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Electrophysiology > Lecture 2 > Flashcards

Lecture 2 Flashcards

1
Q

What are the three things we need to record far-field responses?

A
  1. Spatial summation
  2. Synchrony
  3. Open-Field orientation
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2
Q

What imaging technique uses the right hand rule?

A

MEG

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3
Q

How much is being measured through ABR?

A

± 0.2 µV

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4
Q

What range can computer’s pick things up in? What happens if things are outside this range?

A
  • A range of ± 10 V
  • Anything greater than this will be clipped (heavily distorted)
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5
Q

How much amplification would be appropriate?

A

100,000 x .2 µV = 20 mV

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6
Q

What does ABR come packaged in? How much noise are we often dealing with?

A

Noise (often dealing with 20µV of noise)

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7
Q

How much is being measured in MLR?

A

± 0.4 µV

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8
Q

How much is being measured in LLR?

A

± 4.0 µV

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9
Q

MLR is spread over ____ and LLR is spread over ____

A

50ms, 500ms

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10
Q

How to prevent shocking the system

A
  1. Dryness is most risky—can use humidifier if air is dry!
  2. Leather-souled shoes are better than rubber-souled shoes (insulating).
  3. When in doubt, always discharge before touching equipment by touching metal (e.g., table leg).
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11
Q

Explain common-mode voltage (line noise)

A
  • 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
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12
Q

What are strategies to minimize common-mode/line noise before doing test?

A
  1. differential amplification
  2. balanced impedances
  3. electrode care (placement, braiding)
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13
Q

What are strategies to minimize common-mode/line on the computer?

A
  1. averaging
  2. filtering
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14
Q

Explain differential amplification

A

Differential amplification is amplifying the difference between two things

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15
Q

What are the the two electrodes we are finding the difference between?

A
  • Active (non-inverting)
  • Reference (inverting)

response = (active) - (reference)

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16
Q

What is the ground?

A

the person

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17
Q

What does the ground do?

A
  • 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
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18
Q

How to reject common mode?

A
  • You need a balanced amplifier
  • common mode rejection ratio (CMRR)
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19
Q

What is the common-mode rejection ratio?

A

10000/1(80dB) to 100000/1(100dB)

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20
Q

What is coupling?

A

Connecting the electrodes to the peron

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21
Q

What do you need for coupling to take place?

A
  • 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)
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22
Q

What is the impedance of the epidermis (outer layer) before being exfoiliated?

A

25 kΩ (x2 = 50)
amplifier input impedance must be much higher than sum of all skin contact impedance

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23
Q

skin potentials more likely with ____ impedance

A

high

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24
Q

What is the target impedance?

A

below 5 kΩ

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25
Q

What is the electrode made out of? What filter does it use?

A
  • 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
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26
Q

What types of electrodes are there?

A
  • Silver/Silver-Chloride (most common for high end systems)
  • Silver
  • Tin
  • Gold (often in the clinic)
  • Platinum
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27
Q

High-impedance amplifier reduces ____

A

Distortion

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28
Q

For good CMR, it is also necessary to have balanced ____

A

amplifier-person connections

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29
Q

electrode impedances should be similar for all electrodes - within ____

A

1 kΩ

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30
Q

Explain eletrode-skin impedance
1. what two things are essential
3. why does it need to be low

A
  1. Must be balanced (within 1 kΩ) for good CMR
  2. Must be much lower than input impedance of amplifier (usually below 5 kΩ)
  3. 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
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31
Q

Explain the two main things with electrode placement

A
  1. 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
  2. electrode wires should be braided
    - this minimizes current induction on the wires
    - very inexpensive way to improve recording
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32
Q

What is electrode montage?

A

where we place the electrodes

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33
Q

Ideally, we want an electrode on either end of a ____

A

dipole

34
Q

If looking for something on the AN, place electrodes ____

A

horizontally (ear to ear)

35
Q

If looking for something on the lateral lemniscus, place electrodes ____

A

vertically

36
Q

ABR is usually done ____

A

horizontally

37
Q

Where is the active (non-inverting) electrode placed?

A

On top of head

38
Q

Where is the reference (inverting) electrode placed?

A

Lower then the active (mastoid)

39
Q

What are Jasper’s sites?

A
  • nasion (above nose between eyebrows)
  • inion (bump on back of head)
  • preauricular points (indentation above tragus on the ear)
40
Q

nasion to inion is what type of measurement?

A

front to back

41
Q

preauricular points are what type of measurement?

A

side to side

42
Q

____ is exactly halfway

A

Cz (vertex)

43
Q

What is the way the electrodes should be positioned?

A
  • 10% labels (10-10 system)
  • The major points are 10% up from the bottom then 20% up
44
Q

Typical Montage: Clinic

A

Active: vertex or Fpz (forehead)
Reference: mastoids, earlobes, ear-canal (EcochG), back of the neck
Ground: somewhere convenient

45
Q

The amplified signal of an ABR is a voltage ____

A

fluctuation

46
Q

how do you turn this into numbers to be stored and analyzed on a computer?

A

Sampling rate and resolution

47
Q

What are the basics of digitization?

A

sampling rate and resolution

48
Q

What is sampling rate?

A

The rate at which you take measurements

49
Q

What is resolution?

A

The level of precision (the amount of data it can store)

50
Q

How is resolution measured?

A

Measured in binary digits or ‘bits’

51
Q

How many levels does a 3 bit system have?

A

8

52
Q

How many levels does a 24 bit system have?

A

A lot

53
Q

The more bits, the more ____

A

precise

54
Q

How much resolution and amplification is needed for ABR?

A
  • You need enough amplification and enough bit depth to quantize the stimulus
  • Usually 100 000x and 16 bit
55
Q

What is aliasing?

A

Misrepresenting the frequency (you cant recover the signal)

56
Q

How do you avoid aliasing?

A

to avoid aliasing (misrepresenting frequency), must sample at least 2x the highest frequency present in the signal

57
Q

How do you represent amplitude?

A

to represent amplitudes properly, should sample at least 5x the highest frequency of interest (the part you care about)

58
Q

How do resolution and sampling rate work together?

A

The higher sampling rate and resolution the better

59
Q

Explain low-pass filter

A
  • 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
60
Q

Explain high-pass filter
- what does this reduce?

A
  • 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)
61
Q

If low pass filter is at 3000 Hz we need ____ Hz to avoid aliasing (minimum)

A

6000

62
Q

When do we low-pass filter?

A

must low-pass filter BEFORE digitization to make sure that sampling rate is twice as high as any frequencies present (to prevent aliasing)

63
Q

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

A

6000, 15000

64
Q

What 3 things can filters affect?

A
  1. amplitude
  2. phase
  3. delay
65
Q

What are the 2 best filters to maintain phase?

A
  1. online digital
  2. bessel filters
66
Q

What 6 things do you need for ABR?

A
  1. Environment – quiet, comfortable and relaxing, preferably not too warm and humid (sweating is bad) or dry (static is bad)
  2. Preparing skin – low and balanced impedances
  3. Electrodes placed correctly, not hanging in face, wires not near wires with current or wall voltage
  4. Amplifier settings correct
  5. Filters set appropriately for response
  6. Sampling rate high enough
67
Q

____ gets smaller with averaging and the ____ gets larger

A

Noise, sum

68
Q

____ of the noise is proportional to the reciprocal of the ____ of the number of trials

A

amplitude, square root

69
Q

Averaging Increases ____

A

SNR

70
Q

What should you focus on first with averaging?

A

Achieving low noise first

71
Q

Averaging works for ____ responses

A

time-locked

72
Q

Averaging Increases ____

A

Resolution

73
Q

What are responses from an ABR like?

A

Most responses are very small (e.g., ABR is half a microvolt, noise is 20 times larger), therefore responses are mostly noise (before averaging)

74
Q

Larger amplitude trials are ____

A

Noisier

75
Q

Smaller amplitude trials are ____

A

Quieter

76
Q

Typically rejection set to ____µV for ABR

A

20—30

77
Q

What are 3 sources of noise?

A

electrical
myogenic
ocular (large myogenic)

78
Q

What are 6 ways to minimize noise?

A
  • use shielded components
  • braid electrode leads
  • balance electrode impedances
  • remove sources of contamination
  • subject should be still and RELAXED
  • blinking between trials, fixation
79
Q

How do you make someone relax/sleep?

A
  • 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
80
Q

What is another way to make someone relax?

A

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

81
Q

A gentler alternative to melatonin is ____

A

essential oils

82
Q

What are enough averages to achieve a good SNR?

A

2:1 or 6dB

Electrophysiology (8 decks)

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