Subliminal advertising: The voice within

A funny thing happened at a concert hall in Liverpool recently, when two artists attempted to implant a subliminal message in the minds of their audience. Iain Forsyth and Jane Pollard refuse to reveal the message, nor did they attempt to discover what effect it had on listeners. Instead, their aim was to recreate the atmosphere of 19th-century seances.

This pair of music-obsessed conceptualists came to fame with re-creations of seminal gigs, David Bowie's last appearance as Ziggy Stardust and a legendary performance by US rockabilly fiends The Cramps in a mental institution. Their interest in spiritualism came from those projects, Pollard explains. "We had talked about our re-enactments as a channel through which you raised the spirit of the past, then we realised the terminology we used came from spiritualism, so we looked at psychic research."

They noticed the invention of the telephone and gramophone tied into attempts to reach voices from the ether, thanks to the pseudoscientific connotations that Victorians attached to spiritualism. Forsyth and Pollard decided a modern equivalent would be conspiracy theories that we are being brainwashed via subliminal messages in mainstream media. A recent twist is suggestions that during the Gulf War the US army had used mind-control techniques to cause Iraqi soldiers to surrender.

These were based around the use of silent sound, broadcasts at frequencies either just below or above the range of human hearing. So with help from electronic specialists, the duo devised an infrasound machine to lower the pitch of their own voices. Instead of hearing what they said, the audience were able to feel them speak.

"When we tried it, we felt some pressure in the back of our skulls," Pollard explains. "Not that we're interested in the results, but we wanted the audience to imagine the potential, so we've worked with experts to make something viable."

Such is the power of their impressive machine, with its Fantastic Voyage-style dials and enigmatic symbols, that Forsyth and Pollard published a disclaimer. Pregnant women, people with heart conditions and under-18s were refused entry. Those who attended were warned they might feel anxious or light-headed. All their work is now on show at a Liverpool gallery.

Alongside them, Spiritualized's mainman Jason Pierce performed a piece he had composed alongside a string quartet and other musicians to mask the message. By embedding it within music, the artists followed attempts by advertisers and film-makers to suggest ideas by hiding images in their works.

As a term, subliminal advertising was coined in the Fifties when "Drink Coca-Cola" messages would flash by during movies. Despite public outcry over such techniques and being subsequently banned, psychologists were unable to build a case for the efficacy of such subliminal persuasion. Attempts to persuade Canadian viewers in 1958 to phone in to a TV show fell on blind eyes as well as deaf ears - not that this has calmed many people's worries over hidden messages, especially in rock music.

Here, artists have developed the technique of backmasking to hide messages, where sounds are recorded backwards to sound nonsensical when records are played normally. Reverse recording was first used by The Beatles' on the B-side "Rain", with John Lennon's vocals recorded backwards. He originally took credit, though George Martin later claimed it was his idea.

First to actually hide a message was Frank Zappa, who recorded backwards his songs that contained swear words to avoid censorship by his publishing company. Yet play any sung or spoken words backwards and you will inevitably find some hidden meanings.

At first, this was grist to the mill of Beatles conspiracy theorists who were convinced Paul McCartney was dead. Sure enough, they found such references as "turn me on dead man" when they played backwards "Revolution No 9" and "I'm So Tired" from the band's so-called White Album.

Problem is, it is difficult to find a particular message until someone tells you what to look for, when it leaps out of the gibberish. Not that this held back American fundamentalist Christians, who were convinced that bands had hidden messages promoting sex and drugs. Most infamous was Led Zeppelin's "Stairway To Heaven", which campaigners claimed contained the backmasked line "Here's to my sweet Satan", a charge denied by singer Robert Plant.

Matters came to a head in the mid-Eighties, when British metallers Judas Priest were sued following a suicide pact between two schoolboys in Nevada. One of the boys survived; their families' claimed that a Priest album contained hidden messages, with the words "Do it" audible when the record was played backwards. The case was dismissed after the band's defence called scientists that had already proved backmasking was ineffective. Psychologists John R Vokey and J John Read studied 300 volunteers, many of whom even found it hard to make out phrases, let alone show signs of their behaviour being altered.

By this time, groups were putting in messages for fun. Pink Floyd were serial offenders, with the line "That was pretty avant garde, wasn't it?" hidden away on Ummagumma's surreal excursion "Several Species..." On The Wall, the track "Empty Spaces" advises its listeners, "...congratulations. You've just discovered the secret message."

Christian-baiters Slayer are one of many hard rock bands to insert anti-Church messages in their music, while Cradle of Filth recited the Lord's Prayer backwards. A hint, perhaps, that fundamentalist Christians are still worried about witches that mocked their religion by reading prayers backwards.

No one has had more fun with this, though, than Ozzy Osbourne, who himself was in court over an alleged subliminal message in his 1980 album Blizzard of Oz. When the case was finally closed in 1988, Ozzy could actually insert a backmasked message in a more recent track, the line "Your mother sells whelks in Hull", a comment that sounds like a parody of one of The Exorcist's more potty-mouthed quotes.

Forsyth and Pollard's messaging system is more sophisticated than backmasking, though their use of the technique is just as playful as either Ozzy or Zappa. For those who love to bait rock's censors, the fun could begin again.

'Silent Sound' is on show as part of the Liverpool Biennial at Greenland Street, Liverpool (0151-706 0600), to 26 November