Spatiotemporal dynamics of the BOLD fMRI signals: Toward mapping submillimeter cortical columns using the early negative response

The existence of the early-negative blood-oxygenation-level-dependent (BOLD ) response is controversial and its practical utility for mapping brain fun ctions with columnar spatial specificity remains questionable. To address t hese issues, gradient-echo BOLD fMRI studies were performed at 4.7 T and 9. 4 T using the well-established orientation column model in the cat visual c ortex. A robust transient early-negative BOLD response was consistently obs erved in anesthetized cat (-0.35 +/- 0.09%, mean +/- SD, n = 8 at 2.9 +/- 0 .5 sec poststimulus onset for 4.7 T, TE = 31 ms; -0.29 +/- 0.10%, n = 4 at 3.0 +/- 0.8 sec poststimulus onset for 9.4 T, TE = 12 ms). In addition to i ts temporal evolution, the BOLD response also evolved dynamically in the sp atial domain. The initially spatially localized early-negative signal appea red to dynamically drain from the active sites toward large vessels, follow ed by a wave of the delayed positive signal, which exhibited similar spatio temporal dynamics. Only the early-negative BOLD response within 2 sec of th e stimulus onset (not the entire dip) yielded columnar layouts without diff erential subtraction. The functional maps of two orthogonal orientations us ing the first 2-sec dip were indeed complementary. On the other hand, the d elayed positive BOLD response appeared diffused and extended beyond the act ive sites. It was thus less suitable to resolve columnar layouts. These res ults have implications for the design and interpretation of the BOLD fMRI a t columnar resolution. Magn Reson Med 44:231-242, 2000. (C) 2000 Wiley-Liss , Inc.