Poor sound localization in turn affects the ability to understand speech in the presence of background noise.
Research has shown spatial hearing loss to be a leading cause of central auditory processing disorder (CAPD) in children.
This is the interaural time difference (ITD) cue and is measured by signal processing in the two central auditory pathways that begin after the cochlea and pass through the brainstem and mid-brain.
With some individuals, for a range of different reasons, maturation of the two ear spatial hearing ability may simply never happen.
[15] Transcallosal interhemispheric transfer of auditory information plays a significant role in spatial hearing functions that depend on binaural cues.
[16] Various studies have shown that despite normal audiograms, children with known auditory interhemispheric transfer deficits have particular difficulty localizing sound and understanding speech in noise.
[17] The CC of the human brain is relatively slow to mature with its size continuing to increase until the fourth decade of life.
[19] The medial olivocochlear bundle (MOC) is part of a collection of brainstem nuclei known as the superior olivary complex (SOC).
[20] In a quiet environment when speech from a single talker is being listened to, then the MOC efferent pathways are essentially inactive.
The output of the right ear is therefore dominant and only the right hemispace streams (with their direct connection to the speech processing areas of the left hemisphere) travel up the auditory pathway.
[23] With adults with a mature CC, an attention driven (conscious) decision to attend to one particular sound stream is the trigger for further MOC activity.
[24] The 3D spatial representation of the multiple streams of the noisy environment (a function of the right hemisphere) enables a choice of the ear to be attended to.
Dichotic listening tests can be used to measure the efficacy of the attentional control of cochlear inhibition and the inter-hemispheric transfer of auditory information.
Dichotic listening performance typically increases (and the right-ear advantage decreases) with the development of the Corpus Callosum (CC), peaking before the fourth decade.
[26] Dichotic listening tests typically involve two different auditory stimuli (usually speech) presented simultaneously, one to each ear, using a set of headphones.
[27] The activity of the medial olivocochlear bundle (MOC) and its inhibition of cochlear gain can be measured using a Distortion Product Otoacoustic Emission (DPOE) recording method.
Further research is needed to explore the apparent ability of music to promote an enhanced capability of speech in noise recognition.
Although further research is needed, there is a growing number of studies which have shown that open-fit hearing aids are better able to preserve localisation cues (see, for example, Alworth 2011)[35]