Opportunity to Participate

• EEG study of auditory attention – takes about 2 hours

• Sign up on sheet or email matthew.tata@uleth.ca

• Two extra points added to your final grade!

Functional Imaging

• blood flow overshoots baseline after a brain region is activated

• More oxygenated blood in that region increases MR signal from that region (other regions de-phase faster)

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

• How do we create those pretty pictures?

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

• 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

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

Experimental Design in fMRI

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

Active Rest Active Rest

60 sec 60 sec 60 sec 60 sec

Sig

nal

Experimental Design in fMRI

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

Active Rest Active Rest

60 sec 60 sec 60 sec 60 sec

Sig

nal

Experimental Design in fMRI

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

visual input

Moving Blank Stationary Blank

40 sec 40 sec 40 sec 40 sec

Experimental Design in fMRI

• Voxels in Primary cortex tracked all stimuli

Experimental Design in fMRI

• Voxels in area MT tracked only the onset of motion

Experimental Design in fMRI

• 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

PET: another way to measure blood Oxygenation

• Although oxygenation is measured differently, the logic of PET and fMRI are similar: compare active and “rest” conditions

Advantages of fMRI

• All techniques have certain advantages

• A good scientist leverages these advantages

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 PET

3. Structural MRI is usually needed anyway

4. No radioactivity in MRI

5. Better temporal resolution in MRI

Advantages of PET

• Advantages of PET:1. Quiet

2. 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