Could a drug-free mind ever imagine sailing the oceans in a yellow submarine, or talking to a girl with kaleidoscope eyes? That was the question asked by a Scottish scientist who spoke at Liverpool's famous Cavern Club last week in an attempt to divine whether The Beatles would have been as good if they had not indulged in illegal substances.

Professor Judith Pratt, of the Strathclyde Institute of Pharmacy and Biomedical Sciences, examined the effects drugs have on the way individuals perceive the world before relating it back to The Beatles's evolution from innocent boy band to spaced-out hippies, a trip that saw their music evolve from pop to Technicolor psychedelia and eventually into heroin-influenced blues.

The event was not a normal scientific symposium: after her talk, the audience stood up and sang along to Hey Jude.

Pratt said: "What would their songs be like if they hadn't been exposed to drugs? Taking drugs certainly had a positive influence on The Beatles's songwriting, although that's not to say you should encourage people to take them. The boost in creativity comes from the way drugs changed their perception. But it's a moot point whether they would have made the same music if it weren't for what they were taking."

She focused on two drugs: LSD and cannabis. Smoking marijuana, Pratt claimed, gave them their first boost in creativity. The possible reasons why cannabis didn't cause psychosis in the Fab Four include the relative weakness of the drug, compared to the hyper-strength "skunk" smoked nowadays, but also because the herbal variety contains a substance called cannabidiol, which some scientists think could work to prevent psychosis.

But it was LSD that really sent The Beatles rocketing off the planet. Pratt said that under its influence they recorded Tomorrow Never Knows, inspired by the Tibetan Book Of The Dead, and much of the music of their psychedelic phase, including I Am The Walrus, the Magical Mystery Tour EP and the Sergeant Pepper's Lonely Hearts Club Band album, which notoriously includes Lucy In The Sky With Diamonds, widely interpreted, though never confirmed by its composer John Lennon, as a reference to LSD.

Pratt said: "They might not have gone into that more spiritual spiral if they hadn't taken LSD."

She was unable to conclude whether The Beatles would have been as good, or at least as adventurous, had they never been introduced to marijuana by Bob Dylan in 1964, and insisted there was "not an absolute, direct link" between the drugs and their greatest work.

One pop star cast aspersions on the idea that The Beatles's success was down to their choice of substances. Jackie McKeown, lead singer of the indie band 1990s and former bandmate of Franz Ferdinand stars Alex Kapranos and Paul Thomson in Yummy Fur, told the Sunday Herald he believed The Beatles's music was down to the company they kept and the musical climate of the time.

He said: "The Dylan effect is more pronounced than the weed. You've got these songs like Hide Your Love Away, which sounds really stoned, but also sounds like Dylan.

"A few years later, it's Sergeant Pepper. But are they really being influenced by LSD, or is it Jefferson Airplane, The Byrds and The Grateful Dead? There's nothing on Sergeant Pepper you wouldn't have heard in San Francisco six months earlier. All the lyrics are like plastic, mind-expanding dogs in the sky' kind of stuff, which is basically Beat poetry set to music."

He did concede that one drug, heroin, had had the positive effect of wiping away the "excess" of their psychedelic period and led to the pared-down, bluesy rock of the "White Album".

Jim Gellatly, a former DJ on Xfm Scotland, said it didn't matter whether The Beatles were influenced by drugs, because the music was always good, unlike some of the more overblown music that followed them.

He said: "Look at Pink Floyd's music. You need to take drugs to listen to it and they needed to take drugs to make it."