What is the primary method by which fMRI scans monitor activity in the brain?

fMRI, or functional magnetic resonance imaging, primarily utilizes magnetic fields and radio signals to monitor brain activity. This technique is based on the principle of blood oxygenation level-dependent (BOLD) contrast, which measures changes in blood flow associated with neural activity. When specific brain regions are more active, they require more oxygen, leading to increased blood flow to those areas. The fMRI detects this change in oxygenated and deoxygenated blood by using magnetic fields to create detailed images of brain activity in real-time.

This method allows researchers to observe which parts of the brain are involved in different cognitive tasks, emotional responses, or sensory processing, making fMRI an invaluable tool in both research and clinical settings for understanding brain function. The other methods listed, such as electrodes on the scalp or X-ray technology, do not provide the same non-invasive insights into brain activity and structure as fMRI does. Thermal imaging does not measure brain activity directly; rather, it captures temperature variations on the surface of the body. Thus, the use of magnetic fields and radio signals distinctly identifies fMRI's approach to mapping brain function.