Temporal variability infMRI response to visual stimulation in human brain
In this study, we implemented a new method, based on the Hilbert Transform, to measure the temporal delay of FMRI responses and used it to estimate the statistical distribution of response delays evoked by visual stimuli (checkered annuli) within and across voxels. We assessed delay variability among different cortical sites and between parenchyma and blood vessels (identified by 2DTOF angiography and normalized signal change). Overall, 86% of all responsive voxels in visual cortex showed activation in phase with the stimulus while the remaining voxels showed anti-phase, suppressive responses. Delays for activated and suppressed voxels differed by 0.8 seconds on average. Parametric maps on computer models of the flattened cortical surface revealed the pattern of activated and suppressed voxels was dynamically induced and depended on stimulus size. The average delay of responses in blood vessels was 0.8 to 2.1 seconds longer than in parenchyma (p < 0.01). However, both parenchyma and blood vessels can give rise to responses with long delays. We developed a model to identify and quantify different components contributing to variability in the empirical delay measurements. Within-voxel changes in delay over time were fully accounted for by the effects of empirically measured FMRI noise. Across-voxels, the delay variance was also due to FMRI noise but, in addition, reflected temporally fixed differences among brain sites. The contribution of FMRI noise to the delay variance depended on the power at the stimulus frequency and accounted for as much as 47% of the variability. White noise models significantly underestimate the FMRI noise effects.
Ziad Serhal Saad,
"Temporal variability infMRI response to visual stimulation in human brain"
(January 1, 2000).
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