Can a blind man see colours?
October 29th 2008 20:15
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From: # NewScientist.com news service
# Ewen Callaway
Who needs psychedelic drugs when you've got a good magnet? A world-famous patient with a brain injury that restricts his vision can see coloured stars in his blind field – but only when stimulated with an electromagnetic coil.
The patient – known as GY – suffered damage to the primary visual cortex of his brain – a region essential to vision – at the age of 7. The area is split into left and right halves, which detect vision in the right and left fields, respectively.
A chance participation in a university study later revealed that GY could detect things in the blind part of his visual field, launching a cottage industry probing his brain and vision. Tests showed that GY could at some level detect colour without being consciously aware of it.
"This patient has been studied very often because his lesion is very clear," says Juha Silvanto, a neuroscientist at the University of Essex, UK. "He's a celebrity. He travels all around the world."
Seeing stars
Though the impairment has left GY with a reduced field of view, he sees well otherwise. Just as full-sighted people don't see the gap behind them as a black area, he doesn't see his blind field at all, Silvanto says.
To test whether GY can experience conscious colour in his missing visual field, his team used a technique called transcranial magnetic stimulation (TMS) that excites or inhibits brain activity using an electromagnetic coil.
Applied to another part of the visual cortex, TMS can create the sensation of seeing flashes of light – called phosphenes. "If you hit the back of your head against the wall and you see stars, you're basically activating these neurons without any visual input," Silvanto says.
To create coloured flashes with TMS, the researchers showed GY and several control subjects a red or green computer screen before zapping their brains with the harmless magnetic field.
Seeping colours
In the fully-sighted subjects, the colour of the display determined the colour of their stars.
This was also true for GY in his intact visual field. But he also saw coloured stars when Silvanto's team flashed a magnet over the damaged half of his visual cortex. Yet this occurred only if he viewed the colour monitor in his visual field.
A split-screen experiment – green to his blind field and red to his visual field – created the sensation of red stars in his blind field of view. In normal patients, this experiment produced red and green stars.
"We don't exactly know what exactly is happening," Silvanto admits. But it seems likely that activation of GY's damaged half of his cortex seeps into the undamaged portion.
"This finding suggests that the conscious perception of colour may not require the primary visual cortex, at least in this patient," says Tony Ro, a neuroscientist at City University of New York.
Ro has previously shown that normal subjects in which this part of the visual cortex has been temporarily knocked out can nevertheless detect colours unconsciously.
Green and red flashes derived from strapping magnets to the skull will probably never replace psychedelic drugs for resolute trippers. However, Silvanto hopes to induce multiple colours and shapes in patients like GY to determine what other visual features can bypass the primary visual cortex.
Journal reference: Current Biology (DOI: 10.1016/j.cub.2008.08.016)
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