The idea that neural synchrony relates to consciousness is over two decades old (Crick & Kock, 1990). The neural binding hypothesis is that when synchrony binds nerves together, if some nerves represent a tree trunk and others its leaves, they represent a tree when they fire together. 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.
This suggests that synchrony causes observation rather than just nerves firing.
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), suggesting that the observation of odors depends on nerve synchrony.
d. Are transient. Neural synchronies can be brief and hard to detect (Singer et al., 1997), just as conscious observations are fleeting moments.
e. Accompany cognitions. Beta/gamma brain waves correlate with cognitive functions like attention, memory, sensory integration and motor coordination (Uhlhaas, 2009) p8, so synchrony is also related to higher cognitive functions.
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 assumes that only humans are conscious but if consciousness is any ability to observe, then anaesthetized animals, insects, and primitive brain areas have less consciousness not none at all. If our consciousness began at the cell level, even insects are conscious on their scale.
Brain synchronies also build-up in a time-frame that reflects the chronology of a conscious 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 human perception itself.
Sense input triggers local synchronies in primary sensory areas, then long-range synchronies add emotions, memory, and language to give a global conscious experience. There is agreement that neural synchrony relates to consciousness, but how it does so is unclear.