Evidence of auditory plasticity in people with unilateral hearing loss

Get Complete Project Material File(s) Now! »

Auditory Pathway Anatomy and Physiology

Within the central auditory system, the cochlear nucleus (CN) on each side of the brainstem receives inputs from the ascending auditory fibres of the ipsilateral ear (F. E. Musiek & Baran, 1986; Pickles, 2015). The CN represents the first relay station within the central auditory pathway where auditory inputs signals are directed toward different ascendent tracts; most of the information cross the midline reaching the contralateral cortex (D. Moore, 1991; Pickles, 2015)
The ipsilateral and contralateral pathways provide a better representation of both ears, allowing better preservation within the auditory system of the temporal cues in the incoming acoustic signal (Pickles, 2015). Rosenzwieg (1951) studied in cats the representation of sound at the cortical level to monaural/binaural stimulation using an electrophysiological approach, using clicks as stimuli. To monaural presentation, contralateral ear hemisphere responses were larger in amplitude, thus the hemispheric contralateral representation of the signal reaching the stimulated ear was larger than the ipsilateral representation.
Within the central auditory system (CAS), most neurons are able to respond to stimulation of either ear. Excitatory input normally comes from one ear whereas the other ear can provide excitatory or inhibitory input. The contralateral ear pathway is normally referred as “excitatory dominant” whereas the ipsilateral ear can provide either excitatory or inhibitory input (D. Moore, 1991). The interaction between inputs to the two ears modulates activity within the auditory pathway. This interaction plays an important role for some auditory abilities such as sound localisation where variations of inter-aural cues are relevant, especially for sound localisation in the horizontal plane (D. Moore, 1991).

Contralateral Hemisphere Dominance

In normal hearing individuals, monaural sound presentation produces bilateral hemisphere activation, however this is stronger in the hemisphere contralateral to the ear stimulated. Contralateral pathways have a larger number of fibers compared to ipsilateral pathways (Adams, 1979; Coleman & Clerici, 1987; R. Moore & Goldberg, 1963). This is known as contralateral dominance or preference (Firszt, Ulmer, & Gaggl, 2006). Thus the neural representation of the stimulated ear is stronger contralaterally at cortical levels (Rosenzweig, 1951). This contralateral dominance can be found within the auditory system for structures above the CN (Popelář et al., 1994). Kimura (1967) proposed that in the normal system with bilateral inputs the ipsilateral pathways are suppressed, leading to stronger contralateral activity.
It is possible to observe an asymmetry of the brain hemisphere activity to monaural stimulation using a range of paradigms including electroencephalography (EEG) (Butler, Keidel, & Spreng, 1969; Connolly, Manchanda, Gruzelier, & Hirsch, 1985; Khosla et al., 2003; Näätänen & Picton, 1987; Ponton et al., 2001), magnetoencephalography (MEG) (Pantev, Ross, Berg, Elbert, & Rockstroh, 1998; Reite, Zimmerman, & Zimmerman, 1981; B.
Ross, Herdman, & Pantev, 2005) and functional magnetic resonance imaging (fMRI) (Jäncke, Wüstenberg, Schulze, & Heinze, 2002; Langers, van Dijk, & Backes, 2005; Scheffler et al., 1998; Suzuki et al., 2003). This dominance is evident as larger amplitude/shorter latency (asymmetry and asynchrony respectively) or higher activity for recordings are seen over the hemisphere contralateral to the stimulated ear. Factors such as stimulus level, frequency, type the stimulus (e.g., tones versus speech), ear of stimulation and anatomical variations across participants may influence the degree of contralateral dominance observed in humans (Firszt et al., 2006; Hine & Debener, 2007).

READ CHILDREN IN ARMED CONFLICT SITUATIONS

CHAPTER 1: Introduction
CHAPTER 2: Consequences of Unilateral Hearing Loss in Adults and Children
Introduction
Prevalence
Definition for unilateral hearing loss
Educational, language and cognitive consequences.
Academic outcomes of children with UHL.
Speech and language outcomes of children with UHL
Cognition
Listening effort..
UHL effects on auditory performance .
Binaural summation.
Head shadow effect
Binaural release from masking
Sound localisation.
Speech perception
Temporal processing in UHL
CHAPTER 3: Evidence of auditory plasticity in people with unilateral hearing loss.
Introduction .
Auditory Pathway Anatomy and Physiology
Contralateral Hemisphere Dominance
Evidence of plasticity in UHL: Electroencephalography (EEG) and Magnetoencephalography (MEG) studies
Contralateral dominance index
EEG studies
MEG studies .
Evidence of Functional and Structural Changes within the Cortex due to a UHL: Neuroimaging Studies
Functional magnetic resonance imaging (fMRI).
Diffusion tensor imaging (DTI)
T1-weighted studies
General Conclusions
CHAPTER 4: Methodology for the Behavioural and Electrophysiological Assessment of Auditory Function in People with Unilateral Hearing Loss .
Behavioural Assessment .
Introduction
Sound localisation
Test Protocol.
Speech recognition in noise.
Test Protocol
Self-reported hearing performanc
Conclusion
CHAPTER 5: Impact of unilateral hearing loss on behavioural and evoked potential measures of auditory function in adults.
CHAPTER 6: Longitudinal study of an adult with single sided deafness: case study .
CHAPTER 7: Effects of unilateral hearing loss on cortical auditory function and listening abilities in children
CHAPTER 8: Cortical auditory evoked potential (CAEP) and behavioural measures of auditory function in an child with a single sided deafness.
CHAPTER 9: Discussion.
Appendices