Neuroscientists have gained new insight into how the brain perceives the world by using high resolution high field MRI to study activity in different layers of the visual cortex in great detail.
The visual cortex is the part of the brain that processes sight by receiving feedforward input from the eyes.
But feedback is also required from parts of the brain that conceptualise and contextualise in order for us to fully comprehend what we are seeing.
Scientists have so far been impeded by the integration of both signals within the six different layers of the cortex.
Researchers at the University of Glasgow have come up with a solution by taking advantage of the fact that input from the retina is mapped out in the visual cortex, the University said in a release.
Much like light entering the lens of a camera hits a specific portion of a sensor to form a pixel, so too light entering the eye has a corresponding portion in the visual cortex, which when applied to magnetic resonance images, is called a voxel.
In order to study the feedback signal, the researchers showed subjects a picture - for example a car - part of which was obscured by a white square.
This enabled them to identify and isolate the area of the brain that responded only to the occluded portion of the scene, and thus quieten the feed-forward signal.
But even in the absence of sensory input, the visual cortex communicates with other brain areas.
By measuring the activity in this part, the researchers were able to see where feedback and feed-forward activity took place across the six different layers of the cortex as the brain tried to complete the picture by inferring what the whole scene looked like.
The ability to measure layer specific signals in humans was made possible by 7 Tesla MRI techniques pioneered at the University of Minnesota's Center for Magnetic Resonance Research.
Professor Essa Yacoub and colleagues have developed techniques that allow visualisation of human brain activity at sub-millimeter spatial resolutions and with high degrees of accuracy.
Such a capability was previously only possible with invasive studies in animals.
The result, published in the journal Current Biology, reveals the layered cortical organisation of external versus internal processing streams during perception, with activity during normal visual stimulation peaking in mid-layers and contextual information peaking in superficial layers.
Professor Lars Muckli, of the Institute of Neuroscience & Psychology, said: "Understanding the brain's feedback system is important if we are to develop more powerful computers and artificial intelligence systems, but it might also help us to better understand mental illnesses such as schizophrenia and autism.
