MRI and fMRI: Brain Imaging Techniques

MRI and fMRI: Brain Imaging Techniques

• MRI and fMRI provide valuable insights into brain function, allowing researchers to observe and measure neuron responses in specific brain regions.
• MRI, originally known as Nuclear Magnetic Resonance (NMR), measures the magnetic properties of atomic nuclei (specifically protons).
• Protons spin on an axis, creating tiny magnetic fields. When placed in a strong magnetic field, these internal magnetic fields align either parallel or anti-parallel to the external field.
• The difference in the populations aligned parallel vs. anti-parallel provides the signal MRI measures.
• Stronger magnetic fields (e.g., 7T, 10T) yield more detailed information.
• Advanced MRI techniques like Diffusion Tensor Imaging (DTI) map the diffusion of water in white matter tracts. This reveals the direction of neural fiber pathways, which is crucial for understanding connectivity.
• Brain atlases aid in identifying anatomical regions in individual participants, allowing researchers to correlate structure with function and to compare groups or conditions.
• Studying groups with different experiences (e.g., taxi drivers vs. controls) reveals potential structural differences in the hippocampus.
• DTI can also assess the integrity of white matter tracts. Damage to the ventral visual pathway (as seen in prosopagnosia) is associated with decreased tract integrity.

PET: Positron Emission Tomography

• PET measures metabolic activity in the brain by detecting radioactive emissions.
• A common PET method involves injecting radioactive oxygen-labeled water into the participant.
• The unstable isotope releases positrons; their collision with electrons generates gamma rays.
• PET scanners detect these photons' paths, allowing for calculation of their origin, therefore the locations of active brain regions.
• PET enables the visualization of blood flow and metabolic activity in the brain. Activity is measured by subtracting baseline measurements from measurements taken with stimulation; regions active during the stimulation have higher counts.
• PET can track the binding of specific molecules (e.g., to amyloid plaques in Alzheimer's disease by injecting patients with a specific PET dye).

fMRI: Functional Magnetic Resonance Imaging

• Functional magnetic resonance imaging revolutionized cognitive neuroscience.
• fMRI measures changes in blood oxygenation (BOLD signal) associated with neural activity.
• Neural activity consumes oxygen, which triggers an increase in blood flow to supply more oxygen, hence the BOLD signal.
• Blood oxygenation level-dependent (BOLD) signal provides a measure of neural activity.
• fMRI experiments often employ block designs.
• Block designs present one condition followed by another for a certain time (e.g., 40 seconds), enabling tracking of activity changes across different conditions.

Visual Mapping using Imaging Techniques

• The response patterns of different brain areas (e.g., primary visual cortex, area MT, and area V4) vary significantly depending on the stimulus (e.g., static vs. moving images, color).
• The V5/MT area in the brain responds only to moving stimuli
• Area V1 shows consistent responses during stimulation-periods, while area MT increases only during motion.

Face Recognition and Other Cognitive Functions

• The fusiform gyrus is a brain area particularly active during face recognition.
• Damage in this area (e.g., from stroke) can cause prosopagnosia (face blindness).
• fMRI results show good correlation with neuropsychological findings of face processing.

Event-Related Designs

• Event-related designs are used to study responses to brief events quickly, without lengthy scanning times.
• They vary the timing and order of events to optimize statistical power in detecting responses to individual events.
• Multivoxel pattern analyses (MVPA) of fMRI data can involve analyzing patterns of activation across many voxels in response to stimulus categories, helping to extract more detailed information.

Resting State fMRI

• Researchers use resting-state fMRI to monitor brain activity while participants are at rest.
• This method can identify brain networks and how they fluctuate in response to cognitive processes while they are not actively engaged with tasks.

Mind Reading with fMRI

• Researchers are investigating the possibility of interpreting thoughts and intentions based on observed patterns of brain activity. Tasks such as motor imagery can be distinguished from other tasks.