IBM, with support from DARPA , has a program called SynAPSE . Its goal is to construct a computer with components similar to those of the human brain. This synthetic brain could fit inside a robot head. It would have 10 billion "neurons" with one hundred trillion "synapses" or links between neurons. Each neuron will send a signal along its "axon" when the sum of its inputs across synapses from other neurons reaches a threshold, and then reset itself just like human neurons.

Of course, a natural neuron is a living cell made up of biological molecules such as proteins and DNA, whereas computer "neurons" are built of metals such as silicon. Despite this difference in material composition, the IBM "neuron" is designed to be the functional equivalent of a human brain neuron.

For a simpler example of this important concept of functional equivalence, consider hip replacement surgery (a common, highly successful procedure). The titanium in an artificial hip joint is quite different from the living cells making up the original hip joint. But the artificial joint is functionally equivalent: it interacts with the remaining bone structures just like the original, enabling the patient to walk even better than before.

Just as a titanium hip joint can successfully interact with natural bone structure to restore the function of a hip, so computer chips and brain tissue can interact or interface to restore specific functions of the brain. Such an interface is called a neural prosthesis. Microchips can be engineered so that their low-voltage electrical currents translate into the natural electrochemical currents that flow in and between neurons, and vice versa.

There is a growing volume of research in what is called cognitive neural prosthetics . The goal here is a device that would replace damaged brain tissue and restore higher-level functions such as memory or planned movement. As envisioned by a team of researchers in an influential 2005 article ("Restoring Lost Cognitive Function"), the silicon "neurons" in the prosthesis "would have functional properties specific to those of the damaged [brain] neurons and would both receive as inputs and send as outputs electrical activity to regions of the brain with which the damaged region previously communicated."

The scientists working to create cognitive neural prostheses and those involved in IBM's SynAPSE program are enthusiastic about the good their projects would achieve. Cognitive neural prosthetics would be able, for instance, to help people with paralysis from spinal injuries. The IBM Research website tells us that brain-like computers will "learn and interact naturally with people to extend what either humans or machine could do on their own." What's not to like?

But there's a hint of a big problem in the title of this article in the Journal of Neural Engineering (6/15/11): "A cortical neural prosthesis for restoring and enhancing memory." As the title suggests, the technology for restoring brain functions by implanting neural prostheses is also a technology for enhancement even of normal brains, for indefinite expansion of sensory, intellectual and motor capacities.

As these enhancement technologies emerge, who should get them? Should they be commercially available to those who can afford them? Will they result in a class of superhumans with a higher social or political status? Or should enhancements be treated as a public good available to all at whatever cost?

If we extrapolate from the development of the kind of technology sought in cognitive neural prosthetics and in the SynAPSE program, we can see a striking convergence. On the one hand, we will have enhanced humans with not only increased brain power, but also benefitting from the steady progress in making synthetic body parts that outperform what they replace. On the other hand, we will have computer systems with brain-like components giving them perceptual, motor and cognitive processes that used to be exclusively human.

At what point would the enhanced humans become so differentiated from our species that they no longer belong to it? Would they have more in common with robots whose heads enclose brain-like computers of the sort envisioned at IBM?

In thinking about these questions, we need to discard the sharp distinction that traditional thinking makes between life or consciousness and machines. The notion of a living and thinking machine still seems repugnant or contradictory to many. However, the development of nanotechnology is undermining this intuition.

A machine (e.g. a clock or engine) is a system of physical parts that interact to carry out a specific function. Until recently, humans made machines by assembling visible parts such as gears or bolts composed of massive numbers of imperceptibly small molecules numbered in the trillions of trillions.

With the advent of microelectronics in the mid-20 th century, humans could assemble machines with parts measured in microns (millionths of meters). In the 1980s, the invention of the scanning tunneling and atomic force microscopes allowed engineers to image and manipulate individual atoms and molecules. Atoms range between .1 and .5 nanometers in diameter (a nanometer is one billionth of a meter). The science and engineering of components at this scale is called nanotechnology. Without it, IBM's SynAPSE program and the research on cognitive neural prosthetics would be impossible.

We are now in the early stages of assembling individual atoms and molecules into nanomachines that operate in a nanoscale environment. Living cells are nanomachines engaged in ceaseless metabolic activities, taking apart and building molecules such as sugars, fats and proteins. Today's children are taught to understand cells as living, self-maintaining machines. They learn that their thinking is carried out by an organ composed of billions of living machines. The wall between machine and consciousness is gone.

As we apply our technological prowess to the machinery of the human body and mind, we need to realize that we really are going down a path that has been until now regarded as science fiction. Do we today have the wisdom to settle on a good direction for our society at this tipping point in human history? Or will we just let the chips fall where they may?