Methods for studying the brain

Question 1

What are 4 methods commonly utilised by cognitive neuroscientists?

Answer 1

Psychophysics - measuring limits of perceptual systems
Electrophysiology
Neuroimaging - structural or functional
Psychopathology - when the brain doesn’t work quite right

Question 2

How can lesions in patients assist in the study of functional specialisation?

Answer 2

Damage can often be restricted to a particular brain area –> specific behavioural deficits, which can be narrowed down using specific tasks and then, through “reverse engineering” i.e. comparing to healthy people, can identify that an area very likely to be involved in a behaviour

Question 3

What is a key problem with patient studies?

Answer 3

Patients able to be used for double dissociations are rare - have to rely on case studies i.e. not generalisable
Damage e.g. from stroke is often rather widespread - might be greatest in one area but then be spread slightly across multiple other areas producing different associated deficits

Question 4

How can we overcome the problems with patient studies?

Answer 4

Animal studies - produce controlled and isolated lesions and see how they affect behaviour

Question 5

What must be considered in animal research?

Answer 5

Primate brains are obviously structurally more similar to human brains
Rat brains are similar in terms of underlying subcortical structures i.e. similar in terms of very basic functioning such as memory.
As such, if a study can be done using a rat it becomes unethical to use a primate

Question 6

How does TMS work as a technique?

Answer 6

Creation of temporary virtual lesions using magnetic induction - electric current passed through a stimulation coil generating a magnetic field which then induces a secondary electrical current in the brain which interferes with normal activity

Question 7

What are the 3 key parameters in the TMS process?

Answer 7

Magnitude - increasing the primary current increases field produced and thus increases secondary current and makes “lesion” more severe
Time course - a TMS pulse lasts for <1sec and effect lasts even less before reversing
Localisation using structural MRI

Question 8

What are 6 advantages of using TMS as a method over methods like patient studies?

Answer 8

Brief and reversible
Non-invasive
Can use within-subjects design and group studies
Specificity

Question 9

What are 3 potential risks with TMS?

Answer 9

Small risk of seizures with repeated exposure
Some minor discomfort such as facial twitching
Cannot be used on subjects with pacemakers

Question 10

How do EEGs work?

Answer 10

Measure dendritic currents in neuronal populations non-invasively using electrodes placed at different points on the scalp
Measures ADDITIVE ACTIVITY i.e. activity of multiple neurons in a given area acting in synchrony

Question 11

How do we code the localisation of an EEG response?

Answer 11

Mapping system:
F=frontal, P=parietal, O=occipital, T=temporal and C=central
Odd numbers left of midline, even right of midline, and z for on midline

Question 12

What is an event-related potential?

Answer 12

The “event” is the precise onset time of the stimulus
Activity following the event is plotted as a characteristic waveform (electrode potential in mV as a function of time in ms)
Way of mapping time course of EEG signal - careful control is essential to get timing precise

Question 13

What are the features of the ERP waveform?

Answer 13

Positive peaks marked as P and negative peaks marked as N (positive values are actually on bottom of Y-axis)
Peaks are numbered and peak timings are also written

Question 14

What are comparison sites?

Answer 14

EEG measures are obtained by comparing voltage between different brain areas - use an area where activity is likely and an unrelated area unlikely to show any activity
Subtract the second from the first

Question 15

How can we increase the signal:noise ratio for the EEG?

Answer 15

EEG waveforms represent activity across the whole brain so need to carefully isolate the signal of interest
Average the EEG over many presentations of a stimulus, do the same for many participants and then average all of those average values (so peaks on the waveform are the averages of averages and 0 is the absence of any baseline activity prior to stimulus presentation)
The background oscillatory activity will not be synchronised with the onset of event-related potentials so these fluctuations should be averaged out

Question 16

How do PET scans work?

Answer 16

Measure BLOOD FLOW using radioactive isotopes (commonly oxygen-15)
Has a very short half life - broken down in the brain as it uses oxygen and positrons are emitted which are the basis of gamma rays (picked up the create the image)
More active brain regions use more oxygen –> more positrons emitted and greater signal recorded

Question 17

What are the measurements for PET accuracy?

Answer 17

Temporal resolution is approx. 30 seconds

Spatial resolution is approx. 10mm

Question 18

When can PET scans be useful?

Answer 18

CLINICALLY - Can detect changes indicating disease onset before any anatomical changes are present e.g. early dementia
EXPERIMENTALLY - Measuring effects of drug on brain

Question 19

How does fMRI work?

Answer 19

On the behaviour of hydrogen atoms in a magnetic field
Hydrogen atoms contain one proton and no neutrons
Protons have weak and random magnetic fields which are easily aligned to an external magnetic field
Brief radio pulse induces “spin” in the aligned protons (spin by 90 degrees), thus changing the magnetic field and producing the MRI signal
Protons relax after, return to aligned state and a new pulse can be introduced (a new “slice” of image)

Question 20

How does the fMRI technique allow for discrimination between brain tissue types?

Answer 20

Variation in the rate at which protons return to their aligned state

Question 21

Why is fMRI more useful than structural MRI?

Answer 21

Gives us idea of what is happening in the brain as a function of time i.e. through the rate at which protons are returning to their aligned state

Question 22

What can the MRI signal provide information about?

Answer 22

Concentration of deoxy-Hb in the blood and the temporary changes associated with cognitive processing
Cerebral blood flow and neuronal activation are coupled –> BOLD signal (blood oxygen level dependent contrast)

Question 23

What is the basis of the BOLD response?

Answer 23

How much oxygen different parts of the brain require during different tasks
More oxygen used, more deoxy-Hb leaving that tissue - strong parametric properties so distorts the local magnetic field
High levels of oxygen produce bright white signal, while distortion of magnetic field by deoxy-Hb creates black image

Question 24

What is fMRI measuring?

Answer 24

The oxygen the brain has already used, as opposed to PET which measures oxygen being used

Question 25

What is the haemodynamic response function and what are its 3 phases?

Answer 25

The way the BOLD signal evolves with time in response to increase in neural activity

1) Initial dip - neurons consuming oxygen so deoxy-Hb rises and BOLD signal decreases i.e. MRI image darker
2) Overcompensation (peak) - Increased O2 consumption means blood flow to the area increases above use, so BOLD increases
3) Undershoot - Blood flow and O2 use decrease before returning to normal, potentially due to relaxation of venous system causing temporary increase in deoxy-HB again

Question 26

How does the magnitude of these changes vary with magnet strength?

Answer 26

1st magnets (1.5T) produce changes of 1-3%
New generation (3 and 7T) produce bigger changes (and so more accurate detection)

Question 27

What is the accuracy of fMRI like?

Answer 27

Temporal - a few seconds so fairly good

Spatial resolution - 1mm (also depends on magnet strength - stronger magnets give more accurate data)

Question 28

What is the accuracy of EEG as a technique?

Answer 28

Bad spatial resolution but good temporal i.e. good for knowing WHEN something is happening

Question 29

What are the different image types able to be produced using MRI?

Answer 29

T1-images = variation in rate protons return to relaxed state shows different types of tissues 
T2-images = when in the misaligned state, the MR signal decays because of local interactions with surrounding molecules; deoxy-Hb produces distortions in this component and this is the basis of fMRI

Question 30

Why can EEGs not be used to measure activity at sites such as the thalamus?

Answer 30

Populations of neurons must be aligned in parallel orientation for EEG to work, so they summate rather than cancel out
Neurons are not arranged this way in the thalamus

Question 31

What can be said about the oscillation of the EEG signal?

Answer 31

Signal tends to oscillate at different rates, also called frequency bands
7-14Hz = alpha waves
15-30Hz = beta waves
This oscillatory activity forms much of the background “noise” on an EEG