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. Poor sound localization in turn affects the ability to understand speech in the presence of background noise.Cameron S and Dillon H; The Listening in Spatialized Noise – Sentences Test: Comparison to prototype LISN test and results from children with either a suspected (central) auditory processing disorder of a confirmed language disorder; Journal of the American Academy of Audiology 19(5), 2008
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.Frisina D and Frisina R; Speech recognition in noise and presbycusis: relations to possible neural mechanisms; Hearing Research 106(1-2), 1997 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).
Localization mechanism 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.Dobreva M, O’Neill W and Paige G; Influence of Aging on Human Sound Localization; Journal of Neurophysiology 105, 2011 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.Besta V, Carlile S, Jin C and Van Schaik A; The role of high frequencies in speech localization; Journal of the Acoustical Society of America 118(1), 2005 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.Della Penna S, Brancucci A, Babiloni C, Franciotti R, Pizzella V, Rossi D, Torquati K, Rossini PM, Romani GL; Lateralization of Dichotic Speech Stimuli is Based on Specific Auditory Pathway Interactions; Cerebral Cortex 17(10), 2007. 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.At A, Spierer L, Clarke S; The role of the right parietal cortex in sound localization: a chronometric single pulse transcranial-magnetic stimulation study; Neuropsychologia 49(9), 2011 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.
An auditory space representation enables attention to be given (conscious top-down driven) to a single auditory stream. A gain mechanism can be employed involving the enhancement of the speech stream, and the suppression of any other speech streams and any noise streams.Kerlin J, Shahin A and Miller L; Attentional Gain Control of Ongoing Cortical Speech Representations in a “Cocktail Party”; Journal of Neuroscience 30(2), 2010 An inhibition mechanism can be employed involving the variable suppression of outputs from the two cochlea.Srinivasan S, Keil A, Stratis K, Osborne A, Cerwonka C, Wong J, Rieger B, Polcz V, Smith D; Interaural attention modulates outer hair cell function; Eur J Neurosci. 40(12), 2014 Some of those with spatial hearing loss are unable to suppress unwanted cochlea output.
Those individuals with spatial hearing loss are not able to accurately perceive the directions different sound streams are coming from and their hearing is no longer 3-dimensional (3D). Sound streams from the rear may appear to come from the front instead. Sound streams from the left or right may appear to come from the front. The gain mechanism can not be used to enhance the speech stream of interest from all other sound streams. Those with spatial hearing loss need target speech to be raised by typically more than 10 dB when listening to speech in a background noise compared to those with no spatial hearing loss.Glyde H, Hickson L, Cameron S, Dillon H; Problems hearing in noise in older adults: a review of spatial processing disorder; Trends in Amplification 15(3), 2011
Spatial hearing ability normally begins to develop in early childhood, and then continues to develop into early adulthood. After the age of 50 years spatial hearing ability begins to decline.Cameron S, Glyde H and Dillon H; Listening in Spatialized Noise - Sentences Test (LiSN-S): Normative and Retest Reliability Data for Adolescents and Adults up to 60 Years of Age; Journal of the American Academy of Audiology 22, 2011 Both peripheral hearing and central auditory pathway problems can interfere with early development. With some individuals, for a range of different reasons, maturation of the two ear spatial hearing ability may simply never happen. For example, prolonged episodes of ear infections such as “glue ear” are likely to significantly hinder its development.Farah R, Schmithorst V, Keith R, Holland S; Altered white matter microstructure underlies listening difficulties in children suspected of auditory processing disorders; Brain and Behavior 4(4), 2014
Corpus callosum
Many neuroscience studies have facilitated the development and refinement of a speech processing model. This model shows cooperation between the two hemispheres of the brain, with asymmetric inter-hemispheric and intrahemispheric connectivity consistent with the left hemisphere specialization for phonological processing.Bitan et al.; Bidirectional connectivity between hemispheres occurs at multiple levels in language processing, but depends on sex; Journal of Neuroscience 30(35), 2010 The right hemisphere is more specialized for sound localization,Spierer et al.; Hemispheric competence for auditory spatial representation; Brain 132, 2009 while auditory space representation in the brain requires the integration of information from both hemispheres.Grothe et al.; Mechanisms of Sound Localization in Mammals; Physiol Rev 90, 2010
The corpus callosum (CC) is the major route of communication between the two hemispheres. At maturity it is a large mass of white matter and consists of bundles of fibres linking the white matter of the two cerebral hemispheres. Its caudal and splenium portions contain fibres that originate from the primary and second auditory cortices, and from other auditory responsive areas.Lebel C, Caverhill-Godkewitsch S, Beaulieu C; Age-related regional variations of the Corpus Callosum identified by diffusion tensor tractography; Neuroimage 52(1), 2010 Transcallosal interhemispheric transfer of auditory information plays a significant role in spatial hearing functions that depend on binaural cues.Hausmann M, Corballis M, Fabri M, Paggi A, Lewald J; Sound lateralization in subjects with callosotomy, callosal agenesis, or hemispherectomy; Brain Res Cogn Brain Res 25(2), 2005 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.Bamiou D et al.; Auditory interhemispheric transfer deficits, hearing difficulties, and brain magnetic resonance imaging abnormalities in children with congenital aniridia due to PAX6 mutations; Arch Pediatr Adolesc Med 161(5), 2007.
The CC of the human brain is relatively slow to mature with its size continuing to increase until the fourth decade of life. From this point it then slowly begins to shrink.Sala S, Agosta F, Pagani E, Copetti M, Comi G, Filippi M; Microstructural changes and atrophy in brain white matter tracts with aging; Neurobiology of Aging 33(3), 2012 LiSN-S SRT scores show that the ability to understand speech in noisy environments develops with age, is beginning to be adult like by 18 years and starts to decline between 40 and 50 years of age.Glyde H, Cameron S, Dillon H, Hickson L, Seeto M; The effects of hearing impairment and aging on spatial processing;; Ear & Hearing 34(1), 2013
Roles of the SOC and the MOC
The medial olivocochlear bundle (MOC) is part of a collection of brainstem nuclei known as the superior olivary complex (SOC). The MOC innervates the outer hair cells of the cochlea and its activity is able to reduce basilar-membrane responses to sound by reducing the gain of cochlear amplification.Cooper N, Guinan J; Efferent-Mediated Control of Basilar Membrane Motion; J. Physiol. 576.1, 2006
In a quiet environment when speech from a single talker is being listened to, then the MOC efferent pathways are essentially inactive. In this case the single speech stream enters both ears and its representation ascends the two auditory pathways. The stream arrives at both the right and left auditory cortices for eventual speech processing by the left hemisphere.
In a noisy environment the MOC efferent pathways are required to be active in two distinct ways. The first is an automatic response to the multiple sound streams arriving at the two ears, while the second is a top-down corticofugal attention driven response. The purpose of both is an attempt to enhance the signal to noise ratio between the speech stream being listened to and all other sound streams.Smith D and Keil, A; The biological role of the medial olivocochlear efferents in hearing; Front. Syst. Neurosci. 25, 2015
The automatic response involves the MOC efferents inhibiting the output of the cochlear of the left ear. 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.Bidelman G and Bhagat S; Right-ear advantage drives the link between olivocochlear efferent 'antimasking' and speech-in-noise listening benefits; NeuroReport 26(8), 2015 With children the underdeveloped Corpus Callosum (CC) is unable, in any case, to transfer auditory streams arriving (from the left ear) at the right hemisphere to the left hemisphere.Kimura D; From ear to brain; Brain Cogn. 76(2), 2011