Background: Whatever mental states one is experiencing, the brain is constantly active and demonstrates surprisingly regular spatial patterns, which are characterised by the presence of intrinsic functional connectivity networks (ICNs). Among them, the default mode network (DMN) has generated substantial interest and discussion for its distinct features and function. Accumulating empirical evidence from healthy participants and clinical cohorts suggests that the spatiotemporal integrity of the DMN is important for normal brain function and consciousness. Although the DMN has been intensively studied during resting states, evidence about its involvement in tasks is relatively sparse and largely inconsistent. In addition, a mechanistic understanding of how the DMN supports information processing towards conscious representations is still lacking. By studying the DMN’s function during normal conscious processing, as well as its dysfunction during altered states of consciousness, I hope to achieve a better understanding of the neural correlates of consciousness.

Experiments and results: A series of experiments have been carried out and detailed in four thematically connected chapters. My first experimental chapter (Chapter 2) addresses the controversies in the literature on the spatial extent of the DMN, and its involvement in executive function, by employing a Neurosynth meta-analysis. In Chapter 3, I investigated how the DMN interacts with other ICNs during two cognitive control tasks by focusing on a core region of the DMN the precuneus (PCu), which was found to be functionally divided into distinct subregions during cognitive control task execution. The results support the hypothesis that the PCu mediates the interaction between the internally-oriented DMN and externally-oriented networks. In Chapter 4, utilising a bistable visual task, I further elucidated the functional relevance of the PCu in visual awareness. I found that the DMN’s intrinsic dynamic states may influence information encoding in the primary visual cortex through the PCu. In Chapter 5, I utilised the PCu subregions identified in Chapter 3 and investigated how their relationships with the DMN and the fronto-parietal control network (FPCN) are altered during drug-induced altered states of consciousness. The results suggest that the brain’s functional architecture, which is characterised by the seemingly contraposed relationship between the DMN and FPCN, might be mediated by a connectivity gradient of the PCu with the rest of the brain. Importantly, I found that during altered states of consciousness the functional connectivity gradient of the PCu attenuates.

Conclusions and implications: The DMN and FPCN have been claimed to support internally and externally oriented information processing respectively. The main findings from my investigations can be supported by the theoretic framework of predictive coding, which proposes that consciousness is a process of external states being constantly interpreted through the integration with an internal model of the world, while the internal model of the world being constantly updated by the new experiences. Therefore, consciousness might be supported by the PCu (and by extension the DMN), whose functional subdivisions were found to mediate integration and differentiation between internal and external information processing. To fully understand the mechanism, we need to explore the fundamental principles of brain organisation by which large-scale ICNs can emerge from the underlying whole-brain structure and local neural dynamics. This leads to an avenue of interdisciplinary studies in my future research.