The idea that neural synchrony relates to consciousness is over two decades old (Crick & Kock, 1990). The neural binding hypothesis is that synchrony binds nerves together, so if some nerves represent a tree trunk and others its leaves, when they fire together, they represent a tree. Evidence that neural synchronies correlate with consciousness includes that they:
a. Accompany face recognition. In face recognition, when distant nerves synchronize with no phase lag, the face is recognized (Rodriguez et al., 1999).
b. Are independent of nerve firing rate. Studies of the visual cortex find that the:
“… selection of responses for further processing is associated with enhanced synchronization rather than increased firing.” (Singer, 1999) p62.
c. Represent odors. Different smells produced odor-specific synchronies in locust olfactory nerves that differed for different smells but not for the same smell (Laurent et al., 1996).
d. Are transient. Neural synchronies can be brief and hard to detect (Singer et al., 1997).
e. Accompany cognitions. Beta/gamma brain waves correlate with cognitive functions like attention, memory, sensory integration and motor coordination (Uhlhaas, 2009) p8.
Studies from insects, cats, monkeys and humans agree that:
“… synchronization affects communication between neural groups.” (Fries, 2015) p220.
The result is a mood of optimism that consciousness is coming within the realm of science:
“Beliefs about the basis of subjective experience have slowly evolved, from mystical notions of the soul and a disembodied mind to acceptance of the proposal that consciousness must derive from neurobiological processes.” (John, 2005) p143.
Critics of consciousness by synchrony theory note that unconscious insects, anaesthetized animals and subcortical structures also have synchronies, so why aren’t they also conscious? This limits consciousness to humans but if consciousness is any ability to observe, then anaesthetized animals, insects and lower brain parts have less consciousness not none at all. If consciousness evolved from the cell level, not only humans are conscious.
Neural synchronies correlate not only with consciousness but also build-up in a time-frame that reflects the chronology of an experience (John, 2005):
1. 50 milliseconds: P1 waves occur as nerve synchronies in primary sensory cortex areas register input features.
2. 130 milliseconds: N1 wave synchronies link the cortex to the thalamus/limbic system.
3. 210 milliseconds: P2 waves link higher cortex layers to the thalamus/limbic system.
4. 300 milliseconds: Sustained P300 gamma oscillations synchronize the frontal and parietal lobes with zero-delay in what is considered the basis of the perception itself.
Sense input triggers transient local synchronies in primary sensory areas, then long-range synchronies add emotions, memory, language and planning to give a conscious experience. There is agreement that neural synchrony relates to consciousness, but not how it actually does so.