Can this really be London?

You stand at a dizzy altitude, overlooking an out-of-focus cityscape that could be London, Paris, Berlin ... any city. You reach out to steady yourself, and then suddenly you are on the Eiffel Tower.

Or rather, you're not. You're actually inside a circular group of screens the size of a small room, in front of a touch-sensitive computer console, experiencing Graham Ellard and Stephen Johnstone's installation Passagen.

"Interactivity" is the buzz-word of the moment, and nowhere more so than at Liverpool's Video Positive Festival. But Passagen stands apart from other interactive works featured this year. It not only attempts to simulate urban reality but also tries to make the jaded metropolitan see the city through new eyes.

"We wanted people both to be, well, seduced by the installation - to enjoy its pleasures - and to stand back from it," says Johnstone. "The images don't pretend to be anything other than representations."

That is how Johnstone and Ellard's inspiration, the German philosopher Walter Benjamin, viewed the city. According to Johnstone - who bears a startling resemblance to a slimmed-down Dan Aykroyd - Passagen "celebrates the city" just as Benjamin did: by attempting to "draw out those latent dreams of the city and make them a bit more concrete". But Benjamin also believed it was necessary to stand back from the "modern" - ever-more- ambitious architecture, gadgets and machinery - and question the thinking behind them.

As such, his unfinished Passagen-Werk (found, according to myth, in a suitcase he had with him at his suicide in 1940) was intended to wake people up from their "urban dream". His method, followed by Ellard and Johnstone more than half a century later, was to reveal the history beneath the surface dazzle.

And so, after Passagen's initial cityscape - London, Paris or Berlin, depending which panel you press - you speed down to street level, floor numbers flashing by. Then you follow as a man hurries down the steps into the underground system. As you enter, a measured European voice announces: "In the labyrinth, every step you take is on named ground."

Viewers are urged to look beyond the things that busy residents take for granted, and examine their surroundings as a child or tourist visiting the city for the first time. What is the "angel" of the underground? What is Karl Marx's connection with the Berlin U-bahn? Attention is drawn to small details such as the hole in the station map where a thousand fingers have jabbed their location. The underground is full of stories: each flashing station sign contains one.

Press Invalides in Paris and a snatch of old film shows a French driver in Venezuela describing how a ticket to Pigalle is the most valuable thing he has. "A metro ticket can take you farther than you think," he exclaims.

Passagen isn't merely a collection of ideas or a statement of facts. "We didn't want to give a history lesson," says Johnstone. The installation offers a tantalising visual and aural experience. Familiar urban images are seen in different, sometimes impossible ways. Try to find Passagen's view over Paris for real and you will be disappointed: between you and the world is a large metal grille. To create the Paris cityscape, Ellard and Johnstone had to put the camera through the grille at the Eiffel Tower and then use digital processes to combine 24 separate photographs.

"The city is a place of wonderment," says Johnstone. "But a lot of the time we're too busy - or just too frightened - to appreciate that."

Passagen forces the viewer to slow down and adopt another outlook: the installation can make the mean streets around you seem shockingly fresh, exciting and mysterious once more.

'Passagen' is at the Liverpool Tate Gallery until 4 June, as part of the Video Positive Festival. Admission is free. Festival hotline: 0151 709-2663.

Toby Manning