."
Cameron and Dillon (2014) report "spatial release from masking" is synonymous with "spatial processing." They describe spatial processing akin to …while trying to listen to speech in noisy environments, auditory processes within the brain help us focus on the target sound while suppressing competing sounds from different locations. Indeed, they report the target sound might "pop out" (i.e., spatial release from masking) if we’re able to spatially process these acoustic sounds maximally. However, for children with SPD, the target sounds don’t pop out, and in fact, they likely hear an ongoing, monotonous "jumble" of sounds. They report "We are certainly not suggesting that SPD is the only cause of difficulty understanding speech in background noise fir children with normal hearing thresholds, but it is an important cause." Indeed, Cameron and Dillon report "For many children, it (SPD) is the only cause."
They report that for children, the ability to use spatial processing to effectively follow the teacher’s voice amongst other voices and background noise "is critical to their ability to understand speech in the classroom." Cameron and Dillon state interaural timing differences (ITDs) are theoretically "0" when the sound originates directly in front of the child (O degrees azimuth). However, when sounds originate from 90 degrees to either side, the ITD may be 0.7 msecs, leading to an interaural phase delay. With regard to interaural loudness differences (ILDs), they report new research indicates that for people with normal hearing, ILDs are the dominant mechanism enabling spatial processing.
Of importance, they report SPDs might be very well managed via an improved signal-to-noise ratio (SNR) provided through FM, and arguably, remote microphones. Importantly, multiple studies have shown SPDs can be diagnosed using the LiSN-S (Listening in Spatialized Noise – Sentences) and aural rehabilitation (for SPD) has been successfully implemented (with long lasting, apparently permanent carryover) with the LiSN & Learn training program, available via NAL (National Acoustics Laboratories)
For More Information, References and Recommendations:
Cameron S, Dillon H. (2014) Remediation of Spatial Processing Issues in Central Auditory Processing Disorder (Chapter 8, pages 201 to 224) in: Chermack and Musiek (eds) text: Handbook of Central Auditory Processing Disorder, Comprehensive Intervention, Volume 2, Second Edition. Plural Publishing.