fmri and fnirs

  • ECOG vs. fMRI and fNIRS

    • ECOG: Primarily used in clinical settings
    • fMRI & fNIRS: Mainly for research; not as widely used in general clinical care

  • Principles of MRI

    • Hydrogen atoms: Contain one proton, randomly aligned until exposed to magnetic field
    • After exposure: Atoms align; radio pulse knocks them off alignment
    • Relaxation: Protons release energy, which is measured in fMRI

  • Structural MRI vs. Functional Imaging

    • Structural MRI: Differentiates between white matter, gray matter, and liquid
    • Diffusion Tensor Imaging (DTI): Measures water diffusion; used for assessing axon integrity and directionality
    • Structural measures provide associations between structure and function
    • fMRI provides dynamic, real-time data related to blood oxygenation

  • Blood Oxygen Level Dependent (BOLD) Signal

    • The BOLD signal depends on oxygen levels in the blood, mapping neural activity by detecting changes in magnetic fields
    • Oxygenated (oxyhemoglobin, red) vs. deoxygenated blood (deoxyhemoglobin, blue) have different magnetic properties
    • Increased blood flow occurs during neural activation due to metabolic demands, known as neurovascular coupling

  • Neural Activity and Hemodynamic Response

    • Neuron activation increases metabolic demand for oxygen
    • Action potentials: Sodium-potassium pump plays a critical role in the need for oxygenation, consuming ATP
    • Delay in response occurs (5-15 seconds) post-neural activity due to physiological processes
    • Hemodynamic Response Function: Characterized by initial oxygen dips followed by peaks in oxygenated blood flow
    • Changes measured in fMRI involve assessing patterns over time, not immediate spikes in electrical activity

  • Spatial and Temporal Resolution

    • fMRI has better spatial resolution than EEG, capable of deep brain imaging
    • Temporal resolution is slower than EEG, with a delay due to neurovascular coupling processes
    • Tasks can influence BOLD signals; baseline activity exists across the entire brain

  • Data Analysis in fMRI

    • Task comparisons are essential; subtract BOLD responses from different tasks to capture neural activity changes
    • Statistical tests (t-tests) determine if observed changes in BOLD signals are significant
    • All brain areas may be active, but specific task-related activity is highlighted

  • Functional Connectivity Studies

    • Assess correlations of BOLD signals between different brain regions
    • High correlations suggest structural connectivity; used to infer brain networks
    • Example: Language processing examined in infants using fMRI connected regions associated with wider speech processing

  • Advantages and Limitations of fMRI

    • Advantages: Good spatial resolution, capable of capturing deep brain structures
    • Disadvantages: Poor temporal resolution, expensive, complications due to metal in the body

  • Functional Near-Infrared Spectroscopy (fNIRS)

    • Noninvasive, cost-effective, and portable
    • Can measure blood oxygenation, less movement artifact interference than fMRI, but similar low temporal resolution
    • Uses light absorption rates of oxygenated vs. deoxygenated blood to assess neural activity
    • Example Study: Investigated bilingual processing using fNIRS to visualize neural activity during language tasks.