The device, developed at a research centre owned by Evanston's Northwestern University, consists of the brain stem from the larva of a lamprey, a bloodsucking fish, attached by electrodes to an off-the-shelf Swiss robot.

In an arrangement reminiscent of the genesis of the Daleks, the living brain floats in a container of cool, oxygenated salt fluid.

Placed in the middle of a ring of lights, the robot's sensors detect when a light is switched on. It sends signals to the lamprey brain, which returns impulses instructing the robot to move on its wheels towards the light.

When all the lights are off, the robot stays still. When one of the robot's eyes is masked, the disembodied brain is temporarily confused, but learns to compensate.

One of the researchers, Sandro Mussa-Ivaldi, said the work was a step forward in neural engineering. "There's an element of uniqueness in what we've done, particularly in the fact we've created a closed loop system, where the lamprey brain and the robot are exchanging information," he told the Guardian.

Scientists are exploiting the immature lamprey's instinct to keep itself oriented the right way up in the water. In a cyborg arrangement, that translates into seeking light.

The marriage of baby bloodsucker and Swiss engineering has little chance of conquering the universe as yet. Scientists can only keep the brains alive for a few days and are unable to stabilise them long enough to see whether they can remember anything.

But they hope their work will ultimately lead to the creation of advanced, brain-controlled prostheses for people whose normal ability to control their limbs has been disrupted by a stroke or Parkinson's disease.

"The focus of our work is not so much to create a cyborg as to create a tool for investigating the organisation of the brain," said Dr Mussa-Ivaldi.

Other scientists are already moving towards the practical application of microelectronics to help the disabled.

In Atlanta, scientists have implanted a tiny glass electrode in the cerebral cortex of a quadriplegic patient and coaxed neurons to grow inside. By attaching a transmitter, the patient was able to move a cursor on a computer screen by thought alone.

The creation of the cyborg brings closer the advent of machines with animal parts. Advances in miniaturised electronics have inspired other scientists to try to develop devices with living biological components.

The Washington Post reported that an Iowan entomologist, Tom Baker, has attached moth antennae, capable of detecting the smell of high explosives, to an electronic device which reads variations in the nerve signals sent out by the antennae when they pick something up.

But the electronics are not sophisticated enough to distinguish one smell from another - so as yet the half-moth, half-chip machine isn't much use for its intended purpose, sniffing out land mines.

Dr Mussa-Ivaldi said cyborgs were, in a sense, already all around us. "People wearing prostheses could be considered cyborgs," he said. "Some think that when we're attached to our internet connections, we're cyborgs."