Opportunity to Participate EEG studies of vision/hearing/decision making – takes about 2 hours...

Opportunity to Participate

• EEG studies of vision/hearing/decision making – takes about 2 hours

• Sign up at www.tatalab.ca– Keep checking back there for

more time slots

• Two extra points added to your final grade!

http://www.tatalab.ca/

Structural and Functional Imaging

• This is a Functional MRI Image !?


• This is a structural MRI image (an “anatomical” image)


• What you really want is both images co-registered


• What you really want is both images co-registered

But what is this really an image of!?

MRI Image Formation

• First you need a scanner:

– The first MRI scanner

MRI Image Formation

• Our Scanner

Structural and Functional Imaging• 2-dimensional structural and functional images are comprised of pixels

Pixels


• Brain scan images (CAT, PET, MRI, fMRI) are all made up of pixels (stands for picture elements)

• But of course the brain is 3-D not 2-D

Pixels


• “Slices” are assembled into “volumes”

Pixels


• Volumes are composed of “volume elements” or voxels

Voxels


• Another thing you want: the ability to tell other people where something is– “the activity was centered on voxel #653” will not

work in a scientific journal


• MRI anatomical spaces– Talairach Space:

• Based on detailed analysis of one elderly woman• Talairach & Tournoux (1988)

– Montreal Neurological Institute Template (MNI)• based on average of 152 different brains, each normalized to Talairach

space• advantage: gyri and sulci are more representative• disadvantage: it’s blurry

– MNI “Representative Brain”• the one brain from the 152 in the MNI Template set that is most like the

average• advantage: it’s not blurry• disadvantage: it’s still just one person’s brain


• Reasons for normalizing to standard stereotaxic space (templates)


1. structural and functional volumes may not otherwise be coregistered due to• movement• distortion

2. results can be described in standard coordinates

3. data across sessions can be averaged

These pertain to a single subject’s data


4. Volumes will not match because of variability across individuals

5. data across participants can be averaged

These pertain to dealing with multiple subjects

The Talairach Coordinate System

ACPC


AC - PC line defines y-axis

+y

-y


x-axis perpendicular to interhemispheric plane

+y

-y

-x

+x


z-axis perpendicular to x-y plane

+y

-y

-x

+x

+z

-z


• Cortical Flattening– Software such as

BrainVoyager can “inflate” the cortex like a balloon so that sulci and gyri are “flattened”

– functional data can be transformed with the same complex function

– functional and structural data can be overlaid so that distribution on cortical sheet can be visualized

Functional Imaging

• Oxygenated hemoglobin is diamagnetic - it has no magnetic effects on surrounding molecules

• Deoxygenated hemoglobin is paramagnetic - it has strong magnetic effects on surrounding molecules!

Hemoglobin

Functional Imaging

• blood flow overshoots baseline after a brain region is activated

• More oxygenated blood in that region increases MR signal from that region

Functional Imaging

• It is important to recognize that fMRI “sees” changes in the ratio of oxygenated to deoxygenated blood - nothing more– BOLD: Blood Oxygenation Level Dependant

contrast

Functional Imaging

• How do we create those pretty pictures?

• We ask the question “When the subject engages in this cognitive task, where does blood oxygenation change significantly?” “where does it change randomly?”

Experimental Design in fMRI

• Experimental Design is crucial in using fMRI

• Simplest design is called “Blocked”– alternates between active and “rest” conditions

Active Rest Active Rest

60 sec 60 sec 60 sec 60 sec


• A voxel in tissue insensitive to the task demands shows random signal change



Sign

al


• A voxel in tissue that responds to the task shows signal change that matches the timecourse of the stimulus



Sign

al


• A real example of fMRI block design done well:– alternate moving, blank and stationary visual input

Moving Blank Stationary Blank



• Voxels in Primary cortex tracked all stimuli


• Voxels in area MT tracked only the onset of motion


• Voxels in area MT tracked only the onset of motion• How did they know to look in area MT?

PET: another way to measure blood Oxygenation

• Positron Emission Tomography (PET)• Injects a radioisotope of oxygen• PET scanner detects the concentration of this isotope as it

decays

Advantages of fMRI

• Advantages of MRI:1. Most hospitals have MRI scanners that can be

used for fMRI (PET is rare)2. Better spatial resolution in fMRI than PET3. Structural MRI is usually needed anyway4. No radioactivity in MRI5. Better temporal resolution in MRI

Advantages of PET

• Advantages of PET:1. Quiet2. A number of different molecules can be labeled

and imaged in the body

Limitations of fMRI

• All techniques have constraints and limitations

• A good scientist is careful to interpret data within those constraints

Limitations of fMRI

• Limitations of MRI and PET:1. BOLD signal change does not necessarily mean a

region was specifically engaged in a cognitive operation

2. Poor temporal resolution - depends on slow changes in blood flow

3. expensive

Opportunity to Participate EEG studies of vision/hearing/decision making – takes about 2 hours...

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