Evoked Responses Quiz Overview Flashcards
What is the basis of the electroencephalogram (EEG)?
Our brain is constantly working, giving out small measurements. These can be reordered as potential differences.
neural activity in response to specific types of sensory stimulation can be extracted from the EEG, giving much smaller measurements for us to see. we can only see them because …..
they are amplified
in order to view evoked responses, what two things need to be present
signal averaging and amplification
evoked response/evoked potentials (ER/EP)
measures the electrophysiologic responses of the nervous system to a variety of stimuli and in theory any modality can be tested.
we are looking at neural responses. remember, it is a functional test so we are looking at the functional integrity of the neural function
somatosensory evoked responses (SERs)
generated by the stimulation of afferent peripheral nerve fibers by either physiological or electrical means
-measuring the electrical difference
-most common sites are wrist, knee or ankle
-useful for people with neurologic diseases, prognosis of comatose patients, IOM
-needle electrodes can be placed right on the nerve causing you to see a larger response than electrodes
cortical evoked responses
late evoked responses are generally used for studying higher cortical functions (alzheimer)
-considered experimental and not used widely
how has the usage of evoked potentials/responses changed with the introduction of advances in imaging (i.e. MRIs)
the usefulness of ERs/EPs has decreased and most questions regarding structures are better answered by an MRI however in some cases both an MRI and evoked potentials may be complementary
-MRI is a structural test
-ERs/EPs are a functional test
auditory evoked response (AERs)
brain waves or electrical responses that are generated when the auditory system is stimulated by sound, but can also be through electricity or a mechanical stimuli. can use clicks, tones, and speech sounds.
clicks for AERs
abrupt onset, very short duration and broadband
-not a frequency specific signal as it contains energy from various frequencies, but most energy is within 1 and 4 kHz
tone bursts for AERs
short duration
-frequency specific with more energy hanging around the frequency of interest
what is the general rule with AERs and their stimulus intensity with response
the louder the stimulus intensity, the larger the AER response will be
what happens during an AER
-sound is presented through some sort of transducer
-activity from the cochlear and brain evoked by the sound is picked up by electrodes
-this evoked activity that is conveyed from the auditory structures through body tissue and fluids to the surface electrodes and from here it goes through the wire and through the technology to be analyzed
- takes about 1 second to see AER post stimulus
What is a DIN Pin?
the electrical connector that plugs into the box or pre amp. they were standardized for analog signals.
how can we be sure that the response is coming from the CANS if the electrodes are placed far from the generator site
the stimulus that is evoking the response is sound so we know that the response is from within the auditory system
What are neural generators
-the earliest response with the shortest latencies that are generated by the inner ear and auditory nerve.
later responses relect activity in the CANS
-these have smaller potentials whereas the later responses have larger potentials
FLAG***
microvolts
the small voltage that is used to measure brain activity making up the auditory evoked response
-activity measures from the higher regions of the CANS, such as the cerebral cortex, is larger in size than activity from the cochlea and auditory nerve
FLAG ***
Tell me about evoked vs. non-evoked potentials?
remember, these electrical potentials can be recorded both in response to stimulus and in ongoing manner without presence of external stimuli
-evoked is those such as the ECochG or ABR
-non evoked is those such as the EEG
auditory evoked potentials (AEPs)
represents electrical responses of the nervous system to externally presented stimuli; most will be hidden in the EEG response within the brain
-in simple words, represents brain waves generated in response to sound
what are AEPs generated by
generated by action potentials arising from many neurons within a specific region
how can we classify AEPs
characteristics determined by external or internal processes, based on the time epoch, based on relation of electrodes to generator site and based on structures in the auditory system that generates them
classification: characteristics determined by external or internal processes
exogenous : does not have to hear the signal
-earlier responses are this type
endogenous : must hear the signal
-all cortical potentials and later responses are this way
classification : time epoch
the time interval after which the stimulus occurs
-very early (0-1.5 msec) (ECochG)
-early (1.5-12 msec) (ABR)
-middle (12-50 msec) (MLR)
-slow (50-300 msec) (ALR)
-P300 (300+ msec) (P300)
classification : based on relation of the recording electrodes to generator sites
near field : electrodes are in close proximity
-uncommon as the electrode needs to be directly on the nerve
far field : electrodes are some distance away
-typically what is used as the electrodes will be placed on the skull
classification : based on structures
the receptor potentials are from the cochlear hair cells and the neurogenic potentials are from the 8th nerve and/or brainstem
early latency AEPs are best generated with __________whereas the late latency responses are best generated with ___________
very brief transient ; clicks
what are the four categories of factors that can impact the AEP measurement
stimulus, acquisition, non-pathological and waveform analysis
in regards to the stimulus, what are some aspects that can impact the recording
type, duration/rise time, polarity, intensity and rate
impacting the measurement, stimulus : type
clicks will allow for more neurons to be activated for a longer period of time, allowing for a larger response
-however at low levels tone bursts may want to be used to look at those responses if the patient has a step HF loss
as a result of many studies, what method was found to be most effective with AEPs
the blackman setup, being a 202
-2 ms onset with no plateau then 2 ms offset
impacting the measurement, stimulus : duration/rise time
relating to how long we leave it on plateau for and how long it stays on until its turned off
-standard click is 0.1 msec
the longer the duration of the stimulus, the _____ frequency specific it will be (more like a pure tone)
more
the shorter the stimulus, the ______ frequency specific it will be
less
impacting the measurement, stimulus : polarity
this relates all to how the initial direction of the pressure wavefront is and how that in turn impacts the auditory system
-there is rarefaction, condensation and alternating
rarefaction polarity
initial direction causes the pressure wavefront to move away from the eardrum
-results in outward movement of the stapes footplate and then an upward motion of basilar membrane which depolarizes the hair cells
-this causes the latencies to be slightly shorter and the amplitude to be slightly higher
condensation polarity
initial direction causes the pressure wavefront to move towards the eardrum
-results inward movement of the stapes footplate and then a downward motion of the basilar membrane, leading to the hair cells not being depolarized right away
-causes the latencies to be slightly longer and the amplitude to be slightly smaller
alternating polarity
stimulus pressure wavefront is alternated between both rarefaction and condensation
-sums up the two polarities and is routinely used during ECochG
-with ANSD, we get a flat line as they are reversals
are the differences in latencies within rarefaction and condensation clinically significant?
no
impacting the measurement, stimulus : intensity
with intensity, it is important to know that there is an inverse relationship between intensity and latency
-there is an increase in latency as the intensity decreases
-there is a decrease in latency as the intensity increases
intensity with AEPs can have many unites, but the typical one we see with ABRs is dB nHL. what is important to remember with this unit
there is around a 10 dB change between this and the actual threshold
-as this is based solely on ABR norms and not pure tone norms
what are 3 factors that impact the actual intensity and frequency information that reaches the cochlea
acoustic properties of the transducer, volume of the ear canal and middle ear transmission characteristics
impacting the measurement, stimulus : rate
how many stimulus is presented per second
-generally will run them around 27.7 per second
interstimulus interval (ISI)
the time interval between successive stimuli presentations
-needing to ensure that it is in accordance with the neuron rest periods
-can identify by doing 1 second / rate
if the ISI time exceeds the refractory period, then …….
the neural can fully recover and will be responsive for the next stimulus
if the ISI is shorter than the refractory period, then …..
some stimuli will not contribute to the response because they are presented during the recovery period
-this gives a poor response
fast responses such as the ECochG or ABR occur within a brief period (5-6 msec) and therefore require a __________
brief ISI
in regards to acquisition, what are some aspects that can impact the recording
instrumentation, electrodes, filters, amplifier, artifact rejection and signal averaging
impacting the measurement, acquisition : instrumentation
common aspects is stimulus generator, electrodes, filters, amplifiers, signal average, response delay, response processing and a means to print or display results
-inserts are recommended as they increase interaural attenuation, they attenuate environmental noise and they prevent ear canal collapse
impacting the measurement, acquisition : electrodes
these are the sensing device that detects the bioelectric activity and sends it to the pre amplifier
-they are all designed to conduct electrical activity
two types of electrodes
disc/cup electrode: metal coated with a hole in the center to release the conducting paste, useful for very low frequency
disposable electrode : self adhesive backing that comes in different sizes and shapes
other uncommon ones include ear clip or canal electrode
non inverting vs. inverting electrode
non inverting: the signal is coming in and is going to be left alone
inverting: the signal is coming in and gets flipped 180 degrees before it reaches the processor
10-20 convention with electrodes
what is used for electrode placement where the letter tells us the location and number indicates the side
-odd is left, even is right
-A is auris (ear), M is mastoid, C is cortex and F is forehead
-those with the letter z indicate that they are on the midline
impacting the measurement, acquisition : filters
remove part (or parts) of something from the whole and within the AEP measurement they reject electrical activity at certain frequencies while passing energy at other frequencies
filtering is used to ….
eliminate as much internal noise as possible and to eliminate as much external electrical noise as possible
types of filters
high pass : rejects lows, allows highs to pass
low pass : rejects highs, allows lows to pass
bandpass : rejects energy below and above a certain cutoff, allows those between the cutoffs to pass
band reject : rejects a very specific range between two cutoffs (not typically used)
how can inaccurate amount of filtering impact a recording
-too much can eliminate the AER response
-too little can lead to high noise levels and poor AER recordings
impacting the measurement, acquisition : amplifiers
device that increases the strength of a signal and is critical in the AEP system
-important as the signals that we are capturing are from the cochlea or 8th nerve and are very small, so without the amplifier we could not see them
amplifiers for AEPs need 2 things, these are
input impedance : trying to figure out how clean the measurements are by running an impedance measure
common mode rejection (CMR) : the process of identifying what is common between electrodes and throwing out those common aspects so all that is left is what is different (the true response)
-thought behind this is that those common signals is the noise in the environment
impacting the measurement, acquisition : artifact rejection
this is what takes anything that is not a true response, so an artifact, and removes it from the recording while keeping a running count of how many are thrown out
-what helps create a clean and clean and true signal
what are artifacts within the AEP recording
electrical activity that is not part of the auditory response and should therefore not be included in the analysis of the response
what are the 3 common sources of artifact
electrical (activity within the circuit), electromagnetic (generated from an external or non patient source) or electrophysiologic (originating from the patient such as myogenic)
impacting the measurement, acquisition : signal averaging
converts the analog electrical activity from the amplifier into a series of numerical values and these values are then processed by the computer to generate the averaged response
-this is what cleans the signal
-a way to improve the SNR in a way (increasing the size of the response while reducing the size of the noise)
impacting the measurement, non-pathologic : age
general principle is that shorter latency responses mature at an earlier age than longer latency responses
-earlier evoked potentials mature first (ECochG before ABR which is before the mids and before the higher cortical ones)
with acquisition, what is the importance of a response delay/trigger
this is what indicates at which point the actual response occurs
-this marks the beginning of the recoding
impacting the measurement, non-pathologic : state of arousal
does not matter for ABRs but for mids and lates that is a different story
what do we mean by science vs. art
science is the number we get, the facts we get directly from the recording whereas the art means the morphology the assumptions we make based on the appearance
in regards to non-pathological, what are some aspects that can impact the measurement
age, body temperature and state of arousal
in regards to waveform analysis, what are some aspects that can impact the measurement
present landmarks, latency, amplitude, morphology and polarity direction
impacting the measurement, non-pathologic : body temperature
patients at risk for temperature aberrations include those with infection (high temperature) and those in a come or under the effects of anesthesia or alcohol (low temperature)
impacting the measurement, waveform analysis : latency
time interval between the exact moment of a stimulus presentation and the appearance of a change in the waveform
-expressed in msec
impacting the measurement, waveform analysis : interaural difference of ratio
wanting to see that there is symmetry between both sides
impacting the measurement, waveform analysis : amplitude
finding for each wave
-can do so by measuring the voltage between the peak of a wave and the trough (volley)
-another is through the calculation of the voltage difference between the peak and the preceding trough
-this is calculating the difference between peak voltage of the wave and some measure of baseline voltage
absolute latency vs. interpeak latency
absolute : whole time of recording
interpeak : between the peaks
valuable with determining if there is a space occupying lesion
impacting the measurement, waveform analysis : morphology
the pattern or overall shape of the recording
clinical utility of AEPs
evaluation of hearing sensitivity, evaluation of CANS for pathology, evaluation of the CANS including auditory processing, evaluation of children with language/cognitive/other developmental disorders, monitoring effectiveness of intervention
-they show high correlation to physiological changes in the auditory pathway
why use AEPs
how to prepare for conducting the test
patient instructions, patient arousal state, patient medications, makeup/hair/clothing and then electrode placement and impedances
what populations of people can AEPs be recorded on if they cannot have valid threshold testing
newborns, difficult to test children, patients with developmental disorders, patients with cognitive impairments, persons with false or exaggerated HL, very sick patients/unconscious patients/sleeping patients, patients who are anesthetized and comatose patients
advantages with using AEPs
steps for electrode preparation
-skin surface will be scrubbed with a mildly abrasive substance (NuPrep) in order to remove dead skin, debris and oil
-alcohol wipes can be used to remove some things prior to recording however it takes time to dry
mounting the electrodes
disposable electrodes : pre gelled and no further gel is required
-do not press them in the middle as the gel will then be dispersed
-gently pull on the electrode a few seconds after application and it should remain adhered to the skin
reusable electrodes : should always apply a conductive electrode gel (10-20 paste) and then use medical tape to hole them in place
importance with electrode placement
ensure consistent placement in the accurate position
-secure and consistency attachment throughout the test session
desired impedance levels
-should be within 3-5 kOhms (down to 1 for reusables)
-should not differ by more than 2 kOhms
electrode impedances
a small current is applied to one electrode and the amount of cross over is determined in a second electrode to determine interelectrode impedance
-low and balanced impedances contribute to high quality AER by limiting internal amplifier noise, reducing effects of external electrical interference and maintain higher common mode rejection ratios
if impedances are too low this means
there is not a direct connection between the two electrodes and may lead toa short circuit at the amplifier output
what are some ways to lower impedances
pressing on the electrode for several minutes, moving the electrode slightly to get better contact, adding more electrode conducting paste, securing the electrode snugly with additional tape and removing the electrodes with higher impedances and re-prepping the skin
what does it mean when there is high impedances
can occur if there is a fault electrode or if no electrode is plugged into the receptor on the pre amp
ways to reduce impedance and improve recordings : electrodes
-braid electrodes or tap them to help prevent noise from getting in the recording
-do not mix electrode type
-all electrode wires should run towards the top of the head with the transducer wires running down (they should not mix)
ways to reduce impedance and improve recordings : test environment
-turn off any unnecessary monitors or light
-ensure you are using the designated and grounded electrical outlet
-if chair is operated electrically, unplug it
-always choose the largest size of ear tip
what do we mean when we say electrode montage
this is when we use the 10-20 convention, basically another way to talk about the electrode placement
our reference electrode will be a ___________ electrode
inverting
