Functional Magnetic Resonance Imaging (fMRI)

Functional Magnetic Resonance Imaging (fMRI)

1. Imaging the STRUCTURE of the brain

   • Increase accuracy in mapping lesions and also allow us to find location of lesions while n individual is still alive (don't have to wait until they're dead)
     • Images make complex patterns of data
       • Mapping VARIABILITY in structure
         • Colour coded maps for statistical significance of GYRAL / SULCAL VARIABILITY
           • Mapping CHANGE over time
             • -Grey matter volume -Myelination of the brain

2. Functional MRI

   • The Blood Oxygen Level Dependent (BOLD) Response (the basis of fMRI)
     • Relative levels of de/oxyhaemoglobin change in regional cortical activity
       • Momentary decrease in blood oxygenation after neural activity increases. This is known as the "Initial Dip" in the haemodynamic response function (HRF)
         • Blood flow increases to a level where it OVERCOMPENSATES for the increased demand. Regional blood oxygenation actually increases following neural activation
           • De / oxyhaemoglobin has DIFFERENT MAGNETIC PROPERTIES
             • Local field strength is affected by relative levels. This affects the local signal in the image
               • The image is far removed from neural events

3. MRI

   • Uses magnetic field and radio energy to produce an image
     • A large magnet aligns nuclei that have asymmetry in their magnetic properties e.g. H2O. We then pulse them with radio frequency. Nuclei absorb and re-emit radio frequency energy to give signal

4. Experimental Logic

   • Cognitive Subtraction - reaction time experiments
     • Measure time for a process to occur by COMPARING two reaction / response times
       • T1 : Hit button when see light T2 : Hit a button when the light is green but not red T3: Hit left button when light is green and hit the right button when the light is red
         • From this, we can see how long a psychological process takes e.g. discrimination between two different coloured lights
     • HOWEVER, we are making an ASSUMPTION OF PURE INSERTION - that inserting a component process into the task can be done without disrupting the other components
       • TASK DIFFICULTY may differ between conditions - resulting in changed attention

5. Experimental Design

   • Block design fMRI
     • 1 task = 30 sec then 30 sec rest etc.
       • Lots of brain images
         • Subtract brain activity in rest condition - find difference between two conditions
   • Event Related fMRI
     • Random presentation of stimuli. Allows retrospective coding of events (e.g. memory experiments)
   • fMRI Preprocessing and Analysis
     • It is complicated and requires many steps
     • Block model and convolve it with HDR.

6. What to the blobs represent?

   • Clusters of significant statistics
     • Shows areas where the signal was significantly predicted by the model
       • The activations are DISTANTLY RELATED to underlying neural events
         • This has helped us find functional regions e.g fusiform face area
           • Functional - connectivity analyses: calculate correlations between activations in different areas
             • DYNAMIC CAUSAL MODELLING : looking at models of networks - which best fits the data

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