Task 5 fMRI

Question 1

structural imaging

Answer 1

based on unique tissue properties

Question 2

Blocked design:

Answer 2

• recorded neural activity is integrated over “block” of time during which pp is presented a stimulus /performs certain cognitive task
• recorded pattern is compared to other blocks recorded while doing same/different/no stimulus

Question 3

Event-related design

Answer 3

• across experimenta; trials, BOLD response is linked to specific events (e.g., stimulus presentation)
  –> can be used at great range of experiments

Question 4

Temporal resolution

Answer 4

accuracy with which one can measure when a cognitive event is occurring

Question 5

Spatial resolution

Answer 5

accuracy with which one can measure where a physiological change is occurring

• unit: voxel (volume element) representing minimum unit of brain tissue sampled in each image
• increased voxel size = lowered spatial resolution but increase in amount of active tissue detected
• decreased voxel size = increased spatial resolution –> reduced sensitivity to BOLD effect but more spatially specific information

Question 6

Measures to reach higher resolution

Answer 6

• jittering: use of different delays between start of sampling of brain volume images rel. to start of stimulus presentation to subject
• parallel imaging: spatial coding of signals from coil sensitivity profiles

Question 7

Process of MRI

Answer 7

1. protons are oriented randomly and spin at certain speed (precession)
2. strong magnetic field (B0) is applied –> protons orient parallel or anti-parallel to B0
3. brief radiofrequency pulse (RF) is applied –> knocks proton orientation to original orientation (“longitudinal magnetization”) and “inphase” precessions (“transverse magnetization”)
4. recovery: protons are pulled back into magnetic field
   - time when 63% of longitudinal magnetization is recovered: T1 time
   - time when 63% of transverse magnetization is recovered: T2 time
   - T1 and T2 is unique to tissues

Question 8

Time sequence of MRI

Answer 8

1. 90 degree RF pulse –> rapidly decaying signal
2. 180 degree refocusing pulse: synchrony due to differences in precession speed (T2 effects) –> energy is released (“echo”)
3. echos are collected by scanner –> creates anatomical image

Question 9

Advantages of MRI

Answer 9

• completely safe and allows repeated measures
• provides good spatial resolution: folds of individual gyri can be discerned and allows discrimination between white and grey matter
• can be adapted to fMRI

Question 10

Advantages of MRI compared to CT

Answer 10

• no use of ionizing radiation
• better spatial resolution
• better discrimination between grey and white matter

Question 11

Computerized tomography (CT)

Answer 11

• constructed acc. to amount of x-ray absorption in different types of tissues
• bone absorbs the most while CSF absorbs the least –> ventricles appear black, brain matter intermediate, skull appears white

Question 12

CT applications

Answer 12

• used to diagnose tumours
• identify haemorrhages and other brain abnormalities

Question 13

PET (Mechanism)

Answer 13

• radioactive tracer is introduced into bloodstream
• tracer isotopes are unstable -> rapidly decay into stable form by releasing positron from their nuclei
• when positron collides with an electron: two gamma rays (photons) are released
• radioation emitted from tracer is monitored by PET instrument: gamma ray detector

–> reconstruction of the image of distribution of blood flow: more blood flow = more radiation

Question 14

PET (applications)

Answer 14

• Pittsburgh compound B (PiB): biomarker for AD: binds to βA

Question 15

Advantages of PET

Answer 15

• radiolabelled pharmacological agents can be used to trace very specific pathways
• less susceptible to signal distortion around air cavities

Question 16

Limitations of PET

Answer 16

• risk: use of radioactive tracer: exclusion of pregnant women, contraceptive coils, children, cochlear implants, pacemakers
• poor temporal resolution: it takes 30 s for tracer to enter the brain and 30 s for radiation to peak –> critical window
• poor spatial resolution: about 10 mm
• constrained by rate of decay of radioactive agent (minutes)
• difficult to interpret because of massive data sets
• required blocked design

Question 17

Process of fMRI

Answer 17

• radiowaves cause protons in hydrogen to oscillate
• detector measures local energy fields that are emitted as protons return to orientation of magentic field
  (same as MRI)
• fMRI focuses on magnetic properties of deoxygenated vs. oxygenated hemoglobin –> fMRI detectors measure ratio of oxygenated to deoxygenated hemoglobin: blood oxygen level-dependent (BOLD) effect

Question 18

Hemodynamic response function (HRF)

Answer 18

1. initial dip: neurons consume oxygen –> rise in amount of deoxyhemoglobin –> reduction of BOLD signal
2. overcompensation: blood flow in region increases due to increased consumption of oxygen –> BOLD signal increases significantly
3. undershoot: blood flow and oxygen consumption dip before returning to baseline –> relaxation of venous system

Question 19

pre-processing

Answer 19

1. Stereotactic normalisation: each brain is fitted to normal standard brain –> correction for individual differences
   - brains are divided up into voxels
   - each voxel is given a 3D spatial coordinates: Talairach coordinates
   - every coordinate can be mapped onto corresponding x y z coordinates on any other brain
2. Smoothing
   - enhancing signal-to-noise ratio and increasing spatial extent of active regions
   - spreading raw activation level of any voxel to neighbouring voxels
   - small spatial distortion can produce spurious results

Question 20

Berman’s fMRI approaches

Answer 20

Localization: establishing correlations between brain and behaviour

Commonalities: examine overlapping/non-overlapping patterns of brain activity –> building up a view of shared and distinct processes among psychological tasks

Documenting individual differences: permits to understand consistencies and inconsistencies in human behaviour

Testing psychological models of behaviour

Question 21

Applications of fMRI

Answer 21

• describe netwoks associated with particular cognitive operations and relationships among nodes within networks

Question 22

Advantages of fMRI over PET

Answer 22

• less expensive
• easier to maintain
• no radioactive tracer: repeated testing becomes possible
• spatial resolution is superior –> high resolution anatomical image obtained with MRI

Question 23

Signal to noise ratio

Answer 23

• can be enhanced by smoothing: spreading activity across voxels –>