On the Borders of Consciousness

Abstract

The neural mechanism by which the brain creates conscious awareness remains unknown. The present thesis employs electroencephalography to investigate these neural substrates of conscious awareness through an investigation of the distinctions between neural activity associated with conscious awareness and neural activity which is not accompanied by conscious awareness. The temporal dynamics and the complexity of content during the unconscious processing of information are assessed in the first two chapters using the masked presentation of word stimuli. Results reveal that abstract information is extracted from unconsciously presented stimuli more rapidly than is usually associated with the neural indices of the conscious representation of information. It is also shown that the delay between the processing of different elements of word stimuli is such that some form of stable reentrancy is likely established during unconscious neural activity. The third experiment investigate the oscillatory event related beta desynchrony (ERD) preceding movement with and without awareness of the impending movement. The results show that beta ERD, unlike the evoked EEG response, reflects the awareness of the intention to move. It is argued that beta ERD allows the establishment of reverberating neural assemblies that are thought to be necessary for conscious representation. The final experiment uses a binocular rivalry paradigm to investigate the role of synchronous oscillations in determining the contents of consciousness. It is argued on the basis of this chapter that synchrony reflects the reorganisation and coordination of neural activity but is not, in itself, a mechanism for the binding of neural assemblies. The results are discussed in relationship to the distinction between conscious and unconscious cognition existing across a spectrum rather than representing qualitatively different neural states.