Functional MRI mapping of stimulus rate effects across visual processing stages

Functional magnetic resonance imaging (fMRI) was used to record cortical activation across multiple stages in the visual system during single character visual search and reversing checkerboard stimulation. Scanning used T₂-weighted, gradient echo sequences with late echo times (TE = 36 ms) with a voxel size of 0.94 * 1.88 mm in-plane resolution, 4–5 mm deep, on a conventional scanner. A scout experiment recorded six slices to identify major regions of activation. Two slices were selected for extensive assessment. Character stimuli activated small (average 16 mm²), reliable, statistically defined regions of activation in the calcarine fissure, superior occipital cortex, and fusiform-lingual gyrus. The results include: (1) for character search, the MRI signal change increased linearly from 2.1 to 3.1% for stimulation from 1 to 8 Hz; (2) the character rate effect was equivalent across three levels of the visual system; (3) the checkerboard stimuli showed broader, more intense primary visual activation and less intense secondary visual activation than did character search. Issues relating to fMRI signal variability across the imagining plane, statistical data analysis, signal sensitivity, statistical power, fMRI experimental protocols, and comparisons with positron emission tomography (PET) data are discussed. © 1994 Wiley-Liss, Inc.