This article will focus on fMRI imaging.
fMRI, or functional magnetic resonance imaging, uses MRI equipment. fMRI is sensitive to the amount of deoxyhemoglobin in the blood. We know that when neurons consume oxygen, the neurons convert oxyhemoglobin to deoxyhemoglobin. Now, deoxyhemoglobin has magnetic properties, and these magnetic properties therefore introduce distortions in the local magnetic field. This distortion can be measured, and the measurement therefore gives an indication of how much deoxyhemoglobin in is in the blood. This technique is termed BOLD, for blood oxygen-level-dependent contrast.
The BOLD signal evolves over time in response to an increase in neural activity. This is called the hemodynamic response time, or HRF. HRF has three phases:
- The initial dip: as neurons consume oxygen, there is a small rise in deoxyhemoglobin, which results in a reduction of the BOLD signal.
- Overcompensation: in response to the increased consumption of oxygen, blood to a brain region increases. The increase in blood flow is greater than the increase in blood consumption, and therefore, the BOLD signal increases. This is the component that the fMRI normally measures.
- Undershoot: Blood flow and oxygen consumption dip before returning back to original levels.
The temporal resolution of fMRI is several seconds. This is better than PET scans, but it is still slow compared to the speeds at which cognitive processes take place.