revision epa Flashcards
Describe Ohm’s Law
and
its equation
There will be opposition to the flow resistance of current when a voltage is applied = electrical impedance (ohms(Ω).
CURRENT = VOLTAGE / IMPEDANCE or alternatively, VOLTAGE = CURRENT x IMPEDANCE
What are the two components of Ohm’s Law and what do they mean?
- Capacitive component: Large for low frequencies and reduces for high frequencies (like stiffness reactance in acoustics),
- Inductance component: small for low frequencies and increases for high frequencies (like mass reactance in acoustics)
Provide an equation for Power
POWER = (CURRENT)2x IMPEDANCE.
Difference btw near field and far-field recording?
Near field testing unable to be completed, particularly in animals which obtains are larger voltage as the electrode is closest to the site of generation.
In humans use far-field recording. In far field electrode will ‘see; a small version of the electrical activity generated at the site of neural tissue in the auditory pathway.
Describe the electrode Montage for the four electrodes?
· Non-inverting (+ve) electrode is placed high on the forehead or vertex
Inverting electrode (-ve response, downward deflection is placed ipsilateral mastoid or earlobe (side receiving the acoustic stimuli)
• Describe the considerations for recording electrical activity from living tissue
Near filed and Far field testing
Near field testing unable to be completed, particularly in animals which obtains are larger voltage as the electrode is closest to the site of generation.
In humans use far-field recording. In far field electrode will ‘see; a small version of the electrical activity generated at the site of neural tissue in the auditory pathway.
Explain Time Domain Averaging Techniques, and what it is used for?
Time Domain Averaging technique
- Used to eliminate extraneous noise (such as the EEG) from the recording so the results are accurate.
- Involves presenting multiple signals many times and taking an average the response
- TDA will average any part of the stimulus that is synchronised and reduce components not synchronised.
- This works in the way that any BGN (such as EEG ) is random and not synchronised (unlike the ABR waves occurring at particular intervals). by averaging a signal with equal positive and negative voltages (extraneous noise) it will tend towards zero.
Why is the skin scrubbed before electrodes are positioned on the client?
- To reduce the impedance of the skin to
Explain the Differential Amplification signal processing technique?
Taking the signal between the electrodes and subtracting this result from the response before amplifying it.
- Looks at two separate signals and takes the difference of them. This means that external interference that is similar in both signals will cancel out, leaving the voltages that are different at the non-inverting and inverting electrodes (internal physiological signals) to be amplified.
- Dependent on the skim impedance !!**
How does the polarity of an acoustic stimulus (rarefaction and condensation) affect the transducer?
A rarefaction polarity initially causes the pressure of the transducer to be negative, in turn causing the transducer diaphragm to deflect away from the tympanic membrane. When a condensation polarity is produced, the pressure of the transducer is positive, in turn causing the transducer diaphragm to move towards the tympanic membrane.
What frequencies is latency prolonged?
Lower frequencies. The lower the frequencies, the longer the latency as the travelling wave must travel further through to the apical portion of the cochlea.
(Extra Info)
▪ The travelling wave delay is approximately equal to the period of the signal
E.g. a 4 kHz wave has a period of 0.25 ms, so its travelling delay is also 0.25 ms. Conversely, a 250 Hz travelling wave will have a delay of 4 ms. Therefore, a higher frequency will have a shorter travelling wave delay, whereas a lower frequency will have a longer travelling wave delay.
Why are three electrodes placed onto the skin despite only two being necessary to measure an electrical signal?
Having three electrodes allows the differential amplifier to subtract out external interference.
In a differential amplifier, two signals are measured: one between the non-inverting and common electrodes, and the other between the inverting and common electrodes. These two signals are subtracted, which will tend to cancel signals that are similar (in phase) at the two electrode locations. This reduces the effect of external interference.
By scrubbing the skin with paste before placing electrodes, what are we trying to achieve?
To reduce skin impedance from 106 ohms to less than 5 kohms.
Also ensures accuracy when determining differential amplification.
(More info)
The electrical impedance of dry skin is very high, and this will make it difficult to measure small physiological responses. The typical scrub paste used contains an abrasive that removes the top layer of dead skin cells (high impedance), and a conductive saline solution (sodium chloride) that promotes an electrical connection between the skin and the electrode
The software provided a warning that the non-inverting electrode impedance was not ideal. What is this likely to mean?
The electrode impedance is too high
The aim is to lower impedance, so it is likely that a non-ideal impedance is too high. It is unlikely that the lead wire is broken, as this would prevent the test from running at all.
What could be the effect of an electrode impedance that is too high?
The differential amplifier will not work effectively, and the responses could be contaminated by external electrical signals
The differential amplifier relies on low and preferably similar impedances at all electrodes. If these conditions are not met, external interference may be increased in amplitude.
You have two stimulus rates to choose from: 11.1 Hz and 55.5 Hz. Which will you choose and why?
In a clinical setting, the best option is to use a slower stimulus rate (in this case, 11.1 Hz). Slower presentation rates ensure better neural synchrony.
You have two transducers to choose from: headphones and insert earphones. Which will you choose to use and why? Are there any considerations you need to factor in?
You could choose either, but the best acoustic properties occur with insert earphones, as the frequency response is better, and less ringing is created by the transducer. If you use insert earphones, you need to ensure that you factor in any insert earphone delays (due to the length of the transducer tubing).
The Audiologist would like to know frequency-specific information from their ABR. Before you place the electrodes on your colleague, you need to make a few decisions.
You have two stimulus types to choose from: click and tone burst stimuli. Which will you choose and why?
Given that the ‘patient’ would like frequency-specific information about the ABR, you would select tone burst stimuli, which will give you more specific information about the ABR at different frequencies.
Typically, click stimuli would be used to generate an ABR response, which is representative of high-frequency hearing levels.
Why does stimuli need to be brief? In ABR
To generate synchronous firing (many neurons firing) for successful observation of electrical activity
What are three methods to evaluate the location of ABR generators?
Simultaneous near and far-field recording-electrodes close to the site generation, high amplitudes
Lesion studies in animals – Lesion at particular site in brainstem and AMR recorded will show changes/losses
3D Imaging Techniques - (only one in humans possible). MRI, PET non-invasive, showing active brain regions.
What are some of the slower potentials (low frequency) thought to be generated by?
Dendrites
