Last week Daedalus proposed taping a sequence of electrodes on the skin above a nerve trunk (the spinal cord, say). A voltage spike stepped upwards along the array at a specific velocity would, on travelling-wave-tube principles, fire the one fibre in the trunk with that exact transmission velocity. The array could thus address individual fibres, and induce controllable ‘virtual sensations’.

He now muses that a voltage spike stepped down the spinal cord at a specific velocity would induce a pulse in the tching downgoing motor nerve. It would drive some muscle somewhere in the body. Detailed experiments would be needed to discover what nerve-fibres with what transmission velocities served which muscles; but then a controlled pattern of downgoing voltage spikes could drive just the right muscles in the correct sequence to induce a specific ‘synthetic action’.

The obvious beneficiaries are dedicated joggers and exercisers. Once equipped with electrode arrays taped over the relevant nerves and driven from a portable computer pack, they would need no will-power. They could simply relax and let themselves be jogged along automatically by their own muscles. Assembly-line workers and others engaged on dreary repetitive tasks would also welcome nerve automation. They would find themselves repeatedly ‘going through the motions’ without thought or effort. Their automatic routines, optimized and pre-recorded, would be repeated with wonderful fidelity. Users could, of course, still exercise judgement, and vary their actions by added conscious intention.

Synthetic action would also be a splendid form of practical training. Skills such as typing or playing the piano could be programmed into an electrode array driving the subject's muscles, and replayed repeatedly. He would learn the ‘feel’ of the skill, and would soon be able to carry it out by himself.

The only snag is that a downgoing voltage spike, intended to activate a specific muscle, might also induce a downgoing impulse in some sensory nerve whose traffic goes the other way. This would cancel its upcoming signals and induce a selective anaesthesia. Nerve-automated personnel may be plagued with strange repetitive patterns of numbness. On the other hand, surgeons would welcome a tightly targeted local anaesthesia that could be instantly switched on and off.