experimental design in fmri

Experimental Design in fMRI

• A real example of fMRI block 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?

Structural and Functional Imaging• What you really want is an image, not a squiggly line• Make a map of a statistic (like t-score or z-score) that

describes how well each voxel tracked the cognitive task:• Set all “non-significant” voxels to be transparent

Structural and Functional Imaging• What you really want is an image, not a squiggly line• Make a map of a statistic (like t-score or z-score) that

describes how well each voxel tracked the cognitive task:• Set all “non-significant” voxels to be transparent

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

Electrophysiology

Neurons are Electrical• Remember that Neurons have

electrically charged membranes

• they also rapidly discharge and recharge those membranes (graded potentials and action potentials)

• Review relevant textbook sections if this isn’t familiar to you

Neurons are Electrical

• Importantly, we think the electrical signals are fundamental to brain function, so it makes sense that we should try to directly measure these signals

– but how?

Subdural Grid

• Intracranial electrodes typically cannot be used in human studies

Subdural Grid

• Intracranial electrodes typically cannot be used in human studies

• It is possible to record from the cortical surface

Subdural grid on surface of Human cortex

Electroencephalography and the Event-Related Potential

• Could you measure these electric fields without inserting electrodes through the skull?