Consciousness is part of the natural world. It depends, we believe, only on mathematics and logic and on the imperfectly known laws of physics, chemistry, and biology; it means there's no reason why consciousness can't be reproduced in a machine—in theory, anyway.
In humans and animals, we know that the specific content of any conscious experience—the deep blue of an alpine sky, say, or the fragrance of jasmine redolent in the night air—is furnished by parts of the cerebral cortex, the outer layer of gray matter associated with thought, action, and other higher brain functions. If a sector of the cortex is destroyed by stroke or some other calamity, the person will no longer be conscious of whatever aspect of the world that part of the brain represents. For instance, a person whose visual cortex is partially damaged may be unable to recognize faces, even though he can still see eyes, mouths, ears, and other discrete facial features. Consciousness can be lost entirely if injuries permanently damage most of the cerebral cortex, as seen in patients like Terri Schiavo, who suffered from persistent vegetative state. Lesions of the cortical white matter, containing the fibers through which parts of the brain communicate, also cause unconsciousness. And small lesions deep within the brain along the midline of the thalamus and the midbrain can inactivate the cerebral cortex and indirectly lead to a coma—and a lack of consciousness.
To be conscious also requires the cortex and thalamus—the corticothalamic system—to be constantly suffused in a bath of substances known as neuromodulators, which aid or inhibit the transmission of nerve impulses. Finally, whatever the mechanisms necessary for consciousness, we know they must exist in both cortical hemispheres independently.
Much of what goes on in the brain has nothing to do with being conscious, however. Widespread damage to the cerebellum, the small structure at the base of the brain, has no effect on consciousness, despite the fact that more neurons reside there than in any other part of the brain. Neural activity obviously plays some essential role in consciousness but in itself is not enough to sustain a conscious state. We know that at the beginning of a deep sleep, consciousness fades, even though the neurons in the corticothalamic system continue to fire at a level of activity similar to that of quiet wakefulness.
Data from clinical studies and from basic research laboratories, made possible by the use of sophisticated instruments that detect and record neuronal activity, have given us a complex if still rudimentary understanding of the myriad processes that give rise to consciousness. We are still a very long way from being able to use this knowledge to build a conscious machine. Yet we can already take the first step in that long journey: we can list some aspects of consciousness that are not strictly necessary for building such an artifact.
Remarkably, consciousness does not seem to require many of the things we associate most deeply with being human: emotions, memory, self-reflection, language, sensing the world, and acting in it. Let's start with sensory input and motor output: being conscious requires neither. We humans are generally aware of what goes on around us and occasionally of what goes on within our own bodies. It's only natural to infer that consciousness is linked to our interaction with the world and with ourselves.
Yet when we dream, for instance, we are virtually disconnected from the environment—we acknowledge almost nothing of what happens around us, and our muscles are largely paralyzed. Nevertheless, we are conscious, sometimes vividly and grippingly so. This mental activity is reflected in electrical recordings of the dreaming brain showing that the corticothalamic system, intimately involved with sensory perception, continues to function more or less as it does in wakefulness.
Neurological evidence points to the same conclusion. People who have lost their eyesight can both imagine and dream in images, provided they had sight earlier in their lives. Patients with locked-in syndrome, which renders them almost completely paralyzed, are just as conscious as healthy subjects. Following a debilitating stroke, the French editor Jean-Dominique Bauby dictated his memoir, The Diving Bell and the Butterfly, by blinking his left eye. Stephen Hawking is a world-renowned physicist, best-selling author, and occasional guest star on “The Simpsons,” despite being immobilized from a degenerative neurological disorder.
So although being conscious depends on brain activity, it does not require any interaction with the environment. Whether the development of consciousness requires such interactions in early childhood, though, is a different matter.
How about emotions? Does a conscious being need to feel and display them? No: being conscious does not require emotion. People who've suffered damage to the frontal area of the brain, for instance, may exhibit a flat, emotionless affect; they are as dispassionate about their own predicament as they are about the problems of people around them. But even though their behavior is impaired and their judgment may be unsound, they still experience the sights and sounds of the world much the way normal people do.
Primal emotions like anger, fear, surprise, and joy are useful and perhaps even essential for the survival of a conscious organism. Likewise, a conscious machine might rely on emotions to make choices and deal with the complexities of the world. But it could be just a cold, calculating engine—and yet still be conscious.
Psychologists argue that consciousness requires selective attention—that is, the ability to focus on a given object, thought, or activity. Some have even argued that consciousness is selective attention. After all, when you pay attention to something, you become conscious of that thing and its properties; when your attention shifts, the object fades from consciousness.
Nevertheless, recent evidence favors the idea that a person can consciously perceive an event or object without paying attention to it. When you're focused on a riveting movie, your surroundings aren't reduced to a tunnel. You may not hear the phone ringing or your spouse calling your name, but you remain aware of certain aspects of the world around you. And here's a surprise: the converse is also true. People can attend to events or objects—that is, their brains can preferentially process them—without consciously perceiving them. This fact suggests that being conscious does not require attention.
One experiment that supported this conclusion found that, as strange as it sounds, people could pay attention to an object that they never “saw.” Test subjects were shown static images of male and female nudes in one eye and rapidly flashing colored squares in the other eye. The flashing color rendered the nudes invisible—the subjects couldn't even say where the nudes were in the image. Yet the psychologists showed that subjects nevertheless registered the unseen image if it was of the opposite sex.
As it turns out, though, being conscious does not require self-reflection. When we become absorbed in some intense perceptual task—such as playing a fast-paced video game, swerving on a motorcycle through moving traffic, or running along a mountain trail—we are vividly conscious of the external world, without any need for reflection or introspection.
Neuroimaging studies suggest that we can be vividly conscious even when the front of the cerebral cortex, involved in judgment and self-representation, is relatively inactive. Patients with widespread injury to the front of the brain demonstrate serious deficits in their cognitive, executive, emotional, and planning abilities. But they appear to have nearly intact perceptual abilities.
Finally, being conscious does not require language. We humans affirm our consciousness through speech, describing and discussing our experiences with one another. So it's natural to think that speech and consciousness are inextricably linked. They're not. There are many patients who lose the ability to understand or use words and yet remain conscious. And infants, monkeys, dogs, and mice cannot speak, but they are conscious and can report their experiences in other ways.
So what about a machine? We're going to assume that a machine does not require anything to be conscious that a naturally evolved organism—you or me, for example—doesn't require. If that's the case, then, to be conscious a machine does not need to engage with its environment, nor does it need long-term memory or working memory; it does not require attention, self-reflection, language, or emotion. Those things may help the machine survive in the real world. But to simply have subjective experience—being pleased at the sight of wispy white clouds scurrying across a perfectly blue sky—those traits are probably not necessary.
To be conscious, you need to be a single integrated entity with a large repertoire of states. Let's take this one step further: your level of consciousness has to do with how much integrated information you can generate. That's why you have a higher level of consciousness than a tree frog or a supercomputer. To make a machine conscious, two complementary strategies come to mind: either copying the mammalian brain or evolving a machine. Research groups worldwide are already pursuing both strategies, though not necessarily with the explicit goal of creating machine consciousness.
Can Machines become Conscious?
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