EEG/VEP and fMRI responses to periodic visual stimulation are reported. The
purpose of these experiments was to look for similar patterns in the time
series produced by each method to help understand the relationship between
the two. The stimulation protocol was the same for both sets of experiments
and consisted of five complete cycles of checkerboard pattern reversal at
1.87 Hz for 30 s followed by 30 s of a stationary checkerboard. The fMRI da
ta was analyzed using standard methods, while the EEG was analyzed with a n
ew measurement of activation-the VEPEG. Both VEPEG and fMRI time series con
tain the fundamental frequency of the stimulus and quasi harmonic component
s-an unexplained double frequency commonly found in fMRI data. These result
s have prompted a reappraisal of the methods for analyzing fMRI data and ha
ve suggested a connection between our findings and much older published inv
asive electrophysiological measurements of blood flow and the partial press
ures of oxygen and carbon dioxide. Overall our new analysis suggests that f
MRI signals are strongly dependant on hydraulic blood flow effects. We dist
inguish three categories of fMRI signal corresponding to: focal activated r
egions of brain tissue; diffuse nonspecific regions of steal; and major cer
ebral vessels of arterial supply or venous drainage. Each category of signa
l has its own finger print in frequency, amplitude, and phase. Finally, we
put forward the hypothesis that modulations in blood flow are not only the
consequence but are also the cause of modulations in functional activity, (
C) 1999 Academic Press.