The nerve-traffic of the body is very hard to interrogate or subvert. A nerve trunk may contain hundreds of fibres; an electric shock can fire them all together, but cannot address them individually.

Daedalus now has a scheme to do it. Each fibre, he points out, has its own speed of pulse transmission. Imagine, he says, a sequence of electrodes placed along the subject's arm, for example, above the median nerve. Apply a small voltage spike to each electrode in succession, so that the stimulation travels up the arm at a specific velocity. If the individual pulses are too weak to fire any single fibre, the subject will feel no shock. But suppose the voltage spike steps along at the exact transmission speed of one specific fibre. A matching disturbance will be launched up that fibre, and will be amplified by each successive electrode in travelling-wave-tube manner, until it becomes a fully self-sustaining nervous impulse. Other fibres with different impulse velocities will not keep pace with the moving stimulation, and will not be fired. The subject will feel the sensation carried by that fibre alone — a tickle in his thumb, say.

This elegant technology opens the way for true ‘virtual sensations’ superimposed on the normal sensory traffic of the nervous system. Once the velocities of the fibres in a nerve trunk have been mapped, a computer-controlled electrode array above that nerve could fire just the fibres needed to generate any desired sensory illusion. The long run of the spinal cord, which carries so much nervous traffic, is ideal for travelling-wave-tube stimulation. Aldous Huxley's ‘feelies’ — cinema with sensations — would become possible, computers could communicate with their users by tactile signals, and real telephone sex would be feasible at last.

Virtual sensation, however, will be most useful in tactile education. Doctors, mechanics, cooks, violinists and so on, all have to learn special sensory skills. A set of travelling-wave nerve programs, recorded from experts in the field, could teach the novice the actual feel of a fast pulse, a slack bearing, a too-sticky cake mix or a delicately sustained bow stroke. But sight and hearing, the most important senses, are badly suited to the technique. Their nerves run too deep in the skull to be reached by surface electrode arrays. Nerve-induced virtual television and audio, by-passing the eyes and ears and piped in 24 hours a day, are (perhaps mercifully) unfeasible.