What Brain Plasticity Reveals About the Nature of Consciousness: Commentary

What is consciousness? Consciousness continues to be an illdefined concept, so I shall start by discussing how the term is used in this special issue. As discussed in Overgaard and Overgaard (2010), there is an important distinction between level of consciousness and content of consciousness. Level of consciousness refers to a dimension that varies from coma at one extreme, through sleep and, at the other extreme, alert wakefulness. Philosophers call this creature consciousness since it applies to the whole creature (Rosenthal, 2009). Level of consciousness is of particular relevance to the studies of patients in persistent vegetative state discussed by Laureys and colleagues (Demertzi et al., 2011). However, most of the contributions to this special issue are concerned with the content of consciousness. Alert wakefulness is characterized by consciousness of specific mental states. The states that we are aware of are the contents of consciousness. Philosophers call this state consciousness. This is somewhat confusing, given that, when people talk about altered states of consciousness, they are usually referring to different levels of consciousness rather than different contents of consciousness. So I will continue to use the terms level and contents of consciousness. In studies of the neural correlates of consciousness there is great interest in contrasting the neural activity associated with stimuli that influence the contents of consciousness with the neural activity associated with the same stimuli when they affect behavior in the absence of any change in the content of consciousness (Frith et al., 1999). When a stimulus elicits neural activity and affects behavior it does not necessarily follow that we are aware of that stimulus. A certain minimal level of consciousness is necessary for there to be any contents of consciousness, but the level of consciousness does not determine what the contents of consciousness will be. As is demonstrated in this special issue, brain plasticity has an important role in determining the contents of consciousness. There is also a reflective aspect of consciousness which is modeled by Cleeremans (2011) and which Allen and Williams (2011) suggest may be uniquely human. Are conscious mental states thoughts about thoughts? Is consciousness by its very nature reflective? There is clearly a relationship between this aspect of consciousness and metacognition. I shall return to consideration of this relationship at the end of this introduction.