Theories of cortical folding/Brain imaging methods

Question 1

Theories of cortical folding

Answer 1

1. Limited volume of the skull
2. Axonal Tension Hypothesis
3. Radial Expansion Hypothesis
4. Differential Tangential Expansion Hypothesis

Question 2

Limited volume of the skull

Answer 2

the cortex, which expands during development, is forced to fold due to the limited size of the skull.
–> Largely disproved.

Question 3

Axonal Tension Hypothesis

Answer 3

axons of strongly connected areas pull cortex together to create a gyrus.
–> Largely disproved.

Question 4

Radial Expansion Hypothesis

Answer 4

differential intracortical growth, whereby the superficial layers of the cortex expand more than the internal layers, causing the cortex to “buckle”.
–> Recent evidence against this theory.v

Question 5

Differential Tangential Expansion Hypothesis

Answer 5

as the cortex expands during development, a tangential pressure builds that can only be relieved by out-of plane folding. Regional differences in expansion across the cortex create variability in folds (e.g., depth, width, etc.). —-> More evidence suggests this may play a role.

Question 6

Magnetic Resonance Imaging (MRI)

Answer 6

• Involves the use of a magnetic field to image the brain
• Different types of images depending on your research
question and the image acquisition parameters that will
be used
• Standard for looking at the morphology of the brain is
using T1-weighted structural images: the cortex appears
darker than the white matter
• The strength of the magnet will determine your spatial
resolution (1.5T, 3T, 7T)

Question 7

Functional magnetic resonance imaging (fMRI)

Answer 7

• The idea is that more active areas require more
blood for glucose and oxygen consumption
• Oxygenated iron in the blood has different
magnetic properties compared to deoxygenated
iron
• Therefore, can measure the changes in blood flow - BOLD signal (Blood Oxygen Level Dependent)
• Remember this technique is not a direct measure
of neuronal activity
• The functional activations are overlaid onto
structural MRI images for anatomical context. The
activations are illustrated as heat maps – regions in
red were the most active.

Question 8

Task-based fMRI

Answer 8

measure changes during a specific behavioural task compared to a control task

Question 9

Resting state fMRI

Answer 9

measuring brain activity at rest

Question 10

standard brain atlas of Talairach and

Tournoux

Answer 10

• created by fitting a single brain into a stereotaxic space (i.e. a specific coordinate system). Brodmann’s
cytoarchitectonic areas were then overlaid
based on general location.
• This atlas was originally created for neurosurgeons who needed to find a way to relate brains of different shapes and sizes.

Question 11

MNI stereotaxic space

Answer 11

• A standard imaging space created at the Montreal
Neurological Institute (MNI) that is used around
the world
• Generated by averaging dozens of MRI scans
together to create a “template” that represents the
“average brain”
• Standard coordinate system (slightly different from
that of Talairach and Tournoux) so we can use MNI
coordinates when describing a region
• Registering a subject’s MRI scan to MNI space
(translation, rotation, scaling) is essentially
performing a spatial normalization
• This allows for us to make comparisons across
subjects whose brains differ in overall shape and
size

Question 12

Studying brain connectivity

Answer 12

Use of gold-standard tracers in animal models
• In humans:
1. gross dissection (post-mortem)
2. diffusion MRI --> in vivo
3. resting state fMRI --> in vivo

Question 13

Use of tracers in animal models

Answer 13

• radioactive amino acids can be injected and are absorbed by the neuron and incorporated into the cell’s proteins and transported down the axon to the terminal boutons. Can follow the individual axons from point A to point B.
• Fluorescent labelled tracers also make use of the axon transport system. Cannot see the individual axons but see the neurons at the injection site and the termination site. Can determine direct connectivity – inject tracer into area A and see it appear at area B.

Question 14

Diffusion MRI (dMRI)

Answer 14

• dMRI = uses the natural diffusion of water molecules passing along axons to reconstruct white matter pathways in the living brain
• There are many issues with this method, including the fact that it is difficult to tease apart pathways that are close together or that cross one another
–> Can really only visualize the major highways

Question 15

Resting state fMRI to find connectivity

Answer 15

A subject goes into the MRI scanner and lies at rest
(often fixating on a screen) and you look at areas
that are activated at the same time – these areas are
interpreted as “functionally connected”