Synchronization between Background Activity and Visually Evoked Potential Is Not Mirrored by Focal Hyperoxygenation: Implications for the Interpretation of Vascular Brain Imaging

We performed an electroencephalography and optical topography study simultaneously exploring electrophysiological and vascular response magnitude as a function of stimulus frequency. To elicit a response in the visual cortex, subjects were exposed to flicker frequencies varying from 1 to 25 Hz (1 Hz steps, eyes closed). Extending the standard view to compare magnitudes of the evoked neuronal to the evoked vascular response, we additionally investigated modulations of α-power, a marker of “background” EEG activity. The results show two discrepancies between the electrophysiological and vascular response: (1) VEP and α-power exhibit a discontinuous peak when stimulating at the individual α-frequency (IAF) (∼10–11 Hz), indicating resonance between background oscillations and evoked response; this is not mirrored by the vascular response. (2) The vascular response, in contrast, steadily increases up to a maximum at 7–8 Hz and slightly decreases with higher frequencies. This continuous frequency dependence is partly reflected by the decrease in α-power up to frequencies of 8–9 Hz and a slight increase in α-power beyond the IAF resonance. Although indicating an inverse relationship between α-power and vascular response, the frequency dependence of the evoked response does not show such a correlation. Thus, electrophysiological resonance between an individual's α-frequency and isofrequent stimulation is not mirrored by the vascular response. Also, spontaneous background EEG activity is an important modulator of the vascular response magnitude. We discuss these deviations from a simple one-to-one translation between evoked potential and vascular response amplitude in the light of questions concerning synchronization, attenuation, and induction of background oscillations such as the α-rhythm.