Somatosensory Mapping of Human Cortex with Visual Deprivation

As its name implies, Magnetic Resonance Imaging uses strong magnetic fields to create images of human active brain. Functional MRI is a recent brain imaging method grown to become the dominant brain mapping method to study localised brain activity in humans.
The static magnetic field created by an MRI scanner is expressed in units of Tesla. Scanners used for fMRI are typically within the range of 1.5 to 4.0 Tesla, with even stronger field of 7.0 Tesla. For comparison, the earth magnetic field is approximately 0.00005 Tesla. To create images, the scanner uses a series of changing magnetic gradients and oscillating electromagnetic fields known as a pulse sequence.
Depending on their frequency, energy from the electromagnetic fields may be absorbed by atomic nuclei.
FMRI is based upon the nuclear magnetic resonance (NMR) phenomenon which was discovered in the 1940s. For MRI, scanners are tuned to the frequency of hydrogen nuclei, which are the most common in the human body, such as the brain, due to their prevalence in water molecules. These nuclei interact with radio waves in a magnetic field (nuclear resonance).This property of nuclei can be used to generate signals within a magnetic field and to form images. After it is absorbed, the electromagnetic energy is later emitted by the nuclei, and the amount of emitted energy depends on the number and type of nuclei present (Huettel et al., 2004).
MRI of the brain can be so precise to detect the difference between grey and white matter of the brain. Nearly all fMRI studies rely on an endogenous measure known as blood-oxygenated-level dependent (BOLD) contrast. The idea of using blood properties as indices of brain functions is more than a century old. Early research found that deoxygenated hemoglobin is paramagnetic (having the property of being attracted to a magnetic field). Important studies demonstrating the feasibility of BOLD contrast were conducted by Ogawa and colleagues using both test tube and animal models (1990). These results, in conjunction with the increased prevalence of MRI scanners and the development of high-speed pulse sequences, set the stage for the growth of fMRI in the early 1990s.