When your left world disapears: Hemispatial neglect.
I wanted to discuss a curious neurological disorder that was one of the main reasons I got into neuroscience, hemispatial neglect. Hemispatial neglect often occurs after a stroke. After the stroke, the entire left side of your world simply disappears. Anything with two sides, you just ignore the left. The food on your plate, you only eat the food on the right? You'll never put on the left leg of your pants. Asked to draw a clock? You will only draw the numbers 1 to 6, or cram all the numbers onto the right side. Try to draw a flower you will only draw half of it.
Figure 1 Top: A collection of clocks drawn from patients with hemispatial neglect after strokes. All these patients were only able to draw the right side of the clock, completely unable to realize they are missing the left side. The data can be found in the survey article by Chen and Goedert. Bottom: An example of a flower and a clock from a Swedish stroke patient who is suffering from neglect after a stroke. The example is from Dr. Gerafi's Thesis on Hemispatial neglect
Perhaps what is most curious about hemispherical neglect is only the left side of the world disappears. It is exceedingly rare for the right side of the world to disappear after the stroke. This is because for some reason the right side of your brain is capable of attending to both the right and left side of the world, while the left side of your brain only attends to the right side of the world. Thus if the patients stroke is on the right side of your brain your entire left world will disappear, while the stroke on the left side, leaves the right brain in tact, and thus both regions still function. It is important to stress that the patients are not blind, they simply do not think the left part of their world exists, even though they can see it. It is especially puzzling because it effects their memory as well.
Perhaps what is most curious about hemispherical neglect is only the left side of the world disappears. It is exceedingly rare for the right side of the world to disappear after the stroke. This is because for some reason the right side of your brain is capable of attending to both the right and left side of the world, while the left side of your brain only attends to the right side of the world. Thus if the patients stroke is on the right side of your brain your entire left world will disappear, while the stroke on the left side, leaves the right brain in tact, and thus both regions still function. It is important to stress that the patients are not blind, they simply do not think the left part of their world exists, even though they can see it. It is especially puzzling because it effects their memory as well.
Whats really strange about this, is left and right is relative, if you turn around your left and right switch. In a famous experiment by Bisiach and Luzzatti, they brought Italian neglect patients to the famous Piazza del Duomo. These patients lived in the area, and where very familiar with the court yard. When standing on one side of the courtyard they could only name the buildings on the right side. When the patients walked to the other side of the courtyard, their right and left sides switched. However, when asked what buildings they see, they could only name the buildings on the right. They completely forgot the buildings they just saw!
A picture of Piazza del Duomo from a great website on hemispatial neglect (in Italian). The website has many interesting examples of hemispatial neglect not covered here.
The brain region responsible for attention appears to be at the boundary between the temporal and parietal lobes (in the so called temporal-pareital junction TPJ). Interestingly, the right TPJ is very important in combining sensory information, with internal memory states and information about your bodies current position. When a stroke damages this area, it seems there is an inability to combine sensory information with body position. As a result it becomes impossible for one to pay attention to the left side of the world. Therefore, as strange as it sounds, the left side of your world simply disappears.
The brain region responsible for attention appears to be at the boundary between the temporal and parietal lobes (in the so called temporal-pareital junction TPJ). Interestingly, the right TPJ is very important in combining sensory information, with internal memory states and information about your bodies current position. When a stroke damages this area, it seems there is an inability to combine sensory information with body position. As a result it becomes impossible for one to pay attention to the left side of the world. Therefore, as strange as it sounds, the left side of your world simply disappears.
Author: Alexander White
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