A little more than a year ago I wrote in one of these columns about the way that we are all, bit by bit, becoming cyborgs: humans, but humans changed to improve or restore some of our body functions with man-made additions or replacements.

The list already is an impressive one. Almost all of us have dental fillings or a crown on a tooth. Many of us wear eyeglasses or contact lenses. The replacement of an eye lens by an artificial one to treat cataracts is an operation so routine that it is performed on an outpatient basis, with few or no aftereffects (a friend of mine had a lens replaced one morning and went to a college class the same afternoon). Older people benefit from pacemakers, hearing aids, and artificial knee, hip, or shoulder joints. Heart surgery now employs "stents," tubes of inert material used to widen coronary arteries narrowed by the build-up of plaque.

All these devices are designed to correct defects present at birth, or developing with increasing age. They are not trying to improve over what Nature can do. But of course, fiction writers have been unable to resist the temptation to take the process a step farther. How might a fusion of flesh and blood with mechanical or chemical components lead to something with superhuman powers? In particular, fiction has speculated on cyborg warriors, enhanced to fight some future war. Do such creations form part of a probable future?

Today I want to examine some human characteristics - call them weaknesses, if you like - and compare them with what is already possible with today's technology.

Let's begin with something that made a big difference to a warrior a thousand years ago, but is relatively unimportant today: physical strength. We could, if we wish, certainly replace a real arm by a prosthetic one able to exert a force of a few tons (remember the Six Million Dollar Man?). Unfortunately, our real-world superman still can't lift a car. The arm is strong enough, but the rest of the body is flesh and bone. When you try to use your bionic arm to raise the car, your legs collapse and the muscles of your body tear loose.

Suppose that we opt not for strength, but for speed? The same problem arises if we reduce human reaction time too far. Our natural reaction speed is close to the limit of what our bodies can stand. The pulled muscles of Olympic sprinters and other star athletes attest to this. Unless our cyborg operates with great care, he'll spend most of his time rubbing in liniment.

Our senses offer better room for improvement. We can offer increased visual acuity, though that has a natural limit dictated by the size of the pupil of the eye. There is more scope for change in the range of wavelengths to which our eyes are sensitive. The color span of the rainbow, from red to violet, corresponds to only a factor of two in wavelength. We know that some insects see using ultraviolet light (many "plain white" flowers reveal intricate designs in the ultraviolet). We can build sensors that produce images at these shorter wavelengths. At the other end of the spectrum we can build devices that are sensitive far into the infrared, all the way to "thermal" images that respond to the different temperatures of objects in the environment. Why not replace the eyes of our cyborg soldier with super-eyes, sensitive all the way from short ultraviolet to long- wavelength infrared?

We have no idea how to do this. In particular, we can't interface these artificial eyes with human optic nerves. However, I assume that some day that problem will be soluble, just as I am confident that in another generation we will be able to re-grow nerve cells and restore full body control to paraplegics. That does not mean I expect to see the Army After Next employing teams of soldiers with enhanced artificial eyes. It is so much easier, and more versatile, to carry around the equivalent of miniaturized night-vision glasses, to see in poor light or no light, and through fog and haze.

The same is true of hearing. A human can hear sounds between about 20 cycles a second and 20,000 cycles a second. A bat hears up to maybe 100,000 cycles a second and uses those high frequencies in echolocation of position and in "seeing" insect prey. Some of those insects, in self-defense, have become sensitive to the same frequencies. We can do even better and make an "artificial ear" able to pick up signals anywhere in the range from a few cycles a second to a million cycles or more. Again, we have to ask if it would make sense to put this in a human. Wouldn't it be better to miniaturize the instrument we need, and package it into a small black box that our soldier can carry or leave behind, depending on his or her mission?

As for smell, in humans that sense has so far atrophied that most animals would claim we can't smell anything at all. We can't do it yet, but I expect that twenty years from now you will be able to buy a small and inexpensive device with a dog's ability to pick up and discern among odors. It should be possible to build that capability into a human nose.

If we choose to, that is. Do you really want to smell everything? Wouldn't it be better to package all of our improvements - in strength, in speed, in hearing, in sense of smell, and in visual acuity and wavelength range - into a wholly artificial entity, a robot rather than a cyborg? The result would be superior to an enhanced human in almost every way: strength, senses, tolerance to acceleration and radiation, operating rate (including an on/off capability), range of sizes, necessary support systems, and mean time to failure.

The one area in which the human remains superior is not in hardware, nor is it in software; it is in the performance of the three-pound lump of wetware that sits between your ears. And for that, we can as yet offer no cyborg equivalent, still less a cyborg improvement.

Note: I did not make up the Army After Next; it is a real and ongoing project to foresee military needs a generation from now. I participated in one of their brainstorming sessions, but I was not an insider. I realized this when one of the speakers referred to "increased conflict in urban theaters" and my first mental picture was of fighting in the stalls while "King Lear" continued on stage

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