Our conscious experience is familiar and intimate yet remains largely a mysterious product of the brain. Everyone has an idea of what it means to be conscious, but like thinking and intelligence, consciousness is easier to identify than to define. Definitions range from a mere manifestation of complex thought processes to more slippery notions that see it as the subjective experience of awareness or the “inner self.”

Despite the difficulty of defining consciousness, scientists generally agree that it is a process, not a thing. And consciousness is probably not a single process but several, such as are associated with seeing, talking, thinking, emotion, and so on.

Consciousness is not unitary but can take various forms. A person is not necessarily equally conscious at all stages of life. We don’t think of a newborn baby as being conscious in the same way that a healthy older child or adult is. Indeed, we might say that part of the maturation process is becoming fully conscious. The level of consciousness even changes across the span of a day as we pass through various states of drowsiness, sleep, and waking. One trait that characterizes consciousness, then, is its constant variability.

Countless people, including neuroscience researchers, have wondered why we experience consciousness, the mind’s level of responsiveness to impressions made by the senses. The simplest explanation is that consciousness provides an adaptive advantage. Either our construct of the sensory world or our selection of behavior is enhanced by being conscious. Consider visual consciousness.

According to Francis Crick and Christof Koch (1998), an animal such as a frog acts a bit like a zombie when it responds to visual input. Frogs respond to small, preylike objects by snapping and to large, looming objects by jumping. These responses are controlled by different visual systems and are best thought of as reflexive rather than conscious. And these visual systems work well for the frog. So why would humans need to add consciousness?

Crick and Koch suggest that reflexive systems are fine when their number is limited, but as their numbers grow, reflexive arrangements become inefficient, especially when two or more systems conflict. As the amount of information about an event increases, it becomes advantageous to produce a single complex representation and make it available for a sufficient time to the parts of the brain—such as the frontal lobes—that choose among many possible action plans. This sustained, complex representation is consciousness.

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Of course, to survive we must retain the ability to respond quickly and unconsciously when we need to. This reflexive ability exists alongside our ability to process information consciously. The ventral visual stream is conscious, but the dorsal stream, which acts more rapidly, is not. Athletes model the unconscious action of the online dorsal stream. To hit a baseball or tennis ball traveling at more than 100 miles per hour requires athletes to swing before they are consciously aware of actually seeing the ball. Conscious awareness of the ball comes just after hitting it.

In a series of experiments, Marc Jeannerod and his colleagues (Castiello et al., 1991) found a similar dissociation between behavior and awareness in healthy volunteers as they make grasping movements. Experiment 15-5 illustrates a representative experiment. Participants were required to grasp one of three rods as quickly as possible. A light on a rod indicated it to be the correct target rod on any given trial.

On some trials, unknown to the participants, the light jumped from one target to another. Participants were asked to report whether such a jump had occurred. As shown in the Results section of the experiment, although participants were able to make the trajectory correction, they were sometimes actually grasping the correct target before they were aware it had changed.

On some trials, the extent of dissociation between motor and vocal responses was so great that, to their surprise, participants grasped the target some 300 milliseconds before they emitted the vocal response. As with baseball players, their conscious awareness of the stimulus event occurred only after their movements took place. No thought was required to make the movement, just as frogs catch flies without having to think about it.

Unconscious movements are different from those consciously directed toward a specific object, as when we reach into a bowl of jellybeans to select a candy of a certain color. In this case, we must be aware of all the different colors surrounding the color we want. Here the conscious ventral stream is needed to discriminate among particular stimuli and respond differentially to them. Consciousness, then, allows us to select behaviors that correspond to an understanding of the nuances of sensory inputs.

Consciousness must be related in some way to neural system activity, particularly in the forebrain. One way to investigate these systems is to contrast two kinds of neurological conditions.

In the first condition, a person lacks conscious awareness of some subset of information, even though he or she processes that information unconsciously. Examples include blindsight, visual form agnosia, implicit learning in amnesia, and visual neglect (discussed in Section 15-2). Another example is obsessive-compulsive disorder, in which people persist in some checking behavior—to see that the stove is off or the door is locked—even though they have already checked many times.

All these phenomena show that stimuli can be highly processed by the brain without entering conscious awareness. This is quite different from the second neurological condition, in which people are consciously aware of stimuli that are not actually there. Examples include phantom limbs and the hallucinations of schizophrenia. In both, consciousness of specific events, such as pain in a missing limb or hearing voices, exists even though these events clearly are not “real.”

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We can draw two conclusions from these contrasting conditions. First, the representation of a visual object or event is likely to be distributed over many parts of the visual system and probably over parts of the frontal lobes as well. Damage to different areas not only produces different specific symptoms, such as agnosia or neglect, but can also produce a specific loss of visual consciousness. Disordered functioning can induce faulty consciousness, such as hallucinations. Second, because visual consciousness can be lost, it follows that parts of the neural circuit must produce this awareness.

In Section 15-1, we appointed the neuron the unit of thinking. It is unlikely, however, that the neuron can be the unit of conscious experience. Instead, consciousness presumably is a process that emerges from neural circuits, with greater degrees of consciousness associated with increasingly complex circuitry.

For this reason, humans, with their highly complex brain circuits, are often credited with a greater degree of consciousness than other animals have. Simple animals such as worms are assumed to have less consciousness (if any) than dogs, which in turn are assumed to have less consciousness than humans. Brain injury may alter self-awareness in humans, as happens in contralateral neglect, but unless a person is in a coma, he or she retains some conscious experience.

Some people argue that language fundamentally changes the nature of consciousness. Gazzaniga, for one, believes that the left hemisphere, with its language capabilities, acts as an interpreter of stimuli (see Section 15-4). He maintains that this ability is an important difference between the functions of the two hemispheres.

Yet people who are aphasic have not lost consciousness. Although language may alter the nature of our conscious experience, equating any one brain structure with consciousness seems an unlikely hypothesis. Rather, viewing consciousness as a product of all cortical areas, their connections, and their cognitive operations holds more promise.

We end our discussion of thinking on an interesting, if speculative, note. David Chalmers (1995) proposed that consciousness includes not only the information the brain receives through its sensory systems but also the information the brain has stored and presumably the information the brain can imagine. In his view, consciousness is the end product of all the brain’s cognitive processes.

An interesting implication of Chalmers’s notion is that as the brain changes with experience, so does the state of consciousness. As our sensory experiences become richer and our store of information greater, our consciousness may become more complex. From this perspective, some advantage may attend growing old.

Consciousness

Before you continue, check your understanding.

Question 1

Question 2

Question 3

Question 4

Answers appear in the Self Test section of the book.