Sound localization in the vertical plane should not be affected too much, while localization in the horizontal plane is impaired in unilateral deafness.
The two most important mechanisms used to localize sounds can be grossly divided into localizing sounds in the horizontal (azimuth) and vertical plane (elevation).
In the horizontal plane it is the exquisite capability of the auditory system to distinguish between slight intensity, latency and phase differences between the two ears that allow the brain to deduce where a sound is coming from. For low frequencies it is mainly the inter-aural time and phase differences that play a role. For high frequencies, the difference in sound level between the ears becomes more important (head-shadow effect). For example, if a sound is coming from the left, the left ear will receive the highest sound level. The sound stimulus will also have a shorter latency because of the intervening head tissue, and a phase difference due to the time lag (Van Wanrooij & Opstal, 2007).
Head shadow effect at high, but not at low frequencies. Source: University of Edinborough
In the vertical plane and front-back discrimination it is the analysis of spectral shape cues that lie at the basis of directional hearing. The spectral cues arise from direction-dependent reflections within the pinna (described by so-called head-related transfer functions, see below figure graphs). The pinna provide a monaural localization cue for sound frequencies exceeding about 3–4 kHz (Van Wanrooij & Opstal, 2007).
Head-related transfer function due to the pinna. Source: University of California
However, this hard distinction between elevation and azimuth directional analysis is somewhat simplified, as it appears that elevation is also analyzed in terms of binaural cues, and azimuth is also identified using monaural cues. Hence, all available cues seem to be integrated for both vertical and horizontal localization. Nonetheless, the two distinct pathways are the primary and preferred ways of the auditory system to determine elevation and azimuth. However, when one ear is plugged, or in unilateral deaf people, the textbook binaural hearing in the horizontal plane may be substituted by monaural cues, and I quote from (Van Wanrooij & Opstal, 2007):
[L]isteners rely on monaural spectral cues for sound-source azimuth localization as soon as the binaural difference cues break down.
Therefore, to wrap up, unilateral deafness degrades directional hearing and in particular in the horizontal plane. However, the use of monaural cues may still allow localization of sounds in the horizontal plane in unilaterally deaf subjects, albeit less effectively than normal-hearing subjects using binaural hearing (Van Wanrooij & Opstal, 2007).
- Van Wanrooij & Opstal, J Neurophysiol 2007;97:715-26