Hi Anita,

Let me give you my opinion here, apart from Gang's advice. The magnitude of the signal changes greatly depend on BOLD sensitivity and magnetic field strength. Clearly, we expect larger signal changes at 7T than at 3T, and at 3T than at 1.5T, and so on. Furthermore, the BOLD sensitivity varies across regions depending on the magnitude of the change in transverse relaxation R2* (i.e. Delta R2*) due to the BOLD effect and also depend on the echo time. For instance, sensorimotor and visual cortices usually exhibit higher BOLD sensitivity than frontal areas.

To sum up, it depends on the task and field strength. For instance, we observed very large signal changes (> 5%) at 7T in a simple event-related visually cued finger tapping task.

An excellent reference is

van der Zwaag W, Francis S, Head K, Peters A, Gowland P, Morris P, Bowtell R. (2009) fMRI at 1.5, 3 and 7 T: characterising BOLD signal changes. Neuroimage. 2009 Oct 1;47(4):1425-34. doi: 10.1016/j.neuroimage.2009.05.015.

Hope this helps,
Cesar