Functional magnetic resonance imaging (fMRI) is a new and exciting method that allows investigators to determine which areas of the brain are involved in a particular task. The magnetic resonance signal is made sensitive to blood flow changes and a complex statistical model is used to determine the regions of activation. The fMRI signal is a result of blood oxygen level dependent (BOLD) contrast. The concentration of oxygenated blood determines the fMRI signal. This study investigates how this vascular response varies within and across subjects.

For years it has been assumed that the vascular response remains constant throughout the human brain, an assumption which may not be accurate considering the diverse vascular architecture. Furthermore, it has also been generalized that the vascular response is nearly identical across subjects. Study results have uncovered similarities in different cortical regions within the individual brain (Figure 1), as well as dissimilarities (Figure 2).

Figure 1: Individual normalized hemodynamic response. This hemodynamic response curve is normalized to demonstrate that there are no differences in the rise and fall slopes. In the non-normalized curves, the non-primary areas show a much lower level of response and a lack of post-stimulus undershoot (see below).

Figure 2: Group average hemodynamic response curves generated from 19 volunteers. Note the amplitude and post-stimulus undershoot differences in these curves.