Even though the search for the neural correlates of consciousness is still an unresolved challenge of astonishing complexity (Crick 1994), the continuous efforts to crack the mystery are not expended in vain. Each year brings an increasing number of cognitive neuroscientific studies that reveal yet another piece of the puzzle of the neural basis of subjective experience. However, it often seems that individual findings are too diverse and sparse to form a coherent picture. In addition to the fundamental problem of the binding of conscious experiences (Singer 2001), we increasingly face the problem of how to bind the findings of consciousness-related studies. The present target paper by Prof. Wolf Singer serves such a discovery-binding function, bringing very diverse findings into a unified picture of how the neural correlates with the subjective.
In an impressively erudite manner, Singer (this collection) integrates a very broad range of anatomical and functional findings of the organisational principles of the brain, concluding that the high-level cognitive functions are supported by densely coupled, recurrent neural networks, interacting under the principles of non-linear dynamics. In the proposed framework, perception is treated as an active process, whose self-organisation is initially determined by genes, and later modified by post-natal development, learning, social interactions, and cultural influences. At the neuronal networks level, high-level integration and communication are achieved through synchronisation of oscillations in different electroencephalography (EEG) frequency bands, the most notable of which is gamma band (>30Hz) synchronisation (Engel et al. 1999). Given that an association between the widespread gamma-band synchronisation and conscious awareness is found in rather different experimental paradigms, such as visual masking (Melloni et al. 2007), binocular rivalry (Doesburg et al. 2009), and attentional blink (Gross et al. 2004), gamma synchronisation is often regarded as the main NCC (Singer this collection).
Yet the candidature of gamma synchronisation as the correlate of consciousness is challenged by some findings from research into the behavioural states of the brain. If gamma-range activity correlates with consciousness, it should diminish when consciousness ceases. Contrary to this, gamma band activity seems to increase rather than decrease in response to certain general anaesthetics, such as ketamine (Steriade et al. 1996). Furthermore, gamma synchronisation seems to be absent in some conscious brain states. For instance, it has been reported that large-scale neocortical gamma-band coherence is virtually absent during rapid eye movement (REM) sleep in cats (Castro et al. 2013), a state typically marked by the most intense dreaming in humans (Hobson et al. 2000) as well as in felids (Jouvet 1979).
The target paper briefly mentions neural mechanisms supporting overall brain states, but dismisses them as modulatory systems that are too general to be considered the NCC (Singer this collection). In the following, I will argue that the relation between the neural mechanisms of the contents and states of consciousness is not straightforward, and that the puzzle of the neural mechanisms of consciousness cannot be completed without studying the neural mechanisms of conscious states.