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The Many Brain Areas Needed for Consciousness
How do we go about exploring the correspondence between mind and brain, between the mental world and the physical one? One way is to examine the brain status of people who have “diminished” states of consciousness—people who are anesthetized or even in comas. In each case, we can ask: What is different in the brains of these individuals that might explain why they’re not conscious?
Research on this topic conveys a simple message: Many different brain areas seem crucial for consciousness, and so we can’t expect to locate some group of neurons or some place in the brain that’s the “consciousness center” and functions as if it’s a light bulb that lights up when we’re conscious and then changes its brightness when our mental state changes.
In fact, research in this arena suggests a distinction between two broad categories of brain sites, corresponding to two aspects of consciousness (Figure 6.6). First, there is
the level of alertness or sensitivity, independent of what the person is alert or sensitive to. We can think of this as the difference between being dimly aware of a stimulus (oran idea, or a memory) and being highly alert and totally focused on that stimulus. This aspect of consciousness is disrupted when someone suffers damage to certain sites in either the thalamus or the reticular activating system in the brain stem—a system that controls the overall arousal level of the forebrain and that also helps control the cycling of sleep and wakefulness (e.g., Koch, 2008).
Second, our consciousness also varies in its content. Sometimes we’re thinking about our immediate environment; sometimes we’re thinking about past events. Sometimes we’re focused on an immediate task, and sometimes we’re dreaming about the future. These various contents for consciousness require different brain sites, and so cortical structures in the visual system are especially active when we’re consciously aware of sights in front of our eyes; cortical structures in the forebrain are essential when we’re thinking about some stimulus that is no longer present in our environment, and so on (Figure 6.7).
In fact, this broad distinction between the degree of awareness or sensitivity and the content of consciousness may be useful for us in thinking about variations in consciousness, as suggested by Figure 6.8 (after Laureys, 2005; also Koch, 2008). In dreaming, for example, we are conscious of a richly detailed scene, with its various sights and sounds and events, and so there’s a well-defined content—but our sensitivity to the environment is low. By contrast, in the peculiar state associated with sleepwalking, we’re sensitive to certain aspects of the world—so we can, for example, navigate through a complex environment—but we seem to have no particular thoughts in mind, so the content of our consciousness is not well defined.
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