Spatial hearing loss

Spatial hearing loss
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Spatial hearing loss, refers to a form of deafness that is an inability to use spatial cues about where a sound originates from in space. This in turn impacts upon the ability to understand speech in the presence of background noise.

People with spatial hearing loss have difficulty processing speech that arrives from one direction while simultaneously filtering out 'noise' arriving from other directions. Research has shown spatial hearing loss to be a leading cause of central auditory processing disorder (CAPD) in children. Children with spatial hearing loss commonly present with difficulties understanding speech in the classroom. Spatial hearing loss is found in most people over 70 years of age, and can sometimes be independent of other types of age related hearing loss. As with presbycusis, spatial hearing loss varies with age. Through childhood and into adulthood it can be viewed as spatial hearing gain (with it becoming easier to hear speech in noise), and then with middle age and beyond the spatial hearing loss begins (with it becoming harder again to hear speech in noise).

Sound streams arriving from the left or right (the horizontal plane) are localised primarily by the small time differences of the same sound arriving at the two ears. A sound straight in front of the head is heard at the same time by both ears. A sound to the side of the head is heard approximately 0.0005 seconds later by the ear furthest away. A sound halfway to one side is heard approximately 0.0003 seconds later. 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. Some of those with spatial hearing loss are unable to process ITD (low frequency) cues.

Sound streams arriving from below the head, above the head, and over behind the head (the vertical plane) are localised again by signal processing in the central auditory pathways. The cues this time however are the notches/peaks that are added to the sound arriving at the ears by the complex shapes of the pinna. Different notches/peaks are added to sounds coming from below compared to sounds coming from above, and compared to sounds coming from behind. The most significant notches are added to sounds in the 4 kHz to 10 kHz range. Some of those with spatial hearing loss are unable to process pinna related (high frequency) cues.

By the time sound stream representations reach the end of the auditory pathways brainstem inhibition processing ensures that the right pathway is solely responsible for the left ear sounds and the left pathway is solely responsible for the right ear sounds. It is then the responsibility of the auditory cortex (AC) of the right hemisphere (on its own) to map the whole auditory scene. Information about the right auditory hemifield joins with the information about the left hemifield once it has passed through the corpus callosum (CC) - the brain white matter that connects homologous regions of the left and right hemispheres. Some of those with spatial hearing loss are unable to integrate the auditory representations of the left and right hemifields, and consequently are unable to maintain any representation of auditory space.

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