For the first time, researchers have visualised the effects of LSD on the human brain, suggesting that people under the influence of the drug see things from their imagination rather than from the outside world.
The findings show what happens in the brain when people experience the complex visual hallucinations that are often associated with LSD state.
In a series of experiments, scientists from Imperial College London have gained a glimpse into how the psychedelic compound affects brain activity.
Researchers administered LSD (Lysergic acid diethylamide) to 20 healthy volunteers and used various leading-edge and complementary brain scanning techniques to visualise how LSD alters the way the brain works.
A major finding of the study is the discovery of what happens in the brain when people experience complex dreamlike hallucinations under LSD, researchers said.
Under normal conditions, information from our eyes is processed in a part of the brain at the back of the head called the visual cortex.
However, when the volunteers took LSD, many additional brain areas - not just the visual cortex - contributed to visual processing, they said.
"We observed brain changes under LSD that suggested our volunteers were 'seeing with their eyes shut' - albeit they were seeing things from their imagination rather than from the outside world," said Robin Carhart-Harris from Imperial College.
"We saw that many more areas of the brain than normal were contributing to visual processing under LSD - even though the volunteers' eyes were closed. Furthermore, the size of this effect correlated with volunteers' ratings of complex, dreamlike visions," said Carhart-Harris.
The study also showed what happens in the brain when people report a fundamental change in the quality of their consciousness under LSD.
"Normally our brain consists of independent networks that perform separate specialised functions, such as vision, movement and hearing - as well as more complex things like attention," said Carhart-Harris.
"However, under LSD the separateness of these networks breaks down and instead you see a more integrated or unified brain," he said.
The results suggest that this effect underlies the profound altered state of consciousness that people often describe during an LSD experience, researchers said.
"It is also related to what people sometimes call 'ego-dissolution', which means the normal sense of self is broken down and replaced by a sense of reconnection with themselves, others and the natural world," Carhart-Harris said.
"In many ways, the brain in the LSD state resembles the state our brains were in when we were infants - free and unconstrained. This also makes sense when we consider the hyper-emotional and imaginative nature of an infant's mind," he said.
The findings were published in the journal PNAS.
