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Advanced Neuroimaging Methodologies: fMRI Dynamics

Manual: General · Subject: Psychology

Delve into the physics, preprocessing, and statistical modeling of functional Magnetic Resonance Imaging.

Modeling the BOLD Signal

Hemodynamic Convolution

Functional MRI relies on the Blood-Oxygen-Level-Dependent (BOLD) signal, an indirect measure of neural activity. Because the vascular response is sluggish, the expected BOLD signal y(t)y(t) is modeled by convolving the stimulus onset function s(t)s(t) with the Hemodynamic Response Function (HRF) h(t)h(t): y(t)=(s∗h)(t)+ϵ(t)=∫0ts(τ)h(t−τ)dτ+ϵ(t)y(t) = (s * h)(t) + \epsilon(t) = \int_{0}^{t} s(\tau)h(t-\tau)d\tau + \epsilon(t)

Standard fMRI Preprocessing Pipeline

Slice-Timing Correction
Adjusts for the fact that different slices of the brain are acquired at slightly different times.
Realignment
Corrects for participant head motion during the scan.
Coregistration
Aligns the low-resolution functional images with the high-resolution anatomical T1 image.
Normalization
Warps individual brains into a standard template space (e.g., MNI space) for group comparisons.
Smoothing
Applies a Gaussian kernel to increase Signal-to-Noise Ratio (SNR) and account for anatomical variability.
ℹ️

The Multiple Comparisons Problem

A standard fMRI scan contains over 100,000 voxels. Running a t-test on each voxel requires strict correction for multiple comparisons to avoid massive false positives. Common methods include Family-Wise Error (FWE) correction via Random Field Theory or False Discovery Rate (FDR).

Which fMRI preprocessing step aligns functional images to the participant's own high-resolution anatomical T1 image?

What acronym describes the general statistical framework used to analyze fMRI time-series data at the individual subject level?

fMRI vs. EEG

Functional MRI (fMRI)

  • High spatial resolution (millimeters).
  • Low temporal resolution (seconds) due to sluggish BOLD response.
  • Measures hemodynamics (indirect neural correlate).

Electroencephalography (EEG)

  • Low spatial resolution (centimeters) due to volume conduction.
  • High temporal resolution (milliseconds).
  • Measures direct electrical activity (post-synaptic potentials).