Riley's early pictures, hard-edged, pathologically exact and, for the most part, painted in black and white, stand straight-backed against the gallery's walls like a ward full of preternaturally strict matrons. You half expect to hear the squeak of rubber-soled shoes and the alarming announcement that Dr Riley will see you now.
This clinical air is not entirely fanciful. Although the term was not coined until the New York Museum of Modern Art's "Responsive Eye" exhibition held in 1965, Riley's paintings of the early 1960s were busily helping to invent what would come to be known as Op Art.
If all artists are concerned with illusionistic effects, Op (as in optical illusion) artists are preoccupied with their medical repertoire: namely, illusions that gull the senses into thinking that lines, shapes and colours on a flat canvas are moving about or even generating bursts of light or colour.
Stand in front of Riley's Fission (1963), for example, and you will suffer the worrying delusion that its black-and-white dots, painted to give the impression of twin drums, rather like those you would find on an old-fashioned mangle, are revolving in slow opposition so as to suck you in. Other pictures destabilize the viewer by appearing to wobble, ooze or vibrate according to the movement of the watching eye.
Turn your head towards Current (1964) too quickly, for instance, and you will jump back in the belief that it is falling off the wall; look down at your feet while standing in front of the picture and it will dance about in your peripheral vision.
Other Rileys play tricks with colour perception. Breathe (1966) is clearly painted in black-and-white, and yet your delinquent visual synapses will insist that they can see an area of pale amber about two-thirds of the way down where they know perfectly well that there cannot be one. And so on.
If Riley's images are difficult to pin down in a spatial sense, though, you will have no trouble at all locating them in time. To walk into London's Serpentine gallery is to re-enter the precise moment at which the works were made; anyone who ever owned a psychedelic poster, shopped at the fashion store Biba, or watched the opening credits of The Avengers series will feel a warm sense of homecoming in this show.
Perhaps because of the economic certainties of the time, the 1960s were obsessed with the phenomenological uncertainty of believing one's eyes. It can be no coincidence that Riley's early Op paintings, the rediscovery of Aldous Huxley's book, The Doors of Perception, and also the recreational use of lysergic acid diethylamide (LSD), all date from the same epoch.
The connection is not one that has served Bridget Riley especially well, however. Her imagery seems so very much of its day that it is difficult now to remember that it actually pre-dated it: that her work helped shape the 1960s rather than the other way around. This is a shame, because the pictures at this show are more than just funfair mirrors or souvenirs of swinging London.
The two most obvious qualities of Riley's early paintings - their wobble factor and the way that they look like Biba window displays - are equally red herrings. Of course, Blaze 4 (1964) does fool the eye, and it is difficult to dispel a faint sense of having been gypped when her pictures don't "work" - when Turn (1964), say, just won't sway around no matter how you shake your head at it. But both of these pictures are also about a number of things that have nothing to do with their pseudo- kinetic properties.
One of these things is control. What makes Riley's work so uniquely frightening is not that her paintings cause our eyes to see things that they know aren't there but that they create their disorder out of utter regimentation. Look closely at a picture like Cataract 3 (1967) and you'll find that every perfectly spaced line, each apparently mechanically measured undulation, has been clearly painted by hand.
That Riley has always used paper templates and fleets of assistants to make her work is neither here nor there; the thing that really strikes you about Cataract 3 is the pathological order of its artistry. See the work in photographic reproduction - a medium that, on the face of it, might appear to be more suited to its mechanical aesthetic - and the effect is lost. It is the perversity of its being a painting on canvas that makes it the disturbing (and brilliant) work that it is.
Riley's pictures are also painterly in another sense. Shortly before she set out on her parallel careers as Op artist and Sixties chick, the young painter made a copy of Seurat's Le Pont de Courbevoie from the Courtauld Institute's collection. This was an act more of dissection than of straight homage. What presumably intrigued her were the mechanics by which Seurat managed to create an idea of luminosity out of what were actually nothing more than juxtaposed points of colour.
Still, the action does suggest something about Riley that comes through loud and clear in this Serpentine show: namely, that she sees herself as both a serious painter and as one working in a great tradition of illusionistic painting. Look at her early work and you cannot help feeling that she is right. There is, certainly, a great deal more to Bridget Riley than meets the eye.
`Bridget Riley: Paintings from the 1960s and 1970s': Serpentine Gallery, W2 (0171 402 6075), to 30 August. Sponsored by Bloomberg NewsReuse content