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Auditory Perception. Erin Juurakko, Audiologist. Road Map:. Review pathway of sound The audiogram Loudness and pitch perception and the impact of hearing loss Clinical applications and intervention for hearing loss Cortical development and reorganization. Let’s Recap:.
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Auditory Perception Erin Juurakko, Audiologist
Road Map: Review pathway of sound The audiogram Loudness and pitch perception and the impact of hearing loss Clinical applications and intervention for hearing loss Cortical development and reorganization
Once the acoustic information has been represented in terms of neural activity, the hard part can begin… The task of analyzing information and separating and identifying the different signals is performed by the brain.
Pitch Perception: • Pitch can be considered the perceptual equivalent of frequency. Pitch is the auditory sensation that can be described as on a scale from low to high. The sensation of pitch depends mainly on the frequency content of the sound stimulus, but it also depends on the sound pressure and waveform of the stimulus. Information about frequency is carried by Place coding Temporal coding (Phase locking)
Phase Locking: https://www.youtube.com/watch?v=nAqfwuKbomo
Loudness Perception: • Loudness is the perceptual quantity most related to sound intensity. Loudness refers to the subjective magnitude of sound, as opposed to pressure, intensity, power or level, which refer to the physical magnitude of sound
Information about intensity is carried by Rate Encoding Spread of Excitation
Impact of Hearing Loss on Perception: With sensorineural hearing loss the site of damage can be the OHCs, IHCs, or AN/pathway up to the brain. IHC Damage: IHC damage results in loss of sensitivity to sounds, which increases the absolute threshold (and shows up as a threshold elevation on the audiogram). With extensive damage, pieces of sound may be missing all together making speech unclear.
OHC Damage: OHC Damage is more complex and leads to Loss of sensitivity resulting in threshold elevation Abnormally rapid loudness growth Reduction in frequency selectivity
Hearing loss has a huge impact on the perception of speech- the most important stimulus in individual’s everyday lives. With the most common configuration of hearing loss, perception of phonemes containing high-frequency energy (e.g. fricatives) is more affected than vowels. https://www.starkey.com/hearing-loss-simulator#!/hls/page/1
Auditory Nerve Damage: Noise exposure may do more than just damage haircells. Prolonged release of the neurotransmitter glutamate by IHCs can damage postsynaptic neurons leading to degeneration.
It’s possible to have clinically normal hearing and “pass” an audiometric screening yet have a huge reduction in effective neural transmission from cochlea to brain. This reduction in transmission leads to problems processing complex sounds and decreased performance on discrimination tasks such as detecting speech in background noise. HIDDEN HEARING LOSS Therefore, it is important for audiologist’s to look beyond the audiogram
Early Hearing Detection and Intervention Programs (EHDI): Children born in Ontario have their hearing screened at birth. The prevalence of congenital hearing loss is about 1.5 to 2 per 1000, with the predominant cause being genetic. EDHI programs screen hearing at birth and refer to diagnostics when there is a refer result
How is hearing loss diagnosed in children? Otoacoustic Emissions (OAE)- detect outer haircell activity and are a good measure of cochlear health (although does not provide information about health of IHCs). Normal hearing is a safe assumption in a “well baby” population Evoked Cortical Potentials- provide measure of action potentials at junctions along the auditory pathway and provide information about the auditory stimulus at various points along the central auditory pathway, from cochlea to cortex • Tells us: • Is a signal generated? • Is the signal maintained?
These measures have made it possible to detect hearing loss at a young age and implement appropriate interventions in a timely manner. This equipment has also made identification of Auditory Neuropathy Spectrum Disorder (ANSD) possible…
ANSD: Describes a type of hearing loss in which sound does not reach the brain in a normal way, causing the sound reaching the brain to be of poor quality and difficult to understand. ANSD is considered to exist when OAEs are present and the ABR is absent or abnormal. OAEs indicated cochlear function and the ABR indicates higher central auditory function. Intervention may involve hearing aids or cochlear implants.
Hearing Aids: Treat mild to severe degrees of hearing loss Sounds are amplified and compressed independently in unique frequency bands based on the individual’s degree and configuration of hearing loss Hearing aids restore sensitivity to quiet sounds and restore normal growth of loudness However, hearing aids are limited in that they don’t restore impaired frequency selectivity in an impaired cochlea- therefore they are not very effective for challenges with understanding speech in noisy environments Microphone systems can assist with listening in noise challenges.
Cochlear Implants: Surgically implanted device for severe to profound hearing losses. The system bypasses the outer, middle, and inner ear to stimulate the auditory nerve directly with electrical impulses.
Different cochlear implant manufacturers use different signal processing strategies to convert sounds picked up by the speech processor microphone into electrical signals that can be sent to the implant electrodes. These strategies generally involve sound being converted to a digital signal and then being divided into different frequency components that are stored in filter banks. The sound energy in each filter bank is measured, adjusted, and sent to corresponding electrodes.
Speech Processing in CIs: • The goal of speech processing strategies is to provide a richer, more detailed hearing experience for the patient • The envelope of speech is the outline of the speech signal that reflects changes in loudness or amplitude over time • The fine structure contains more detailed information and reflects changes in pitch or frequency • Historically CI processing strategies have focused on the envelop
Slow, Broad Changes or Details: Auditory Chimeras Coarse Detail (Envelope) Sentence #1 Chimera #1 Fine Detail (Fine Structure) Coarse Detail (Envelope) Chimera #2 Sentence #2 Fine Detail (Fine Structure)
What Happens When the Details are Missing? Coarse Detail (Envelope) Sentence #1 Chimera #1 Noise + Coarse Detail (Envelope) Chimera #2 Sentence #2 Noise
If your brain can hear the general pattern, you can understand • Fine-structure is not necessary for understanding speech • However, without fine structure information, there is limited performance in complex listening environments such as noisy, reverberant spaces • Therefore as implant technology progresses, providing the patient with as much speech information as possible is the goal
Sensitive Period for Cochlear Implantation: Speech outcomes Harrison, 2005
