Nov. 3, 2008 A researcher from the University of Essex has succeeded in inducing the experience of colour in the blind part of a partially sighted patient’s visual field.
Using a magnetic coil to stimulate the patient’s brain, Juha Silvanto demonstrated for the first time that it is possible to experience visual sensations of colour in an area of blindness caused by a cortical brain lesion.
Dr Silvanto, a lecturer in visual perception from the Department of Psychology, explained: ‘We demonstrated that the undamaged side of the patient’s brain had taken over the function of colour perception.
‘This technique demonstrates that it is possible to restore visual awareness but we don’t yet understand the neural mechanisms. If further research enables us to uncover this process, it opens up the possibility of therapeutic treatment.’
Working with Alan Cowey from the University of Oxford and Vincent Walsh from University College London, Dr Silvanto tested a patient who had suffered damage to the primary visual cortex of his brain in a childhood accident. The lesion to this part of his brain considered essential for vision has left him with a reduced field of vision on his right side.
The researchers demonstrated that the colour shown to the patient’s intact (left) visual field determined what colour he saw in his blind field. By first presenting a red stimulus to his intact visual field, this created the sensation of seeing red also in the blind field when his visual cortex was subsequently stimulated using magnetic stimulation.
The partially sighted patient had previously been unable to experience visual perception in his blind field for more than 40 years.
While this was a short-term effect, it opens up the prospect of developing longer-lasting therapy to recover visual function, for example in stroke patients.
In the portion of his visual field controlled by the damaged part of the brain, GY has a condition called blindsight. This phenomenon can occur when people do not consciously see as a consequence of a V1 lesion. However, when forced to guess which way a moving object they "observed" was traveling, for instance, they get it right most of the time. In other words, despite the fact that they do not experience vision, they nonetheless continue to detect things around them.
In the new study, the researchers applied a method called transcranial magnetic stimulation (TMS) to GY's primary visual cortex. By stimulating both the normal and the damaged hemispheres of the brain, the method can induce visions of flashes of light (or phosphenes) in the blind fields of people like GY.
This method has previously been used in a general way to stimulate entire brain regions. In the new study, Silvanto's team developed a more targeted method to activate particular neurons with TMS by taking advantage of a trend that had been seen before: TMS preferentially activates those neurons that were less active to begin with.
They asked GY to look at a screen in the color red for a time; this adapts the brain to the color red, leaving the neurons responsible for the experience of red to become less active. They then applied TMS to GY's damaged and intact visual processing centers. The result: he saw the color red.
"This was the first time this patient [consciously] experienced a colored visual percept in his blind field," Silvanto said.
"In summary," the researchers wrote, "our results show that in the absence of V1, color perception may be possible via the intact hemisphere."
The more targeted TMS method the team developed is also an important technical advance for cognitive neuroscience, Silvanto added. "Now we can target the stimulation at specific populations of neurons," he said. "It makes the resolution of the technique much higher."
The researchers include Juha Silvanto of Beth Israel Deaconess Medical Center and Harvard Medical School, and University of Essex; Alan Cowey of University of Oxford; and Vincent Walsh of Institute of Cognitive Neuroscience, University College London.
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