Week 7: Recording AEPs Flashcards
what are the major stimulus parameters in measurement of AEPs (7)
type duration rate of presentation polarity intensity transducer masking *the consequences of choosing an inappropriate stimulus parameter vary greatly
stimulus type for AEPs
- click stimulus
- –100 microseconds of full on stimulus
- –commonly ised for ECochG and ABR
- –flat spectrum to an extent
- –associated with the basal part of the cochlea
- chirp stimulus
- –presents frequencies in sequence from low to high and can be broad band or narrow band
- –CE-chirp= gives greater response amplitude because all frequencies hit brainstem at the same time
- tone burst or tone pip stimulus
- –based on the length of rise-plateau-fall time in msec or cycles
- –frequency specific with energy around the fundamental frequency
- –commonly used in ABR
- –give info on HL configuration
gating
*how the rise and fall time creating the envelope is constructed
—blackman is the best to use with tone burst
(controls most of the splatter)
—-modulating rise/fall times=gating
—triangular= linear envelope
stimulus intensity of AEP stimulus
*latency decreases and amplitude increases with greater stimulus intensity
*intensity does not affect all APs in the same way
*5 references may be used to describe intensity f stimulus:
dB SPL, dB peSPL, dB HL, dB SL, and dB nHL
calibration of brief stimuli
- done with oscilliscope to measure voltage of the click and increase intensity level of 1000 Hz pure tone until = voltage and that is the pe SPL
- difference between dB nHL and dB SPL= about 30 dB
stimulus presentation rate
*is a parameter that must be selected by the clinician in any AEP measurement
*fast rate to stress the aud nerve vs slow rates for more robust responses
no single correct stimulus rate is appropriate for all pts under all test circumstances
stimulus polarity
- condensation= positive polarity
- rarefaction= negative polarity
- –there is some difference in latency between these two because negative is excitation and positive is inhibition
- rarefaction= switches between condensation and rarefaction and then averaging
- **polarity of acoustic stimulus depends on whether the diaphragm of the transducer moves outward or inward in producing the sound
transducers
- a device for converting energy from one form to another
- used in AEP receive an electrical signal and produce a sound that is often presented as an air-conducted stimulus
- –some are done with bone conduction
insert earphones for AEP
- very important to use a set of insert earphones compatible with auditory evoked system (selected by manufacturer)
- advantage of inserts:
- –increased intra-aural attenuation
- –increased reduction of ambient noise
- –prevent ear canal collapse
- –increased pt comfort
- –reduce chance for stimulus artifact
supra-aural earphones
- correct placement of the earphone-cushion is important
- –ensure that the earphone diaphragm is directly aligned with the EAC and pressed against the pinna with no air leaks
- –instruct the pt not to move the headset
- –higher chance for stimulus artifact
stimulus artifact
- headphone use electromagnetic field to move diaphragm, when the headphone is near or on the electrode, it can pic that up
- inserts on the other hand have a long tube and you can place the transducer away
- –25 cm long tube
- shielding= trying to reduce this more (tin foil on transducers)
- alternating click polarity is helpful to reduce hat at times
- control runs= of you pinch the tube and dont allow signal through to EAC and still see sine wave response, it is a stimulus artifact because ear isnt hearing it
bone-conduction oscillator
- the ABR is commonly recorded with bone stimulation
- ensure the bone oscillator is placed on the mastoid bone
- artifact an maximum outputs are challenges to BC
acquisition factors of AEPs
- electrode type
- electrode location
- analysis time (epoch)
- averaging (sweeps=number of stimuli presented)
- amplification
- filter settings
- —the type of AEP and it’s success is determined by these factors
electrodes basics
- sensors that are placed on the skin or in some other location as close as clinically possible to the generator of the response
- plug into preamplifier, and the response is compared between several electrodes
- made up of different materials and come in different shapes and sizes (metal, disposable)
- conventional is the disc (cup)
- –gold, silver, or silver coated using silver chloride
- –earclip electrodes are a variation of the disk electrodes
- ear-canal electrodes (tiptrode or TM/tymptrode)
- disposable single use electrodes are usually made of foam or cloth
- –snap electrode or entire lead
electrode placement basics
based off of the international 10-20 system which is a road map allowing us to describe electrode locations *has 4 starting points ---nasion= nose bridge ---inion= bump at back of skull ---pre-auricular point both left and right **c=central **z=midline **f=frontal **t=temporal **o=occipital **a=earlobe **m=mastoid odd #=left; even #=right
