Evaluation of the early response in fMRI in individual subjects using short stimulus duration.

Optical imaging studies have provided evidence of an initial increase in deoxyhemoglobin following the onset of neuronal stimulation/activation and demonstrated that this initial increase could be spatially more specific to the site of neuronal activity. These studies also raised the possibility of improving the specificity of fMRI by selective mapping of this early response. Previous MR studies reported the observation of this early response but were limited in scope and not in full agreement. This paper presents a more extensive study that (a) demonstrates the initial signal decrease in individual subjects and (b) examines its dependence on stimulus duration and subject. Binocular visual stimulation experiments were performed on 14 subjects using echo-planar imaging (EPI) with high temporal resolution. An initial signal decrease was consistently observed in regions that were more localized than those displaying the delayed positive response. In agreement with previous fMRI and optical imaging findings, the maximum signal decrease was 1-2% and occurred at approximately 2 s after the onset of the stimulus, depending on the subject. For stimulus longer then 3.0 s, the temporal dynamics and the amount of signal change of the early response was essentially independent of the stimulus duration, while the delayed response and the post-stimulus undershoot increased both in terms of magnitude and rise time as the duration of the stimulus increased; this observation is concordant with the recent optical imaging study.