Neuroimaging and Map Plasticity in Humans - L2 David McGonigle

How do MRIs work?

Uses magnetism to image body structure:

Patient placed within large static magnet

Then smaller magnets change fields in different directions to manipulate image.

T1 vs T2 Images

Two types of contrast:

T1 grey matter appears darker than white matter

T2 white matter appears darker than grey

No one contrast is better for all images

What is BOLD contrast + what are they used for?

Blood Oxygenation Level Dependent contrast.

The magnetic properties of oxygenated and deoxgynated blood differ which allows

fMRIs to distinguish between them.

Early fMRI experiment (Kwong et al, 1992)

Showed patients an extreme visual stimulus (illusion/pattern) and looked for activation in visual areas in the brain. = higher voxel signal (voxel is like a pixel but for pictures with volume, e.g. an illusion), when shown stimulus vs when no stimulus.

What is MEG? How is different from fMRI?

MagnetoEncephaloGraphy - directly measures the actual magnetic fields from neurons, rather than indirectly measuring the difference between oxygenated and deoxygenated blood. (like fMRI)

Advantages of MEG over fMRI

MEG is silent

Direct measurement of neuronal signal

Disadvantages of MEG compared to fMRI

Extremely sensitive = heavily shielded room is needed to prevent other magnetic and electrical fields interfering

Becomes less sensitive the deeper into the brain you go, whereas fMRI is equally sensitive at any point in the brain.

Example of fMRI being used to investigate cortical maps for somatosensory cortex:

The Cutaneous Rabbit Illusion: tapping in two places on the arm in quick enough succession can feel like a tapping in between them as well, like a rabbit running up your arm. fMRI showed that the area of the cortex associated with the intermediate (illusory) area was activated even though it had not actually been tapped.

What are JVP domes used for?

to measure tactile discrimination/ spatial resolution = how close do two points have to be to no longer be able to discriminate between them - more accurate than two-point discrimination using simpler tools (may be issues with putting both points down at the same time)

• used on the fingertips

Study of frequency vs orientation discrimination using brain scanning (Sathian et al ‘Feeling with the Mind’s Eye)

Used PET scans to find differences in spatial frequency discrimination vs orientation discrimination during a JVP dome task.

Found that orientation discrimination is more reliant on visual processing (we form an image in our minds of the orientation of what we have just felt)

But frequency discrimination (groove width/ texture) does not require visualisation (more linked to only somatosensory)