Root of Thought: Reflections on Neuroscience

Root of Thought: Reflections on Neuroscience

by Kong MD Henry Kong MD, Henry Kong MD

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Overview

Consciousness is a clever adaptation for life as a social ape. But like all adaptations, it has its limits. Lion claws and butterfly wings are made of tissue and proteins that require proper diet and maintenance to keep their integrity. Likewise, a fully functioning conscious system is a phenomenon that requires a well-nourished neural substrate to keep it going. It is important to point out that a healthy brain is necessary but not sufficient for consciousness, because not all of the brain is conscious. There are great swaths of brain that are crucially important for maintaining the vegetative functions of homeostasis such as breathing, heartbeat, temperature,
and sleep. With a hundred billion neurons and a hundred trillion synapses, the brain is the most complex adaptation known. Yet we know shockingly little about how it enables the human mind to become conscious, make decisions, believe in God, and behave morally. However, recent discoveries in cognitive neuroscience, behavioral genetics, and evolutionary psychology are beginning to revolutionize old conceptions of nature and nurture, reason and passion, and automatic versus willfully chosen actions. Root of Thought will take readers on a remarkable journey through the landscape of the mind in search of its biological basis.

Product Details

ISBN-13: 9781450222679
Publisher: iUniverse, Incorporated
Publication date: 05/28/2010
Pages: 212
Product dimensions: 6.00(w) x 9.00(h) x 0.48(d)

About the Author

Henry Kong, MD was born in Seoul,
South Korea, in 1968. He has been intrigued by the mysteries of the mind for as long as he can remember. He studied biology at MIT, philosophy at Oxford, and medicine at Rutgers. He is an internist in private practice in Toms River, New Jersey.

Read an Excerpt

Root of Thought

Reflections on Neuroscience
By Henry Kong

iUniverse, Inc.

Copyright © 2010 Henry Kong M.D.
All right reserved.

ISBN: 978-1-4502-2267-9


Chapter One

Consciousness

'You', your joys and your sorrows, your memories and your ambitions, your sense of personal identity and free will, are in fact no more than the behavior of a vast assembly of nerve cells and their associated molecules. Francis Crick The Astonishing Hypothesis

Stephen Colbert: In five words or less, how does the brain work? Steven Pinker: Nerve cells fire in patterns. The Colbert Report

How does it feel?

Close your eyes. Clear your thoughts. What comes to mind? The fading orange of the setting sun, perhaps, or the window frame, or your fingers holding the pages of this book. Try as you might, these pictures cannot be separated from one another in space and time. The details fill in and melt together effortlessly. Your attention captures a vivid here and know that just as quickly slips away.

The image in your mind's eye is neither frozen nor flat; it is a space with depth, length, and height. Colorful shimmering impressions move in and out of focus. The leaves of a sycamore tree rustle in the evening breeze. Its shadow lengthens towards the cornfield and the farmhouse beyond. It's getting harder to make out the details in the gloaming, but you know they're out there.

Sit up, walk to the window, unlock the hinge, and lift. It's stuck. Pull harder; more tugs until it suddenly gives. Slide it up past peeling paint chips. Now step back, close your eyes, and concentrate again. What fills your senses? The sound of birdcalls, the smell of freshly-mowed grass, the heat on your fingers all share space in your mind's eye, but not all at once. Now open your eyes. You may recall reading a book like this once before, when you were in college. But the scent of a late summer evening suddenly brings forth memories from many years ago. Was that you?

* * *

All organisms have been naturally selected over millions of generations to survive and reproduce in their own particular niche. In the case of animals, evolution has invented sensory systems that enable monitoring of the external world in real time and motor systems that allow self-propulsion. The behavior of these input/output systems is largely hardwired, either at birth or shortly thereafter, through the elaborate unfolding of genetic programs. Every organism has an identical set of genes (about 23,000 in the case of humans, and 18,000 in the fruit fly) in each of its cells. This genetic ensemble contains the recipe for sculpting the entire creature. But not all genes are expressed at the same time or in the same cell. Gene expression is regulated by complex interplay of the environment with other genes, which themselves are usually controlled by yet other genes, such that their protein products are made at specific sites and times in the animal's body and lifespan. The result is the differentiation and development of specialized cells such as liver cells, blood cells, and muscle cells, and the organization and growth of those cells into specialized tissues and organs such as skin, heart, and brain.

At the genetic level, lower animals such as nematode worms and fruit flies are nearly as complex as human beings. The number of genes they possess and the way they are regulated are not very different from us. In fact, thanks to the conservative nature of evolution by natural selection, most human genes have counterparts in insects and worms. This makes them ideal model organisms for scientific and medical research into things like cancer, heart disease, and diabetes. Having evolved from a common ancestor hundreds of millions of years ago, humans and all the 'lower creatures' on earth truly are cousins.

At the level of the brain and mind, however, things begin to diverge. Insects and worms have neurons quite similar to ours. But because of their small size, their brains are necessarily less complex. As a result, their behaviors are rather stereotypical. Whether these creatures of habit have minds is difficult to answer (I would guess no), but we can be fairly certain that if they do, they would be markedly impoverished compared to ours.

As we proceed to the higher organisms (vertebrates, mammals, monkeys, and apes), there is a trend towards greater neurological complexity. All complex biological behavior is the product of brain activity, via the neuromuscular interface. Increasing neural complexity is almost always accompanied by increasing behavioral complexity. Higher animals do not necessarily act predictably. Their behavior is flexible and adaptable to the environment. They learn. And one species of primate in particular, Homo sapiens, has learned to reflect upon its own behavior. This ability is made possible by a generous grant from the human cerebral cortex.

Perception

Behavioral flexibility depends on the proper functioning of many parts of the brain. At the very beginning of this process is sensory perception. Organisms act in response to what they take to be 'out there'. Neuroscientists now have a rather good picture of at least the first stages of perception. We know that touch, balance, hearing, smell, sight, and taste are mediated by minute receptors in the skin and joints, in structures deep inside the ear and nose, in the retina at the back of the eye, and in taste buds on the surface of the tongue. These receptors translate or transduce phenomena in the outside world like heat, gravitational acceleration, odor molecules, and waves of sound and light into patterns of electrical activity on the surface membrane of sensory nerves. Perception occurs as the information conveyed by these patterns is relayed to higher and higher processing centers in the central nervous system.

Let's take the often-used example of vision. Imagine light coming from the outside world passing through a lens and focused into a tiny upside-down image on a thin curved sheet of retinal nerve cells in the back of the eye. There, specialized receptor cells, the rods and cones, contain chemicals that change shape when energized by absorbing light. These chemical reactions produce changes in the electrical potential of these cells that are transmitted to neurons connected to them. If sufficiently stimulated, these neurons will produce electric spikes, or action potentials, which spread down their axons and are passed on to other neurons that may be linked to them. They are, in effect, tiny electrical wires made of organic molecules rather than copper. The activities of thousands of nerve cells are channeled into multiple bundles of neural code streaming from the two eyes through the arching optic nerves. These nerves cross the optic chiasm, pass through a relay station known as the lateral geniculate nucleus of the thalamus, and arrive at an area at the back of the brain called V1, the primary visual cortex. The perceived visual image that was initially projected onto a thin sheet of cells on the surface of the retina is transferred to another thin sheet of cells at the back of the brain. But that image is no longer made of light, but by a representational digital code contained in millions of neurons. Objects to the right side of your nose (your right visual field) stimulate the left side of the retina on both eyes and eventually end up on your left visual cortex. Objects on the left side end up on the right visual cortex. This cross-over phenomenon, known as decussation, is a curiously common motif found throughout the nervous systems of all higher organisms. Other sensory modalities such as touch, pain, and hearing (but not smell), as well as the control of motion of one side of the body are also processed in the opposite side of the brain.

The picture we see out there now exists in the form of a partially processed buzz of electrochemical activity spread out over millions of nerve cells and their billions of connections, called synapses, scintillating with the bursts of millions of action potentials. Believe it or not, there is order in all this noise. Back in the 1960's, the great pioneers of modern neuroscience, David Hubel and Torsten Wiesel, discovered that patterns of neural activity in cats and monkeys actually code for simple features like edges, movements, and colors in particular parts of the visual field. Each neuron in the visual pathway has a receptive field, a region of space within which a stimulus will produce a response in the neuron. Retinal cell receptive fields are sensitive simply to spots of contrasting light, for example, a dark spot surrounded by light, or a light spot surrounded by darkness. A single cortical cell receives information from several retinal cells and summates the information from their simple fields. As a result, cortical receptive fields are more elaborate; they depict lines and edges rather than spots. Cortical neurons still further up the line then summate the information from the lower level cortical cells, producing even more complex receptive fields. Some of these cells respond to motion, orientation, and shape. Information is organized through the hierarchical architecture of neural connectivity.

The visual field is mapped point by point onto the surface of the visual cortex. Columns of cells in the visual cortex are dedicated to analyzing different features of visual stimuli such as which eye is seeing them, what their colors are, and how their edges are oriented. Neuroscientists have now mapped the primary visual cortex of animals such as cats and monkeys, and it is possible to visualize in stunning full color the activity of the cortical columns in living animals by a technique called optical imaging.

This neural activity is still at the early stages of visual processing. More interesting stuff happens further upstream. The axons of the V1 neurons are intimately connected to many neurons in other cortical and subcortical (evolutionarily more ancient neuron clusters beneath the cerebral cortex) areas. In the 1970's, the neuroscientists Leslie Ungerleider and Mortimer Mishkin observed that the higher order processing of visual input is roughly divided into two pathways: a dorsal stream (so called because its nerve fibers run along the top, or dorsal, part of the brain), and a ventral stream (whose fibers run along the lower, or ventral, part of the brain). The dorsal stream specializes in the location of objects in space, allowing one to perform such tasks as grasping a moving branch or catching a passed football, while the ventral stream deals preferentially with the recognition of objects. Thus the dorsal system has come to be known as the 'where' or 'how to' pathway, and the ventral system as the 'what' pathway. Though somewhat underappreciated at the time, Ungerleider and Mishkin's discovery has turned out to be one of the landmark advances in modern neuroscience with implications extending far beyond perceptual psychology.

There is now much evidence supporting this elegant division of visual perception. Perhaps most impressive are the psychological double dissociation cases of brain injury patients who have deficits in either the processing of motion (such as perceiving the flow of water) or in the perception of form (such as recognizing simple geometric shapes), but not both. These patients were subsequently found to have anatomic lesions in either the dorsal or ventral systems, respectively.

The other sensory systems are designed similarly. Sounds, touch, temperature, tastes, and smells are all deconstructed and then recreated in their respective areas of the brain. A different area of cortex is allocated for each modality. This deconstruction is followed by integration in association areas of the cortex where combinations of features are bound together to create a fairly accurate facsimile of the outside world. How does an object maintain its apparent integrity even though its features take separate routes through the perceptual system? This question has come to be known as the binding problem. It is one of the great unsolved questions in neuroscience today. There are actually three separate binding problems. More on this later.

The products of our perceptions are not, of course, the real world; they are working approximations of external reality. A typical theater stage set consists of numerous cheap props. Taken alone and offstage, they look artificial. But taken together and under proper lighting, they imbue a scene with meaning and believability. So it is with the mind's eye. We do not see a circle of blackness where our natural blind spot (the part of the visual field where the optic nerve leaves the retina) is. We are prone to visual illusions that trick us into thinking an object is darker when seen against a light background than if it were set against a darker one. The moon looks much larger on the horizon than it does high up in the sky. This is because our senses have evolved shortcuts to get the job done. Where there is a lack of information, such as the blind spot, the brain simply 'fills in' the gap. Our minds abhor a vacuum; as far as they're concerned, absence of information does not equal information of absence. When accurate perception would require a great deal of computation, such as figuring out the absolute brightness or the actual size of an object, or the stuff that's hidden behind the blind spot, the brain simply makes its best guess. This is a recurring theme that we shall see in various guises throughout this book: the mind is a grab bag of tricks designed by natural selection to get our genes into the next generation. It is created by a process that has no goals or foresight. In the words of the legendary biologist and neuroscientist, Francis Crick, 'God is a hacker.'

Perceptual coherence is an area of intense and exciting research for neuroscientists. An object in the world, such as a wedge of Swiss cheese, activates different sensory neurons specializing in perceiving the sizes and patterns of the holes in the cheese, the smell of the cheese, the color of the cheese, and so on. These cells are activated simultaneously and pass on information to downstream neurons in the association cortex. The simultaneous activity arriving at the association areas acts to bind these disparate properties of the cheese together, thus integrating the perception into a coherent interpretation.

This approximate picture we see in our mind's eye is a remarkably veridical one. The sights and sounds we see and hear seem to emanate from things out there rather than from phenomena in our heads. The explanation is that evolution has made our mental approximations of external reality match very tightly to the physical world, at least in those dimensions that are important for our survival. The view directly down from my girlfriend's 15th floor window above the East Village really does appear to be approximately 150 feet above the East Village. If it did not, perhaps due to a genetic glitch affecting the development of my brain's perceptual circuits, I might be prone to step out into the abyss, forfeiting my chance of passing on any possible altitude misperception genes to my progeny. On the other hand, equally 'real' phenomena like ultraviolet light, magnetic field intensity, relativistic time dilation, or the simultaneous visual perception of all six sides of a cube are inaccessible to our naked senses. This is because evolution has neither felt the pressure to select nor had the biological substrate with which to create the means for sensing them in our species.

Our perception of a coherent reality is a product of complex and creative neural ensembles that simultaneously deconstruct and integrate sensory input. Neurological deficits can interrupt this process. Damage to sensory receptors at the bottom of the pathway causes primary defects such as congenital red-green colorblindness or conductive hearing loss. Damage to later stages of sensory conduction is responsible for cortical blindness or sensory neural hearing loss. Damage to even more upstream areas in the association cortex can result in more specialized deficits. Examples include the previously mentioned inability to perceive motion (optic ataxia) due to lesions in the dorsal visual stream, the inability to perceive shape (apperceptive agnosia) from lesions to the ventral visual stream, and even the inability to perceive faces (prosopagnosia) from damage to the face processing area of the fusiform gyrus.

(Continues...)



Excerpted from Root of Thought by Henry Kong Copyright © 2010 by Henry Kong M.D.. Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents

Contents

Foreword....................ix
Introduction....................xi
Chapter 1 Consciousness....................1
Chapter 2 Decision Making....................47
Chapter 3 Religion....................83
Chapter 4 Politics....................140
Conclusion....................195

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