Your brain has a problem. An engineering problem.

Here's the problem: every second you are awake, your brain must transform raw sensory data from your eyes, ears, nose, tongue, and skin, and physiologic data about your blood chemistry, hormones, and the states of tension in your muscles, into a picture of the moment that instigates and controls the things you do to stay alive. Picture a black box into which an ocean of data flows every moment, and from which an infinitely nuanced range of outputs emerges.

What happens in the black box? If you were trying to program a computer to do all this, you'd never come close. But not only does your brain do this seemingly without effort or strain-it teaches itself how to do it. Learning to draw information from experience is not as simple as filling an empty vessel with knowledge--we not only lack knowledge at birth, we lack the vessel to hold it.

How, for example, do we transform a fleeting retinal flicker into the image of a soaring bird? The short answer is, we can't, without the help of the other elementary aspects of mental life that provide information about the internal state of the body and about the dynamic state of our muscles. Nor can we interpret hunger pangs as a yen for pistachio ice cream, based solely on the quality of the visceral sensations. Nor can we differentiate picking up a baby from lifting a sack of potatoes, using only the sequence of motions we undertake. Somehow we must learn how to identify what we perceive, pursue what we need, and act with purpose.

Although we aren't born with a vessel that can hold knowledge, some structure is there from the outset. Unlike a computer, which doesn't care where in its gigabytes of capacity it stores information (you can even move it to another disc), our brains come embedded with sense-specific virtual maps. A speck of light in one corner of your visual field tends to activate a small set of neurons in a particular spot of your brain. The first bit of information we can derive from sensation thus consists of the locations within the brain, down to specific sets of neurons that immediately become active at an encounter with a specific stimulus.

The final bit of information output consists of the specific arrays of cortical neurons in the motor strip that, at any given moment, send signals out to the muscles they control. Similar to the virtual map of the world associated with sensation, there is a virtual map of our musculature within the motor strip, as specific neurons feed specific muscles.

The fact that we assign any significance at all to our world has everything to do with our present state of arousal-not merely sexual arousal, but the degree of activity in the neural networks of the brain. The body becomes aroused by a variety of circumstances--strong appetites, pain, fear, pharmacologic agents--but this general sort of arousal conveys very little information other than the fact that the body wants something badly. The specific objective we will pursue, or object we will invest with significance, derives from the state of appetite or a specific physiologic need. The map of our appetites exists in part within the cortex (specifically, the insula), in part within the architecture of the hypothalamus, and in part within the chemical or neuronal signals of a specific need in the body system, as for food, water, warmth, companionship, stimulation, and so on.

So, once again, how do the powers of perception and action develop in the absence of an organized curriculum? How does the mind teach itself to perceive objects and perform purposeful actions? The mind must organize itself according to the world it encounters. Repeated exposure to patterns of sensation and signals of bodily need, at a time when one is also performing a series of motions, establishes a linkage in the mind (stored in the brain's neuronal networks) between the things one senses, feels, and does frequently. A perception acquires meaning as an object-a soaring bird, pistachio ice cream, a "DON'T WALK" sign-because one tends to do certain things to or with or because of it. Motions become linked into coherent programs of action like watching, walking, grabbing, or drinking when they tend to produce predictable changes in sensory patterns and arousal states, repeatedly. The qualities of these objects and actions stem directly from the form and degree of appetite and arousal that drove the action and found the object salient.

In upcoming entries, I will dissect each of these elementary functions of mental life, and discuss how they can go wrong in mental illness.

About the Author

Dean F. MacKinnon, M.D.

Dean F. MacKinnon, M.D. studies and treats affective disorders and teaches medical students at the Johns Hopkins University School of Medicine.

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