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Experiment allows scientists to 'read' volunteers' thoughts

Neuroimaging technique gives a new insight into the location and nature of human memory.

Scientists have read the minds of healthy volunteers using a brain scanner to detect what they were thinking. By placing the volunteers in the scanner after they had been shown three film clips, the researchers were able to tell which clip they were recalling.

The advance brings a step closer the prospect of a "thought machine" to detect what a person is thinking from their brain activity pattern. But the technique is still at an early stage of development and its capacity to discriminate between "thoughts" is limited.

Scientists have searched for evidence of memory traces for almost a century. Although their biological existence is accepted, their precise mechanisms, location and nature remain a mystery.

Eleanor Maguire, professor of neuroimaging at University College, London, has previously shown it is possible to tell where a person is standing in a virtual reality room by using a brain scanner to detect the pattern of their thoughts. She has also shown that a small area of the brain at the back of the hippocampus was enlarged in male taxi drivers who had done "The Knowledge" – memorising the maze of London streets. These studies focused on spatial memory, the most basic sort.

The results of the latest study take the research further by showing that episodic memory – of the everyday events that make up the autobiography of our lives – can be tracked in the same way even though they are more complex. They demonstrate that these memories are stable and trigger the same brain activity each time they are recalled, making it possible for them to be identified and correctly interpreted on each occasion.

Professor Maguire said: "We've been able to look at actual memory traces for a specific episodic memory. We found that our memories are definitely represented in the hippocampus. Now we've seen where they are, we have an opportunity to understand how memories are stored and change through time. We are not at the point of being able to put people in a scanner and read their thoughts. But we can predict from their brain activity what they are thinking and remembering. The more we understand about how memories are stored, the more we can understand about how people [with brain injuries] can be rehabilitated."

For the study, 10 volunteers were shown three short film clips, lasting seven seconds each. They showed different actresses performing three tasks – posting a letter, throwing a coffee cup in a bin, and getting on a bike. The volunteers were then placed in a functional magnetic resonance imaging (fMRI) scanner and asked to recall each clip in turn. This was repeated many times and the scans were analysed to detect patterns in the brain activity associated with each clip. In the final stage of the experiment the volunteers were returned to the scanner and asked to recall the clips at random. The researchers found they were able to tell which clip they were thinking about from the pattern of their brain activity.

Although patterns in individual volunteers' brains varied from one another, they showed remarkable similarities in the parts of the hippocampus that were active. The findings are published in Current Biology. "We have documented for the first time that traces of individual rich episodic memories are detectable and distinguishable in the hippocampus. Now that we have shown it is possible to directly access information about individual episodic memories in vivo and noninvasively, this offers new opportunities to examine important properties of episodic memory," the researchers conclude.

Visible recall: How the experiment worked

*Volunteers watched three seven-second film clips of a woman posting a letter, throwing a cup in a bin or getting on a bike, and were asked to recall one of them while in the brain scanner.

*The researchers were able to tell which of the three clips they were recalling by observing their brain activity.

*The brain scan of one volunteer showed where the memories were laid down in the hippocampus – the brightest spots indicate where the memories of the three clips were most distinct from one another.