Consciousness continued: what vegetative patients can tell us about it.
A recent New Yorker article discusses patients in a vegetative or minimally conscious state and what the recent research about their consciousness status can inform us about consciousness in general. The article starts with the much publicized research of Owen’s group that found that a woman, in a vegetative state, responded to verbal instructions and could imagine playing tennis. It also discusses what blindsight and neglect can tell us about (un)consciousness.
In the nineteen-eighties, researchers determined that patients who had the syndrome—now called “neglect”—could process some objects in the left field of vision. In one experiment, a patient was shown two pictures of a house. The images were identical except that, in one, flames were emerging from a window on the left side of the façade. The patient said that she couldn’t see any difference between the images, but, when she was asked which house she would want to occupy, she almost always chose the one that was not on fire. “This is more complex than blindsight, because it means that the patient was unconsciously able to interpret and understand the symbolic meaning of the pictures,” Naccache said. “It is a powerful experiment to demonstrate that unconscious perception and unconscious cognition can reach upper levels of the brain.”
The article then goes on to discuss Naccache’s theories and here I can see parallels to both Greenfield’s (sustained representation) and Koch’s views (focus on content, an ‘ignition’ and networks).
“When we are conscious, the key property is our ability to report to ourselves or to others the content of the representation—as when I say, for example, ‘I am perceiving a flower,’ or the fact that I am conscious of speaking with you now on the telephone,” Naccache told me. “You have patients who are conscious, or who are able to make reports, but you can prove that some stimuli escaped their conscious reports, as in the case of blindsight or neglect. You can study the neural fate of these representations by showing that, even if the stimuli were not reported by the subject, they were still processed in the brain.”
Naccache believes that consciousness also requires an ability to sustain a representation over time, which Owen’s patient clearly was able to do. “In assessing apparently vegetative patients who are unable to speak, and thus report, the direction of research should be to look for sustained representation,” he said. “If we can prove by neuroimaging techniques that this person is able to actively maintain a given representation during tens of seconds, it provides strong evidence of conscious processing.”
Naccache has recently incorporated a third neurological feature into his definition of consciousness: broadcasting. In a person who is conscious, he explained, information entering the brain is processed in a few areas and then distributed—or broadcast—to many others. “It’s as though there is a kind of ignition in the brain, and then information is made available to a very rich number of regions,” Naccache told me. “And that makes sense, that the information is initially represented locally and then made available to a vast network, because the person has this ability to maintain the representation within the network for a long time.
The article also covers Giacino’s work that supports more of Greenfields views with consciousness dependent on levels of arousal (which may map to the quantity of neuronal assemblies of Susan).
The woman had what Giacino calls a “drive disorder,” in which a patient is unable to speak, move, or, possibly, think unless physically stimulated—by touch. Doctors believe that such disorders are caused by damage to the limbic lobes or to other parts of the brain that trigger and sustain behavioral responses. Some patients with drive disorders respond to drugs that increase brain levels of dopamine, a neurotransmitter that is associated with arousal. “Imagine if the woman were in a nursing home,” Giacino said. “Somebody would stop by for three minutes, check her bedpan, and present simple commands like ‘Squeeze my hand,’ ‘Close your eyes,’ and ‘Open your mouth.’ She is not going to do any of those things, but she clearly had a significant amount of preserved function. It had to be harnessed externally.” At J.F.K. Johnson, patients with drive disorders receive behavioral and drug therapy. (Some patients improve, but prospects for recovery are largely determined by the extent and nature of the damage to the drive system.)
Giacino applied Deep brain stimulation to one such patient and got spectacular results.
The researchers speculated that, because of damage to the man’s frontal lobe, thalamus, and brain stem—areas involved in regulating arousal—the nerve signals in his brain were muted. As Nicholas Schiff, a neurologist at Weill Cornell Medical College who led the study of the man’s brain, put it, “It’s as if a radio were turned to such a low volume that you couldn’t hear the music distinctly.” He added, “The scans confirmed our expectation that this patient had a greater capacity for language than he demonstrated.”
The researchers described implanting electrodes in the man’s thalamus, which, by stimulating the brain tissue, had enabled him to regain considerable physical and mental function. “Deep brain stimulation can promote significant late functional recovery from severe traumatic brain injury,” they wrote. When the electrodes were turned on in the man’s thalamus, his speech improved, his movements became more fluid, and he was able to chew and swallow. When the researchers turned off the electrical stimulation, the man soon relapsed.
This is close to associating arousal with a minimum quantity to synchronous firing of neuronal assemblies; but what I most like is the ignition analogy of Naccache.
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