Improved spatial localization of post-stimulus BOLD undershoot relative to positive BOLD

Area of Science:

• Neuroimaging
• Functional Magnetic Resonance Imaging (fMRI)

Background:

• The negative blood oxygenation level-dependent (BOLD) signal, known as post-stimulus BOLD undershoot, is a common observation in fMRI studies.
• The spatial characteristics of this post-stimulus BOLD undershoot have remained largely unknown, limiting its application in precise brain activity mapping.

Purpose of the Study:

• To investigate the spatial characteristics of the post-stimulus BOLD undershoot in functional magnetic resonance imaging (fMRI).
• To determine the utility of the post-stimulus BOLD undershoot for improving the spatial localization of fMRI signals to active cortical sites.

Main Methods:

• Gradient-echo BOLD fMRI was performed in isoflurane-anesthetized cats at 9.4 T in response to visual stimuli.
• Cortical cross-sections were imaged to analyze the spatial distribution of the BOLD signal and cerebral blood volume (CBV) responses.
• The relationship between the magnitude of post-stimulus BOLD undershoot and echo time (TE) was examined.

Main Results:

• A robust post-stimulus BOLD undershoot was consistently observed, lasting over 30 seconds after stimulation cessation.
• The magnitude of the undershoot was primarily driven by T2* changes, as indicated by its linear dependence on echo time.
• While prolonged CBV elevation was localized to the middle cortex, the post-stimulus BOLD undershoot was detected throughout the cortex, with the largest signal in the middle cortical layer (layer 4).

Conclusions:

• The post-stimulus BOLD undershoot, despite contributions from pial vessels, is a valuable tool for enhancing the spatial resolution of fMRI.
• This undershoot signal can be effectively used to improve the localization of fMRI activity to specific cortical regions, particularly the metabolically active middle cortical layers.