EEG and ERP

Question 1

How did ancient Egyptians and ancient Greeks measure the brain?

Answer 1

• Ancient Egyptians (1700 BCE) mind located in heart - “memorise by heart”
• Ancient Greeks (6-500 BCE) mind located in brain
• Ancient Greeks (400 BCE) mind located in heart no cigars for Aristotle
• Ancient Greeks thought human body sacred Not fans of dissection (publicly)
• The Egyptians loved it!
  2-3 BCE identified different regions of the brain
  Live brains
• But most of what they learned was later lost or destroyed bc of wars

Question 2

How was dissection done from 0-1700 CE?

Answer 2

~ 1 (Greece) – animal dissection reveals central nervous system
~ 1000 CE (Spain and Iran) - brain injury surgeries reveals link between brain problems and cognitive problems
~ 1200-1300 CE (Europe) - first textbooks to illustrate the brain
~ 1500-1600 CE (Europe) - human dissection reveals different brain regions
~ 1700s CE (Europe) – brain injuries reveal different brain regions associated with different cognition

Question 3

How was brain activity measured before the 1800s?

Answer 3

Invasively
Up until the 1800s, brain research done on dead animals, dead humans, live humans with brain damage
Not pleasant for anyone involved
Did not tell us about living working brains
In the 1800s, we started to develop non-invasive brain measures

Question 4

What is phenology (early 1800s)?

Answer 4

Skulllumps = size of brain areas = character + thoughts + emotions

Question 4

What is phenology (early 1800s)?

Answer 4

Skulllumps = size of brain areas = character + thoughts + emotions

Question 5

What is Craniometry (early-mid 1800s) ?

Answer 5

Skull size = intelligence

Question 6

What is Pneumoencephalography (late 1800s) ?

Answer 6

Not measure ppls brain, actually kind of look at brain
Turn someone upside down
Drill holes in their skull
Drain out cerebrospinal fluid
Do x-rays

Question 7

What is EEG?

Answer 7

Continuous recording of electrical potentials generated by brain cells
Measured using scalp electrodes
EEG draws trace, not very useful unless we’re looking for really big changes like in epilepsy

Question 8

What is ERP?

Answer 8

Average electrical potential triggered by a particular event
Measured using scalp electrodes

Question 9

What is transcranial doppler ultrasound?

Answer 9

Velocity of blood flow through blood vessels in the brain
Gross measure of brain fction
Measure rate of blood flow, can see lateralisation

Question 10

What is TMS?

Answer 10

Magnetic pulse forces groups of brain cells to fire
Artificial temporary “lesion”

Question 11

What is fNIRS?

Answer 11

Emitter optode injects near-infrared light into head - passes through skin/tissue/bone - absorbed by red blood cells
Detector optode measures non-absorbed (reflected) light
Shows changes in concentration of oxygen in blood cells when they fire
Find out what part of brain is analysing the stim we present it

Question 12

What are fixation related potentials?

Answer 12

ERPs combined with eyetracking
Average electrical potential generated by brain cells in response to the eyes fixating on a particular area in a visual scene

Question 13

What is tDCS?

Answer 13

Scalp electrodes used to send constant low direct electrical current into brain makes neurons fire more
Increase (anodal) or decreases (cathodal) neural excitability

Question 14

What do ERPs measure?

Answer 14

If all cells in the cortex were aligned then electrical charge would travel in the same direction.
But there are folds in cortex so some brain cells are 90 degrees to others
This means positive charge of some cells is cancelled out by negative charge of others
So we mostly measure ERPs from large groups of cells that fire at the same time and are well aligned
Not measuring all cells in the brain

Question 15

How are EEGs and ERPs measured? (+ stimulus type)

Answer 15

Wires coming from cap are going into boxes, electrical activity (analogue) is being digitized in one box and amplified in another bc the signal is so tiny that it has to be amplified so we can see it

Can use any stimulus that can be:
• presented in relative isolation
• repeated many times (50+ in adults and 100+ in children)
• separated by a large enough gap to avoid overlap in ERP responses

Question 16

How do we process the EEG to produce and ERP?

Answer 16

Cut the EEG into chinks that surround the onset of the stim, start to average the chunks together, the noise cancels itself out (bc its random) and the signal starts to emerge

Question 17

How do we measure the amplitude? (3)

Answer 17

Peak amplitude: look at the peak and look at the most neg value within a fixed interval
Mean amplitude: not just peak, choose an interval and take the mean value of the peak
Mean area: all the area underlying the peak
Choice depends on who you’re testing, what you’re testing and how reliable your data is

Question 18

How do we measure the latency?

Answer 18

Not as reliable
At what point in time did the peak occur

Question 19

How o we measure morphology?

Answer 19

Not just how big peaks are and what time they happen but also what shape they are

Inter class correlations: Correlations that show how similar 2 waves are
+ morphology + sensitive - not specific

Question 20

What are 6 advantages and one disadvantage of measuring peaks?

Answer 20

+ direct measure
+ good for timing of processes
+ can measure at multiple points in time
+ good for detecting atypical morphology
+ can be quick (if only one condition)
+ often reliable (if enough epochs) 50+ in adults 100+ in children
- does not necessarily reflect one brain response
Lots of things are happening in the brain at the same time

Question 21

What are components? (+ 1 advantage and 3 disadvantages)

Answer 21

2 conditions: 1 that has the process of interest and one that doesnt and then subtract them, the waveform should represent the brain process we’re interested in
+ one brain response = one cognitive process compare two conditions
- indirect measure
- takes longer (if 2+ conditions)
- less reliable if one condition has fewer epochs

Question 21

What are components?

Answer 21

2 conditions: 1 that has the process of interest and one that doesnt and then subtract them, the waveform should represent the brain process we’re interested in
+ one brain response = one cognitive process compare two conditions
- indirect measure- takes longer (if 2+ conditions)
- less reliable if one condition has fewer epochs

Question 22

Why are ERPs bad at location?

Answer 22

ERPs are measured from electricity
Electricity is good because it:
• travels at the speed of light
• reflects brain firing in (practically) real time
• so has excellent temporal resolution/timing
Electricity is bad because it:
• spreads out from its source, so an ERP generated in one part of the brain can be detected all over the place
• travels the path of least resistance, so it smears each time it hits a surface
This makes it hard to pinpoint the location of firing brain cells

Question 23

What is the inverse problem?

Answer 23

If you knew the locations and orientations of a set of dipoles (sources), you could use a set of equations to predict the distribution of voltages at the scalp
This is called the forward problem = relatively easy to solve

If you knew the distribution of voltages at the scalp, you could not use a set of equations to predict (with any certainty) the locations of orientations of a set of dipoles because an infinite number of these could produce any one distribution of voltages at the scalp
This is called the inverse problem = very hard to solve
You can use constraints to try and reduce the number of likely of dipoles, but there is currently no widely-accepted mathematical technique that identifies the source of ERPs with a small and well-justified margin of error
This also affects MEG, so ERP and MEG are not great at location
MEG isnt as bad bc mag activity travels in a straight line