Why is there consciousness? It is not necessary for biological survival. Insects, worms, fish, and many other simple organisms survive just fine without conscious feelings. In fact, they exhibit complex behavior. Salmon, for example, migrate from fresh water to oceans and then manage to find their way back to their original freshwater location to reproduce, all without consciously experiencing their trip.

Let’s address this question by exploring the biology of consciousness. We’ll first look to the past and ask why consciousness evolved. Then we’ll turn to the present to see how activity in the contemporary human brain gives rise to conscious experience.

Ever since the insights of Charles Darwin, scientists have recognized that organisms evolve through natural selection. Characteristics that are advantageous in a given environment—in other words, that help organisms to survive and reproduce in that environment—become more frequent from one generation to the next. A Darwinian perspective, then, raises the question: What advantage is there to consciousness? How does it help organisms survive and reproduce?

THE EVOLUTIONARY ADVANTAGE CONSCIOUSNESS. One evolutionary advantage of consciousness involves decision making. Simple organisms lacking consciousness make simple decisions; they respond reflexively to specific stimuli that trigger specific responses. Organisms with consciousness, however, can do something more complex: combine different pieces of information and decide how to act based on this combination (Merker, 2005).

The ability to combine information promotes survival. Imagine an organism that has not eaten for some time and thus is low in bodily energy. Suppose it smells food in the vicinity but also hears a predator in the distance. What does it do? Ideally, the organism would combine two pieces of information, weighing one (presence of food) against another (possibility of predator). If it felt extremely hungry, the food was near, and the predator sounded far away, it would eat. If the predator sounded closer and the organism didn’t feel too hungry, it would flee. To combine the information, the organism needs to bring the various sounds, smells, and feelings of hunger together in the same mental place (Merker, 2005). Consciousness is that place. Conscious organisms, then, can make intelligent decisions that combine multiple pieces of information.

THE ORIGINS OF CONSCIOUSNESS. Consciousness is old. Mammals have it, and they have existed for at least 100 million years (Baars, 2005). Evidence suggests that consciousness is even older than that—going back perhaps 300 million years (Cabanac, Cabanac, & Parent, 2009). Key evidence comes from research on lizards.

If you handle a lizard, its heart rate increases, which suggests that the lizard is consciously aware of the handling (Cabanac & Cabanac, 2000). A lizard’s brain has a cortex, which is needed for conscious experience. Other organisms with an evolutionary history older than lizards (e.g., amphibians, such as frogs) lack a cortex and do not react physiologically when handled, which suggests that they lack consciousness (Cabanac et al., 2009). Lizards have existed for about 300 million years. This combination of facts suggests that consciousness is roughly that old.

Let’s turn from the past to the present—from the evolution of consciousness to today’s brain and how it creates conscious experience. How does activity in masses of cells in your head cause you to experience pleasure, pain, love, and hate?

FROM BRAIN MATTER TO CONSCIOUS EXPERIENCE. No question in science is more difficult to answer. In fact, it is so difficult that some argue it is unanswerable. Mysterianism is a theoretical claim that human beings lack the mental capacity to ever figure out how brain mechanisms produce conscious experience; it will always remain a mystery (McGinn, 1999).

The mystery lies in the stark difference between biological matter and conscious experience. If you look into the brain, you find cells and neurotransmitters. They function according to normal laws of physics. If you reflect on your conscious experience, you observe something completely different: thoughts and feelings that seem disconnected from ordinary physical matter. Consider these questions: How much does your current conscious experience weigh? About how big is it? These questions don’t even make sense. Conscious experience exists, yet it lacks physical properties such as mass and size. This disconnect between physical brain matter and conscious experiences makes it difficult even to imagine how the physical action of brain cells could create consciousness.

BRAIN SYSTEMS REQUIRED FOR CONSCIOUSNESS. Scientists cannot, at present, fully answer the difficult question of exactly how brain functioning produces conscious experience. Nonetheless, they have made progress on a slightly easier question: What subsystems of the brain are necessary for an organism to have conscious experience? Research identifies two critical aspects of brain functioning (Alkire, Hudetz, & Tononi, 2008; Edelman & Tononi, 2000).

A famous case of an individual who lost consciousness supports the conclusion that intact thalamo-cortical circuits are required for conscious experience. In 1975 Karen Ann Quinlan, a young woman in Pennsylvania, experienced respiratory-system failure after consuming a combination of alcohol and drugs. Subsequently, she lost consciousness and never regained it, living in a vegetative state for a decade. (Her case became famous because it highlighted ethical questions involving the rights and care of patients in that state.) After her death, an autopsy revealed that the respiratory failure had caused damage to her thalamus; other parts of her brain were intact (Kinney et al., 1994). The loss of cortical connections through the thalamus thus appeared to be the cause of her permanent loss of consciousness. This autopsy, combined with contemporary research on brain mechanisms in consciousness, has an implication for the patient you read about in this chapter’s opening story. It suggests that, despite the considerable brain damage he experienced, thalamo-cortical connections in his brain were relatively intact.

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THALAMUS, CORTEX, AND CONSCIOUSNESS. How do scientists know that thalamo-cortical circuits are so important? Research with anesthetic drugs provides evidence. General anesthetics, of the sort used in surgical procedures, cause people to lose consciousness. By administering them while taking images of people’s brains, researchers can identify brain changes that accompany the loss of consciousness.

Findings indicate that anesthetics induce unconsciousness by reducing activity in the thalamus. Consciousness ceases when thalamo-cortical circuits no longer integrate information that is processed at multiple sites in the cortex (Alkire, Haier, & Fallon, 2000). The fact that unconsciousness and reduction of activity in thalamo-cortical circuits occur simultaneously implies that these circuits are key to consciousness (Alkire et al., 2008).

Brain research thus shows that the integration of activity in large numbers of separate brain regions is needed for conscious experience. Interestingly, this biological fact is consistent with psychological experience. Psychologically, our experiences generally are coherent and unified. If a large green garbage truck whizzes past you, your sensory receptors receive different types of information: a flash of green light, a whiff of garbage, the sound of a truck engine, a breeze on your skin. Consciously, however, you experience one thing: a truck whizzing by. If someone asks, “What just happened?” you probably will not say, “There was a bright green light. I smelled some garbage. There was the sound of an internal combustion engine. And I felt a breeze.” Instead, you’ll report one coherent conscious experience: “A garbage truck.”

At a biological level, then, thalamo-cortical circuits integrate activity in multiple brain regions (Edelman, 2003). At a psychological level of analysis, the conscious mind combines separate pieces of sensory and perceptual information. This enables people to have unified, coherent conscious experiences (Figure 9.4).

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