Converging Intracranial Markers of Conscious Access

TLDR

The study used visual masking and iEEG in ten patients to compare conscious versus nonconscious processing of briefly flashed words. Nonconscious processing of masked words produced early gamma activity without long‑distance coherence, whereas conscious processing of unmasked words showed sustained prefrontal voltage, increased gamma power, beta‑band phase synchrony, and long‑range Granger causality, indicating a convergent distributed state that informs theories of conscious access.

Abstract

We compared conscious and nonconscious processing of briefly flashed words using a visual masking procedure while recording intracranial electroencephalogram (iEEG) in ten patients. Nonconscious processing of masked words was observed in multiple cortical areas, mostly within an early time window (<300 ms), accompanied by induced gamma-band activity, but without coherent long-distance neural activity, suggesting a quickly dissipating feedforward wave. In contrast, conscious processing of unmasked words was characterized by the convergence of four distinct neurophysiological markers: sustained voltage changes, particularly in prefrontal cortex, large increases in spectral power in the gamma band, increases in long-distance phase synchrony in the beta range, and increases in long-range Granger causality. We argue that all of those measures provide distinct windows into the same distributed state of conscious processing. These results have a direct impact on current theoretical discussions concerning the neural correlates of conscious access.