Task-evoked deactivations: dissociation between BOLD fMRI and FDG

Area of Science:

• Neuroimaging
• Cognitive Neuroscience
• Metabolic Imaging

Background:

• Task-evoked decreases in blood-oxygenation-level-dependent (BOLD) signals are common in functional magnetic resonance imaging (fMRI).
• These "deactivations" are often observed in the default mode network (DMN), a network of brain regions typically active during rest.
• The underlying metabolic reasons for these fMRI deactivations are not fully understood.

Purpose of the Study:

• To investigate the metabolic basis of task-induced BOLD fMRI deactivations.
• To simultaneously measure BOLD fMRI signals and cerebral glucose consumption (CMRglc) during cognitive tasks.

Main Methods:

• Utilized simultaneous PET/MRI to measure BOLD fMRI and CMRglc.
• Assessed visuomotor and language tasks in 22 cognitively unimpaired older adults.
• Analyzed signal changes in task-relevant regions and the DMN.

Main Results:

• Positive BOLD responses correlated with increased CMRglc.
• Regions with negative BOLD responses (fMRI deactivations) did not show decreased CMRglc; instead, glucose consumption often increased.
• The posterior cingulate cortex exhibited elevated CMRglc alongside negative BOLD signals.
• Whole-brain normalization suggested relative CMRglc reductions were due to smaller increases compared to the global average.

Conclusions:

• BOLD fMRI deactivations can coexist with stable or even elevated cerebral glucose consumption.
• Findings challenge the assumption that fMRI deactivations directly represent reduced neural or metabolic activity.
• Suggests a complex relationship between BOLD signals and brain metabolism during cognitive tasks.