What if ... the human brain could be hacked into?

Information that kills. It's a cliche of ghost and horror stories -- the apparition so fearful that the mere sight makes the victim drop dead from sheer terror. There are also ancient myths of creatures like the basilisk and cocktrice, which it's death to look at unless you use a mirror as a kind of visual condom. Could the high-tech equivalent ever stare lethally out of our computer displays?

SF writers rather like the idea of informational weapons, probably because we're a weedy bunch who spend our lives tapping at keyboards rather than developing healthy muscles that would allow us to sock foes on the jaw. 'Black ice' software in William Gibson's SF is a famous example. His cyberspace hackers connect their brains directly to the net, and it's unsurprising that multinationals come back with security programs that hack right back.

Apart from the small snag of their not having been invented yet, direct neural net connections don't seem so attractive these days. For a start, you might catch Melissa. More interestingly, Fred Hoyle's 1957 novel The Black Cloud suggests there's knowledge we don't dare know, thoughts we mustn't think. The Black Cloud is an intelligent gas cloud in space which helpfully lets Earth scientists download its advanced knowledge ... whereupon their brains overheat with the bogglement of it all, and the result is dead scientists.

Likewise, Piers Anthony's Macroscope features interstellar transmissions booby-trapped with 'destroyer' information so that anyone bright enough to decode them will rapidly become an ex-genius, either a corpse or a vegetable. By bizarre coincidence, Macroscope appeared in October 1969 and so did the first episode of Monty Python's Flying Circus, featuring informational carnage as soldiers helplessly laugh themselves to death in the World's Funniest Joke sketch. (Not a new idea. US writer Oliver Wendell Holmes's comic verse about a near-fatal joke, 'The Height of the Ridiculous', appeared over a century ago.)

When I planned a story about this kind of offbeat weapon, I started from the mathematical end. Douglas Hofstader's Godel, Escher, Bach: an Eternal Golden Braid explains Godel's theorem in terms of the construction of a logical Trojan Horse that crashes the system of mathematical proof itself. Elsewhere I'd read a lot about the imperfections of pattern-recognition software. These ideas came together in a blaze of implausibility.

Human optic nerves and brains do a fantastic amount of processing of what isn't a terribly good image on the retina. (As someone said, imagine designing a high-performance optical system using no glass, only jelly.) Suppose there could be a Trojan Horse image which crashes our internal pattern recognition and makes the brain's operating system fall over? A scientific report invented for the resulting story went on about 'Godelian shock input', data incompatible with internal representation.

The practical result -- still fictional, and I hope it stays that way -- was BLIT images, named for the Berryman Logical Imaging Technique supposedly used to generate them. Get an eyeful of one of these, and your brain stops. (I also imagined some lesser ones that merely produce convulsions and other jollies.) Terrorists lay down their guns, since now they can bring death to the streets with just a stencil and an aerosol paint-spray. Of course they have to wear distorting goggles to protect themselves from the deadly fractal cockatrices they're painting by night on hoardings, shop windows, Tube carriages....

Oddly enough, that short story 'Blit' (readable on the Infinity Plus SF web pages, www.infinityplus.co.uk) and its successors touched a nerve with other SF writers. Bruce Sterling, one of the original cyberpunks, sent a fan letter. Two more authors whose work I enjoy went further and wrote me into novels. Greg Egan's Permutation City has a character frightening off spies who might be monitoring her terminal, by typing 'Whoever you are, be warned: I'm about to display the Langford Mind-Erasing Fractal Basilisk ...'

Ken McLeod got a bit more personal when he mentioned the 'Langford visual hack' in The Cassini Division ... whereupon the heroine thinks: 'And yes, I know the Langford hack is just a viral meme in its own right, replicating down the centuries like an old joke, wasting resources every time we act on the insignicant off chance that if someone could think of it, somehow it could be done. What kind of twisted mind starts these things?'

I hope Ken's heroine is right to believe BLIT images are impossible. I really do.