Detection of the early decrease in fMRI signal in the motor area.

The initial decrease in BOLD signal at stimulus onset (i.e., the initial dip) has generated a great deal of interest because of its potential for providing more spatially specific mapping. Despite a number of experimental fMRI studies demonstrating its existence in the visual cortex, the initial dip has not been reported in other areas. The present work examined the initial dip in the motor area. Using a visually-guided finger-tapping paradigm, the dip was detected in both motor and visual areas simultaneously. The dip in the motor area was found to exhibit characteristics similar to those revealed by visual stimulation studies. The motor dip peaked approximately 2 sec after stimulus onset and reached an amplitude that was roughly 0.3 times of the positive amplitude. Furthermore, the dip in the motor area was more localized and less sensitive to large vessels, indicating an improvement in spatial specificity despite the relatively low spatial resolution used in this study. These data indicate that the initial dip is a general phenomenon that can be employed for more spatially specific functional mapping, although its full utility in humans remains to be further demonstrated. Magn Reson Med 45:184-190, 2001.