Over at the Mind blog, Deric has just posted a very fascinating exchange of opinions amongst Christof Koch and Susan Greenfield regarding the neural correlates of consciousness. the exchange seems to be from Scientific American, and in it both Christof and Susan put forward their views on the NCC and then have minor quibbles over particular conscious experiences / phenomenon. While Koch believes that consciousness of a percept arises from activation of a unique set of neurons, that form assemblies and networks and the neurons themselves differ from normal neurons (he implicates a strong role for layer 5 pyramidal neurons in Frontal cortex, having synapses with occipital visual areas, as being crucial for a conscious percept) , Susan insists that consciousness arises because of synchronous firing of neuronal assemblies - and the degree of consciousness depends on the number of neurons involved in that synchronous assembly (that lats up to millisecond intervals) . Thus while Koch focuses more on content of consciousness (and despite Susan's criticism that is an important area of investigation) and how it arises from a qualitative phenomenon (different types of neurons - pyramidal- involved), Susan focuses more on degrees of consciousness and takes consciousness to be a qualitative phenomenon. I, like Deric , find merit in both arguments.
Physiologically, the likely substrate for NCC is a coalition of pyramidal neurons--a type of neuron that communicates over long ranges--within the cerebral cortex. Perhaps only a million such neurons--out of the 50 billion to 100 billion in our heads--are needed to form one of these coalitions. When, say, Susan enters a crowded room and I see her face, a coalition of neurons suddenly chatters in concert for a fraction of a second or longer. The coalition reaches from the back of the cortex, where representations of visual stimuli are first processed, into the front of the cortex, which carries out executive functions such as providing perspective and enabling planning. Such a coalition would be reinforced if I paid attention to the stimulus of her image on my retina, which would strengthen the amplitude or the synchrony of the activity among the select neurons. The coalition sustains itself and suppresses competing coalitions by feeding excitatory signals back and forth among the neurons in the back and front of the cortex. If, suddenly, someone calls my name, a different coalition of neurons in the auditory cortex arises. This coalition establishes two-way communication with the front of the brain and focuses my consciousness on the voice, suppressing the earlier coalition representing Susan's face, which fades from my awareness.
This notion about networks of neurons has received a boost from recent results by researchers at the Mount Sinai School of Medicine, Columbia University and the New York State Psychiatric Institute, working under Stuart C. Sealfon of Mount Sinai and Jay A. Gingrich of Columbia. Sealfon's and Gingrich's teams have demonstrated in genetically modified mice that hallucinogens--such as LSD, psilocybin (an ingredient of mushrooms) and mescaline--act on a type of molecule, called a serotonin receptor, found on the pyramidal cells that cluster in layer 5. The hypothesis that the mind-bending effects of hallucinogenic compounds come from activation of one receptor type on a specific set of neurons--rather than from "messing up" the brain's circuits in some holistic manner--can be further tested with molecular tools that can toggle layer 5 pyramidal cells on and off until the exact set of neurons being affected is identified.
My own starting assumption is that there is no intrinsic, magical quality in any particular brain region or set of neurons that accounts for consciousness. We need to identify a special process within the brain. And to be a truly robust correlate of consciousness, this neuronal process must account for a variety of everyday phenomena, including the efficacy of an alarm clock, the action of anesthetics, the distinction of dreams from wakefulness, the existence of self-consciousness, the possible difference between human and animal consciousness, and the possible existence of fetal consciousness. A more plausible view of consciousness is that it is not generated by a qualitatively distinct property of the brain but by a quantitative increase in the holistic functioning of the brain. Consciousness grows as brains grow.
The central problem is that models developed by Llinas and others conceive of consciousness as an all-or-nothing condition. They fail to describe how the physical brain can accommodate the ebb and flow of a continuously variable conscious state. I favor an alternative. For more than a decade, scientists have known that the activity of tens of millions of neurons can synchronize for a few hundred milliseconds, then disband in less than a second. These "assemblies" of coordinating cells can vary continuously in just the right space and timescales for the here-and-now experience of consciousness. Wide-ranging networks of neurons assemble, disassemble and reassemble in coalitions that are unique to each moment. My model is that consciousness varies in degree from one moment to the next and that the number of neurons active within an assembly correlates with the degree of consciousness present at any given time.
This neuronal correlate of consciousness--the transient assembly--satisfies all the items on the shopping list of phenomena above. The efficacy of an alarm clock is explained as a very vigorous sensory input that triggers a large, synchronous assembly. Dreams and wakefulness differ because dreams result from a small assembly driven by weak internal stimuli, whereas wakefulness results from a larger assembly driven by stronger external stimuli. Anesthetics restrict the size of assemblies, thus inducing unconsciousness. Self-consciousness can arise only in a brain large and interconnected enough to devise extensive neuronal networks. The degree of consciousness in an animal or a human fetus depends on the sizes of their assemblies, too.
Take a look at the original debate as there is an even more more stimulating point-counterpoint section too. Sphere: Related Content