Functional magnetic resonance imaging (fMRI)
Also known as: fMRI, functional MRI, BOLD fMRI
In one sentencefMRI uses MRI to track blood-oxygen changes (the BOLD signal) that follow neural activity, showing noninvasively which brain regions are active.
The BOLD signal
When neurons become active, more oxygen-rich blood flows to the area than is actually consumed. Deoxygenated hemoglobin distorts the local magnetic field, so when its share drops the MRI signal rises slightly.
This is the blood-oxygen-level-dependent (BOLD) signal. Seiji Ogawa and colleagues reported the contrast in the rat brain in 1990, laying the groundwork for fMRI.
What it can show
fMRI images the whole brain at millimeter resolution without surgery, which makes it one of the most widely used tools for studying human memory, language and emotion. Because the blood-flow response lags neural activity by seconds, its time resolution is lower than that of EEG, which works on a millisecond scale.
What the signal really reflects
BOLD is an indirect measure of neural activity. In 2001 Nikos Logothetis and colleagues reported in monkeys that BOLD tracks local field potentials more closely than neurons’ spiking output.
Which types of neurons produce the signal was long hard to test directly. The 2010 optogenetic fMRI study showed that driving excitatory neurons alone is enough to generate BOLD signals, opening a causal approach to the question.
Interpreting results
Research fMRI results are often group averages. A region “lighting up” cannot by itself tell what an individual is thinking or how they are doing.
Related entries
- Optogenetic fMRI (ofMRI) — Optogenetic fMRI (ofMRI) drives specific neurons with light while measuring whole-brain responses with fMRI, mapping the circuits those cells recruit.
- Optogenetics — Optogenetics delivers light-sensitive proteins to chosen neurons so researchers can switch those cells on or off with light, with millisecond precision.
- Neural circuit — A neural circuit is a group of neurons connected by synapses that work together to carry out a specific function such as sensory processing, movement or memory.
References
- Brain magnetic resonance imaging with contrast dependent on blood oxygenation. Ogawa S, Lee TM, Kay AR, Tank DW. PNAS 87, 9868–9872 (1990)
- Neurophysiological investigation of the basis of the fMRI signal. Logothetis NK, Pauls J, Augath M, Trinath T, Oeltermann A. Nature 412, 150–157 (2001)
- Global and local fMRI signals driven by neurons defined optogenetically by type and wiring. Lee JH et al. Nature 465, 788–792 (2010)