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49 Cards in this Set

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evoked potentials
represent electric responses of the nervous system to sensory stim, consist of sequence of deflections or waves
characteristics of auditory evoked potentials
latency
amplitude
spectrum
variability
electrode position
strength of evoked potential =
picking up activity from multiple neurons
EEG
recording of random and spontaneous bioelectric activity generated by the CNS w/o sensory stim (way of measuring changes in brain activity)
EEG fq
less than 20 Hz
EEG amp
20-50 uV (can be up to 300 mV)
Different types of waves (list)
delta
theta
alpha
beta
delta waves
<4 Hz, largest, slowest, associated with sleep
theta waves
4-8 Hz, largest and associated with sleep
alpha waves
8-13 Hz, indication of wakefulness, seen best w/ eyes closed, can block by mental activity; recorded best over parietal and occipital lobes
beta waves
>13 Hz, seen over frontal regions and other regions during intense mental activity, smallest amplitude
diagnostic uses of EEG
1.determine brain lesions (brain lesions will affect EEG but not Eps b/c EEG=ongoing and not stim locked)
2. determine sleep disorders
How to determine if change in EEG is b/c of sensory stim (ex. aud) or random
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
what is another way to look for commonalities
draw baseline through middle of individual traces then positive and negative amplitudes are measures as successive small intervals after onset of signal
advent of computers
-computer digitizes electric activity from brain,
-stores positive and negative voltages at successive time points
- and then averages them electronically
what do you use to measure voltage changes to aud stim
electrodes
how long is analysis window or epoch
seldom longer than 500 msec
what are patterns of configurations of waves/peaks dependent on
-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)
why were ABRs discovered relatively late
b/c low amp in relation to ongoing EEG activity and other and evoked potentials
ABR
most widely used aud evoked potential
types of auditory evoked potentials (list)
electrocohleography
short latency
middle latency
long latency
sonomotor AEPS
postaricular AEP
electrochochleography
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)
short latency
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
middle latency
transient middle latency AEP (10-80 ms)
steady state middle latency AEP: 40 Hz AEP
long latency
100-1000 msec after an acoustic even
-cortical responses
-cognitive potentials
-event-related potentials
sonomotor AEPs
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
classification of auditory evoked potentials (list)
evoked vs. nonevoked
farfied vs. nearfield
endogenous vs. exogenous
example of evoked and non evoked
evoked=evoked from stim
nonevoked=muscle activity
far field vs. near filed
far field=from scalp (most AEPs)
near field=EchochG, intraoperative monitoring
endogenous
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
exogenous
depend on physical feature of stim, if change some acoustic parameter of stim some responses will be sensitive to this
Characteristics of AEPs: latency
(also characterize different latencies)
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
what happens in general for later latencies
larger amplitudes in general
amplitudes of:
subcortical potentials vs. cortical potentials
subcortical: <10mV
cortical: >10 mv
amplitudes of:

ABR
MLR
LLAEP
ABR=.2 mV
MLR=1 mV
LLAEP=1-10 mV
number of averages needed for

ABR
MLR
LLAEP
ABR =2000
MLR=1000
LLAEP =100
spectrum frequency
the shorter the latency the higher the fq spectrum
spectrum frequency of:

EEG
ABR
MLR
LLAEP
EEG=1-15 Hz
ABR=higher than MLR and LLAEP
LLAEP=.1-100 Hz
characteristics of aud evoked potentials:

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

ELECTRODE POSITION
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
-
characteristics of aud evoked potentials:

SUSCEPTIBILTY TO STATE CHANGES
the shorter the latency the less susceptible to changes in subject state
characteristics of aud evoked potentials:

RATE OF MATURATION
shorter latency the more rapidly maturation proceeds, ear matures before the brain, more peripheral pathways mature before more central pathways
two major applications of ABR testing
1. identification of neurological abnormalities
2. estimation of hearing sensitivity
when would u choose ABR over behavioral measures?
-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
is ABR a test of hearing?
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
how can ABR test neural abnormalities
ABR most robust in identifying 8th nerve tumors >1cm in diameter, less w/ demyelinating diseases (MS)
ABR and intraoperative monitoring
useful in monitoring activity of surgical removal of tumors affecting 8th nerve
how can an ABR test hearing sensitivity
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
clinical applications of ABR
-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