© Copyright 1999-2005 by Ralph E. Kenyon, Jr .
Mobile organisms move. Why? They move in response to stimuli - stimuli from without and from within. A mobile organism that doesn't move dies. In order to survive, an organism must engage in the "right amount" of activity. Too little activity and too much activity are both troubling to all mobile life. Too little activity and the organism finds neither food nor mate. Too much activity and the organism exhausts its ability to maintain its health, resulting in metabolic and reproductive failure.
Stimuli from without take three forms. Those that are to be approached, those that are to be avoided, and those that are to be ignored. No stimulus initially falls in the latter category. There are basically two reasons for approaching a stimulus. It leads to a source of energy, or it is leads to a source of reproduction. There are two reasons for avoiding a stimuli. It leads to a drain of energy, or it leads to a threat to reproduction. If a stimulus neither supplies nor drains energy, and it neither offers nor threatens reproduction, then the organism must learn to ignore it in order not to use energy that would otherwise be available for seeking food or reproduction or fleeing threats to life or procreation.
Stimuli from within also take three forms. They prompt an organism to increase its movement, they prompt an organism to decrease its movement, or they do not provide sufficient stimulus to effect either. Increase in movement may be directed towards or away from an external stimulus. Decrease in movement is primarily energy conserving. As an organism lives, it must maintain a fine balance between conserving energy and obtaining energy. Energy obtained is consumed in maintaining itself, in reproduction and avoiding threats to both.
Stimuli cannot be responded to without senses. Organisms have internal and external apparatus for sensing their condition and for detecting circumstances that may need to be responded to. Examples include sources of energy, energy drains, potential mates, the need for energy, the need to conserve energy while maintaining structure, and more. An economic principle - minimizing the cost to obtain a benefit - permeates all mechanisms for governing when to increase or decrease motion. In an environment with limited energy, natural selection favors organisms which use energy efficiently.
In the context of a dynamic organism in its environment, the sensory processes must "recognize", that is, permit the organism to respond to, circumstances that the movement toward or away from provides a benefit to or reduces the cost to the organism. At the lowest levels in the phylogenetic scale, these distinctions are "hard wired" into the structure and functioning of the organism. However, at higher levels, process and structures provide for "soft wired", or learned responses. For example, in the presence of a noxious chemical some bacteria just move more rapidly - increasing the chances that they move away from the bad stuff. At this level the detection system cannot even distinguish which direction to go (or even control the direction). Faster movements gets enough of them away for the species to survive.
At higher levels in the phylogenetic scale more complex structures can sense in which direction it is "better" to go as well as actually control that direction. The metaphor of the "ghost in the machine" has common currency in our culture. When Descartes tried to explain it he still clung to the Christian doctrine of dual substances - spirit and body. This left Descartes with a model of a man containing "little men" (homunculi) inside. Today we laugh when confronted with Descartes homunculus as a model for brain function. The philosophical absurdity of this "circular" structure adds to our mirth. However, there is an element of structure here which isn't far from the truth.
In order for an organism to be able to have a sense of direction, it must be able to map a representation of the significant environmental circumstances in relation to its own body. The mapping must provide for the dual function of approach to beneficial circumstances as well as retreat from detrimental circumstances. I will make an analogy with a somewhat familiar and easily conceived experience - that of a rowboat with two sets of oars big enough for four people.
Think, for a moment, about any row-boat. For simplicity, we will presume that we can only stroke the oar so as to move the boat forward. This is consistent with having legs or arms or cilia or fins, etc., that work better in one direction than the other. It is a simpler and more efficient design. With this arrangement we can get forward motion as well as the ability to turn to one side. If you stroke the oar on the port (left) side faster than the oar on the starboard (right) side, the boat will turn to the right. Conversely, if you stroke the oar on the starboard side faster, the boat will turn to the left. One person in a boat would tend to remind us of Descartes homunculus - a brain within a brain. To get away from the absurdity of the homunculus, let this boat have 4 people - two on each side. Each person will serve the function of a lookout and a rower. Each lookout corresponds to a sensor. Each rower corresponds to an actuator. There will be no central "brain" coordinating their efforts. The overall design of the system will provide for apparently intelligent action on the part of the organism (the rowboat and its crew) in spite of this lack of a central coordinating authority. As each person does his job, the boat will move in an intelligent manner. Here's how.
The aft-most person is looking out for things that are to be avoided. He or she looks out over the side where he or she sits. If he see something to be avoided he rows faster, in proportion to how close it is. The fore-most person looks across the boat for things on the other side that are desirable. He or she also rows faster when he or she sees something to be desired. Nobody talks to anybody else. Each does his or her own job as he or she sees fit. The forward or "approach" lookouts tell themselves as rowers to row faster when they see something "good" on the opposite side of the boat. The aft or "avoidance" lookouts tell themselves as rowers to row faster when they see something "bad" on the same side of the boat. As each does his job, the boat will turn and approach something good or turn and retreat from something bad. Let's look at the details of the process.
The port forward person rows faster when he or she sees something good on the starboard side, while the starboard forward person rows faster when he or she sees something good on the port side. Conversely, the aft starboard person rows faster when he or she sees something bad on the starboard side, while the aft port person rows faster when he or she sees something bad on the port side. What happens to the boat when something good appears on the starboard side? Faster rowing on the port side turns the boat right - toward this good stuff. As the boat turns to face the good stuff both sides row at the same speed, and the boat approaches the good. What happens to the boat when something bad appears on the starboard side? The faster rowing starboard side turns the boat left - away from the bad stuff. As the boat turns directly away from the bad stuff, both sides row at the same speed, taking the boat directly away from the bad. If we have one person on each side doing both jobs, we can achieve the same effect with 4 eyes and two oars - 4 sensors and two actuators. These are two eyes watching for good, one on each side, and two eyes watching for bad - again, one on each side.
I will refer to an eye that is watching for something good as an approach sensor, and an eye watching for something bad as an avoidance sensor. The approach sensor activates the actuator on the opposite side. This is called a contralateral connection. The avoidance sensor activates the actuator on the same side. This is called an ipsilateral connection. The dual functions of approach and avoidance - benefit and cost - can be effected by two overlapping mappings between sensors and effectors. An operational characteristic of this system is that the organism moves in such a way as to bring a desirable into front center or to bring an undesirable into rear center. The contralateral mapping brings the desirable object into front center; the ipsilateral mapping brings the undesirable object into rear center.
|Why does the left half of the brain control the right side and
An answer to this question involves understanding the overall response between sensors and propulsors in the generalized mobile living organism in its environment. Typically an organism needs to face something to recognize it. If something is detected on one side, the legs on the opposite side must move in order to turn the organism to face the object. (Legs evolved to work better in one direction.) By the same token, detecting something dangerous or painful on one side of the organism requires the legs on the same side to move in order to turn the organism away from the danger. Danger needs to be responded to faster, so the shorter (neurological) distance of a same-side or ipsilateral connection is more advantageous to survival over the long run. In rudimentary nervous systems the sensory analysis or identification processes must be closer to the dangerous stimulus for faster avoidance response, but that same (identification and response) process must also be used to activate the propulsive mechanisms on the opposite side to achieve turning towards. So it's reasonable to hypothesize that contralateral neurological structures had a survival advantage extremely early in evolution, way before brains evolved, thus committing the development of higher organisms to "cross-wired" brains.
In order for organisms to "seek out" benefits, they require an internal source of stimulus. Some of this is provided by spontaneous discharges of nerve cells as they charge to capacity without external stimulus. (A nerve cell will normally spontaneously fire after a certain period of time.) In addition to this there is the activation system which broadcasts stochastic noise throughout the nervous system. One can think of this as like a throttle that keeps the engine idling or revs it up when needed.
Internal sensors that detect the organism's need for sustenance or rest will increase or decrease the activation system. With an increase, more motion is stimulated - thus increasing the likelihood that what is needed will be found. With a decrease, less motion is stimulated - allowing the organism to rest, or digest its meal, etc.
Without a minimum level of nervous system activity the organism will simply die. In order to survive it needs a certain level of activity and stimulation. In the absence of input from external senses, the activation system will prompt the organism into action. Restlessness due to boredom - cabin fever - is an example of the process functioning at our levels. We also have a very complex system of levels of needs that interact dynamically. Without enough input stimulation, the organism begins to move around. This moving around produces more inputs. For the lowest level organisms, those not capable of sensing or controlling direction, this is the only mechanism, but that in itself is not enough for higher organisms.
For direction control, the organism must be capable of recognizing a beneficial or detrimental circumstances. When motion in the environment brings the organism into proximity with such circumstances, the ramp architectures direct the motion of the organism so that it approaches or retreats. Some recognition is hard-wired in. We retreat from circumstances that cause pain and approach those that give pleasure. With the remainder our sensory system must be capable of making distinctions. It must distinguish between that which is known - and classified in one of the three previously mentioned categories - and that which is unknown. Unknown circumstances must be approached with caution - preparation for retreat - approached in order to determine if it is good, bad, or to be ignored.
At any given time, the vast majority of sensory input for an organism will be in the classification of to be ignored - or presently to be ignored. Our senses are tuned to respond more to those things which are currently of interest - to approach or to flee. (Do you want to approach - and consume - food right after a Thanksgiving dinner?) The tuning of our senses is affected by our changing internal state. Hunger makes food interesting. A full tummy makes food not only uninteresting, but to be avoided, "No, I couldn't possibly eat another piece of pie, let's retire to the sitting room." - flee this food.) Only a small portion of the environment produces different - novel - stimulation. Most of our circumstances are familiar. Things don't change much. But when something changes, we often notice it, particularly if it has good or bad consequences for us. However, too much novelty in the input overwhelms the organism's ability to determine in which direction to go. We experience excessive levels of stimulation as anxiety. Too many new things all at once requires too much attention and uses up too much energy. Too many new inputs over-stimulate our nervous systems. Over-stimulation prompts retreat or shut-down.
It was discovered and reported that birds and other organisms use a fundamental binary structure in their communication systems. Higher frequency sounds are used to signal submission or a wish to approach. Lower frequency sounds are used to signal aggression or threat. Compare the yip or wine of a submissive dog to the growl of an aggressive one. By slowing down bird-calls, it was discovered that they all follow the same pattern. One might think that higher frequencies provide more stimulation, but the reverse is actually true. Because of the nature of sounds, it is easier to selectively respond to higher frequencies than to lower ones. Lower frequency sounds tend to activate many sensors - providing higher stimulation - than higher frequency sounds which tend to activate only selective sensors. (Another factor is the fact that a larger animal is more likely to have a slightly lower pitch in its voice, so lower pitch is also an indicator of possible larger size, and larger size is more likely to win out in a contest for dominance. In experiments it has been noted that Chickadees ignore calls which have been technologically lowered in frequency, but approach those that have been raised.) The universal binary approach-avoid communications distinction gets encoded as a result of less-more stimulation where over stimulation prompts anxiety and flight.
Our comfortable level of stimulation is a balance between too little - boredom - and too much - over-stimulation. Too little stimulation is bad for survival; we evolved to keep active, and we experience boredom - "I gotta get out and DO something!". With too little sensory stimulation our internal activation system provides the dominant inputs. But this has no real direction. Think of the sensation one experiences when one has drunk too much coffee. We feel jittery, but with no direction. What happens is we respond more quickly to changes in external stimuli. Boosting the activation system during times of hunger or fear can result in a more timely response when the opportunity to capture food or the need to flee presents itself. Internal sensors - hunger detectors - can boost the system as can external sensors when a predator is spotted - adrenaline response. If we are well fed, sexually satiated, well rested, and in no danger, a restlessness due to the dominance of the activation system over all other inputs will be felt. The few species that achieve this state with any regularity play - cats, otters, ferrets. But this serves a long term purpose of learning. It improves the efficiency of living.
Too much stimulation is bad for survival; it creates anxiety by threatening to overwhelm our coping mechanisms. "I can't handle this right now!" Under-stimulation is characterized by a lack of novelty in our environment. Over-stimulation is characterized by an abundance of novelty in our environment. Under-stimulation produces boredom and stimulates the organism to move around. Over-stimulation produces anxiety and stimulates the organism to "run away".
How can exposure to something different become non-stimulating? Consider for a moment the ticking of a clock. First we hear it loud. Then it's just sort of "there". While we are aware of it, we don't pay any attention to it. Later we simply no longer hear it at all. The process is called "habituation" and is well documented. Organisms rapidly learn to ignore repetitive stimuli that provide neither pain nor gain. It's a waste of energy to pay attention to something that offers no immediate benefit or danger. The rule of economic efficiency of benefit versus cost immediately applies.
In order to reproduce we need to be able to identify or recognize a potential mate, and a potential mate must be genetically compatible - a member of the same species. We must be able to recognize such an opportunity. We must be able to recognize members of our own species. Other organisms which "look just like us" qualify. Those which are too different do not. The ability to recognize a potential mate depends upon our seeing one that is "not very different" from ourselves. Too many differences motivate us to avoid. (Too many similarities motivate us to avoid also. - natural selection favors population groups which don't inbreed too much.) A potential mate must be similar enough to be recognized as the same species, but different enough to provide a good genetic complement. Again, this is a "middle" ground in the level of novelty or stimulation.
|This page was updated by Ralph Kenyon on 2009/11/16 at 21:33 and has been accessed 21088 times at 92 hits per month.|