My sincere thanks go to Valdas Noreika for having identified with succinct clarity the weaknesses in our current attempt to identify the neuronal correlates of consciousness (NCC). I would have sincerely appreciated these comments before finalising my manuscript, as they would have forced me to distinguish more clearly between the neuronal underpinnings of the conscious state and the neuronal correlates of conscious versus unconscious processing.
Noreika is absolutely right in pointing out that the search for the mechanisms permitting access to conscious processing falls short of identifying the NCC proper and, likewise, that the determination of variables required for the maintenance of a conscious state is insufficient if pursued without considering the contents of conscious processing. The mere fact that one can distinguish between the “conscious state” and the conditions required for “conscious processing”, yet also consider both as targets in the search for the NCC, suggests that the explanandum is ill-defined. Presently, both studies devoted to the distinction between conscious and unconscious processing and those investigating the brain states required for conscious processing are considered as investigations of the NCC, although they clearly target different neuronal mechanisms. Thus, studies on consciousness are fraught with the problem of a lack of a clear definition of “the” consciousness for which we wish to find a neuronal correlate. Another problem is that we are still far from fully understanding the neuronal mechanisms underlying higher cognitive functions. Behavioural studies suggest, for example, that perception involves probabilistic Bayesian-matching operations in which sensory evidence is compared with stored knowledge about the probability of occurrence and the features of the respective perceptual objects. However, it is entirely unknown where and how the huge amount of priors are stored, how the specific priors can be retrieved on the fly within the few hundreds of milliseconds sufficient for recognition, and how the matching operations are realized in neuronal networks. Thus, at the present stage it is even impossible to precisely define the signatures of neuronal activity that could be considered the result of a perceptual process or as the neuronal representation of a percept.
In the light of these uncertainties, the distinctions between conscious and unconscious processing or between states compatible with conscious and unconscious processing, respectively, appear to be exploited primarily in order to learn more about mechanisms underlying pattern recognition, decision making, and intentionality, rather than serving the search for the neuronal underpinnings of the ill-defined phenomenon that we address as “consciousness”. In contrast to NCC research, these more humble approaches have been quite successful, probably because the explananda are well-defined and can be operationalised.