This article describes a protocol for functional magnetic resonance imaging (fMRI) to study the visual cortex. It outlines the steps for preparing participants and acquiring both structural and functional images to assess brain activity in response to visual stimuli.
Stabilize the participant's head on the posterior half of the head coil of a Magnetic resonance imaging, or MRI, machine.
Place a mirror to reflect visual stimuli from a screen to stimulate the brain's visual cortex.
The visual cortex processes visual information from the eye's retina.
Acquire structural images of the brain to serve as anatomical references.
Acquire functional MRI images to measure baseline brain activity.
Without stimulation, neurons require less oxygen for metabolism, resulting in deoxygenated blood in the brain.
The magnetic properties of deoxygenated hemoglobin in the blood distort the MRI magnetic field, resulting in a weak signal.
Next, display visual stimuli to stimulate neurons in the visual cortex.
The increased energy demand of the stimulated neurons prompts a flow of oxygenated blood. Lowering deoxygenated hemoglobin levels reduces distortion in the magnetic field, enhancing the signal strength.
Using the blood oxygen level-dependent signal, measure activity in the stimulated visual cortex.
Begin by instructing the participant to fixate on the central cross during the imaging scans. Briefly demonstrate the visual stimulation to familiarize the participant with the procedure. Position the participant comfortably on the scanner table, and immobilize the participant's head in the posterior half of the head coil array with foam padding to reduce motion artifacts. Provide earplugs to protect their hearing.
Use the scanner's positioning system to move the table into the scanner bore. Place the wide view mirror 10 centimeters from the participant's eyes. Adjust the mirror to maintain a consistent viewing angle. Begin fMRI scanning with a localizer scan, incorporating three orthogonal planes and adjusting the scanner for frequency and shimming calibration.
Proceed with an MP-RAGE anatomical scan for enhanced EPI slice positioning. At the start of the fMRI protocol, instruct the participant to fixate on the white cross atop a gray background at the stimuli center for 10 seconds. Create visual stimuli using a compatible program by displaying a series of rotating wedges for 30 seconds.
The wedges should extend to the edge of the screen or mirror, with an 8-hertz contrast-reversing black and white checkerboard pattern. Present the white cross for an additional 10 seconds. Repeat cross-fixation and stimulation display with the second 30-second visual stimulation paradigm, consisting of expanding and contracting rings. Maintain an 8 Hertz contrast-reversing black and white checkerboard pattern at 100% contrast during the experiment. Upon completion of imaging, slide the table out of the scanner bore, ensuring the participant remains still.
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