Visual Neglect

When a patient named Sally had a stroke that damaged her right parietal lobe, she seemed perfectly normal. Though she was paralyzed on her left side, she remained the "talkative, intelligent woman" she was before the stroke.

Yet, soon after she came home, Sally's father began to notice strange tendencies in her behavior. Tendencies that she, who seemed completely cognizant, was completely unaware of. Sally would bump into objects on her left side, consume food only the right side of her plate, and as a whole seemed indifferent to the events that were taking place to her left. She wasn't blind in one eye or unable to "see" what was going on in the left, but unless someone drew her attention towards the left side, she seemed unphased by it. For example, if someone pointed to the remaining food on the left side of her plate, for Sally, it was like seeing it for the very first time.

Sally's condition is dubbed visual neglect. And like Sally, patients who suffer from it have trouble dealing with the "left" (interestingly enough, visual neglect only works in right's favor). They may look to the right constantly, apply makeup only to the right side of their face, or brush only the right-half of their teeth.

Brain areas in parietal lobe are associated with the deployment of attention. When the area is damaged, individuals are unable to "attend" to certain visual input. The authors of the paper liken the scenario to the images with two recognizable forms. Since the brain only allows a small subset of its neurons to be active at any time, there is an "attentional bottleneck," that forces us to be conscious of one pattern at a time. Visual neglect would be an exaggerated example of this.

The condition expresses itself in other ways as well. If tasked with drawing a flower or a house for example, patients may only draw half the flower or house. If tasked with filling in the numbers of a clock, patients may write in only half the numbers--or even more strange, may write in all the numbers but use only half the area of the clock.

As mentioned, in Sally's case the stroke led to the paralysis of her left side. But Sally didn't seem to know this (a condition called anosognosia). If asked to touch her nose with her right hand she did so easily. If asked do the same with her left, she would agree to the task, and promptly lift up her left hand with the right one. She wasn't trying to be funny. As the author explains "even though 'she' (the conscious person) was unaware of the paralysis, some part of the brain 'knew' the left arm was paralyzed."

Yet, the most interesting aspect of visual neglect is how certain patients are that nothing is wrong. When scientists positioned a student holding a pen to Sally's left and held a mirror to Sally right, they wondered if the illusion might correct Sally's neglect. Sally could clearly see the pen in the mirror. She was then asked to take the pencil with her right hand and write her name on a notepad in her lap. Instead of reaching to her left, Sally reached into the mirror, insisting that the pen was "inside the darned mirror." On further attempts, the situation did not improve, instead she would reach behind the mirror, searching for the pen there.

It was as if... Sally was more confident that "left" didn't exist, than that mirrors were mere illusions.

As authors Vilayanur S. Ramachandran and Diane Rogers-Ramachandran write,

"What is surprising is the illusion's resistance to intellectual correction. Her high-level knowledge about mirrors and what they do cannot correct her behavior even after repeated failed attempts to grab the pen. Indeed, it is the other way around: her knowledge of mirror optics has been warped to accommodate the strange sensory world she is now trapped in... "

Source:
"Half a World." Scientific American Mind.
http://www.sciam.com/article.cfm?id=seeing-the-world-in-half-view