Trouble Thinking

July 9, 2010

Hemispatial Neglect: Half the World Ends With You

Filed under: Science — Tags: , , , , — Durandal @ 2:52 pm

You probably have functioning eyeballs. A bold claim, I know, but I didn’t build my reputation by not being the most courageous voice of the 21st century.

You might think your functional eyes mean you have vision covered. Maybe you’re getting cocky about it. Sometimes you look at things just to prove you can. Maybe you sweep your eyes over a beautiful sunset and have a little chuckle about the blind. You’re a monster.

But I’m not here to address your grievous moral failings, I’m here to tell you that there’s more to seeing than sight. That is, seeing isn’t just the ability of your eyes to receive information. Seeing is the world showing up. And for the world to show up, you need to have more than just sight. You need to be paying attention.

Have you ever been so focused on a book that you didn’t notice anyone near you talking? Or so focused on a game or movie that you didn’t see the fire starting slightly to your right?

Or been so focused on the right side of the world that you ignored the left half of everything in existence?

What’s that? That’s silly? No. That’s Hemispatial Neglect. Admittedly, it would be difficult to experience without some brain damage.

Usually caused by a stroke or other form of damage to the right parietal lobe (the bit that tends to handle spatial relations), it causes patients to entirely ignore the left. Left of what? Well, there are two broad types. Patients can ignore things to the left of their own mid-line (egocentric neglect), which amounts to drawing a line down the center of the entire visual field and ignoring the left half. Patients can also ignore the left half of every given thing (allocentric neglect), which would be like drawing a line down the center of every object in sight and ignoring the left half. Interestingly, many times patients will reorient objects, then ignore the left half. That is, if you have them hold a map upside down, they’ll still be unable to see California because it’s on the “left half” of the US even though it’s in the right half of their visual field at the moment.

There’s also some differentiation in terms of axis and range. That is, some people will neglect the left half of a line drawn at a slight angle, or may neglect objects only at a distance or only nearby. There’s also the chance that a patient will display output neglect. That is, some patients become unable to understand that the left side of their own bodies is them, and therefore unable to use limbs that are perfectly healthy. Oliver Sacks wrote about a man who pushed his own leg out of the bed because he thought some prankster had placed a corpse leg in his bed. Which I suppose ignores why his go-to explanation was “weird prank hospital” but whatever.

Some patients won’t shave the left side of their face, or eat the food on the left half of the plate, or will draw things without left sides.

It doesn't help if you're kind of shit at drawing to begin with.

Now, let’s be completely clear: these patients don’t have any issue with their sight. Certainly their eyes are fine. Even in terms of brain damage they don’t have anything that should cause loss of vision, the Parietal lobe isn’t really directly related to sight. So why is it that these patients can’t see the left side of anything?

Well, there’s more to seeing an object than sight. Think of a Where’s Waldo book, for instance. The first time you look at the picture, Waldo is in your sight in the sense that light bounces off of the Waldo in the picture and into your eyes and your brain processes that information. But if someone asked you to point him out, you’d state you can’t see him. That isn’t because there’s a little black bar over Waldo in your vision, or because you have cataracts, it’s because you lack the ability to direct your attention specifically at Waldo. You don’t consider yourself able to see an object until you can pay attention to it. If you can’t pay attention to an object, you can never see it.

An interesting aspect of this inattentional blindness is that the patient, more often than not, has no idea he or she can’t see the left side of things. In fact, in the worst cases the patient may even draw things from memory as though they have no left side. While slightly hilarious, this can be immensely difficult for patients to deal with. It’s difficult to interact with the world without constantly injuring yourself when it’s basically like the Invisible Woman is following you around being a jerk.

Try to shave NOW jackass!

Luckily, it is possible to treat and if you’re fortunate (well, relatively) enough to be affected by it as a child your likelihood of recovering your left-sight is reasonably high.

Treatment usually consists of neurologists and therapists attempting to slowly draw more and more of the patient’s attention to the left side of the visual field as a whole, or individual objects while trying as hard as they can not to yell “LOOK TO THE LEFT YOU ASS THE LEFT SIDE GOOD LORD”.

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