Cerebral Cortex Advance Access published online on December 18, 2007
Cerebral Cortex, doi:10.1093/cercor/bhm229
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Spontaneous Fluctuations in Posterior 
-Band EEG Activity Reflect Variability in Excitability of Human Visual Areas
1 Functional Brain Mapping Laboratory, Department of Neurology, University Hospital Geneva, 2 Deptartment of Fundamental Neuroscience, University Medical School, Rue Micheli du Crest 24, 1211 Geneva 14, Switzerland, 3 Berenson-Allen Center for Noninvasive Brain Stimulation, Harvard Medical School, 330 Brookline Avenue, Boston, MA 02215, USA, 4 Department of Physiology—Faculty of Medicine & Program of Cognitive Science, The Hebrew University of Jerusalem, Jerusalem 91220, Israel
Address correspondence to Gregor Thut, Functional Brain Mapping Laboratory, Department of Neurology, University Hospital Geneva, 24., Rue Micheli du Crest, CH-1211 Geneva 14, Switzerland. Email: gregor.thut{at}medecine.unige.ch.
Neural activity fluctuates dynamically with time, and these changes have been reported to be of behavioral significance, despite occurring spontaneously. Through electroencephalography (EEG), fluctuations in 
-band (8–14 Hz) activity have been identified over posterior sites that covary on a trial-by-trial basis with whether an upcoming visual stimulus will be detected or not. These fluctuations are thought to index the momentary state of visual cortex excitability. Here, we tested this hypothesis by directly exciting human visual cortex via transcranial magnetic stimulation (TMS) to induce illusory visual percepts (phosphenes) in blindfolded participants, while simultaneously recording EEG. We found that identical TMS-stimuli evoked a percept (P-yes) or not (P-no) depending on prestimulus -activity. Low prestimulus -band power resulted in TMS reliably inducing phosphenes (P-yes trials), whereas high prestimulus -values led the same TMS-stimuli failing to evoke a visual percept (P-no trials). Additional analyses indicated that the perceptually relevant fluctuations in -activity/visual cortex excitability were spatially specific and occurred on a subsecond time scale in a recurrent pattern. Our data directly link momentary levels of posterior -band activity to distinct states of visual cortex excitability, and suggest that their spontaneous fluctuation constitutes a visual operation mode that is activated automatically even without retinal input.
Key Words: electroencephalography (EEG) • phosphene • state dependency • transcranial magnetic stimulation (TMS) • visual perception
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