The large-scale integration problem refers to the series of processes whereby the nervous system coordinates activity that is distributed over distant brain regions to produce a unified cognitive moment. One of the most plausible candidate mechanisms behind large-scale integration is the formation of dynamic links among brain regions, links that are mediated by phase synchronization. Phase synchronization refers to the relation between the temporal structures of the neural signals regardless of signal amplitude. Two signals are said to be synchronous if their rhythms coincide.
Neural assemblies — distributed networks of neurons linked by reciprocal connections — provide a conceptual framework for the integration of distributed neural activity. Large-scale integration commonly involves neural assemblies separated by long distances.
Converging evidence indicates that phase synchrony is probably involved in brain integration. Electrophysiological analyses in cats and primates have shown that the emergence of phase synchrony over widespread cortical domain correlates with the occurrence of attentive and perceptuomotor behaviours, as well as during the execution of a learning task. Analogous findings have been made in humans using electroencephalographic and magnetoencephalographic techniques.
Although the evidence for phase synchronization as a mechanism for large-scale integration is well grounded, it is only correlative. Direct proof that changes in synchronous activity can affect behaviour remains to be established in most cases. Similarly, the cellular mechanisms of synchronization, the interplay between slow and fast brain rhythms, and the role of parallel phase synchronization over different frequency bands constitute topics for future research.
The emergence of a unified cognitive moment relies on the coordination of scattered mosaics of functionally specialized brain regions. Here we review the mechanisms of large-scale integration that counterbalance the distributed anatomical and functional organization of brain activity to enable the emergence of coherent behaviour and cognition. Although the mechanisms involved in large-scale integration are still largely unknown, we argue that the most plausible candidate is the formation of dynamic links mediated by synchrony over multiple frequency bands.
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Thanks to Jean-Baptiste Poline for this help concerning metabolic imaging methods. This work was partly supported by the Ministère de l'Education et la Recherche (Action Cognitique) and the Fundacion Puelma (E.R.).
- INVERSE PROBLEM
Mathematical analysis aimed at localizing the neural sources of the electromagnetic field measured at the scalp surface.
- CORTICAL COLUMN
Cylinder of cortex with a diameter up to 1 mm that groups neurons with strong reciprocal connections.
- BETA RHYTHM
Neural rhythmic activity (12–25 cycles per second).
- GAMMA RHYTHM
Neural rhythmic activity (about 25–70 cycles per second).
- TIME–FREQUENCY ANALYSIS
Mathematical techniques used to estimate the spectral components (amplitude, frequency and phase) of short non-stationary signals (for example, Wavelets, ARMA, Hilbert).
Probability for a neuron to spike as a function of the latency of the last spike of a second neuron.
- GO–NO-GO PARADIGM
Task in which the subject must produce a motor response for one class of stimulus while ignoring others.
- BINOCULAR RIVALRY TASK
Task in which each eye of the subject is shown a different image. This results in a bistable visual experience.
- THETA RHYTHM
Neural rhythmic activity (4–8 cycles per second).
- PARKINSONIAN TREMOR
Abnormal rhythmic muscular activity (4–8 Hz) observed in Parkinsonian patients.
- ALPHA RHYTHM
Neural rhythmic activity (8–12 cycles per second).
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Varela, F., Lachaux, JP., Rodriguez, E. et al. The brainweb: Phase synchronization and large-scale integration. Nat Rev Neurosci 2, 229–239 (2001). https://doi.org/10.1038/35067550
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