Further evaluation of the initial negative response in functional magneticresonance imaging

The Initial negative response (i.e., the dip) in the functional MR signal a t stimulus onset has aroused much interest. Such a response is consistent w ith optical imaging data and can be potentially mapped to generate spatiall y more specific maps. However, there are still controversies regarding the exact origin of the initial response. In particular, experimental reports o f its echo-time dependence have been inconsistent. Furthermore, several inv estigators have suggested the possibility of an apparent dip that may arise artifactually when the Interstimulus interval (ISI) is not sufficiently lo ng. The present study investigates the echo-time dependence of the initial response and the effect of the ISI on the initial response. Experimental re sults obtained at TEs of 21, 30, and 45 msec demonstrate that the initial d ip has a TE dependence that is in agreement with a T2* contrast, and thereb y consistent with a blood oxygenation level-dependent origin. At an ISI of 90 sec, a statistically significant initial negative response was detected and shown to be Indistinguishable from that observed at an ISI of 45 sec, w hich was used in our previous studies, indicating that the initial negative response observed at 4 T is not a consequence of short ISI. Magn Reson Med 41:436-441,1999. (C) 1999 Wiley-Liss, Inc.