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The Journal of Neuroscience, May 3, 2006, 26(18):4940-4948; doi:10.1523/JNEUROSCI.3989-05.2006

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

Stefan P. Koch, Jens Steinbrink, Arno Villringer, and Hellmuth Obrig

Berlin NeuroImaging Centre, Department of Neurology, Charité–Universitätsmedizin Berlin, 10117 Berlin, Germany

Correspondence should be addressed to Stefan P. Koch, Berlin NeuroImaging Centre, Department of Neurology, Charité–Universitätsmedizin Berlin, Campus Mitte, Schumannstrasse 20-21, 10117 Berlin, Germany. Email: stefan.koch{at}charite.de

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 -power, a marker of "background" EEG activity.

The results show two discrepancies between the electrophysiological and vascular response: (1) VEP and -power exhibit a discontinuous peak when stimulating at the individual -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 -power up to frequencies of 8–9 Hz and a slight increase in -power beyond the IAF resonance. Although indicating an inverse relationship between -power and vascular response, the frequency dependence of the evoked response does not show such a correlation.

Thus, electrophysiological resonance between an individual's -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 -rhythm.

Key words: flicker; steady-state visual-evoked potential (ssVEP); oscillatory activity; hemodynamic response; near-infrared spectroscopy (NIRS); neurovascular coupling; visual cortex

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Received Sept. 20, 2005; revised March 9, 2006; accepted March 31, 2006.

Correspondence should be addressed to Stefan P. Koch, Berlin NeuroImaging Centre, Department of Neurology, Charité–Universitätsmedizin Berlin, Campus Mitte, Schumannstrasse 20-21, 10117 Berlin, Germany. Email: stefan.koch{at}charite.de

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