Summary: Neuroimaging scans and EEG readings comparing nonconscious and conscious stimuli are compared, showing particular patterns in conscious processes. These findings are in line with predictions made by the Global Workspace Theory of consciousness, in which consciousness is closely related to interaction between specialized modules of the brain.
When a bunch of photons hit your eye, it unleashes a long chain of cause and effect that leads to an image being mapped in your brain. When does that image become conscious?
Conscious and Unconscious Perception1
The most basic method of discriminating between conscious and unconscious information is to ask the subject if they noticed it. However, people can respond to information that they don't report. What does it mean to notice something then?
Merikle et al performed experiments in the 80s which helped to resolve this question. In the Stroop task, people are asked to read words written in a different color than the word. Words written in their color (green) are easier to read than those not in their color (also red). Merikle modified the stroop task, using only two colors (red and green), and using the word to prime subjects to describe the color. As was expected, when "green" comes before a green square, subjects respond faster than with no priming.
However, when the situation is regularly reversed and the "red" prime normally comes before a green square (and vice versa) people also respond faster to similar levels. That is to say, subjects are able to notice that the prime and stimulus are incongruent, and act on that information to respond faster to the stimuli.
When the reversed prime ("red" before green) is flashed for such a short time span that people don't report seeing it, they are unable to use this information to react faster to the green stimulus, and the typical Stroop effect is observed -- being subliminally primed with a congruent color speeds up recognition, being subliminally primed with an incongruent color slows it.
There are methods of interfering with subject's reports (muteness trivially, anosognosia creepily), but for most humans it very closely corresponds to what is normally considered conscious perception.
People's brains respond to information even if the person is unaware of it, but there are measurable differences in perception without awareness and perception with it.
Methods of Manipulating Perception2
Nonconscious stimulation is split up into two categories: subliminal and preconscious. A subliminal stimulus is one in which the bottom up process information is so reduced that people cannot detect it, even if they're paying attention to it. A preconscious stimulus is one that is potentially noticeable (i.e. it's presented in a way that subjects can normally report seeing it), but not reported because of other distractions.
To present a stimulus subliminally you can:
- Mask a stimulus, by presenting it close in time to other unrelated or interfering stimuli. (i.e. a word flashed for 33 ms is noticeable by itself, but not when proceeded and followed by geometric shapes)2,3
- Use dichoptic masking, where you present two different images to each eye, and the subject reports seeing something which is neither of those4
- Use flash suppression, where you show one eye an image and flash shapes in the other eye to interfere with image perception5
To present something preconsciously you can:
- Use inattentional blindness, where you present something that participants aren't focusing on.
- Distract them! Present another stimulus and then quickly follow it with the one that you're interested in presenting preconsciously during their attentional blink.6
Neurological Differences
So what's the biggest difference between when people do and don't report seeing something?
Across various different methods of nonconscious stimulation, a few patterns emerged. When your eyes are stimulated, areas in your visual cortex (in the back of your head) undergo activity to process it regardless of whether or not you report seeing it. When people do report something, much more of the brain "lights up".
This lighting up also corresponds with recurrent processing, and ERP components in the P3b range. Recurrent processing is simply when a signal whips back and forth between specialized subregions, such as when signals from the visual cortex goes to the frontoparietal region then back to the visual cortex.1,7
The idea that conscious access is related to recurrent processing in the frontoparietal region stands up to experimental verification. Researchers are able to interfere with conscious reports of information independently of stimulus identification simply by applying transcranial magnetic stimulation to the prefrontal cortex, without changing the stimulus.8
The idea that conscious access is related to recurrent processing in the frontoparietal region stands up to experimental verification. Researchers are able to interfere with conscious reports of information independently of stimulus identification simply by applying transcranial magnetic stimulation to the prefrontal cortex, without changing the stimulus.8
So basically, consciousness seems to be related to widespread neural activity in cortical areas, as well as recurrent signalling and some particular components of EEG readings. So what?
The Global Workspace Theory1,9,10
The Global Workspace Theory of Consciousness asserts that consciousness is related to information from the various specialized subregions of the brain becoming “globally available” for attention, motor control, and cognitive reference. This explains phenomena like blindsight fairly elegantly, saying that visual information in the scotoma ceases to be conscious information because it ceases to be globally available to the system. Douhane adds that neurons with long axons in the frontoparietal cortex are probably the Global Workspace.1
Baars and Dennett are fond of the theater metaphor of consciousness. There’s a spotlight of attention on the stage, and actors (specialized cortical systems) come into and out of this to play their parts. This group of interacting subagents is actually somewhat close to orthonormal's model for dissolving qualia. Behind the scenes, directors and stagehands (decision processes, attention direction, contextual systems) arrange the scenes. Everywhere we shine the spotlight we see consciousness, because consciousness is attached to the light.
No part of the system is conscious, but there’s a show going on. And that’s what we see.
Next Obvious Question:
Okay, so why does that make us talk about consciousness? Why would we use the first person?
To be continued...
Notes
A huge thanks to John Salvatier for getting me a bunch of the papers and editing feedback and putting up with my previous attempts to write an article like this. Also thanks to mtaran, falenas108, and RS (you don't know him) for reading drafts of this article.
Images are from Zeki 2003 and Dehaene 2011, respectively. I'd be very happy if someone helped me format that to show up with the pictures.
1Merikle & Joordens, 1997
2Dehaene, S., & Changeux, J.-P. 2011
3Breitmeyer & Ogmen, 2007
4Moutoussis & Zeki 2002, Image from Zeki 2003
5Tsuchiya & Koch
6Marti et al 2010
7Lamme 2006
8Rounis et al 2010
9Baars 1997
10Metzinger
References
Baars, B. (1997). In the Theatre of Consciousness: The Workplace of the Mind. New York: Oxford University Press. Retrieved from here
Bruno G. Breitmeyer and Haluk Ogmen (2007) Visual masking. Scholarpedia, 2(7):3330
Dehaene, S., & Changeux, J.-P. (2011). Experimental and theoretical approaches to conscious processing. Neuron, 70(2), 200-27. Elsevier Inc. doi:10.1016/j.neuron.2011.03.018
Kouider, S., & Dehaene, S. (2007). Levels of processing during non-conscious perception: a critical review of visual masking. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 362(1481), 857-75. doi:10.1098/rstb.2007.2093
Lamme, V. A. F. (2006). Towards a true neural stance on consciousness. Trends in Cognitive Sciences, 10(11). doi:10.1016/j.tics.2006.09.001
Merikle, P. M., & Joordens, S. (1997). Parallels between perception without attention and perception without awareness.Consciousness and cognition, 6(2-3), 219-36. doi:10.1006/ccog.1997.0310
Lamme, V. A. F. (2006). Towards a true neural stance on consciousness. Trends in Cognitive Sciences, 10(11). doi:10.1016/j.tics.2006.09.001
Lau, H., & Rosenthal, D. (2011). Empirical support for higher-order theories of conscious awareness. Trends in cognitive sciences, 15(8), 365-373. doi:10.1016/j.tics.2011.05.009
Marti, S., Sackur, J., Sigman, M., & Dehaene, S. (2010). Mapping introspection’s blind spot: reconstruction of dual-task phenomenology using quantified introspection. Cognition, 115(2), 303-13. Elsevier B.V. doi:10.1016/j.cognition.2010.01.003
Metzinger, T. (2003). Being No One. Philosophy, 699. MIT Press.
Moutoussis, K., & Zeki, S. (2002). The relationship between cortical activation and perception investigated with invisible stimuli. Proceedings of the National Academy of Sciences, 99(14), 9527. National Acad Sciences. doi:10.1073/pnas.PNAS
Rounis, E., Maniscalco, B., Rothwell, J., Passingham, R., & Lau, H. (2010). Theta-burst transcranial magnetic stimulation to the prefrontal cortex impairs metacognitive visual awareness. Cognitive Neuroscience, 1(3), 165-175. doi:10.1080/17588921003632529
Tsuchiya, N., & Koch, C. (2005). Continuous flash suppression reduces negative afterimages. Nature neuroscience, 8(8), 1096-101. doi:10.1038/nn1500
Zeki, S. (2003). The disunity of consciousness. Trends in Cognitive Sciences, 7(5), 214-218. doi:10.1016/S1364-6613(03)00081-0