CSD 312 Auditory Nerve & Disorders

Auditory Nervous System

Nerve fibers from the cochlea form a cable and pass through internal auditory canal. 8th nerve then attaches to brainstem at the cerebellopontine angle. Descending (efferent) pathways also exist.

Afferent

Neurons that move toward the brain

Synaptic junctions

There are __________ _________ between hair cells and the auditory nerve which allof for the exchange ofi ons in the cells that allow for action potential. This is inside the cochlea.

Tonotopic Organization

Different parts of the cochlea are responsible for detecting different pitches. High pitch frequencies are at the beginning and lower pitch frequencies are at the apex (inner spiral)

Auditory Cortex

Different areas of the __________ ________ are stimulated based on the frequencies (tonotopic organization of the ________ _________

Temporal

The primary auditory cortex is in the __________ lobe

Primary Auditory Cortex

Filters out background noise and tunes into speech.

Secondary Auditory Cortex

Once sound reaches the brain, it goes to the primary auditory cortex and then the ___________ ______ _______ which functions as the “launching pad”. It sends information to other parts of the brain.

Experiences

Brain activity when it hears a sound or word is influenced on prior language and _________. There are other sensory and bodily responses associated with teh sound which also activate in the brain.

Development of Neurons in the Auditory Cortex

Synaptogenesis - first year of life

Synaptic Overshoot - 2-4 years

Slow Decrease - adolescence

Auditory Maturation

Maturation of Sound Coding → Maturation of selective listening and disocvering new sound → Maturation of Perceptual Flexibility

Maturation of Sound Coding

From full-term birth to 6 months of age

Both middle ear and brainstem auditory pathways

Coarser representation of sounds

Higher frequencies less discriminable (not a specific understanding of sound)

Parentese (higher pitch, dramatic inflection, longer duration of sounds)

teenage

The auditory pathway matures all the way up to _______ years even though the cochlea is fully formed at birth.

Human speech

Our brains are pre-wired to hear ________ _______

Maturation of Selective Listening and Sound Details

6 months to 5 years of age

Cortex and central processing

Infants listen in a broadband way

Difficult to separate target sounds from competing

Children tend to pay attention to global acoustic differences vs. fine acoustic details

Auditory Categories (Objects)

Babies are universal listeners!

Up to 6 months of age, babies have the capacity to differentiate all sound contrasts of all languages. Focusing only on distinctive features of speech sounds can be first obsreved around age 8-12 months.

The brain aims to withstand the enormous variability of the physical world.

Phonetic Sensitive Period

The first year of life.

Later, differentiation of previously non-distinctive features becomes progressively more difficult.

Maturation of Perceptual Flexibility

Age 6 through adolescence

Less consistent than adults in identifying speech sounds

Perceiving speech in difficult listening situations less automatic (more issues with filtering background noise)

Top-down

Perceptual Filling In is strongly related to __________ interactions (hear something → use prior language knowledge to process)

second

Perceptual Filling in can be demonstrated after the __________ year of life

Attention

Perceptual Filling In - In quiet, the auditory cortex is capable of recalling information regardless of ____________

Degraded

Perceptual Filling In - In noise, attention is required to help understand ___________ speech

Acoustic Neuroma

AKA Vesitbular Schwannoma

Slow growing, Benign tumor on the 8th nerve

Symptoms - unilateral tinnitus, unilateral sensorineural hearing loss, subtle balance problems, aural fullness, facial numbness or tingling, (later symptoms - headaches, unsteadiness, vertigo).

Occurs unilaterally except in Neurofibromatosis 2

Acoustic Neuroma Diagnosis

Unilateral hearing loss

Audiometry: pure-ton audiometry - asymmetry, speech recognition - can be poor; rollover, acoustic reflexes - absent or elevaed, reflex decay - positive decay,

ABR

MRI/CT scan

Acoustic Neuroma Observation

Especially in older adults, may opt to monitor tumor with MRI

Patient may never need surgery if tumor is small and doesn’t cause symptoms

Acoustic Neuroma Surgery

Patient usually will have total loss of hearing in affected ear, but is often treatment of choice. Depends on the size of the tumor.

This may be done if the tumor impacts the auditory nerve

Acoustic Neuroma Stereotactic Radiation Therapy

Single, high dose of targeted radiation. Slows the growth of the tumor and may shrink it, but does not make it disappear. Can take up to 18 months to become effective.

About 20% of timors will regrow some time after treatment

Improve

With acoustic neuroma, hearing doesn’t necessarily ________ after surgery but it prevents it from getting worse.

Vestibular Neuritis

Inflammation of the vestibular portion of the 8th cranial nerve from viral infection.

Symptoms - dizziness/vertigo, imbalance, nausea, hearing NOT affected

Treatments - anti-nausea drugs, vestibular suppressants, recovery takes about 3 weeks, vestibular rehabilitation may help this

Auditory Neuropathy / Auditory Dysynchrony

Abnormal 8th nerve function despite normal cochlear function

Potential cause - problem with the connection between the hair cell and the nerve (inner hair cells, synaptic junction, and/or the 8th nerve)

Risk factors - hyperbilirubinemia, exchange transfusion, premature birth, perinatal asphyxia

Treatment - hearing aids, cochlear implants — treatment may change over times

Auditory neuropathy / Auditory Dysnchrony Diagnosis

Need both ABR and OAE

Normal otoacoustic emission (this may go away over time)

Abnormal ABR (little to no response because it measures after the cochlea)

Audiometry - pure ton threshold range from normal to profound, word recognition can range from good to poor but its typically poor

Acoustic reflexes are absent or elevated.

Malingering

Non-organic hearing loss / functional hearing loss

Deliberately faking and/or exaggerating a hearing loss - usually for money (in adults) or attention (in children). It is easier to fake a unilateral hearing loss.

Psychogenic / hysterical/ Conversion Hearing loss

Non-organize hearing loss / functional hearing loss

Underlying psychological problem leads to person subconsciously believe that they have a hearing loss.

Signs of Functional Hearing Loss

Exaggerated listening behaviors, claims to have every symptom on case history, may claim to have had a sudden loss, but cannot pinpoint when it happened

What to do with suspected functional hearing loss

ask for a different behavioral response (e.g., counting

the beeps, yes/no)

• for kids, perform a play task

• ascending method

• explain the discrepancies, putting the blame on your own instructions

• swap transducers

• tell kids to “listen harder”

• perform tests for non-organic hearing loss

• have the person come back for re-testing

• be careful when writing up results

Autoimmune Inner Ear Disease (AIED)

Bilateral progressive sensory hearing loss (over several months) with tinnitus, vertigo, and aural fullness

From antibodies or immune cells attacking the inner ear. This causes inflammatory response in the inner ear. About 1/3 of patients have another autoimmune disorder

Treated with short-term steroid use (gives temporary relief of symptoms)

Autoimune inner ear disease and sudden idiopathic hearing loss

Postnatal causes of hearing loss

Sudden Idiopathic Hearing Loss

Hearing loss occurs suddenly or over the course of a few days. Unilateral in most acses. Will likely also have tinnitus and vertigo.

Possible causes - autoimmune diseases, viral or other infections, vascular disorders, unknown

Treatment (short-term) - vasodilators, steroids

TIME MATTERS

Hearing may or may not return to normal