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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.
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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.
Reference:
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.
-TZR-
Hi. I find your reports very interesting. In regard to having more elaborate ways of studying reports of conscious perception, my preoccuptation may be of some interest. I am interested in observing, subjectively, instances where I am aware that “my” brain has provided to my subjective experience a contrent. For example, if I stare at a 5-letter anagram without using any system to get the anagram, sometimes the answer just pops up (I am aware all my subjective experience is illusionary, that is, it exists but may not be what it seems). The experience is very different, to me, from my experience in figuring out the anagram with a system (separating the consonants and the vowels, etc.) In fact, I can marvel at “my” brain–like feeling a complicated engine at work. I’m looking for a variety of ways I can notice this engine carrying out my adaptation in my daily life. It is fascinating when you can catch its workings, or rather, have a subjective experience, which the engine also provides, of being in touch in this manner.
For Tom Lynch:
a little word of wisdom from William James. He said that introspection is like:
“trying to turn up the gas quickly enough to see how the darkness looks”
In other words, beware of introspection… it can be very tricky.
For TZR:
why do you put detection and identification in the same bag? When I saw the title of your paper I thought that you were going to compare clear detection, unclear detection and no detection… not detection and identification.
Hi Camilo,
Our study relates to how people perceive the stimulus. In general they report either being fully conscious, vaguely conscious or not conscious of the stimulus. No conscious perception is trivial (I’d say). Vague experience is often described as “seeing that something was present but without being able to identify what it was”, while clear perception is “seeing and identifying the stimulus clearly”: “Clear” is, of course, a relative concept since we are using stimulus durations up to about 180 msec.
I would say that the “vague experience” reports, including its description, relates to a person’s ability to detect a stimulus, i.e. detection without identification. Clear experience is both detection and identification.
So I’m not at all sure what you would call clear and vague detection, or how this relates to a clear and vague experience of a stimulus. Could you please explain?
best,
Thomas
There are some situations in which subjects are not sure if they detected something. For example, with very dimm targets, or very brief, or masked targets, among others. Another example, that unfortunately is not amenable for fMRI studies (I would think), is when you experience motion-induced blindness… many times you are not sure if the target disappeared or is still present. It is a very hard task, even though all you are doing is signal the presence or absence of a target. Detection itself can have gradations.
Does it make sense to you?
Camilo