Novel Methods for the Visualization and Analysis of Functional Maps in Cortex

Flat Maps 16 1997 #32] and consequently to a stronger MR signal if a pulse sequence with a high weight on T2* is used, e.g., gradient echo or echo planar imaging (EPI) [Dössel, 2000 #25]. This is due to the fact that T2* becomes larger for an increasing oxyhemoglobin-deoxyhemoglobin ratio. Since the signal increases with activity due to an increase in blood oxygenation, this fMRI signal it called a blood oxygen level-dependent (BOLD) signal. In active regions, the neural activity signal is stronger. The BOLD contrast effect can be ex- ploited to get maps of blood oxygenation levels in the brain. The neural activity maps can be inferred from the blood oxygenation maps. BOLD fMRI is therefore an important tool to learn more about cerebral localization. The signal obtained with the BOLD fMRI method is an indi- Figure 6: Image Pathway A visual stimulus shown to a subject causes neural activity. This leads to a change in the oxyhemoglobin-deoxyhemoglobin ratio, which can be measured (see text); Source: Brian A. Wandell