Scientists at University College London and University of California in LA are now able to tell what images people were looking at or what sounds they were listening to through scans of their brains.
The US team say their study proves brain scans do relate to brain cell electrical activity.
The UK team say such research might help paralysed people communicate, using a “thought-reading” computer.
In their Current Biology study, funded by the Wellcome Trust, people were shown two different images at the same time – a red stripy pattern in front of the right eye and a blue stripy pattern in front of the left.
The volunteers wore special goggles which meant each eye saw only what was put in front of it.
In that situation, the brain then switches awareness between both images, sometimes seeing one image and sometimes the other.
While people’s attention switched between the two images, the researchers used fMRI (functional Magnetic Resonance Imaging) brain scanning to monitor activity in the visual cortex.
It was found that focusing on the red or the blue patterns led to specific, and noticeably different, patterns of brain activity.
The fMRI scans could reliably be used to predict which of the images the volunteer was looking at, the researchers found.