Understanding Spatial Hearing Loss and Its Assessment
Spatial hearing is the ability to use auditory cues to determine the location, direction, and distance of sound in space. Being able to understand speech in a complex, dynamic, and often noisy environment is taken for granted by normal-hearing people.
A normal-hearing person is able to find all sound sources in the environment, focus on what is judged to be the most important sound source, and ignore competing sounds. Their mental resources can be focused on the information conveyed by a foreground speaker, while spending limited resources on surveying speakers and noises that are perceptually placed in the background.
However, for the hearing-impaired listener, these tasks are much more challenging. The organization of a complex sound environment takes a lot of mental capacity, detracting from one’s focus on the information coming from the foreground speaker. Listeners with unilateral hearing loss (UHL) typically have difficulty understanding speech in the presence of competing sound; this is likely due to the lack of access to spatial cues.
Enabling people with hearing loss to be able to better organize the spatial complexity of typical daily life environments has been a key goal in the development of hearing aids.
Environments with spatial complexity are common in the world of today. Imagine sitting at home watching TV. When only the TV is heard, the situation is simple. But, when a car passes by outside, our attention is attracted to it, and we can deduce a lot about the car even though we cannot see it.
What direction is it driving in? Is it headed for our driveway? As we start noticing other things than the TV, the environment becomes complex. From the sounds of the car, we are able to deduce the location of the car every instant, and from that we determine if it is something we need to pay attention to.
Figure 1. Spatial separation.
Key Components of Spatial Hearing
When we perceive sound in a spatially complex environment, the key components of the sound that enable our brain to decode the spatial complexity are the differences in time and the level of the sounds reaching our two ears. These are known as interaural time differences (ITD) and interaural level differences (ILD).
It has long been widely accepted that the interaural time differences, or ITDs, are the key for locating sources in the horizontal plane. This also holds true for familiar situations in which the listeners’ expectations completely match the actual course of events.
However, when there is uncertainty in the environment, which occurs in many daily life situations, the interaural level differences, or ILDs, seem to play an enhanced role. Singh et al have shown that, in a situation where only ITDs were present and the location of the target talker was uncertain (ie, the subject had to listen for a specific cue to find the correct response), average speech recognition was around 20%. When ITDs and ILDs were both present, the performance rose to 50%.
 and Interaural Time Differences (ITD))
Figure 2. Interaural Level Differences (ILD) and Interaural Time Differences (ITD): blue and black lines represent sound at the listener’s right and left ears, respectively. ITDs, or the time differences of sounds reaching the right ear versus left ear, are the key for locating sources in the horizontal plane.
Bandwidth is another important aspect of spatial hearing. The ITDs are predominately a low-frequency phenomenon, being effective only below 1500 Hz, and thus quite robust relative to most types of hearing loss.
Figure 3. Magnitude of the interaural level difference, or the level differences between a speech (blue line, right ear) andnoise (black line, left ear) signal. ILDs first start reaching a magnitude of 5 dB at frequencies higher than about 1000 Hz; above about 6000 Hz, they can reach 20 dB or more.
Consider a situation where a noise source is placed on a listener’s left side and a speech source is placed on the right. The speech reaching the right eardrum is amplified, whereas the noise reaching the right eardrum is amplified differently, creating the so-called “better-ear” effect on the right ear. Therefore, the more audibility is provided at higher frequencies, the more access is provided to locally favorable signal-to-noise ratios (SNR) at high frequencies.
Spatial cues are mediated by the sound sources that are active at any moment in time. Each sound source has its own unique characteristics, including how much energy is present at different frequencies. The frequency response of the hearing aid contributes to determining how we perceive the source.
Hearing aid acoustics change the way sound is perceived. One aspect of this is the output stages, which, in the case of a thin tube BTE solution, impose peaks and notches on the frequency response of the hearing instrument. These peaks and notches can be mistaken for, or can partly conceal, monaural spatial cues that are used to determine whether a sound source is located directly in front of or directly behind the listener.
Figure 4. Frequency response of RITE (blue) versus a typical thin-tube solution (grey).
Methods for Assessing Spatial Hearing Abilities
The assessment of spatial hearing abilities in individuals with UHL is of growing clinical interest, particularly for everyday listening environments. Current approaches used to measure spatial hearing abilities include:
- Spatial Release from Masking (SRM): This measures the improvement in speech recognition thresholds (SRT) when the target and masker are co-located as opposed to when they are spatially separated, utilizing a sound-field setup.
- Binaural Intelligibility Level Difference (BILD): This elicits improved SRTs by presenting target speech 180° out-of-phase to one ear instead of in-phase to two ears, utilizing headphones.
- Listening in Spatialized Noise-Sentences (LiSN-S) test: This measures improvement in SRTs when the target and masker are spatially separated, simulating a three-dimensional auditory environment under headphones.
Spatial Release from Masking is the improvement in speech recognition thresholds (SRT) when the target and masker are co-located as opposed to when they are spatially separated, utilizing a sound-field setup. The LiSN-S test also measures improvement in SRTs when the target and masker are spatially separated. Although similar, the LiSN-S utilizes a more clinically assessable procedure by simulating a three-dimensional auditory environment under headphones. Akin to the LiSN-S, the BILD also utilizes headphones but instead elicits improved SRTs by presenting target speech 180° out-of-phase to one ear instead of in-phase to two ears.
The purposes of one study were (a) to determine if patterns of individual variability were similar across the three measures for 30 adults with normal hearing and 28 adults with simulated UHL and (b) to evaluate the effects of simulated UHL on performance. Results of this study confirmed the three tests were all sensitive measures of binaural hearing deficits in participants with UHL. Although all measures were correlated with each other, only the measures conducted under headphones (BILD and LiSN-S) were influenced by magnitude of asymmetry.
The Speech, Spatial and Qualities of Hearing Scale (SSQ) has recently been used to assess the effects of bilateral versus unilateral hearing aid fitting. In this study, it was demonstrated that the extra benefit of two hearing aids relative to one hearing aid is especially apparent in complex environments where divided attention or rapid switching of a person’s mental focus is necessary, as well as in situations involving discrimination of distance and movement.
Improvements observed with hearing aids are obtained in a number of dimensions relating to listening in complex environments. These range from indoor social settings, where there is little or no change in the physical layout of the situation, to orientating in a rapidly changing outdoor environment.
Auditory selective attention, or the ability to ignore interfering sounds and voices at will, is used whenever communication is taking place in a competing background. A recent study indicates that elderly hearing aid users can have significant residual abilities in this domain, provided that the different talkers differ clearly in terms of their spatial cues.