Abstract
Being conscious is a profound aspect of human existence, and understanding its function and its inception is considered one of the truly grand scientific challenges. However, the nature of consciousness remains enigmatic, to a large part because “being conscious” can refer to both the content (phenomenology) and the level (arousal) of consciousness, and how these different aspects are related remains unclear. To empirically assess the relation between level and content of consciousness, we manipulated these two aspects by presenting stimuli consciously or non-consciously and by using Propofol sedation, while brain activity was measured using fMRI. We observed that sedation affected both conscious and non-conscious processes but at different hierarchical levels; while conscious processing was altered in higher-order regions (the intraparietal sulcus) and spared sensory areas, the opposite effect was observed for non-conscious processing. The observation that Propofol affected non-conscious processing calls for a reconsideration of what kind of information one can gain on “consciousness” from recording neural responses to sedation without considering both (content) conscious and (content) non-conscious processing.
Highlights
The concept of consciousness is multifaceted and can refer to at least two aspects: the content and the level/state of consciousness
The most striking finding in our opinion is that reduced arousal affected non-conscious neural processes, and that this effect differed from the effects of Propofol on conscious neural processes
For non-conscious stimuli Propofol sedation changed the balance of the lateralized response from left- and right-sided stimuli, increasing a leftward bias
Summary
The concept of consciousness is multifaceted and can refer to at least two aspects: the content and the level/state of consciousness. The level of consciousness commonly refers to arousal/wakefulness, and occurs on a continuum e.g., from comatose to fully awake (Laureys, 2005) These two aspects have mostly been investigated separately and there is much debate on how to conceptualize their relation (Bachmann, 2012; Bayne et al, 2016; Fazekas and Overgaard, 2016; Hohwy, 2009; Koch et al, 2016; Laureys, 2005; Overgaard et al, 2006). We included “absent” trials during both levels of sedation, where the subjective visual appearance during the trials was identical to non-conscious trials, but no target stimulus was presented (see Methods for details) This enabled us to isolate the blood-oxygenlevel-dependent (BOLD) signal change related to conscious and non-conscious visuospatial neural processing, and to exclude the general, non-specific effects of sedation, including e.g., basal physiological processes. Neural processes would be affected by a change in arousal regardless of whether stimuli are consciously perceived or not
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