EEG: L5

Question 1

what is EEG?

Answer 1

detecting neural activity by placing electrodes on the scalp

Question 2

role of electrodes

Answer 2

pick up small fluctuations of electrical signals originating from activity of neurons

Question 3

what can we learn from EEG?

Answer 3

something about cognition when people perform tasks

Question 4

EEG is… (2)

Answer 4

1. invasive, however can be intra-cranial by measuring exposed cortex
2. cheap, easy to conduct

Question 5

whats the alpha rhythm

-> initially how many electrodes were used?

Answer 5

when people close their eyes the electrical signal was not constant but it varied with a frequency of 8-13 Hz
- 2 (one at front, one rear & recorded voltage difference between them)

Question 6

Advantages and disadvantages of EEG

Answer 6

• temporal resolution is great (fast signal speed)

- spatial resolution is not good (low number of pixels)

Question 7

Neurophysiology of EEG

1. what does it reflect
2. voltages that arise when…

Answer 7

1. post-synaptic potentials (NOT APs)

2. neurotransmitters bind to receptors on the membrane of the post-synaptic cell

Question 8

what is small dipole?

Answer 8

-when ion channels to open/close -> graded changes in the potential across the membrane

Question 9

What signal do we record?

Answer 9

• many neurons spatially align, their summed potentials add up and create the signal
• > aligned together, oriented same way (either positive or negative), must be clusters of them

Question 10

where does this pooled activity come from?

Answer 10

large cortical pyramid cells

Question 11

what is the functional unit (to be able to measure)

Answer 11

10,000 simultaneously stimulated

Question 12

what determines the sign (+ or -) of the recorded potentials?

Answer 12

their orientation

-> some orientations lead to signals that cannot be recorded

Question 13

Limitations of EEG

1. signals based on?
2. sulci & gyri
3. meninges, CSF & skull

Answer 13

1. based on signals generated in superficial layers of cerebral contex on the gyri (ridges)
2. signals in the sulci are harder to detect than the gyri & may be masked by gyri
3. smear the EEG signal -> difficult to localise the source

Question 14

what is the inverse problem?

Answer 14

-> difficult to localise the source
= scalp configuration of signals cannot be reconstructed due to a lot of noise existing. Its therefore hard to know exactly where the signals we are measuring are coming from -> difficult spatial configuration

Question 15

EEG signal are in relation to?

Answer 15

a reference electrode

Question 16

What is a reference electrode?

Answer 16

a neutral point where an electrode is placed e.g. noise, mastoids

Question 17

what amplitude do EEG signals have?

-> consequence of this

Answer 17

10μV to 100μV = tiny signals

-> must be amplified 1,000 to 100,000

Question 18

what is a typical sample frequency

Answer 18

256-1024 HZ but can be >4000HZ

Question 19

what removes the low & high frequencies which do not reflect brain activity & what HZ are these?

Answer 19

• band-pass filter
  low = <0.5-1 HZ
  high = >35-70HZ

Question 20

processing of data

Answer 20

when studying cognitive processes the raw signal has to be cleaned before it’s interpreted

Question 21

what is artefact rejection?

Answer 21

finding all artefacts that are not brain signals - sweating, electrical noise, eye movements and blinks-> electrodes measure so we can discard

Question 22

what can the eye be regarded as and what impact does it have on EEG signal?
-> how can they be removed?

Answer 22

strong impact as it can be regarded as a dipole itself
= signals will contaminate the signal of interest and be much larger
-> excluding contaminated trials, mathematical algorithms e.g. ICA