Detection of the early negative response in fMRI at 1.5 tesla

Recent experimental studies have revealed an initial decrease in magnetic r esonance (MR) signal that is consistent with optical imaging results. This initial response, thought to arise from a transient increase in deoxyhemogl obin concentration, is probably more localized to the site of neuronal acti vation. However, with MR imaging, this early response has only been demonst rated at high fields. In this paper, the observation of the initial respons e at 1.5 T is reported. Compared with results obtained at 4 T, the present study reveals that the initial response grows more rapidly with the field s trength than the late positive response, suggesting a more microvascular or igin of the initial response. (C) 1999 Wiley-Liss, Inc.