Electrophysiological recordings in monkeys show that the temporal structure of neuronal activity patterns which precede the rise of an evoked stimulus can be correlated with subsequent perceptual decisions or behavioral events (see Engel et al. 2001 for review). These patterns are often expressed as a synchrony in multiple site recordings of local field potentials: a synchrony that is dominant in the gamma frequency range. Using high-resolution EEG and direct estimation of local field potentials in humans, a positive correlation between gamma activity in a frontoparietal network before stimulus arrival and the reaction time to the stimulus was recently shown (Gonzalez Andino et al. 2004). It is assumed that these prestimulus synchronies reflect alerting attention mechanisms and the genesis of prerepresentations that influence stimulus processing through top-down control. These prestimulus differences in alerting attention might also explain the finding of Super et al. (2003) that the firing rate of V1 neurons 100 ms before the stimulus predicted whether or not the monkey consciously perceived a visual stimulus.
In summary, there exists consistent experimental evidence that subtle variations of the functional state of the brain which precede the entry of sensory stimuli influence subsequent event-related information processing and that some of them (like synchronies) may reflect alerting attention mechanisms. These data are in agreement with the critical proposal of the neuronal workspace model that access to consciousness relies upon the ongoing spontaneous activity of the workspace neurons. Still, further studies are needed to test the specific predictions of the formal model at the neuronal level (see Dehaene et al. 2003; Dehaene and Changeux 2005).
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