Aud 720 Exam 2 Part 1

represent electric responses of the nervous system to sensory stim, consist of sequence of deflections or waves

latency
amplitude
spectrum
variability
electrode position

picking up activity from multiple neurons

recording of random and spontaneous bioelectric activity generated by the CNS w/o sensory stim (way of measuring changes in brain activity)

less than 20 Hz

20-50 uV (can be up to 300 mV)

delta
theta
alpha
beta

<4 Hz, largest, slowest, associated with sleep

4-8 Hz, largest and associated with sleep

8-13 Hz, indication of wakefulness, seen best w/ eyes closed, can block by mental activity; recorded best over parietal and occipital lobes

>13 Hz, seen over frontal regions and other regions during intense mental activity, smallest amplitude

1.determine brain lesions (brain lesions will affect EEG but not Eps b/c EEG=ongoing and not stim locked)
2. determine sleep disorders

1. look at EEG traces following reps of identical test signals that are above a threshold and superimpose the,. Traces reveal commonalities that were not apparent when traces are viewed separately
2. If aud stim is not greater than the hearing persons thresholds then will not find commonalities

draw baseline through middle of individual traces then positive and negative amplitudes are measures as successive small intervals after onset of signal

-computer digitizes electric activity from brain,
-stores positive and negative voltages at successive time points
- and then averages them electronically

electrodes

seldom longer than 500 msec

-equipment variables (size of analysis window, width of filter pass band)
-signal variables (spectrum of signal, presentation rate)
-subject variables (age, gender, normal vs. pathologic)
-subject state (awake or sleep)

b/c low amp in relation to ongoing EEG activity and other and evoked potentials

most widely used aud evoked potential

electrocohleography
short latency
middle latency
long latency
sonomotor AEPS
postaricular AEP

1.5-2 msec
3 types:
cochlear microphonic
summating potential (from IHC)
action potential (distal portion of aud nerve, same as wave I of ABR)

1.5-`0 msec
-transient=single response resulting from presentation of
transietnt=single stim (neural units generating these responses are sensitive to onset of stim) ABR occurs fast.
steady state short=responses reflecting repeated or continual stim
FFR

transient middle latency AEP (10-80 ms)
steady state middle latency AEP: 40 Hz AEP

100-1000 msec after an acoustic even
-cortical responses
-cognitive potentials
-event-related potentials

1. myogenic potentials (muscle potnetials-as muscles move they naturally give off electrical potentials)
2. postauricular AEPS-when stim is loud enough, muscle near ear fires in synchrony with acs stim (causes difficulties in determining what is neurally or muscualry generating the potential)
3. neck and scalop muscles can also be activated

evoked vs. nonevoked
farfied vs. nearfield
endogenous vs. exogenous

evoked=evoked from stim
nonevoked=muscle activity

far field=from scalp (most AEPs)
near field=EchochG, intraoperative monitoring

largely independent of physical features of stim but sensitive to context w/in with stim is presented and ability of subj to recognize or attach meaning to context

depend on physical feature of stim, if change some acoustic parameter of stim some responses will be sensitive to this

time it takes from onset of stim to occurrence of response (more important for ABR then for later potentials)
short=0-10 ms
middle=10-80
long=100-1000 ms

larger amplitudes in general

subcortical: <10mV
cortical: >10 mv

ABR=.2 mV
MLR=1 mV
LLAEP=1-10 mV

ABR =2000
MLR=1000
LLAEP =100

the shorter the latency the higher the fq spectrum

EEG=1-15 Hz
ABR=higher than MLR and LLAEP
LLAEP=.1-100 Hz

shorter latency=shorter variablity
-early exogenous potentials are more related to characteristics of stim and are less variable (can be recorded in sleeping/sedated)

degree to which small shifts in electrode alter waveform morphology
-shorter latency the more far field the response
-can record ABR from many different places b/c far-field
-want to max electrode position to pull signal out of the noise
-

the shorter the latency the less susceptible to changes in subject state

shorter latency the more rapidly maturation proceeds, ear matures before the brain, more peripheral pathways mature before more central pathways

1. identification of neurological abnormalities
2. estimation of hearing sensitivity

-neonates, children adults w/ mental handicaps, patients w/ psychiatric disorders whose voluntary behave responses are too erratic, patients involved in litigation whose lack of cooperation negates the results of more conventionally, l audiomet

no it is a test of synchronous neural function; however can use to make inferences about hearing sensitivity based on presence of responses to stim presented at various stim intensities

ABR most robust in identifying 8th nerve tumors >1cm in diameter, less w/ demyelinating diseases (MS)

useful in monitoring activity of surgical removal of tumors affecting 8th nerve

ABR is a test of neural function and depends on neural synchrony, if a patient has a neural abnormality it can affect the ability to obtain synchronous neural responses for ABR and then utility to estimate hearing sensitive is compromised

NOTE: ABR does not represent conscious hearing

-diagnose hearing loss (determine type of loss, degree of loss, config of loss)
-diagnose lesions along aud pathway b/c measuring along aud pathway
-higher level potentials=reveal cortical activity for aspects of speech processing
-assess attention-diff kind of evoked response she person is paying attention to the acoustic event
-cognitive function-window into integrative cortical function