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Being able to read another person's mind is still science fiction. But Frank Tong, a cognitive neuroscientist at Vanderbilt University in Nashville, Tennessee, and his colleague Stephenie Harrison might have brought this fantasy a little closer to reality. Researchers thought that brain areas involved in the earliest stages of visual processing, including the primary visual cortex, could not retain the information they interpret from the signals received from the eye. Using functional magnetic resonance imaging (fMRI), Tong and Harrison have now shown that early visual areas do retain precise visual information about items that are no longer in the visual field — at least for a brief period (see page 632). Tong tells Nature more about the discovery.

What did you actually find?

We showed volunteers two striped patterns in different orientations and then asked them to remember one of the patterns for several seconds while we scanned their brains by fMRI — a technique that measures a signal produced by the increase in blood oxygenation that follows neural activity. By decoding the activity in the visual cortex, we could predict in more than 80% of the tests which of the two patterns a volunteer was remembering.

Were you surprised?

We thought we might find some evidence of visual memory in the visual cortex, but we were surprised to find it when brain activity was extremely low. It could be that when you're thinking about something, it is not at the same degree of vividness as when you are actually seeing it. Also, it could be that neurons in the visual cortex can transmit much information with little activity.

How were you able to interpret the signal?

Usually, fMRI signals are measured using 'voxels', a three-dimensional unit of measurement consisting of a few millimetres along each side. We used pattern analysis to pool the weak information contained in many individual voxels to obtain more robust information across the visual cortex. With this method, we can predict what people are seeing, paying attention to or actively remembering.

Will mind reading be possible some day?

We have a long way to go before these techniques could be applied to, say, a criminal investigation, but the possibility of reading out a person's thoughts does exist. But here we were reading out what our volunteers chose to remember, so people have some control over what thoughts can be read out. Right now, what we are doing is still fairly basic.