Lecture 2 Flashcards
What are the three things we need to record far-field responses?
- Spatial summation
- Synchrony
- Open-Field orientation
What imaging technique uses the right hand rule?
MEG
How much is being measured through ABR?
± 0.2 µV
What range can computer’s pick things up in? What happens if things are outside this range?
- A range of ± 10 V
- Anything greater than this will be clipped (heavily distorted)
How much amplification would be appropriate?
100,000 x .2 µV = 20 mV
What does ABR come packaged in? How much noise are we often dealing with?
Noise (often dealing with 20µV of noise)
How much is being measured in MLR?
± 0.4 µV
How much is being measured in LLR?
± 4.0 µV
MLR is spread over ____ and LLR is spread over ____
50ms, 500ms
How to prevent shocking the system
- Dryness is most risky—can use humidifier if air is dry!
- Leather-souled shoes are better than rubber-souled shoes (insulating).
- When in doubt, always discharge before touching equipment by touching metal (e.g., table leg).
Explain common-mode voltage (line noise)
- Wall voltage = 120,000,000 µV (300,000,000 times larger than ABR)
- If near any source of electricity, ABR can pick this up (need to position correctly)
- Almost always have a bit of common-mode voltage
What are strategies to minimize common-mode/line noise before doing test?
- differential amplification
- balanced impedances
- electrode care (placement, braiding)
What are strategies to minimize common-mode/line on the computer?
- averaging
- filtering
Explain differential amplification
Differential amplification is amplifying the difference between two things
What are the the two electrodes we are finding the difference between?
- Active (non-inverting)
- Reference (inverting)
response = (active) - (reference)
What is the ground?
the person
What does the ground do?
- ground electrode provides the amplifier with a ‘reference voltage’ to prevent amplifier drift (to stay in the voltage range of amplifier; active & reference)
- also helps with common-mode rejection
How to reject common mode?
- You need a balanced amplifier
- common mode rejection ratio (CMRR)
What is the common-mode rejection ratio?
10000/1(80dB) to 100000/1(100dB)
What is coupling?
Connecting the electrodes to the peron
What do you need for coupling to take place?
- a conducting medium (skin doesn’t contact the electrode directly)
- a paste or jelly is used
- electrolyte should come into contact with subcutaneous tissue (exfoliation)
What is the impedance of the epidermis (outer layer) before being exfoiliated?
25 kΩ (x2 = 50)
amplifier input impedance must be much higher than sum of all skin contact impedance
skin potentials more likely with ____ impedance
high
What is the target impedance?
below 5 kΩ
What is the electrode made out of? What filter does it use?
- Metal immersed in electrolyte, ion exchange, two layers (opposite charge) building-up adjacent to the metal
- this acts as a high-pass filter (removes low frequencies)
- depends on particular metal
What types of electrodes are there?
- Silver/Silver-Chloride (most common for high end systems)
- Silver
- Tin
- Gold (often in the clinic)
- Platinum
High-impedance amplifier reduces ____
Distortion
For good CMR, it is also necessary to have balanced ____
amplifier-person connections
electrode impedances should be similar for all electrodes - within ____
1 kΩ
Explain eletrode-skin impedance
1. what two things are essential
3. why does it need to be low
- Must be balanced (within 1 kΩ) for good CMR
- Must be much lower than input impedance of amplifier (usually below 5 kΩ)
- Should be low to reduce effects of skin potentials
- can only be completely removed by puncturing skin
- sweat is an enemy, keep subjects cool
- generally low frequency - can be removed with filtering
Explain the two main things with electrode placement
- wires should be away from devices with current (e.g., ER-3 earphones)
- do not dangle in face!!
- requires some planning
- generally electrodes go towards back of the head - electrode wires should be braided
- this minimizes current induction on the wires
- very inexpensive way to improve recording
What is electrode montage?
where we place the electrodes
Ideally, we want an electrode on either end of a ____
dipole
If looking for something on the AN, place electrodes ____
horizontally (ear to ear)
If looking for something on the lateral lemniscus, place electrodes ____
vertically
ABR is usually done ____
horizontally
Where is the active (non-inverting) electrode placed?
On top of head
Where is the reference (inverting) electrode placed?
Lower then the active (mastoid)
What are Jasper’s sites?
- nasion (above nose between eyebrows)
- inion (bump on back of head)
- preauricular points (indentation above tragus on the ear)
nasion to inion is what type of measurement?
front to back
preauricular points are what type of measurement?
side to side
____ is exactly halfway
Cz (vertex)
What is the way the electrodes should be positioned?
- 10% labels (10-10 system)
- The major points are 10% up from the bottom then 20% up
Typical Montage: Clinic
Active: vertex or Fpz (forehead)
Reference: mastoids, earlobes, ear-canal (EcochG), back of the neck
Ground: somewhere convenient
The amplified signal of an ABR is a voltage ____
fluctuation
how do you turn this into numbers to be stored and analyzed on a computer?
Sampling rate and resolution
What are the basics of digitization?
sampling rate and resolution
What is sampling rate?
The rate at which you take measurements
What is resolution?
The level of precision (the amount of data it can store)
How is resolution measured?
Measured in binary digits or ‘bits’
How many levels does a 3 bit system have?
8
How many levels does a 24 bit system have?
A lot
The more bits, the more ____
precise
How much resolution and amplification is needed for ABR?
- You need enough amplification and enough bit depth to quantize the stimulus
- Usually 100 000x and 16 bit
What is aliasing?
Misrepresenting the frequency (you cant recover the signal)
How do you avoid aliasing?
to avoid aliasing (misrepresenting frequency), must sample at least 2x the highest frequency present in the signal
How do you represent amplitude?
to represent amplitudes properly, should sample at least 5x the highest frequency of interest (the part you care about)
How do resolution and sampling rate work together?
The higher sampling rate and resolution the better
Explain low-pass filter
- Low-pass filter removes high-frequency energy that is not part of the response you are recording (e.g. 1500-3000 Hz for ABR)
- this restricts frequencies to prevent aliasing!
- Maximum setting of low-pass filter is <1/2 of sample rate
Explain high-pass filter
- what does this reduce?
- High-pass filter removes low-frequency energy that is not part of the response you are recording
- this reduces amount of electrophysiological noise in recording (which is greater at low frequencies)
If low pass filter is at 3000 Hz we need ____ Hz to avoid aliasing (minimum)
6000
When do we low-pass filter?
must low-pass filter BEFORE digitization to make sure that sampling rate is twice as high as any frequencies present (to prevent aliasing)
ABR has important energy from 100-3000 Hz
- your sampling rate must be at least ____Hz to prevent aliasing
- your sampling rate should be at least ____Hz to represent the shape correctly
6000, 15000
What 3 things can filters affect?
- amplitude
- phase
- delay
What are the 2 best filters to maintain phase?
- online digital
- bessel filters
What 6 things do you need for ABR?
- Environment – quiet, comfortable and relaxing, preferably not too warm and humid (sweating is bad) or dry (static is bad)
- Preparing skin – low and balanced impedances
- Electrodes placed correctly, not hanging in face, wires not near wires with current or wall voltage
- Amplifier settings correct
- Filters set appropriately for response
- Sampling rate high enough
____ gets smaller with averaging and the ____ gets larger
Noise, sum
____ of the noise is proportional to the reciprocal of the ____ of the number of trials
amplitude, square root
Averaging Increases ____
SNR
What should you focus on first with averaging?
Achieving low noise first
Averaging works for ____ responses
time-locked
Averaging Increases ____
Resolution
What are responses from an ABR like?
Most responses are very small (e.g., ABR is half a microvolt, noise is 20 times larger), therefore responses are mostly noise (before averaging)
Larger amplitude trials are ____
Noisier
Smaller amplitude trials are ____
Quieter
Typically rejection set to ____µV for ABR
20—30
What are 3 sources of noise?
electrical
myogenic
ocular (large myogenic)
What are 6 ways to minimize noise?
- use shielded components
- braid electrode leads
- balance electrode impedances
- remove sources of contamination
- subject should be still and RELAXED
- blinking between trials, fixation
How do you make someone relax/sleep?
- how you talk
- soft lights
- comfort
- a REALLY good chair
- have clean blankets and pillows ready
- let people know what to expect in advance
allow for washroom breaks
What is another way to make someone relax?
Jay Hall has suggested using melatonin
- powerful hormone; can shift body clock
- can interfere with some medication (e.g., blood pressure)
- safety in children not clearly established
- may lead to grogginess on awakening
A gentler alternative to melatonin is ____
essential oils
What are enough averages to achieve a good SNR?
2:1 or 6dB
