The Effects of Hearing Impairment, Age, and Hearing Aids on the Use of Self-Motion for Determining Front/Back Location

Abstract

Background: There are two cues that listeners use to disambiguate the front/back location of a sound source: high-frequency spectral cues associated with the head and pinnae, and self-motion-related binaural cues. The use of these cues can be compromised in listeners with hearing impairment and users of hearing aids.

Purpose: To determine how age, hearing impairment, and the use of hearing aids affect a listener's ability to determine front from back based on both self-motion and spectral cues.

Research design: We used a previously published front/back illusion: signals whose physical source location is rotated around the head at twice the angular rate of the listener's head movements are perceptually located in the opposite hemifield from where they physically are. In normal-hearing listeners, the strength of this illusion decreases as a function of low-pass filter cutoff frequency, this is the result of a conflict between spectral cues and dynamic binaural cues for sound source location. The illusion was used as an assay of self-motion processing in listeners with hearing impairment and users of hearing aids.

Study sample: We recruited 40 hearing-impaired participants, with an average age of 62 yr. The data for three listeners were discarded because they did not move their heads enough during the experiment.

Data collection and analysis: Listeners sat at the center of a ring of 24 loudspeakers, turned their heads back and forth, and used a wireless keypad to report the front/back location of statically presented signals and of dynamically moving signals with illusory locations. Front/back accuracy for static signals, the strength of front/back illusions, and minimum audible movement angle were measured for each listener in each condition. All measurements were made in each listener both aided and unaided.

Results: Hearing-impaired listeners were less accurate at front/back discrimination for both static and illusory conditions. Neither static nor illusory conditions were affected by high-frequency content. Hearing aids had heterogeneous effects from listener to listener, but independent of other factors, on average, listeners wearing aids exhibited a spectrally dependent increase in "front" responses: the more high-frequency energy in the signal, the more likely they were to report it as coming from the front.

Conclusions: Hearing impairment was associated with a decrease in the accuracy of self-motion processing for both static and moving signals. Hearing aids may not always reproduce dynamic self-motion-related cues with sufficient fidelity to allow reliable front/back discrimination.

Figure 1

Mean audiograms for our hearing impaired listeners (N = 37). Individual audiograms are shown in grey, mean in black. The typical profile was that of a sloping loss.

Figure 2

Front/back illusion method. This figure illustrates the method for generating a front-to-back illusion. The head was tracked with infrared cameras measuring the position of retro-reflective markers mounted to a crown. The position of the signal (grey star) was smoothly panned across loudspeakers so as to be always exactly twice the angular position of the listener’s head. At least for low-pass filtered signals, the perceptual location of this moving signal was that of a static signal located at 180° (white star). Although not pictured, a back-to-front illusion can be generated in a complimentary manner.

Figure 3

Self-motion results for normal (A) and hearing impaired listeners (B). The dotted lines are mean proportion of “front” responses for static signals located in the front (open up triangles) and in the back (open down triangles). The solid lines represent illusion conditions, in which the signals were located in back but moved to simulate front locations (filled up triangles) or located in front but moved to simulate back locations (filled down triangles). The strength of the illusion decreased as a function of lowpass filter cutoff frequency for normal hearing listeners (A) but not for hearing impaired listeners (B).

Figure 4

Illusion strength as a function of hearing impairment (A) and age (B) among hearing impaired listeners. The dots represent individual data points for each listener. The solid lines are linear fits to the data, showing no consistent relationship between illusion strength and amount of hearing loss (A) or age (B).

Figure 5

MAMA as a function of hearing impairment (A) and age (B). Similar to the previous figure, the dots represent individual data points for each hearing impaired listener and the solid lines are a linear fit to the data. No consistent relationship was seen between MAMA and amount of hearing loss (A) or age (B).

Figure 6

Self-motion results for listeners wearing hearing aids. Symbols and lines are as seen in Figure 3. Across conditions, listeners wearing hearing aids were more likely to respond with a “front” response as amount of high-frequency energy was added to the signal.

Figure 7

Self-motion results for unilaterally fitted (A) and bilaterally fitted listeners (B). Symbols and lines are as seen in Figure 3. The accuracy of static front/back localization was lower for those fitted with two hearing aids than those fitted with only one. Response to the illusion conditions was not different between the two groups.

Figure 8

Self-motion results for open fit (A) and closed fit listeners (B). Symbols and lines are as seen in Figure 3. Both illusion strength and static front/back localization accuracy were lower for those fitted with two hearing aids than those fitted with only one.