The neurochemical correlate of consciousness: exploring neurotransmitter systems underlying conscious vision

How and where does our brain integrated the information that we get into our eyes into a unifying percept and into a conscious experience? Although different neural correlates of consciousness (NCC) have been proposed, depending on the kind of neural signals recorded, the type of manipulation used, and the interpretation of behavioral results, they all seem to agree that consciousness requires integration of information by means of recurrent processing (RP). But the role of neurotransmitters in the NCC has not received much attention, while neurotransmitters are essential in neural communication. Moreover, many studies investigating the NCC manipulated either the content of consciousness or the state of consciousness. Therefore, in this thesis, I have investigated how these are related. I manipulated both neurotransmitter systems (via e.g. pharmacological interventions) - and hence the state of consciousness - as well as the content of consciousness (via e.g. masking, bistable illusions etc.) while measuring neural activity with EEG and fMRI to assess RP. Our results show that the NMDA receptor is important for integrating information of different brain areas. But, inhibition (via the neurostransmitter GABA) guides this process and is required for the selection process to determine the outcome of the competition for conscious access. Combining manipulations of conscious state and content thus give more direct insights into the neural and pharmacological underpinnings of consciousness. This may open the way for a molecular biological understanding of consciousness and towards a neurochemical correlate of consciousness.