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The Journal of Neuroscience, March 14, 2007, 27(11):2858-2865; doi:10.1523/JNEUROSCI.4623-06.2007

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Behavioral/Systems/Cognitive
Synchronization of Neural Activity across Cortical Areas Correlates with Conscious Perception

Lucia Melloni,^1 * Carlos Molina,^1,2 Marcela Pena,^1,3 David Torres,¹ Wolf Singer,^4,5 and Eugenio Rodriguez^1,4,5 *

¹Laboratorio de Neurociencias, Escuela de Psicología, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, San Joaquin, 8940000 Santiago, Chile, ²Radboud University Nijmegen, 6525 EK Nijmegen, The Netherlands, ³International School for Advanced Studies, Cognitive Neuroscience Sector, 34014 Trieste, Italy, ⁴Department of Neurophysiology, Max Planck Institute for Brain Research, 60528 Frankfurt am Main, Germany, and ⁵Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe University, 60438 Frankfurt am Main, Germany

Correspondence should be addressed to Dr. Lucia Melloni, Brain Imaging Center and Cognitive Neurology Unit, Johann Wolfgang-Goethe Universität, Schleusenweg 2-16, 60528 Frankfurt am Main, Germany. Email: melloni{at}mpih-frankfurt.mpg.de

Subliminal stimuli can be deeply processed and activate similar brain areas as consciously perceived stimuli. This raises the question which signatures of neural activity critically differentiate conscious from unconscious processing. Transient synchronization of neural activity has been proposed as a neural correlate of conscious perception. Here we test this proposal by comparing the electrophysiological responses related to the processing of visible and invisible words in a delayed matching to sample task. Both perceived and nonperceived words caused a similar increase of local (gamma) oscillations in the EEG, but only perceived words induced a transient long-distance synchronization of gamma oscillations across widely separated regions of the brain. After this transient period of temporal coordination, the electrographic signatures of conscious and unconscious processes continue to diverge. Only words reported as perceived induced (1) enhanced theta oscillations over frontal regions during the maintenance interval, (2) an increase of the P300 component of the event-related potential, and (3) an increase in power and phase synchrony of gamma oscillations before the anticipated presentation of the test word. We propose that the critical process mediating the access to conscious perception is the early transient global increase of phase synchrony of oscillatory activity in the gamma frequency range.

Key words: visual awareness; electroencephalography; event-related potentials; gamma-band oscillations; long-range coordination; oscillatory neural synchrony

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Received Oct. 25, 2006; revised Jan. 16, 2007; accepted Feb. 5, 2007.

Correspondence should be addressed to Dr. Lucia Melloni, Brain Imaging Center and Cognitive Neurology Unit, Johann Wolfgang-Goethe Universität, Schleusenweg 2-16, 60528 Frankfurt am Main, Germany. Email: melloni{at}mpih-frankfurt.mpg.de

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Stimulus-related synchronization,'visual binding' and signal-to-noise ratio in the brain.

walter g. sannita

J. Neurosci. Online, 3 Apr 2007 [Full text]

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