Blind Except for Movement: Woman’s Injury Offers Insight into How the Brain Works
Milena Canning can see objects only if they are moving, hinting at the inner workings of our visual system
Milena Canning can see steam rising from a coffee cup but not the cup. She can see her daughter’s ponytail swing from side to side, but she can’t see her daughter. Canning is blind, yet moving objects somehow find a way into her perception. Scientists studying her condition say it could reveal secrets about how humans process vision in general.
Canning was 29 when a stroke destroyed her entire occipital lobe, the brain region housing the visual system. The event left her sightless, but one day she saw a flash of light from a metallic gift bag next to her. Her doctors told her she was hallucinating. Nevertheless, “I thought there must be something happening within my brain [allowing me to see],” she says. She went from doctor to doctor until she met Gordon Dutton, an ophthalmologist in Glasgow, Scotland. Dutton had encountered this mystery before—in a 1917 paper by neurologist George Riddoch describing brain-injured World War I soldiers. To help enhance Canning’s motion-based vision, Dutton prescribed her a rocking chair.
Canning is one of a handful of people who have been diagnosed with the “Riddoch phenomenon,” the ability to perceive motion while blind to other visual stimuli. Jody Culham, a neuroscientist at Western University in Ontario, and her colleagues launched a 10-year investigation into Canning’s remarkable vision and published the results online in May in Neuropsychologia. The team confirmed that Canning was able to detect motion and its direction. She could see a hand moving toward her, but she could not tell a thumbs-up from a thumbs-down. She was also able to navigate around obstacles, reach and grasp, and catch a ball thrown at her.
Scans of Canning’s head showed an apple-sized hole where the visual cortex should be. But the lesion apparently spared the brain’s motion-processing region, the middle temporal (MT) visual area. “All the credit [for Canning’s perception] must go to an intact MT,” says Beatrice de Gelder, a neuroscientist at Maastricht University in the Netherlands, who was not involved in the study.
The next mystery is how information from the eyes gets to the MT without traveling through the visual cortex. “I think of the primary visual pathway as a highway. In Milena’s case, the highway dead-ends, but there are all these side roads that go to the MT,” Culham says. “It’s got to be one of these indirect routes, but we are not yet sure which one.” These side roads most likely exist in all our brains as remnants of the early visual system that evolved to detect approaching threats even without full-fledged sight, Culham says.
Canning is an eager participant in the researchers’ ongoing study. “If I can help them understand the brain more,” she says, “I could understand why I’m seeing what I’m seeing.”
Credit: Illustration by Thomas Fuchs – www.scientificamerican.com