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What happens in the brain when we become conscious of something? What processes and structures are responsible for becoming aware? Is consciousness an either-or process or can we have in-between forms of perception?
We have recently attempted to put those questions into empirical terms. In a study that is now in press in NeuroImage, we asked subjects to report how clearly they saw visual stimuli. The stimuli were simple geometrical shapes (circle, square, triangle) that were presented at different durations, from approximately 16 msec to about 150 msec.
From a previous behavioural study, we have concluded that conscious perception is not an either-or, and that there are instances where subjects report having “vague” percepts. That is, some stimuli are experienced as “something being presented” withouth being able to determine what was presented. Detection without identification.
Question is, how does the brain work under these conditions? In this study we have showed that vague perception shares much of the same network of fronto-parieto-temporal and cortico-thalamo-cortical network as seen during conscious perception. However, we also identify some unique activity in the brain during vague perception, especially in the prefrontal cortex.
I’ll leave this news hanging in the air/ear (…) just for now. When the article is published, I will link to the PDF. I’ll leave it to Mark Christensen, the PI of this project, to put it in his own words. Take also time to look at the image below.
NOTE: Why is this research important to neuroethical consideations? First of all, it demonstrates that specific types of experience are closely related to what happens in the brain. It shows that questions about the mind can be asked — and answered — by neuroscience. And it strengthens our view that conscious vs. unconscious perception is not a clear dichomotic distinction. Rather, we need to make use of more elaborate ways to study reports of conscious perception. Finally, this finding strengthens models suggesting the necessity of a widespread brain network to support consciousness (see also this article, PDF). Knowing what it means to be conscious tells a lot about what it means to be human.
Subjective reports of graded perception
by Mark S. Christensen
In an fMRI, which is to be published in NeuroImage, we have shown that subjective reports of perceptual clarity correlates with graded neural activation within a parietal, premotor, basal ganglia and frontal operculum network.
In a simple visual masking experiment, we asked subjects to report their subjective experience of perceptual clarity of masked visual stimuli on a graded scale ranging from no perceptual experience, over a vague/glimpse like experience, to a clear perceptual experience. This was done during an event-related fMRI experiment.
Within fronto-parietal-thalamic areas where the activity was increased for clear perceptual experiences compared to no perceptual experience, we found a network, where the activation varied in a gradual way, following the subjective report. Furthermore, we found areas in insula and frontal cortex outside the fronto-parietal-thalamic network, where the intermediate, fringe-state showed unique activation.
The results provide the first evidence of sensory fringe states, and that the subjective experience of conscious perception has a counterpart in graded neural activation. Furthermore, they strengthen the scientific value of subjective reports. Finally, we show that within a network for conscious perception that includes the parietal and premotor cortices, the subjective experience resides.
Christensen MS, Ramsøy TZ, Lund TE, Madsen KH, Rowe JB (in press). An fMRI study of the neural correlates of graded visual perception. Neuroimage
The neural correlates of clearly perceived visual stimuli have been reported previously in contrast to unperceived stimuli, but it is uncertain whether intermediate or graded perceptual experiences
correlate with different patterns of neural activity. In this study, the subjective appearance of briefly presented visual stimuli was rated individually by subjects with respect to perceptual clarity: clear, vague
or no experience of a stimulus. Reports of clear experiences correlated with activation in a widespread network of brain areas, including parietal cortex, prefrontal cortex, premotor cortex, supplementary
motor areas, insula and thalamus. The reports of graded perceptual clarity were reflected in graded neural activity in a network comprising the precentral gyrus, intraparietal sulcus, basal ganglia and the insula. In addition, the reports of vague experiences demonstrated unique patterns of activation. Different degrees of perceptual clarity were reflected both in the degree to which activation was found within parts of the network serving a clear conscious percept, and additional unique activation patterns for different degrees of perceptual clarity. Our findings support theories proposing the involvement of a widespread network of brain areas during conscious perception.
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