Electrophysiology Flashcards
Artifact Rejection Level
The level of artifact allowed, (<10%), so that all stimulus is accepted and spurious activity is rejected.
Stimulus within artifact criterion = good sweep
Stimulus exceeds rejection threshold = rejected sweep
Exogenous vs. Endogenous
Exogenous: Sensitive to stimulus manipulation
Endogenous: Sensitive to psychological state of subject
N400
A measure of semantic incongruities e.g. I have my tea with milk and poop
Latency: 400ms
What is the relationship between audiometric thresholds and ABR
In general there is a good correspondence between audiometric threshold and ABR threshold. (match well for high frequency, 500 Hz threshold on ABR is usually worse than audio)
In the presence of a retro-cochlear/neurologic disorders ABR thresholds and PTA do not show a correspondence
Stimulus delivery
Stimulus generation D/A conversion Trigger response in ear Pre-amplification Amplification Filtering A/D conversion Averaging
Cochlear Microphonic
It is an alternating current (AC) receptor potential elicited by sound.
It reflects the instantaneous motion of the cochlear partition in the vicinity of the recording electrode
It reflects the spatial average of the extracellular correlate of the inner (IHC) and outer hair cell (OHC) receptor currents.
Currents have 3 properties when recorded form outside the Cochlear :
1) Dominated by the more populous OHC.
2) Dominated by hair cells located at the base of the cochlear irrespective of stimulus frequency.
3) They reflect both the displacement and the velocity of the cochlear partition.
a) OHCs and displacement of the cochlear partition thought to contribute to the CM at low stimulus intensities
b) IHCs and velocity of the cochlear partition thought to contribute to the CM at higher stimulus intensities
Stimulus polarity
Condensation: Headphone diaphragm displaced outwardly; TM displaced medially
Rarefaction: Headphone diaphragm displaced inwardly; TM displaced laterally
Sampling rate
The number of samples played per second
Increasing sample rate = Increase in wave V latency.
Latencies of earlier components of the ABR are generally unaffected = Increase in inter-wave latency interval
Amplitude of the earlier ABR waves decreases with increasing rate
Impedance
Resistance, opposition to current flow.
It is important to have low and balanced impedance across electrodes
Filtering - effects on waveform
High-pass filter: As the low frequency cut-off increases (narrowing of filter) - ABR amplitude, especially for wave V decreases and latency progressively decreases.
P300
A cochlear potential.
It is sensitive to attention and cognition
Elicited by MMN
Trigger
The response sent to the computer recording the responses so it knows when the stimulus is started
Rate of maturation
Latency decreases with maturation
Myelination
Susceptibility to state changes
The shorter the latency the less susceptible to changes in subject state
ABR is not susceptible to state
Cortical is susceptible to state
Electrode position
The shorter the latency the more far field the response
Variability
The shorter the latency the smaller the variability
Stimulation rate
The shorter the latency the faster the rate
Spectrum
The shorter the latency the higher the frequency spectrum
ABR: 30 - 1500 Hz
MLR: 10 - 100 Hz
Late potentials: 0.1 - 30 Hz
EEG: 1-12 Hz
Amplitude
The shorter the latency the smaller the amplitude
Subcortical: < 10μN
Cortical: >10μN
Latency
Absolute latency: the time between a peak and the stimulus onset
Inter-peak latency: the time between two peaks in an ABR wage form
Trigger
The beginning of the time window. This signals the averager that the stimulus has been sent
Pre-stimulus interval
Period between trigger and stimulus
Enables assessment of non-stimulus related brain activity
ISI - inter-stimulus interval
Time from the offset of one stimulus to onset of the next
SOA - Duration
SOA - Stimulus onset asynchrony
Time from onset of one stimulus to onset of the next
1/Rate & ISI + Duration
Condensation
Headphone diaphragm is displaced outwardly; TM displaced medically
Produces slightly longer latencies. Wave V amplitude tends to be longer
Rarefaction
Headphone diaphragm is displaced inwardly; TM is displaced laterally
Produces slightly shorter latencies with higher amplitude for the early components of ABR
