[ intro ].

Most of the time, physical blindness is like a blindfold:. It keeps people from taking in visual signals at all.

But there's a surprising exception to that rule. In some rare cases, people can lose their vision but still respond to visual cues— except, they do it subconsciously. In other words, they can see without knowing they're seeing.

This condition is called blindsight. And not only does it shed light on how vision works, it also offers some clues to human consciousness. The first person to observe blindsight was a grad student in the late 1960s who was working with a rhesus monkey named Helen.

Helen's primary visual area, known as V1, had been surgically removed so scientists could study that region's role in vision. Generally speaking, whether you're a human or a monkey, damage to V1 makes you blind. And as far as anyone could tell,.

Helen could basically only tell apart light from dark. But! The grad student noticed that if he held up a piece of fruit,.

Helen would look at it and reach to grab it. She could also navigate through obstacles to eat crumbs off the floor. So, at times, she seemed to act like any monkey with typical eyesight.

But other times, like when she got upset, she'd still stumble around like she couldn't see. So in some ways, she was clearly blind. Of course, Helen couldn't tell anyone what her experience was like, so no one knew for sure what was going on— until a similar thing showed up in human patients.

In one famous case, a patient known as DB had brain surgery that accidentally damaged one side of his vision center, so he could no longer see anything to the left of his nose. But, a researcher noticed DB would reach for things outside his field of view, as if he could actually see certain things in his blind spot. So a team of researchers came up with an unusual experiment.

They would shine a circle of light into his blindspot, then ask DB to point at it. And he'd say he couldn't. 'Cause he was blind. But if they asked him to just guess, he was usually right.

The researchers also projected lines on a screen and asked DB to “guess” whether they were horizontal or vertical., And even though he assured them he couldn't see anything, he guessed right more than 80 percent of the time. Which is way better than random chance. And DB wasn't the only person with this apparent superpower.

Another patient named TN had his visual centers damaged from two separate strokes that left him completely blind. But again, scientists suspected there was more to the story. On one occasion, they asked him to walk down a supposedly empty hallway —except, it wasn't empty.

The scientists had filled it with boxes, chairs, a file tray, and all sorts of obstacles. But TN perfectly dodged every single one. Except, afterward, he had no idea that the hallway was anything but empty.

So how on earth can people do this? Scientists have some ideas. First of all, in each of these patients, the same part of the brain was damaged: that V1 region that had been removed from Helen, the monkey.

Scientists think this region is where signals from your eye turn into conscious sight. But…. V1 is just one part of a complex network that gives us our vision.

See, when signals leave your retina and travel through the optic nerve, they don't go straight to V1. Their first stop is a part of the brain called the thalamus, which is sort of like a relay center. From there, most visual signals go on to V1, but two other paths lead signals to different parts of the brain: the amygdala and a region called hMT+.

The amygdala is involved in emotional responses, like when you're scared. And it acts subconsciously. So, for example, you can react to something scary even before you consciously understand what you're reacting to.

Like, if you open your apartment door and find a crowd of people inside, you might jump before realizing it's a surprise party. In other words, even if the signals going to V1 hit a dead end because it's damaged, the amygdala is still getting visual signals— and responding to them— even when you're not consciously involved. hMT+ is another part of the vision system that tracks movement by picking up on things like change and contrast. It also gets signals from the thalamus, and scientists think people may be able to register where things are moving in their visual field, even though it can't see the thing that's moving.

Those are two possible ways people might be able to see when they're blind. But it is possible there are simpler explanations. For example, humans rarely have V1 completely damaged.

And, like DB, they usually have vision in at least part of their visual field. So in some cases, maybe patients are able to make accurate guesses because of light scattering to the parts that can see. There are also some people with blindsight who report seeing… something.

Like waves or shadows. That might influence how they answer. But many patients are just as surprised by their abilities as the experimenters.

The idea of subconscious signals influencing people's behavior isn't new, though— we've known about unconscious perception for decades. Scientists have recorded many examples of people reacting to images or words that flashed in front of them too quickly to register consciously. But one of the interesting things about blindsight is it shows that our conscious experiences are just a small part of the work our brains actually do.

No one knows what makes us conscious, or why we experience life instead of just going through the motions like a robot. But research on blindsight gives us an unusual window into this problem. It shows us that certain brain regions seem to be responsible for our conscious experience of vision, and it also tells us that in a lot of ways, vision can work unconsciously— just a sequence of input and output.

Which brings up an unsettling question:‌. If blind people unknowingly respond to things they don't see, what about people who can see? Do they also respond to things they're not conscious of?

They almost certainly do. We all have unconscious processing going on all the time but, fortunately, we don't usually need to rely on it. And while it might be a little creepy to think that your brain is nudging you along while you're blissfully unaware, it's also reassuring to know it's looking out for you more than you think.

While it may seem odd to you that there are parts of your brain guiding you subconsciously, the real mystery for scientists is why we have consciousness at all. [ outro ].