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> Unconscious states cast light on consciousness, Evidence from persistent vegetative states (PVS)
post Jun 05, 2004, 05:59 PM
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Unconscious states cast light on consciousness
By Bernard J. Baars

There are some striking similarities between unconscious states. For example, deep sleep shows slow, high, and regular electrical waves in the brain. But a similar pattern can often be seen in other unconscious states, like some comas, general anesthesia and the momentary loss of consciousness that affects epileptics. In contrast, conscious states like waking and classical REM dreams show fast, irregular, and short waves. These differences are so obvious that many scientists have thought there must be something fundamental about them. Studying unconscious states may be very revealing of the nature of consciousness.

Steven Laureys is a clinical neuroscientist at the University of Liege in Belgium. His research team has conducted an extensive series of studies of comatose patients, in what is technically called a “persistent vegetative state” (PVS). These patients have lost consciousness for a variety of reasons. Some have had heart attacks, leading to a loss of oxygen and glucose to the brain, others suffered drug overdoses, carbon monoxide poisoning, or a radical drop in blood sugar. None of the patients had local damage in the brainstem regions that are known to cause a loss of consciousness. The question was therefore, “why had they lost consciousness?”

PET scans were conducted during PVS in 35 patients with different medical conditions. All showed global decreases in brain metabolism compared to conscious control subjects. Those patients who recovered consciousness were also studied afterwards; their global metabolic activity increased when they were awake. Most important, unconscious patients showed a significant drop in the “functional connectivity” between many brain areas, in cortex and thalamus (the “gateway” to the cortex). Functional connectivity is a way of talking about the degree to which many brain regions work together. Unconscious patients, therefore, suffered from a loss of coordination between many regions of the cortex.

This is a striking confirmation of other evidence, from completely different sources. For example, in binocular rivalry we compare conscious vs. unconscious streams of information to the two eyes. But subjects are awake. Yet similar findings have emerged. Thus Srinivasan et al (1999) found increased correlations between different brain areas in the conscious stream compared to the unconscious one. The methodology of this study was very different, yet strikingly similar results were found.

There are many ways of talking about the coordination of different brain regions. Different researchers speak of brain “coherence,” “complexity,” “functional connectivity,” and the like. Yet they have very similar fundamental hypotheses. A widespread consensus seems to be emerging that the conscious state involves not some particular piece of cortex, but rather the integration of many different regions. Consciousness is what Sherrington (1906) classically called the “integrative activity of the nervous system.”
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