By Zoe Youngzie Lee
Where is your mind? Is it in your body? Or someplace else? What is your mind made of- the same substance that your body and brain is made out of, or something else? This is René Descartes’ mind-body problem. Descartes himself believed the mind is comprised of some other substance, and that it communicates with the body through an organ called the pineal gland. Not a lot of contemporary philosophers nor scientists support this Cartesian dualism, but the question of connecting the mind and body still exists. “The Hard Problem,” as we now call it, is the question of how physical processes in the brain give rise to subjective experience. (The Hard Problem contrasts with “the Easy Problem”- the question of how we discriminate, categorize, and react to stimuli, such as focusing attention, deliberately controlling behavior, etc.) Investigating the possible neural correlates of consciousness is one way to try to cross the “explanatory gap” between the objective, material brain and the subjective world of qualia.
There is a great deal of ongoing research concerning the neural correlates of consciousness (NCC). After all, the integration of our perceptions—vision, touch, smell—and higher cognition—memory, emotions, inner narrative—seem to comprise what we call “consciousness.” Scientists are in search for the underlying brain mechanisms and activity patterns that make us ‘conscious,’ i.e. aware of our own subjective senses and thoughts, of qualia.
Neural correlates of consciousness, as defined by Christoph Koch, the chief scientific officer of the Allen Institute for Brain Science, are “the minimal set of neuronal mechanism or events jointly sufficient for a specific conscious percept or experience” (Rees et. al, 2002).1 As we have discussed earlier on, it is still philosophically disputable whether any physical activity can give rise to such subjective phenomenon. Therefore, research on the neural correlates of consciousness are empirical investigations to find neural patterns, if any, that correlate with a conscious experience (2002). Most are done by examining which areas of the brain are active during conscious experiences, but inactive during unconscious experiences. Based on many empirical findings, some modern theories of conscious perceptions have been developed.
Local Recurrency Theory (a.k.a. First Order View, the “Hardware” view)
Local recurrency theory, introduced by Victor Lamme, a professor of cognitive neuroscience at University of Amsterdam, mostly deals with our ability to have conscious visual experience (Kouider, 2009).2 Our vision is indeed a critical component of consciousness, as the bridge between the environment and our mind. Our eyes let in and filter innumerable stimuli that we are bombarded with every moment. But among those bazillion components that make up a particular experience, not all of them are processed for introspection.
As visual stimuli first enters through our retina, it goes to the primary visual cortex (a.k.a. the ‘V1’ area). The information is passed on to the extrastriate cortices (V2, V3, V4, and V5 or MT) and the parietal and temporal cortices for further evaluation (of the object’s dimensions, color perception, face processing, etc.). Then, this theory argues, if and when these areas produce recurrent activities as they feed the information back to V1, visual consciousness is achieved. This is followed by recurrent processing in wider areas responsible for attentional, executive, and linguistic functions.
This theory implies that if the recurrent feedback is eliminated, consciousness should also be eliminated. In experiments when the V5 area is stimulated with transcranial magnetic stimulation (TMS), subjects report seeing moving blobs of light (i.e. phosphenes). If the feedback signal from V5 to the primary visual cortex, V1, is disrupted by stimulating V1 right after V5, subjects report seeing less or no blobs of light.
However, the Local Recurrency theory falls short when explaining why this mechanism is applied only to such restricted regions.
Global workspace theory (a.k.a. the “Software” view)
Stanislas Dehaene, professor of experimental cognitive psychology at Collège de France, introduced an alternate theory. The Global Workspace Theory speculates that the specific modules for perceiving color, processing faces, etc. are informationally encapsulated–i.e. each module can only access the input or output of another module but not what is going on inside that module. These separate modules are also activated largely unconsciously and mandatorily, as opposed to voluntarily. Different modules of the brain interact in a global workspace, where they share the information with special workspace neurons as modulators. Workspace neurons, hypothesized to be located mostly in the frontal and parietal areas, break into and connect individual modules with long axons. When the information enter the global workplace, they become conscious, whereas they stay unconscious when the information stays within a module (Kouider, 2009).
In an experiment by Dehaene et al., subjects showed a widely distributed activation in the brain when they consciously processed a non-masked word, while the masked words (unavailable for conscious processing but still unconsciously processed) activated only the visual area but no further (2001).3
For an object to be conscious, however, not only does it have to be explicitly represented with firing neurons and accessed by workspace neurons, but also needs to be given the attention for the conscious processing to be accessible and maintained. If the latter condition is not met, the object will remain in a ‘preconscious’ state (Kouider, 2009).
Although we do not pay attention to the periphery of our visual field, our visual perception is much more like figure (b) rather than (a). Because our vision has a very low spatial resolution and low color sensitivity, this phenomenon requires some explanation. Higher-order theory speculates that consciousness is in the prefrontal cortex, which assesses information if the perceptual signal is reliable–that is, if it is given enough attention. The reason why our visual field is perceived as clearly as it does may be because the prefrontal cortex mistakens the passive perception of our surroundings as a reliable signal due to lack of attention to them (Lau et al., 2011).4 (There are alternative explanations for this phenomenon, such as that our memory from previous visual fixations may be filling in the periphery.) This theory also relates, in part, to Kant’s notion of unity of consciousness.
*Other theories of consciousness include gamma synchrony, and information integration theory, duplex vision theory, etc.
1 Rees, G., Kreiman, G., & Koch, C. (2002). Neural correlates of consciousness in humans. Nature Reviews Neuroscience, 3(4), 261–270. http://doi.org/10.1038/nrn783
2 Kouider, S. (2009). Neurobiological theories of consciousness. Encyclopedia of consciousness, 2, 87-100.
3 Dehaene, S., Naccache, L., Cohen, L., Bihan, D. L., Mangin, J.-F., Poline, J.-B., & Rivière, D. (2001). Cerebral mechanisms of word masking and unconscious repetition priming. Nature Neuroscience, 4(7), 752–758. http://doi.org/10.1038/89551
4 Lau, H., & Rosenthal, D. (2011). Empirical support for higher-order theories of conscious awareness. Trends in Cognitive Sciences, 15(8), 365–373. http://doi.org/10.1016/j.tics.2011.05.009