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Methods of Cognitive Neuroscience > Problem 5 > Flashcards

Problem 5 Flashcards

1
Q

Positron emission tomography

PET

A

Measures local variations in cerebral blood flow that are correlated with metal activity

  1. Radioactive substance is introduced into the blood stream
  2. Radiation is monitored by the PET instrument
  3. A postiron is emitted
  4. If the positron collides with an electron = 2 photons are created, which travel in opposite directions

–> PET Scanner determines where the collision took place

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2
Q

functional magnetic resonance imaging

fMRI

A

Putting a person inside a scanner that performs a series of cognitive tasks whilst BOLD images representing the brain are collected

  1. the images show small changes in the brightness levels of certain brain areas, that reflect brain activity
  2. The areas in which the brightness changes relative to the task can then be determined using statistical analyses

–> exploits the fact that local blood flow increases in active parts of the brain

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3
Q

What is the main difference between EEG and MEG + PET and fMRI ?

A

PET and fMRI do not directly measure neural events

–> they measure metabolic changes that correlate with neural activity

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4
Q

What is the main difference between PET and fMRI ?

A

With fMRI, imaging is focused on the magnetic properties of the deoxygenated form of hemoglobin (deoxyhemoglobin)

–> this form is weakly magnetic in the presence of a magnetic field

THUS: fMRI detectors measure the BOLD effect, which can be used to indirectly measure neuronal activity

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5
Q

Blood oxygen level-dependent effect

BOLD

A

Refers to the value of the ratio of oxygenated to deoxygenated hemoglobin

–> measured by fMRI detectors

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6
Q

fMRI results report an increase in the BOLD effect even though one might expect the proportion of deoxygenated hemoglobin to be greater in the area surrounding active brain tissue.

Why is that ?

A

When a brain region becomes active, the amount of blood that is being directed to that area obvi increases

–> neural tissue is unable to absorb all of the excess oxygen

BUZ: changes in blood flow/BOLD response occur rather slowly

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7
Q

Block design experiment

A

The recorded neural activity is integrated over a block of time during which the participant either

a) presented with a stimulus
b) performs a task

–> the recorded activity pattern is then compared to other blocks that have been recorded

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8
Q

What do PET and fMRI have in common ?

A

They both detect changes in metabolism/blood flow in the brain while the participant is engaged in cognitive tasks

–> can identify brain regions that are activated during these tasks this way + test hypotheses about functional anatomy

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9
Q

Structural imaging

A

Is based on the fact that different types of tissue have different physical properties

–> specialized to visualize these different anatomical properties of the brain in static maps

e.g. CT, MRI

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10
Q

Functional imaging

A

Is based on the assumption that neural activity produces local physiological changes in that brain region

–> this can be used to produce dynamic maps of the moment-to-moment acitivy of the brain when engaged in cognitive tasks

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11
Q

Disadvantages of CT scans

A
  1. Exposure to radiation due to x-ray
  2. Cannot distinguish between grey and white matter
  3. Cannot be adapted for functional imaging purposes
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12
Q

Why is MRI superior to the CT?

A
  1. No use of radiation
  2. Better spatial resolution, so folds of gyri can be seen
  3. Better discrimination between white + grey matter
  4. Can be adapted for use in detecting changes in blood oxygenation (fMRI)
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13
Q

The strength of a magnetic fiel is measured in units called … ?

A

Tesla

–> typical scanners have a field strength between 1.5 and 3 T

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14
Q

What are the advantages vs disadvantages of PET?

A

Advantages
Less signal distortion around air cavities

Disadvantages
1. Worse temporal + spatial resolution than fMRI

  1. Use of radiation
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15
Q

Why is fMRI superior to PET ?

A
  1. No use of radiation

2. High temporal and spatial resolution

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16
Q

Name Problems associated with functional imaging.

A
  1. It is susceptible to error due to the fact that this is a statistical science

–> thus analyzing + interpreting the data is rather difficult

  1. Interpret individual differences
  2. Small spatial distortions can produce spurious results
  3. Each persons head might be aligned slightly differently in the scanner over time
    - -> might lead to false positive results
17
Q

How do functional imaging researchers deal with the problem of individual differences ?

A

By Averaging data over many participants so one is left with regions of activity that are common to most of us

–> for this one first has to perform Stereotactic normalization + smoothing

18
Q

Stereotactic normalization

A

Each brain is mapped unto a standard reference brain

–> done by dividing each brain into thousands of voxels

19
Q

Smoothing

A

Spreads some of the raw activation level of a given voxel to a neighboring voxel

–> the closer the neighbor is the more activation it gets

e.g.: An inactive voxel surrounded by active ones gets switched on and vice versa

20
Q

Preprocessing

A

Refer to the processing stages or measures before a statistical analysis can take place

e.g.: Stereotactic normalization, smoothing

21
Q

What do signal and noise refer to in the context of voxels ?

A

Signal
–> the thing of interest which corresponds to the larger cluster of activity

Noise
–> Isolated voxel

22
Q

Name the 2 main advantages of smoothing.

A
  1. Enhances the signal to noise ratio
  2. Facilitates detection of common regions of activity across individuals

–> as spatial extent of active regions is increased - so when averaging the activity across individuals one is more likely to find common regions

23
Q

Statistical analysis

A

T-test to compare means
–> is the mean activity at a particular voxel in the experimental condition greater than in the baseline condition

BUT: Significance level is too great, thus one uses a correction method to determine a suitable p-value

24
Q

Random field theory

A

Is a correction method that chooses a statistical threshold (p-value) based on spatial smoothness

25
Q

Family wise error

FWE

A

A method for correcting for many statistical comparisons based on the number of tests being conducted

26
Q

False Discovery Rate

A

A method for correcting for many statistical comparisons based on the number of positive results obtained

–> more conservative

27
Q

Excitation vs inhibition

A

Refers to the nature of the mechanism by which neurons communicate

–> increase vs decrease of the activity of a brain region triggered by the activity in another region

28
Q

Explain the question of necessity vs sufficiency

A

This question points out the fact that a region may be active during performance of a task but it might not be critical to that task

–> so active due to a particular strategy the individual adopted

THUS: functional imaging tells us which brain regions are sufficient to performing a task but not really which are crucial or necessary

29
Q

What dictates how the MR signal will behave in a BOLD image ?

A

The proportion of deoxyhemoglobin relative to oxyhemoglobin within a particular voxel

–> high concentration of oxyhemoglobin give a higher signal (brighter image)

30
Q

Hemoglobin

A

Is a protein molecule that carries oxygen

–> deoxyhemoglobin disrupts the T2 signal, whereas oxygenated hemoglobin elicits a stronger signal

31
Q

Why do fMRI detectors measure the BOLD effect ?

A

Because in essence, it measures the time course of the blood flow change process (HRF)

–> amount of blood that is directed to a given area increases if it becomes active

THUS: measures the difference in no oxygen to oxygen as a region becomes active in response to a task

Methods of Cognitive Neuroscience (9 decks)

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