Initial simulations explored the sequence of activity leading to conscious access. When sensory stimulation was simulated as a brief depolarizing current at the lowest thalamic level, activation propagated according to two successive phases (see Figure 7): (1) initially, Alisertib mouse a brief wave of excitation progressed into the simulated hierarchy through fast AMPA-mediated feedforward connections, with an amplitude and duration directly related to the initial input; (2) in a second stage, mediated

by the slower NMDA-mediated feedback connections, the advancing feed-forward wave amplified its own inputs in a cascading manner, quickly leading the whole stimulus-relevant network into a global self-sustained reverberating or “ignited” state. This ignition was characterized by an increased power of local cortico-thalamic oscillations in the gamma band and their synchrony across areas (Dehaene et al., 2003b). This second phase of the simulation reproduces most of the empirical signatures of conscious access: late, all-or-none, cortically distributed potentials involving prefrontal cortex and other high-level GSI-IX in vivo associative cortices, with simultaneous increases in high-frequency power and synchrony (e.g., de Lafuente and Romo, 2006, Del Cul et al., 2007 and Gaillard

et al., 2009). In GNW simulations, ignition manifests itself, at the cortical level, as a depolarization of layer II/III apical dendrites of pyramidal dendrites in a subset of activated GNW neurons defining the conscious contents, the rest being inhibited. In a geometrically accurate model of the pyramidal cell, the summed postsynaptic potentials evoked by long-distance signaling among these distributed sets of active cells would create slow intracellular currents traveling from the apical dendrites toward the cell’s soma, summing up on the

cortical surface Oxygenase as negative slow cortical potentials (SCPs) over regions coding for the conscious stimulus (see He and Raichle, 2009). Simultaneously, many other GNW neurons are strongly suppressed by lateral inhibition via GABAergic interneurons and define what the current conscious content is not. As already noted by Rockstroh et al. (1992, p. 175), assuming that many more neurons are inhibited than activated, “The surface positivity corresponding to these inhibited networks would then dominate over the relatively smaller spots of negativity caused by the reverberating excitation.” Thus, the model can explain why, during conscious access, the resulting event-related potential is dominated by a positive waveform, the P3b. This view also predicts that scalp negativities should appear specifically over areas dense in neurons coding for the current conscious content.