One Man's Blindsight Offers Insights into Enhancing Subcortical Visual Pathways
ARTICLE IN BRIEF
Investigators comment on a case report about a patient who has no active visual cortex, but who had unconscious emotional reactions to facial expressions, an example of affective blindsight.
A remarkable account of blindsight involving a man who stepped around obstacles he claimed he could not see reveals the complexity of the visual system, and points toward potential therapies that would exploit visual pathways not yet completely understood.
The man, known as TN, was blinded by a pair of strokes 36 days apart, each one destroying the occipital cortex of one hemisphere, according a report in the Dec. 23, 2008, Current Biology. Imaging studies, including diffusion tensor imaging (DTI), revealed that TN had no active visual cortex, yet testing revealed that he experienced unconscious emotional reactions to facial expressions, an example of affective blindsight.
Doctors then created an obstacle course by placing boxes and other objects in a long corridor, and asked TN to walk down the hall without his cane. “Astonishingly, he negotiated it perfectly and never once collided with an obstacle,” the authors reported. (A video is available at www.youtube.com/watch?v=GwGmWqX0MnM). Beatrice de Gelder, PhD, a neuroscientist who heads the Cognitive and Affective Neuroscience Lab at Tilburg University in the Netherlands, led the study team.
SIGNALS FROM THE RETINA
How did TN accomplish this feat? The authors discount the possibility of echolocation, since neither TN nor the researcher hovering behind him emitted any sound. And imaging results show no residual vision in the occipital cortex.
But TN's eyes and optic nerves remain undamaged, which means visual signals from the retina probably continue to reach subcortical areas. Signals reaching the amygdala, for example, might explain his affective blindsight, which produced his unconscious emotional reaction to fearful faces, the investigators reported in a February 2003 paper in Brain.
Similarly, signals reaching the superior colliculus, which directs eye movements and apparently directs attention to moving objects in the visual field, appear to play a pivotal role in blindsight, according to a report in the July 2006 Brain.
The middle temporal (MT) area of the visual cortex, located in the superior temporal gyrus and believed to process visual motion, might have contributed to TN's ability to avoid the obstacles, according to Melvyn A. Goodale, PhD, Canada Research Chair in Visual Neuroscience, director of the Centre for Brain and Mind, and Distinguished Professor of Psychology at the University of Western Ontario.
“MT is one of the areas whose input is often spared after damage to the primary visual cortex,” said Dr. Goodale, co-author, with A. David Milner, of Sight Unseen: An Exploration of Conscious and Unconscious Vision (Oxford University Press 2004). “I don't think the detection of obstacles moving on one's retina during walking has to be conscious, however. Motion could be handled by a system that's quite impervious to conscious scrutiny. But sometimes such patients also retain the ability to actually experience visual motion.”
As an example, Dr. Goodale pointed to a case of subconscious vision similar to TN's reported by Gordon Neale Dutton, MD, an ophthalmologist and professor of vision science at Glasgow Caledonian University. He reported in the April 2003 issue of Eye that a woman in her 30s who became blind after bilateral occipital lobe infarctions discovered during an eye exam that she could detect motion.
“Both she and her husband were astonished when she accurately named the colours [sic]of large moving objects, and when she was able to accurately mirror hand movements being made in front of her with movements of her own hands,” Dr. Dutton reported in the paper. “However, she was even more surprised that she was able to walk accurately around a number of objects placed in her path, when she was encouraged to do so and told that she could.”
The patient discovered she could enhance her vision by moving her head from side to side, and by rocking in a rocking chair. Both actions caused objects in her field of vision appear to move.
“She can see rainwater running down a window but cannot see through it,” Dr. Dutton reported. “When her daughter is walking away from her she can see the pony tail moving from side to side but cannot see her daughter. She can see the movement of the water going down the plug hole but she cannot see her child in the bath.”
The woman had experienced such visual phenomena before visiting the ophthalmologist, but discounted them because she believed she was blind. “It seems that the act of drawing her awareness to her own subconscious vision has been the catalyst to the initiation of her (albeit incomplete) functional rehabilitation,” Dr. Dutton reported.
AVENUE FOR REHABILITATION
Enhancing the motion detection abilities of people with blindsight provides a promising avenue of rehabilitation, according to Ladan Shams, PhD, assistant professor of psychology at the University of California-Los Angeles, who has embarked on such research.
“We hope to study brain mechanisms that underlie multisensory motion perception and learning,” Dr. Shams said. “Many neurons in the primary visual cortex are selective for directional motion. MT is interesting because a lot of blindsight patients show activity in this area, even when the area V1 is entirely lesioned.”
Finding such ways to enhance subcortical visual pathways and promote rehabilitation of people with damaged vision “is where the future may lie,” said Steven L. Galleta, MD, Van Meter Professor of Neurology and Ophthalmology, chief of the Neuro-ophthalmology Division, and director of neurological training at the University of Pennsylvania School of Medicine. “If the primary visual cortex can't see, then the question is, can extrastriate areas dedicated to color, motion, and form be harnessed to enable the patient to do better than chance in a visual paradigm such as walking through a maze? There's all sorts of fascinating work to be done.”