I recently went to a party where our host regaled us with a compilation of concert recordings in which famous pianists had suffered from horrible memory lapses. Everyone fell about with laughter at the sound of celebrities going hideously off the rails, but, as a pianist, I found it an uncomfortable experience. The struggles of Curzon, Richter and Rubinstein with memorisation had become a spectator sport.

Playing from memory in public is a fairly recent fashion. Before the late 19th century, playing without the score was often considered a sign of casualness, even of arrogance. The custom of playing from memory developed along with the growth of a body of classics that everyone agreed were worth preserving exactly as their composers had intended. Teachers encouraged students to memorise them. Many young players memorise easily, but it gets harder as time goes on. As the pianist Charles Rosen put it: "With advancing age, memory becomes doubly uncertain; above all, what begins to fail is confidence in one's memory, the assurance that the next note will follow with no conscious effort."

Clara Schumann felt that playing by heart "gave her wings power to soar", but many musicians find it so stressful that they play less naturally than they would with the score. And the pressures are much worse today than they were in Clara Schumann's day. After a century of recording, the record-buying public has been trained to expect perfection, whereas earlier audiences didn't mind if things went occasionally awry.

The burden of memorisation falls particularly on solo instrumentalists. I've always played from memory in solo recitals and concertos, but I play chamber music from the score. Chamber groups are not expected to play from memory; those that do - like the Kolisch Quartet in the 1930s, or the Zehetmair Quartet today - are regarded as spectacular exceptions. Nor are symphony orchestras expected to play from memory. And no one suggests that playing a chamber work or a symphony with music on the stand prevents a performance from being superlative.

Conductors sometimes conduct from memory, but they themselves don't have to make a sound, so many mistakes go unnoticed. Opera singers have to memorise, but they have the help of prompters, discreetly feeding them the next line. Songs have words, and because words are our everyday language, they help singers to memorise. Abstract music is also a language, but one whose immediate meaning is less clear-cut. Scientists now agree that memorising music is more complex than memorising words, and the challenge is multidimensional for those who also have to play instruments.

It's not as if composers require musicians to memorise. In Beethoven's day, his pupil Carl Czerny apparently had such a phenomenal memory that, as a teenager, he could play all his master's works by heart. But Beethoven disapproved, saying it would make him casual about detailed markings on the score. Chopin was angry when he heard that one of his pupils was intending to play him a Nocturne from memory.

Others felt it would be inappropriate to play without a score, Mendelssohn, who had an amazing musical memory, was nevertheless modest about it. When he visited London and took part in a performance of one of his own piano trios, the piano part was missing. "Never mind," he said, "just take any book of music, place it on the music desk, and have someone sit beside me and turn the pages, and then no one need know I play from memory." Liszt, though gazing heavenwards in contemporary drawings of him at the piano, appears to have played only half of his repertoire from memory. And when he played his own compositions, he used the score to demonstrate that these were seriously worked-out pieces, not fleeting improvisations.

The growing taste for watching soloists play from memory has actually narrowed the breadth of the repertoire. Vladimir Horowitz, for example, played a huge number of works at home from the score, but only performed a small repertoire from memory in public. Today many soloists won't commit themselves to more than a handful of works each season, no doubt partly because of the burden of memorisation. In the past few years, I've successfully memorised several solo recital programmes, each lasting about two hours. Had I allowed myself to use music, I could have performed the programmes much earlier, and with equal interpretative power. It was the sheer effort of memorising that added months to the process.

Secure memorisation has several elements that interweave unpredictably for the player. There is analytical memory, an understanding of the music's structure. There's photographic memory, which enables the player to "read" the visualised page when it isn't there. And there's physical memory, perhaps the most dependable kind, but one for which there's no shortcut. In performance, muscle memory will carry you along when something distracts you, or when you have a moment of doubt. Indeed muscle memory can enable you to play the whole piece while thinking about something else entirely (as I discovered when putting in my hours of piano practice as a child). But to develop it you simply have to play the music over and over, for a long period of time.

I recently gave two performances of the Schumann piano concerto. For several weeks before the concerts, I privately played the piece by heart without problems at least once a day and felt very secure. At the first performance, however, with an orchestra of 60 musicians and 2,000 listeners, I had several terrifying moments of insecurity. Worse, they were in places in the piece where I'd never had trouble before. So the following day I hammered those places into my memory. At the second performance, I had another couple of nasty moments - but in completely new places.

Must musicians waste so much of their time and emotional energy on memorisation? If we've prepared the music thoroughly, does playing it from memory really add an extra dimension that is worth all the pain?

· Susan Tomes plays with the Florestan Trio on May 15 at Rodewald Concert Society, Liverpool, and at the Florestan festival, Peasmarsh, East Sussex, from June 7-10