Biology 202
2000 Third Web Report
On Serendip

Plato's myth of the cave provides an intriguing place to begin a discussion of reality and the brain. In this parable, Plato describes individuals chained within a cave who cannot see each other. (1). The only visible thing is the wall of the cave, upon which appear shadows cast by models of animals and objects moving before a fire. One individual escapes from the cave into the light of day and sees the real world for the first time. She returns to the cave with the message that the things they have seen on the cave wall are mere shadows. The real world waits outside the cave.

This paper explores some questions spurred by this course and Plato's parable. What is the relationship between the brain and the "real", or external, world? Can we know the external world? Are we forever confined to the cave?

The best direct evidence for the existence of an external world is the existence of sensory apparatus. The five senses have evolved over millennia because they have enhanced the chances of survival. The senses, particularly vision, enable human beings to gather information from the outside world. The brain interprets this information, and this is what is commonly referred to as "reality". In this sense, the brain IS reality because everything we see, hear, touch, taste and feel is interpreted through the brain. But how well does ordinary reality correspond to the actual world outside of ourselves? How well does the brain interpret the external world?

Seeing Is Not Believing

What information is the brain getting about the external world? It all derives from sensory input. The currency of sensory input is the action potential, or a membrane permeability change that moves along an axon. For example, light that enters the pupil of the eye causes a change in the permeability of the membranes of photoreceptors. That information then travels to the brain via the same chemical changes. The photoreceptors and ganglion cells in the eye discriminate the amount of light coming from different locations in space. But it is the brain that "sees". And what the brain sees does not, in some important ways, reflect the information it receives from the retina. The brain makes a large contribution to what is actually seen. For example, the optic nerve head on the retina has no photoreceptors. The brain has no information about this part of the world and makes up information to fill in this "blind spot." But there are more dramatic examples of how seeing is not descriptive of what is out there.

The ganglion cells in the retina do not faithfully report the intensity or size of light measured by the photoreceptors to the brain. The signal to the brain reports only the changes in light intensity on the retina. This is the result of the so-called lateral inhibition network, which assures that the brain receives information only about areas where the brightness of light changes. The brain fills in all the spaces between the edges. So what we see is not a representation of the external world, but rather the brain's interpretation of signals from the eye. What we see does not represent actual properties of reality. Color, for example, is a category created by the brain, not a property of the outside world.

The lateral inhibition network is a powerful barrier between us and the external world, but it also allows us to see a picture of reality that makes more sense than the information we are getting about it. If the ganglion cells reported light intensity to the brain, our reality would be constantly changing, and our sense of the world would not be stable. The lateral inhibition network is a product of evolution, which implies that it enhances our survival. (The lateral inhibition network in humans, for example, would be of little use to fish in their underwater environment.) And the lateral inhibition network reveals something important about reality - that it has entities that are well defined, at least for human purposes, by their edges.

Sensory Apparatus Is Believing?

Another important element in the discussion about the relationship of the human brain to the external world is illustrated in looking at the sensory apparatus of other animals. The cat's eye is designed to see at low light conditions. It has a special structure in the back of the eye, the tapetum lucidum, which reflects light back outside the eye so it can absorb light a second time, enabling the cat to see in the "dark". Some snakes, such as pit vipers, use sight and a heat-detecting ability to locate prey at night. (2). A concave heat-sensing organ just below the eyes allows them to track their prey with great accuracy. Heat and visual data are sent, via the optic nerve, to the snake's brain, which transposes the two types of data into a single image. Another modification allows the snake to literally "taste" the air. Jacobson's organ is located in the roof of the mouth and can detect minute quantities of chemical substances in the air. Odors are sampled by flicking out the tongue and retracting it across the Jacobson's organ. This organ is crucial in the trailing and recognition of prey. (3).

In fish, the lateral line system is a network of sensory receptors located along the head and sides. The system detects movements and pressure changes in the surrounding water. (4). Many bats have limited sight and navigate primarily through sonar, or echolocation. They produce high-frequency sounds and judge the distances to a target by comparing the original sound to its echo. (5).

These examples illustrate that the relationship between the brain and the external world depends on the types of sensory receptors an animal has. An animal's ability to exploit certain properties in its environment is determined by the sensory apparatus it has. Clearly, what one is, whether it be fish or reptile or human, defines the properties of the external world with which one can interact. One's sensory receptors determine one's reality. Another point illustrated by these examples is that animals rarely rely on a single type of sensory information to determine the nature of its environment. And often a special sensory ability compensates for a sense that may be less developed.

Who has the better picture of reality? Which set of sensory apparatus provides more accurate information about the external world to the brain of its owner? If accuracy is defined as the ability to provide adequate information for an organism's survival, it seems that all extant species receive "accurate" information about the external world, at least for a species' respective needs to survive. Whether this information reflects actual properties of the external world is a more difficult question.

Clearly, human reality is limited by the types of sensory receptors we have and do not have. Yet humans have the ability to detect aspects of reality that our senses alone cannot detect. Radio signals cannot be heard by humans, but we have invented a device that transforms those signals to those for which we do have receptors. We have devices that allow us to detect electromagnetic fields, ultraviolet light, the microscopic world, and the list goes on. It seems that human beings are unique in this ability to transcend the limits of our senses to know, albeit second-hand, other properties of the external world. It appears that human beings may have a more complete, if indirect, picture of the nature of the external world than other animals. It is a picture called science.

Science Is Believing?

Scientific inquiry depends on systematic observation and experiment. Observations can be made directly, through the senses of vision, hearing, taste, olfaction and touch. As discussed, these observations do not in many ways reflect actual properties of reality. Are all scientific hypotheses based on direct observation necessarily suspect, then? Even observations made indirectly, through the use of special equipment (such as a microscope) that extends the range of perception, are filtered through our senses. How can the fallibility of our senses be reconciled with scientific knowledge obtained through these senses? A look at another powerful ability of the brain may help to answer this question.

The brain does not rely on a single source for information. It relates several kinds of input to interpret physical reality. Consequently, the brain often has conflicting or ambiguous information about physical reality. It makes sense of this information in a variety of ways. For example, the brain uses several variables to determine the location of an object in three-dimensional space: binocular cues, eye movement cues, lens accommodation cues, "painter's" cues, and perspective and texture cues. These cues often yield different answers. The brain is designed to resolve these ambiguities. Ordinary reality, then, can be defined as the best sense the brain can make at a given time about fundamentally ambiguous information from the physical world.

To resolve ambiguities, the brain seeks additional perspectives, asks new questions, and makes things up. It will often show alternative possibilities rather than pick an intermediate value among conflicting views. The more different perspectives the brain has on something, the more definite the interpretation. This helps to explain that the picture in the brain is not located in a discrete place; the optic nerve projects its signals to many regions in the brain.

The method used by scientists parallels in important ways the process by which the brain makes sense of sensory input. Scientists first ask a question or identify a problem. They then make educated guesses about the answers by sorting through clues and observations, and combining these bits of information to produce a general statement or hypothesis. They test these hypotheses by seeking additional information, examining alternatives, asking new questions, and, sometimes, making things up. The scientist, using her brain deliberately, ultimately tries to provide the best possible answer to a question using fundamentally ambiguous information.

As with the brain, the more data that supports a particular scientific hypothesis, the more reliable the conclusion. Much scientific knowledge, such as the laws of physics, the periodic table of the elements and the theory of evolution, to name a few, is supported by centuries of experiment that rests largely on indirect evidence, obtained with the aid of special equipment. This helps to reconcile the apparent gap between our senses and the scientific knowledge. In addition, we see the application of scientific knowledge in our daily lives -- airplanes fly, lights go on, gasoline powers automobiles, etc.

Science is a representation of reality that enables us to know more of the external world. Ordinary reality agrees with common sense. However, many conclusions drawn from common sense break down when confronted with conclusions drawn from science. Experiments in relativity and quantum physics, for example, have demonstrated the gap between our common experience of the world and the actual nature of the world. (6).,(7). Neither picture of reality, the ordinary or the scientific, are primary representations, but rather indirect and coded pictures. The tenuous link between what the human brain sees, our "ordinary" world, and what is really out there has been discussed. The human ability to use our senses to, in effect, transcend their limits has enabled us to observe properties of the external world that we cannot detect on our own.

This "scientific" picture of reality is more real than the picture in our brain, in the sense that it reflects more closely the actual properties of the external world. It is this picture of reality that exists outside Plato's cave. And it is through science that we can know, indirectly, that external world. What is most real is, in fact, invisible to the unaided human eye. What are we really looking at when we look at a tree? Through the aid of scientific instruments and experiment, it can be reasonably hypothesized that we are looking at constantly moving, overlapping electron clouds, and organic and inorganic molecules undergoing chemical reactions while obeying certain physical laws. The tree, as it is seen, is an abstraction created by the brain. Only through the aid of instruments can we glimpse, second-hand, the real nature of the external world. Thus, human beings are confined to the cave and must rely on reports from these instruments for information on the real world.

Should we really want to leave Plato's cave to enter the real world? Perhaps the brain's function is not to know absolute reality but to filter only that which is necessary for human survival. But looking at the brilliance of a desert sunset leads one to wonder if that is the only function. And the ability of the brain to think about itself, to transcend itself, to expand its reality, may not only prove unnecessary for human survival, but may ultimately be our undoing. While our sample of reality is a small sample of what is "out there", it is likely that we could not survive in the constant flux of the "real" world. The brain runs crucial interference between us and the real world, and at the same time it paints a picture that is breathtaking in its beauty and complexity.

WWW Sources

2)A website from NOVA with general information on the nocturnal eye and animals that have night vision.

3) Informative article from Encyclopedia Brittanica on the form and function of the snake.

4)Article from Encyclopedia Brittanica on the form and function of fishes.

5) A website on the greater spear-nosed bat, specifically echolocation in the bat.

6)What are black holes and what is the evidence for their existence?

7) Relativity on the world wide web: tutorials, pictures, links.

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