Neural Mechanisms for Conscious Experiences

All conscious experiences are popularly attributed to the workings of the "mind," a word that conjures up the image of a nonneural "me," a phantom interposed between afferent and efferent impulses, with the implication that the mind is something more than neural activity. Most neuroscientists agree, however, that the mind represents a summation of neural activity in the brain at any given moment and does not require anything more than the brain.

Although a good definition of "mind" is not possible at this time, the term includes such actions as thinking, perceiving, making decisions, feeling, and imagining. Thus, we can conclude that "mind" refers to a "process" that gives rise to and includes conscious experiences. The truth of the matter is, however, that physiologists have only a beginning understanding of the brain mechanisms that give rise to mind or to conscious experiences.

Let us see in the rest of this section how two prominent neuroscientists, Francis Crick and Christof Koch, have speculated about this problem. They begin with the assumption that conscious experience requires neural processes—either graded potentials or action potentials—somewhere in the brain. At any moment, certain of these processes are correlated with conscious awareness and others are not. A key question here is: What is different about the processes that we are aware of?

A further assumption is that the neural activity that corresponds to a conscious experience resides not in a single anatomical cluster of "consciousness neurons" but rather in a set of neurons that are temporarily functioning together in a specific way. Since we can become aware of many different things, we further assume that this particular set of neurons can vary, shifting, for example, among parts of the brain that deal with visual or auditory stimuli, memories or new ideas, emotions or language.

Consider perception of a visual object. As was discussed in Chapter 9, different aspects of something we see are processed by different areas of the visual cortex—the object's color by one part, its motion by another, its location in the visual field by another, and its shape by still another—but we see one object. Not only do we perceive it; we may also know its name and function. Moreover, an object that can be seen can sometimes be heard or smelled, which requires participation of brain areas other than the visual cortex.

Neurons from the various parts of the brain that synchronously process different aspects of the information related to the object that we see are said to form a "temporary set" of neurons. It is suggested that the synchronous activity of the neurons in the temporary set leads to conscious awareness of the object we are seeing. (Instead of "leads to," perhaps we should say "corresponds to" or "is." We don't even know the appropriate term here.)

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Consciousness and Behavior CHAPTER THIRTEEN

Consciousness and Behavior CHAPTER THIRTEEN

As we become aware of still other events—perhaps a memory related to this object—the set of neurons involved in the synchronous activity shifts, and a different temporary set forms. In other words, it is suggested that specific relevant neurons in many areas of the brain function together to form the unified activity that corresponds to awareness.

It is possible to record neuronal activity pulsing at a frequency of 40 to 70 Hz—faster even than the beta waves of the EEG. These oscillations may be the electrical record of a synchronous neural set, and they are the focus of a great deal of interest because they may be the clue to the "binding problem," namely, how the brain integrates information that occurs simultaneously in many parts of the brain into a single conscious experience. Such synchronous activity of ensembles of neurons is also implicated in the preconscious decision of whether or not stimuli are even perceived in the first place and in the planning of motor movements before their execution.

What parts of the brain might be involved in such a neuronal set? Clearly the cerebral cortex is involved, although not all parts at once, since removal of specific areas of the cortex abolish awareness of only specific types of consciousness. For example, damage to parts of the parietal lobe causes the injured person to neglect parts of the body as though they do not exist, but other parts are not neglected. Subcortical areas such as the thalamus and basal ganglia may also be directly involved in conscious experience, but it seems that the hippocampus and cerebellum are not.

A critical question is: What binds together the functions of first one set of neurons and then another? By experience, we know that the binding must occur rapidly and that it can be very short-lived. Moreover, although its capacity at any one time may be limited, there is a huge range of combinations possible. In other words, we must postulate a mechanism that can focus our attention on a limited number of things at any one time but, over time, can bring an enormous number of things into conscious awareness.

Saying that we can bind together the activity of one set of neurons and shift the binding to a new set at a later time may be the same as saying we can focus attention on—that is, bring into conscious awareness—one object or event and then shift our focus of attention to another object or event at a later time. Thus, the mechanisms of conscious awareness and attention must be intimately related.

We include Figure 13-8 to indicate in one small way the complexity met when trying to sort out the brain's mechanisms of processing information. As confusing as it seems, the lines on this lateral view of the brain indicate only 15 percent of the possible connections in the cerebral cortex, and this is the brain of a monkey, not a human.

Anterior limbic areas

Visual areas

Anterior limbic areas

Visual areas

Parietotemporal areas

FIGURE 13-8

Possible connections for information processing in the cerebral cortex of a monkey brain. Actually, only about 15 percent of the total possible connections are represented. The small notations in the figure are not important for our purposes.

Reprinted with permission from Malcolm P. Young, "The organization of neural systems in the primate cerebral cortex," Proc. R. Soc. Biol. Sci., 252:13-18, 1993. Fig. 3.

Auditory areas

If Wti'

"¡Mf Sensorimotor cortex

Parietotemporal areas

FIGURE 13-8

Possible connections for information processing in the cerebral cortex of a monkey brain. Actually, only about 15 percent of the total possible connections are represented. The small notations in the figure are not important for our purposes.

Reprinted with permission from Malcolm P. Young, "The organization of neural systems in the primate cerebral cortex," Proc. R. Soc. Biol. Sci., 252:13-18, 1993. Fig. 3.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment