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The human brain and the human mind, which are probably one and the same, have fascinated scientists, philosophers, and most of the rest of humankind for centuries, perhaps much longer. The human brain is a truly remarkable organ that is responsible for our abilities to move and control our bodies and organs; to process visual, auditory, and other sensory information; to perceive; to react; to experience emotions; to remember; to think; to make decisions; to plan; to communicate with each other (by various means such as speech or the written word); to act in a social and responsible manner; and to understand complex phenomena, like scientific theories and the brain itself. The human brain also gives us the sensation that there is a little person inside us, or a "self," who is in control of what we are doing. What is even more remarkable is that the brain is only the size of a small coconut (or a decent sized mango fruit), which you can hold in the palm of your hand, and weighs about 1.5 kilograms (or 3 pounds). In appearance the brain looks like a gray, white, and mushroom colored overgrown walnut, with large wrinkles and numerous networks of blood vessels on its surface. It has the consistency of a jelly or porridge, and falls apart if taken out of the human skull (see figure 1.1).
To really appreciate the splendor of the human brain, the next time you go out into the street or to a shopping center, stop for a moment and observe the people walking around you, talking to each other, avoiding collisions with each other, running businesses, buying goods, driving cars, and flying airplanes. The human brain controls all of these functions and activities, and what is more, the human-created objects around you, like buildings, cars, and roads, the rich and sophisticated human culture and society, and the vast technology have all resulted from the interaction among human minds. On top of all of these things, every person is a conscious, thinking, and planning human being, living in a complex world.
Although the human brain is exceptional in its capabilities, even the brains of animals and insects are quite amazing. The brain of an ant is the size of a grain of sand, yet this enables the ant to move around (quickly) over complicated landscapes, pick up chemical ant-made scents, detect changes in temperature, air movement, and vibrations, search for food to take back to the nest, and interact socially with its own colony. The ability of insects like the bee to fly is also controlled by a tiny nervous system.
Considering the vast and astonishing capabilities of the human brain, it is not surprising that many scientists would assert that it is the most complicated system in the universe. Physicists and mathematicians cannot solve exactly the so-called three-body problem, where there are just three interacting particles. "Chaos" can evolve from such a simple system. In the human brain, there are on the order of a hundred billion to a thousand billion neurons (the basic building blocks of the brain), which are intricately connected and interact strongly with each other. These neurons communicate through small pulses of electricity and chemical currents. Each neuron simultaneously receives and interprets input from thousands to tens of thousands of other neurons, and based on this information decides whether it should itself "fire"-that is, emit an electrical pulse that travels to other neurons-or remain quiescent. Basically if a neuron receives suf.cient excitation from other neurons, counterbalanced by neurons that act on it in an inhibitory role, it will fire.
Each individual neuron performs a very simple and almost trivial function, yet, as a whole, a collection of neurons is able to perform very complex tasks and functions, as outlined above. Brain function is an emergent collective property of a large number of neurons, which seems to encompass more than the sum of its parts. This sort of emergent behavior is a general feature of other large physical, biological, and social systems. Nature abounds with many beautiful patterns, structures, and systems that emerge from rather trivial local interactions. Just look at the way that cells in your body organize themselves to form a complex living creature, how water molecules are organized into clouds, and how society is organized, with its complex legal and financial systems.
In the brain, memory is a representation of the past. Clearly memory serves an important biological function as it gives animals a survival edge in that they can use previous knowledge to better acquire food, or to prevent a certain dangerous situation from reoccurring. In some respects, the way that memory is stored in the brain is similar (but not precisely equivalent) to how .owing water cuts out a network of trenches and rivers in a landscape. When it rains, the water will preferably flow along the same paths that were previously scoured out of the land. In the brain, when a particular channel between two neurons is used often, this channel normally becomes "enlarged" so that it is even more accessible in the future, and conversely if a particular channel is not used, it diminishes in its capacity. This is how learning takes place in a nervous system. By directing the flow of electricity through the same channels as before, the brain is able to reinitiate (or recall) the same electrical patterns, or memory states, that caused this change in the first place. In the human brain there are on the order of one million billion connections between neurons that can be varied in this way. The flow of information (or electrical current) across these junctions, where two neurons (almost) touch each other (called synaptic clefts), is controlled by the transfer of chemicals (called neurotransmitters). The amount of chemical current that is transferred across a synaptic gap is variable, and this is how memory is actually stored in the brain.
(The brain also has a type of memory referred to as "habituation," a process by which the nervous system becomes familiar with a stimulus and responds less often and less vigorously when it is repeatedly and persistently stimulated. In this case, stimulus-receiving sensory neurons actually release less neurotransmitters upon repeated .ring, and have a diminished effect on response-output motor neurons. Repeated use of the same channel causes it to "shrink" instead of enlarge.)
A memory is represented by a certain stabilized pattern of firing neurons. When the brain receives a meaningful input, it processes the input into a stationary state (also called an attractor), where the activation states of the neurons collectively stabilize, or persist in the same state of excitation (or quiescence).
This is how memory is recalled, or how we recognize that an input was familiar to us. Memories are not stored in any one particular neuron, but are distributed over a wide area of the brain involving many neurons, possibly many millions. Different memories may also share certain active neurons in their representations, so in this sense memories overlap each other. This is quite unlike the way memory is stored in a computer, where it has a unique address and a separate location in which it is stored. To retrieve a memory, a programmer just uses its address. In the brain, a memory is retrieved instead by the content of the input. If sufficient information or cues are provided, the brain will be able to retrieve that memory. This is a useful attribute of brain function because one can retrieve a memory with only part of its "address," whereas a computer would not respond if it was not given the entire address precisely. This explains why we are able to recognize people we "know," even though they may look quite different from when we last saw them. Because neurons work together, or collectively, the brain is quite impervious to errors in the input and to noise in general. If a small number of neurons are in the incorrect .ring mode for a particular memory, the other neurons collectively correct those neurons. A computer, on the other hand, stops executing (what it was meant to do) if a single solitary instruction, or a single bit, is wrong.
It is fascinating to consider how a simple process of integrating and .ring neurons can account for not only memory but a diverse range of brain functions such as language, conscious awareness, creativity, the ability to understand the world mathematically, and emotions like joy, sorrow, and anger, to mention just a few. Each neuron by itself is an unintelligent binary device ("on" or "off") in the overall machine, but somehow the neurons combine to generate these incredible properties of brain function. Over the last twenty or thirty years an incredible amount of progress has been made in understanding how the brain works, both experimentally and theoretically. Scientists now have a basic understanding of how memory is stored and retrieved in the brain, and how the brain is capable of doing some of the amazing things it can do. Much of what we know comes from studying simple mathematical models (called neural networks), examining brain-affected or brain-damaged patients, and conducting animal experiments.