Scientific Understanding of Consciousness
Oscillation and Synchronization -- Recent Research
Science, Vol313, 15 September 2006, p.1626
High Gamma Power Is Phase-Locked to Theta Oscillations in Human Neocortex,
We observed robust coupling between the high- and low-frequency bands of ongoing electrical activity in the human brain. In particular, the phase of the low-frequency theta (4 to 8 hertz) rhythm modulates power in the high gamma (80 to 150 hertz) band of the electrocorticogram, with stronger modulation occurring at higher theta amplitudes. Furthermore, different behavioral tasks evoke distinct patterns of theta/high gamma coupling across the cortex. The results indicate that transient coupling between low- and high-frequency brain rhythms coordinates activity in distributed cortical areas, providing a mechanism for effective communication during cognitive processing in humans.
Nature 394, 179-182 (9 July 1998)
Visual synchrony affects binding and segmentation in perception
Marius Usher and Nick Donnelly
The visual system analyses information by decomposing complex objects into simple components (visual features) that are widely distributed across the cortex. When several objects are present simultaneously in the visual field, a mechanism is required to group (bind) together visual features that belong to each object and to separate (segment) them from features of other objects. An attractive scheme for binding visual features into a coherent percept consists of synchronizing the activity of their neural representations. If synchrony is important in binding, one would expect that binding and segmentation are facilitated by visual displays that are temporally manipulated to induce stimulus-dependent synchrony. Here we show that visual grouping is indeed facilitated when elements of one percept are presented at the same time as each other and are temporally separated (on a scale below the integration time of the visual system) from elements of another percept or from background elements. Our results indicate that binding is due to a global mechanism of grouping caused by synchronous neural activation, and not to a local mechanism of motion computation.
The following study report published in Science states that groups of activated neurons synchronize in the gamma-frequency band (30 to 100 Hz), and previous studies have related gamma-band synchronization to several cognitive functions. Yet, if gamma-band synchronization subserves those functions, it must have mechanistic consequences for neuronal processing. It has been shown that the precise timing of pre- and postsynaptic activation determines long-term changes in synaptic strength and that gamma-band synchronization of synaptic inputs directly enhances their effective synaptic strength.
The study further states that synchronization between two groups of neurons is also likely to facilitate interactions between them. Gamma-band synchronization entails rhythmic inhibition of the local network, and the periods between inhibition provide temporal windows for neuronal interaction. Two groups of neurons will therefore probably have a greater influence on each other when their temporal interaction windows open at the same times, i.e., when the rhythmic synchronization within the groups is also synchronized between the groups.
Science 15 June 2007: Vol. 316. no. 5831, pp. 1609 - 1612
Modulation of Neuronal Interactions Through Neuronal Synchronization
Thilo Womelsdorf,1 Jan-Mathijs Schoffelen,1 Robert Oostenveld,1 Wolf Singer,2,3 Robert Desimone,4,5 Andreas K. Engel,6 Pascal Fries1,7
1 F. C. Donders Centre for Cognitive Neuroimaging, Radboud University Nijmegen, 6525 EN Nijmegen, Netherlands.
Brain processing depends on the interactions between neuronal groups. Those interactions are governed by the pattern of anatomical connections and by yet unknown mechanisms that modulate the effective strength of a given connection. We found that the mutual influence among neuronal groups depends on the phase relation between rhythmic activities within the groups. Phase relations supporting interactions between the groups preceded those interactions by a few milliseconds, consistent with a mechanistic role. These effects were specific in time, frequency, and space, and we therefore propose that the pattern of synchronization flexibly determines the pattern of neuronal interactions.
Studies such as these support the hypothesis that synchronization between neuronal functional groups binds together the functions of the groups. The 40 Hz oscillations and synchronization espoused by Llinás are consistent with these research results.
Nature 448, 802-806 (16 August 2007)
Correlation between neural spike trains increases with firing rate
Center for Neural Science, New York University, New York 10003, USA
Courant Institute of Mathematical Sciences, New York University, New York 10012, USA
Department of Mathematics, University of Houston, Houston, Texas 77204, USA
How do cortical cells transform correlation between their synaptic currents into correlation between their output spike trains? We addressed this question by studying pairwise spike train correlations, a strategy that can capture the full statistical structure of a neural network.
Populations of neurons in the retina, olfactory system, visual and somatosensory thalamus, and several cortical regions show temporal correlation between the discharge times of their action potentials (spike trains). Correlated firing has been linked to stimulus encoding, attention, stimulus discrimination, and motor behaviour.
PLoS Biology, September 2007
Brain Dynamics Underlying the Nonlinear Threshold for Access to Consciousness
Antoine Del Cul1,2,3, Sylvain Baillet4,5, Stanislas Dehaene1,2,3,6
1 INSERM, Cognitive Neuroimaging Unit, IFR 49, Saclay, France, 2 Atomic Energy Commission (CEA), NeuroSpin Center, Saclay, France, 3 University of Paris XI, Orsay, France, 4 Cognitive Neuroscience and Brain Imaging Laboratory, CNRS UPR640, IFR 49, Paris, France, 5 University Pierre & Marie Curie, Paris, France, 6 Collčge de France, Paris, France
When a flashed stimulus is followed by a backward mask, subjects fail to perceive it unless the target-mask interval exceeds a threshold duration of about 50 ms. Models of conscious access postulate that this threshold is associated with the time needed to establish sustained activity in recurrent cortical loops, but the brain areas involved and their timing remain debated. We used high-density recordings of event-related potentials (ERPs) and cortical source reconstruction to assess the time course of human brain activity evoked by masked stimuli and to determine neural events during which brain activity correlates with conscious reports. Target-mask stimulus onset asynchrony (SOA) was varied in small steps, allowing us to ask which ERP events show the characteristic nonlinear dependence with SOA seen in subjective and objective reports. The results separate distinct stages in mask-target interactions, indicating that a considerable amount of subliminal processing can occur early on in the occipito-temporal pathway (<250 ms) and pointing to a late (>270 ms) and highly distributed fronto-parieto-temporal activation as a correlate of conscious reportability.
Citation: Del Cul A, Baillet S, Dehaene S (2007) Brain Dynamics Underlying the Nonlinear Threshold for Access to Consciousness. PLoS Biol 5(10): e260
Copyright: © 2007 Del Cul et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.