{♫Intro♫}.

Imagine sitting down to dinner, eating everything on the plate, and feeling half as full as usual. Or maybe going to shave, but weirdly enough, it takes half the time.

Losing half of the world sounds like a weird, abstract dream state. But for those that develop hemispatial neglect, that's exactly what happens, without them even realizing it. This condition isn't totally understood, but research into it has given us some fascinating insights into just how we pay attention.

Hemispatial neglect is a condition that causes people to completely neglect one of their sides. As in, they're completely unable to pay attention to half their world. Usually it's the result of a stroke, and, while estimates vary as to exactly how often it happens, it's relatively common as far as neurological conditions go.

People who live with this condition are typically unable to do anything that requires attention to their neglected side. They might only do their makeup on one side of their face or only read one side of a newspaper. And if people approach them from their neglected side, they might not notice.

Hemispatial neglect doesn't just apply to day-to-day life, either — it can also show up in your memories. For instance, in 2013, researchers in Italy asked 96 patients with hemispatial neglect to picture themselves standing in a familiar public square — and asked them to describe what they would see. The participants successfully described the right side of the square, and totally neglected to mention any of the features of the left side — even though they were just picturing being there.

Then, the researchers asked them to imagine standing on the opposite side of the square and to describe their surroundings again. Now the participants were only able to describe the things on their new right side, even though just moments ago they hadn't registered those things at all. As for everything they'd noticed in the first round — the features that were now on their left — they neglected those entirely.

As dramatic as that sounds, someone living with hemispatial neglect wouldn't easily realize one side had disappeared. But from the outside, the signs are very clear, and doctors can easily test for the condition. One common clinical test is to ask a patient to draw the face of an analogue clock.

If they're neglecting one of their sides, all the numbers end up smushed together on one side or the other. But even though it seems like the brains of people with this condition just block out a part of the world completely, research has shown that, on some level, they are still processing information from their neglected side. Like, experiments have shown that people can process the meaning of a word that's only shown to their neglected side, even if they have no awareness of actually seeing the word.

Meaning, just the fact that they're shown the word will affect their behavior on decision-making tasks, even if they don't know they saw it. So, the problem isn't that the information isn't getting in; the brain is just not paying attention to it. So what makes the brain just… forget a whole side of the world exists?

One clue comes from the fact that hemispatial neglect often happens after strokes and other types of brain damage that affect a part of the parietal lobe called the inferior parietal lobule. This part of the brain is involved with paying attention to where things are. So when it gets damaged, you might be unable to notice things in certain locations.

But there's an unusual twist: Although hemispatial neglect can happen on either side of the brain, it's usually the left side of the world that disappears. And since the right side of the brain controls the left side of the body, it comes from damage to the right parietal lobe. It can happen on the other side, but in general, people with an injury on the left hemisphere recover from hemispatial neglect quickly and without intervention.

On the surface, that seems bizarre. But this mismatch can actually tell us a lot about how our brains manage our attention. Scientists have two main hypotheses about why this might happen.

Although both parietal lobes are involved in spatial awareness — or, telling us where things are in space — the left parietal lobe is also highly specialized for processing language. So, scientists think that it may only have the bandwidth to attend to space on one side: the right. Meanwhile, the right parietal lobe deals with spatial awareness on both sides.

So it can pick up the slack if you damage your left parietal lobe. But it doesn't work the other way around: If you damage your right parietal lobe, there's no backup — you just lose awareness of your left side. At least, that's the idea.

But some clinical observations of patients with hemispatial neglect suggest there's more to the story. Specifically, there appears to be a link between attention and alertness. People with hemispatial neglect have trouble staying alert, and this condition isn't the only time scientists have seen a connection between attention and alertness.

As healthy people start to fall asleep, research has shown that they also begin to neglect their left side. For example, in one study, participants who were asked to identify the direction of a sound as they drifted to sleep typically identified sounds on the left as coming from the right. Scientists are suggesting here that as we drift off to sleep, the left side begins to suppress activity in the right hemisphere that helps keeps us awake — that way we can switch off for the night.

According to this hypothesis, as that activity gets suppressed, the same brain regions involved in hemispatial neglect become less active too, and we lose awareness of our left side. Scientists still don't have a perfect theory nailed down. But cases of hemispatial neglect make it possible to isolate aspects of the way typical brains function.

Most of us go about our lives seeing the world as one big cohesive experience, and can't recognize the different aspects of that experience. But this condition goes to show that your brain is constantly working to maintain one really impressive balancing act. And that when it comes to nailing down all the tricks behind that act, we still have a lot to learn from conditions like hemispatial neglect.

Thanks for watching this episode of SciShow Psych! And a special thanks to this month's President of Space, Matthew Brant, for helping us bring you this episode! Matthew is one of our patrons on Patreon, the wonderful community of supporters helping us make science education on the internet free for everybody.

If you are not a patron but are interested in supporting what we do, you can find out more at patreon.com/SciShow. {♫Outro♫}.