FUNCTIONAL MAGNETIC-RESONANCE-IMAGING (FMRI) OF THE HUMAN BRAIN

Functional magnetic resonance imaging (FMRI) can provide detailed imag es of human brain that reflect localized changes in cerebral blood flo w and oxygenation induced by sensory, motor, or cognitive tasks. This review presents methods for gradient-recalled echo-planar functional m agnetic resonance imaging (FMRI). Also included is a discussion of the hypothesized basis of FMRI, imaging hardware, a unique visual stimula tion apparatus, image post-processing and statistical analysis. Retino topic mapping of striate and extrastriate visual cortex is discussed a s an example application. The described echo-planar technique permitte d acquisition of an image in 40 ms with a repetition rate of up to 2 p er second. However, FMRI responses are slow compared to changes in neu ral activity. Onset of a visual checkerboard test pattern evoked a res ponse that was delayed by 1-2 s and reached 90% of peak in 5 s. Return to baseline following stimulation was slightly slower. Alternating co ntrol (blank) and test (checkerboard) patterns every 20 s induced a cy clic response that was detected in the presence of noise using a cross -correlation technique that was verified by parametric statistics. FMR I revealed retinotopically organized patterns of visually evoked activ ity in response to annular stimuli that increased in visual field ecce ntricity. Retinotopy was also observed with test patterns rotated arou nd the fixation point (center of gaze). Results from repeated tests 1 week apart were highly similar. Compared to passive viewing, an active visual discrimination task enhanced responses from extrastriate assoc iation cortex.