Evoked Responses Quiz Shortened Flashcards
What is an amplifier?
a 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 measurement
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)
-The thought behind this is that those common signals are the noise in the environment
When using amplifiers what would happen if 2 electrodes were places next to each other in the same location?
The result would be nearly a flat line because of the CMR cancelling out the commonalities
What is a filter?
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
What are filters used to do?
eliminate as much internal noise as possible and to eliminate as much external electrical noise as possible
What are the 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. good filter for AER.
band reject : rejects a very specific range between two cutoffs (not typically used in AER recordings)
How can an 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
What is a filter slope?
a specified cutoff frequency. this does not happen abruptly but rather at a slope. can play a critical role in AER recordings particularly in the mid and late AERs
Our reference electrode will be a ___________ electrode
inverting
Exogenous vs Endogenous?
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
Electrode Generator sites: tell me the difference between near field and far field
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
- The closer you are to the signal, the better
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
What is an Auditory Evoked Potential (AEP)?
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
How do we classify AEP’s?
characteristics determined by external or internal processes, based on the time epoch following the stimulus, based on relation of electrodes to generator site and based on structures in the auditory system that generates them
What are electrodes?
A sensing device that detects bioelectric activity and sends it to the pre-amplifier. AERs are recorded by attaching electrodes to the scalp, mastoid, earlobes, external ear canal, or TM. Designed to conduct electrical activity in the frequency range of the evoked responses.
How do you prep the skin for an electrode?
whats the goal?
scrub with mildly abrasive liquid substance to remove dead skin, debris, and oil that might interfere with the electrical activity. the aim is to get the impedance below 5 ohm. alcohol pads may only be used to remove vernix prior to ABR on neonates. if using alcohol, make sure skin is completely dry or impedances might be slightly higher.
gently pull electrode a few seconds after application and it should remain tightly adhered to the skin
How long is the lead wire that connects the electrode to the DIN pin?
Typically 1 meter or 36 inches. A shorter wire (1 ft) reduces electrical interference but is not commonly used.
Tell me about Disc-Type/Cup Electrodes
- conventionally used for AERs
- metal: tin, silver, or platinum
- a hole of about 2 mm in the center to inject/release conducting paste/gel
- Silver chloride coated electrodes are useful for very low frequency.
- best to use electrodes made of the same metal to avoid imbalance
- expected to have a higher impedance than disposable
Tell me about Disposable Electrodes
- commonly used for infants
- not sterile but maybe more hygienic
- skin still needs to be prepared by cream/gel is not required
- connected with a snap lead wire of alligator clip lead wire or the whole electrode and wire can be disposable
- should be possible to achieve impedances b/w 1-5 ohms
What is electrode impedance? What are we shooting for?
impedance is a materials resistance to flow of electrical current. it is measured in ohms. should be measured before and after AER recording if there is reason to suspect change such as excessive patient movement, increased electrical artifact, and when switching b/w ears.
desired impedance is b/w 1 to 5 ohms, 0 is not desirable bc it may lead to a short circuit at the amplifier or reflect a connection b/w 2 closely placed electrodes
how to reduce impedance
turn off unnecessary computer monitors, turn off phones, unplug the chair they are in if it can be operated, always use the largest size ear tip to reduce the risk of stimulus leakage, braid or tape electrode, do not mix electrode types, do not place ground electrodes near the heart, ensure electrodes are symmetrical b/w ears, all electrode leads should run toward the top of the patients head, pressing on the electrode for several minutes, moving it slightly to get better contact, adding more gel, securing it snug with additional tape, and removing electrodes with higher impedance and reprepping skin
what does it mean if the electrode impedance is reading “open”?
means it is greater than 80kOhm. this can happen if there is a faulty electrode or if no electrode is plugged in.
The more soft tissue the ____ the impedance values
better
How do you choose which electrode will be used?
dictated by the type of AER to be recorded and the patient population
What is the difference between non-inverting vs. inverting electrodes?
Non-inverting: the signal is coming in and is going to be left alone (how it comes in is how it stays). Located at Cz.
Inverting: the signal is coming in and gets flipped 180 degrees before it reaches the processor (takes the signal and flips its 180 degrees). Located at earlobe or mastoid.
Tell me about the electrode montage
Some evoked reponses like ABR are recorded with 3-4 electrode sites whereas cortical AER are sometimes recorded simultaneously with 20-30 electrodes facilitated by an electrode cap.
Cz = very top of head
Fz = high forehead
Fpz =how forehead
A1 = right ear
A2= left ear
M1 & M2 = mastoids
Odd #s = left
Right #s = even
What are common sources of artifact? How do you get rid of an artifact?
electrical = activity within the electrical circuit
electromagnetic = generated from an external or non-patient source
electrophysiologic = originating from the patient
To get rid of these, you can remove the source of the artifact (turn off the phone/machine), modify test parameters (increase intensity, change filters) or artifact rejections (any voltage exceeding designated voltage is not sent to the computer)
What is signal averaging?
what is sounds like, it averages the electrical activity. used to increase the size of the AER and reduce the size of the noise.
What is an auditory evoked repsonse (AER)?
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
What are the sounds used for AERs?
clicks, tone bursts, and speech sounds
Tell me about 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
Tell me about 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 is polarity? (stimulus factor)
The initial direction of the pressure wavefront in the stimulus waveform, measured at the face of the transducer. There are 3 types: rarefaction polarity, condensation polarity, & alternating polarity.
What is rarefaction polarity?
- polarity that causes the pressure wavefront of a transducer to move away from the eardrum
- the early components of the AEPs latency is slightly shorter and amplitude is higher for
What is condensation polarity?
- a polarity that initially causes the pressure wavefront of a transducer to move toward the eardrum
- the early components of the AEPs latency may be slightly longer
- wave 5 amplitude tends to be larger for normal hearing subjects
What is alternating polarity?
- a polarity that is alternated on successive trials (between rarefaction & condensation)
- at high intensities, alternating polarities are sometimes used to reduce stimulus artifact
What are the factors that affect AEPs?
stimulus factors, acquisition factors, non-pathologic subject factors, and waveform analysis
What are stimulus factors for AEPs?
stimulus type (click/burst), duration, polarity, intensity & rate.
What are acquisition factors for AEPs?
type of electrode, filter, amplifiers, artifact rejection, signal averaging,
What are non-pathologic subject factors for AEPs?
gender, age, body temp. typically dont make a difference
What is waveform analysis for AEPs?
looking at several factors of a waveform over time: latency, amplitude, morphology, & polarity direction
Tell me about waveform analysis LATENCY
time b/w stimulus presentation and the appearance of a change. measured in msec.
Tell me about waveform analysis AMPLITUDE
the difference b/w the peak of a wave and the following valley. measured in microvolts.
Tell me about waveform analysis MORPHOLOGY
pattern/shape of the waves. morphology can be poor even if latency and amplitude are deemed normal.
Tell me about waveform analysis POLARITY DIRECTION
which way is up? polarity of an AEP depends on the electrode location relative to the generator of the response & which electrode is plugged into the positive and negative volate inputs of the differential amplifier
What are the components of instrumentation?
stimulus generators, electrodes, filters, amplifiers, a signal average with artifact rejection, response delay (trigger), response processing, & a means to print or display test results
What transducer is recommended when conducting AEPs?
Inserts because they make wave 1 more visible, prevent canal collapse, increase interaural attenuation, and attenuate environmental noise
AERs can be used on patients whi cannot be assessed validly with behavioral audiometry such as:
newborns, difficult to test children, developmental disorders, cognitive impairments, false/exaggerated hearing loss, sick or sleeping patients, comatose
AERs may be useful for?
evaluating hearing sensitivity, evaluating CANS pathology, evaluation of CANS including auditory processing, evaluation of children with language cognitive or other developmental disorders, and monitoring effectiveness of intervention such as CIs or HAs
What is the 10-20 International Electrode nomenclature?
each site is located to a given patient reference by well defined anatomic landmarks. this can make the difference b/w recording well formed responses vs non-observable ones
nasion = bridge of nose
inion = occipital protuberance
f = frontal
c = coronal
p = parietal
o = occipital
t = temporal
What is a ground electrode, and where is it located?
necessary in order to record consistent responses n varied test conditions. can be located anywhere on the body
What are the 3 receptacles/jacks an electrode box has?
non-inverting, inverting, and ground = a one channel recording, meaning the inputs for channel 2 are not used.
Two-channel recording = 5 or 6 electrode receptacles, allowing you to perform ipsi and contra recordings simulatenosly. if this has an Cz or Fz site, to avoid placing 2 non-inverting electrodes on a patient (1 for each channel) the receptacles are linked by a jumper cable. one side goes in input 1 for channel 1 and the other side goes in input 1 for channel 2.
how do we recognize an ipsi response from a contra response?
you will have an absent/reduced wave 1 and 2 in a conta response
What is on the x and y axis of an AER?
x axis = latency in ms
y axis = amplitude in microvolts (µv)
What is the interplay b/w latency, amplitude, and intensity?
if you decrease intensity, latency will increase, and amplitude will decrease.
Is short or long more frequency specific?
long
