Among the most sought-after subjects for visual perception research are people with the condition known as hemianopia, in which damage to the primary visual cortex (V1) on one side of the brain blanks out the corresponding half of the visual field in both eyes.
People with hemianopia can react to stimuli in their “blind” fields, even though they deny being aware of the stimuli. This “blindsight” phenomenon implies the existence of neural detours that allow information to flow from the eyes to the higher regions of the visual cortex without traversing the damaged V1.
Because these neural detours usually operate beneath awareness, the properties of blindsight are also a favorite hunting ground for consciousness researchers, who hope to determine precisely which neuronal activities are needed to create a conscious perception.
“Previous studies have looked at what brain areas are activated by the blindsight phenomenon, whereas we’re trying to do the opposite—we’re trying to make those activations reach awareness,” says Juha Silvanto, a researcher at the University of Essex in the United Kingdom.
Silvanto recently conducted a series of experiments with the world’s most researched hemianopia case, a man now in his 40s known as G.Y., whose left-side V1 was destroyed by a stroke following a childhood car accident. The precision with which G.Y.’s V1 was damaged, leaving virtually his entire remaining visual cortex undamaged, has made him uniquely valuable to researchers.
Silvanto and his colleagues found in 2007 that by using transcranial magnetic stimulation (TMS) to excite neurons in G.Y.’s left and right visual cortices at nearly the same time, they could cause him to perceive a bright, arc-shaped object, known as a phosphene, that extends into both his “blind” and intact visual fields. People with undamaged V1s can get a similar perception from TMS stimulation on either side, but “if we just activate G.Y.’s damaged hemisphere he doesn’t perceive anything,” says Silvanto. “The idea is that [with TMS on both sides] the activation originating from the damaged hemisphere becomes part of the process that gives rise to awareness.”
Subsequently Silvanto and his colleagues found that they could make G.Y. see the phosphene in color, even in his “blind” visual field, if they exposed him to the same color beforehand—but the color he perceived in his blind field was the color to which his other, intact visual field had been exposed. They reported these results in the Oct. 28 issue of Current Biology.
Continued exposure to a given color normally reduces the activity of the neurons tuned to that color in a process known as chromatic adaptation; a red square, for example, if shown for 10 seconds, will leave an afterimage that appears green because the neurons tuned to red have been suppressed. Silvanto and colleagues think that when neurons are inactivated in this way, they react again more strongly when stimulated by TMS—thus explaining why G.Y.’s phosphenes appeared in the same color to which his intact visual field had already adapted.
How did the color information from his intact visual cortex enter consciousness as if it had reached his damaged visual cortex? Silvanto and other researchers would like to find out. “We only know that these phosphenes result from the interaction of activity in G.Y.’s damaged hemisphere with activity from his normal hemisphere,” Silvanto says.
Tony Ro, a neuroscientist at City College of New York who has also done research in this area, notes too that it remains unclear what is happening in G.Y.’s brain during his blind-field perception of the color phosphene. But he suggests that the phenomenon may be related to the perceptual filling-in that occurs, for example, in the small blind spot (where the optic nerve enters the retina) in every sighted person’s field of view.
In G.Y.’s blind field, in other words, the colored phosphene “may be an extrapolation or interpretation of information that was seen during chromatic adaptation and is seen in the TMS-induced phosphene in the normal hemifield. If so, this suggests that conscious color perception is relying upon some higher-order brain area that is interpreting neural activity, or the lack thereof, across several lower-level visual areas.”
Ro points out that the TMS in this case was used, on both sides, on the visual area known as V5/MT, which is usually considered responsible for motion perception—yet for G.Y. it seems to have been responsible for color perception too.
“Perhaps most interestingly and importantly,” Ro says, “this finding provides evidence that the conscious perception of color does not require the primary visual cortex, at least in this patient.”
Silvanto emphasizes that when he and his colleagues stimulated both sides of G.Y.’s brain to produce the phosphene, the timing of the magnetic pulses on each side was crucial. “What we want to try next is to get the same outcome when we use visual stimuli instead of magnetic stimulation,” he says. “But it’s important that the information from the damaged hemisphere reaches the intact hemisphere at the precise time.”