Earmold Considerations for Optimal Spatial Hearing in Children with Unilateral Hearing Loss

One of the primary functions of the auditory system is to facilitate spatial hearing. Normal binaural hearing allows listeners to exploit acoustic cues provided by interaural disparities in time and level to identify the location of important sound sources in three-dimensional acoustic space and to enhance speech perception in the presence of competing speech or noise. Psychophysical studies have shown that low-frequency information (below 1500 Hz) codes interaural time disparities (ITD), and high frequency information (above 3000 Hz) codes interaual level disparities (ILD), both of which are critical for sound localization in the horizontal plane. There is however, a great deal of ambiguity in the system in that ITD and ILD cues alone are insufficient to locate sounds originating in the vertical plane. The auditory system resolves the ambiguity by incorporating a monaural spectral cue: the directional-dependent frequency response of the pinna, head and torso, known as the head-related transfer function (HRTF). Listeners appear to incorporate and weight ITD, ILD and HRTF cues to determine the source of sounds, and when conflicting cues are centrally processed, different listeners will weight spatial cues quite differently, learning over time to rely on one cue at the exclusion of the others (Macaulay and Hartmann 2010). Listeners with normal hearing demonstrate exquisite sensitivity to interaural time and level disparities, and as a result, horizontal sound localization error is very low. For adult listeners, measured mean absolute error for broadband stimuli ranges from ~2o to 9o azimuth with greatest sensitivity between -35o and +35o (for reviews see Durlach and Colburn 1978; Middlebrooks and Green 1991). By age 5 or 6 years, children with normal hearing show comparable adult-like horizontal sound localization acuity (Van Deun et al. 2009; Johnstone, Nábĕlek and Robertson 2010). As a result, sound localization acuity measurements provide a robust and reliable assessment of binaural function in both child and adult listeners. Research has shown that unilateral hearing loss (UHL) is particularly detrimental to sound localization acuity (Veihweg and Campbell 1960; Humes, Allen and Bess 1980; Newton 1983; Bess, Tharpe and Gibler 1986; Johnstone et al. 2010). Unilateral hearing loss disrupts spatial hearing because it deprives the listener of critical interaural acoustic cues thereby forcing the listener to rely on conflicting acoustic information. Unfortunately hearing aids can further degrade performance on sound source localization tasks for some listeners with impaired hearing (Hausler, Colburn and Marr 1983; Van den Bogaert, Klasen, Moonen, Van Deun and Wouters 2006; Johnstone et al. 2010). Research with adult bilateral hearing aid users has shown that spatial hearing measurements such as sound localization and “cocktail party” experiments are sensitive enough to measure the effect of hearing aid technology, earmold venting, microphone settings and compression circuitry on binaural hearing in groups of patients (Nobel and Byrne 1990; Byrne, Sinclair and Noble 1998; Noble, Sinclair and Byrne 1998; Keidser et al. 2006; Van den Bogaert et al. 2006; Marrone, Mason and