Despite having healthy eyes, Agnes finds it impossible to see more than one object at a time, while being blind to everything else around her. Her strange condition reveals surprising truths about the ways we all perceive the world.

Look around, what do you see? Children playing outside, your office or a train full of people, perhaps? In order to perceive these complex visual scenes, your brain has to process each individual object – a friend’s nose, a colleague’s ear, the door, a seat, a shoe – and then stitch them all together to create a meaningful world.

But not everyone is able to do this. There are some people in the world whose brains aren’t able to place the pieces of the puzzle together; instead they only process one object, or a part of an object, at a time. Called simultanagnosia, the condition means they quite literally can’t see the wood for the trees.

Now, an unusual case of simultanagnosia has provided a unique insight into the condition, as well as a better understanding of how our conscious and unconscious brain works in harmony to create our reality.

The story starts with Agnes (not her real name). Agnes had to have a neurological exam after a short illness. The results were unremarkable apart from one thing – she had trouble describing what was happening in a variety of images shown to her by her doctor, Joel Shenker, at the University of Missouri, Columbia.

Agnes described the curtains and the window, but could not name anything else in the picture

For instance, one image showed a boy stealing cookies from a kitchen while his mother was busy washing dishes. Agnes described the curtains and the window, but could not name anything else in the picture. After a delay, she looked at the picture again and this time described the child stealing the biscuit, but was unable to describe any other part of the scene, including the curtains, the window or his mother.

In some ways it was a classic case of simultanagnosia: Agnes could perceive individual objects, but could not group several objects in order to perceive a larger scene. But unlike most people with the condition – which normally occurs alongside Alzheimer’s or other kinds of dementia – she had little else wrong with her brain. It meant that for the first time, researchers could explore exactly what was going wrong.

Invisible world

Shenker was fascinated by Agnes. “She would say she could see lots of lines when we showed her the picture of the cookie theft, but that a plate was all she could really make out from those lines,” he said. “In theory if we showed her the picture many times, she might be able to describe the entire scene individually, but could never put it together, she never saw it as a whole.”

Despite her apparent difficulties, Agnes lived alone and was functioning well around her house

What was really surprising was that Agnes lived alone and was functioning well around her house. “In fact she was curious as to why she needed to see me at all,” said Shenker. “She wasn’t bumping into walls and she could find the peas and the carrots and the chicken and make herself whole meals. We didn’t know how she was doing it.”

There was only one logical conclusion, said Shenker. Agnes’s brain must be unconsciously processing information about her world that her conscious mind doesn’t have access to.

Shenker decided to test this theory using a variation of the Stroop test. In its original form, the Stroop test involves a participant being presented with a series of colour names – red, blue, and green, for instance. Each word is typed in red, blue or green coloured ink. Participants can name the colour of the ink faster when the ink and the word match, than when they are mismatched.

In his variation of the test, Shenker used just the first letter of each colour. So the letters ‘R’, ‘B’ and ‘G’ were printed in red, blue or green ink. Just like in the original test, participants were faster at naming the colour of the ink when the letter and the colour matched - a red R or a blue B, for instance - than when they did not.

He then grouped lots of small letters into the shape of a capital letter. For instance, lots of little b’s grouped together to form a capital G.

The arrangement determined how long it took to name the colour of the ink. For example, if both the small and large letters matched the colour of the ink (lots of little g’s grouped into a capital G, printed in green, for example) participants were quicker at naming the colour; if either of the letters - or both of them - were discordant with the colour, they were slower.

When Shenker and his colleague Matthew Roberts, also at Missouri, showed Agnes these letters, she said she could only see the small letters. “We presented her with tiny letter b’s that were grouped together to form a capital G. But she only saw the b’s,” said Shenker. “Even when we got her to trace her finger around the letter G, she couldn’t perceive it, she only saw the smaller b’s.”

But here is where things got really weird. When Agnes named the colour of the ink, her speed was influenced by the presence of the large letter – just as much as other participants.

If, for example, there were lots of little b’s in red ink that formed a large letter R, she was quicker to name the ink than if the same red b’s formed a G. It showed that some part of her brain was processing that large letter even though she couldn’t consciously access that information.

Conscious or not

Our ability to unconsciously process a lot of what goes on around us frees us up to use our conscious mind for the most important stuff at hand. It’s a gift that we rely on, even though we hardly give it a second’s thought.

Shenker gives the example of the “cocktail party effect”. “Say you’re standing having a conversation with George at a party, and there’s lots of other people around and the clinking of drinks. Over in the corner are Sally and David and I ask you ‘what were they saying?’ Chances are you couldn’t tell me anything about their conversation. But if Sally mentions your name, suddenly you hear it. If I were to ask you ‘who said your name,’ it’s likely that you could probably tell me whether it was a male or a female voice, perhaps who it was, and even a few words before your name was said.”

How is that possible? The only way you could have done that is if your brain was actually processing every stream of information in that room. Only when it became necessary to focus on a specific stream of information – triggered by the mentioning of your name – did it pop into your consciousness. “Your brain is taking care of a lot of stuff that you might eventually want to become aware of,” says Shenker.

His tests showed that Agnes’s brain did in fact group objects together and could use this information, without it being sent to her conscious awareness. “The fact that she had no problems going about her life illustrates the extent to which your brain can process things unconsciously, without you ever needing to become aware of them,” says Shenker. “I’ve never encountered a better example of that concept.”

Fire! Fire!

There are many other intriguing examples of how our brains can process information unconsciously, says Kirsten Dalrymple, who studies perception at the University of Minnesota, Minneapolis. Take blindsight. This a condition in which people are blind, yet they can walk around an obstacle course and can guess what item you are presenting to them more accurately than by chance alone. This is because their blindness comes from a problem processing vision in the brain, not their eyes. Despite being unaware of anything in front of them, their healthy eyes are able to pass information to their unconscious brain, which can process it and use it to guide their behaviour. You can see an example of a person with blindsight navigating their way through a room in the video below.

Another great example transpires from people with visual neglect. Damage to their brain means that they are only conscious of one side of their world. For example, if you show someone with left visual neglect a picture of a house with the left-hand side on fire, they would say: “I see a house”. If you show them the same picture, but this time with the right-hand side on fire, they would say: “I see a house on fire.”

However, if you show them a picture of a fire-free house, and an identical picture of a house with its left side on fire and ask them which house they would rather live in, they will choose the house that is not on fire – meaning at some unconscious level they had in fact processed the fire, even though they could not consciously access that information.

Unlike people with spatial neglect, people with simultanagnosia can see both sides of the world, but only attend to a small part of it. Dalrymple calls this their “attentional window”. We all have different attentional windows at different times. If you’re walking down a busy street, for example, your attentional window is large – you are aware of lots of things around you. If you suddenly spot a squirrel by your feet, your attentional window becomes smaller as you focus on the animal. At this point, the other aspects of the street fade into the background. This is a trick that magicians use to make objects disappear – they divert your attention from the whole scene to just a tiny part of it by clicking their fingers or waving a wand, while they remove the object in an area outside of your attentional window.

While it’s not clear exactly what parts of the brain control these attentional windows, it seems that good communication between the parietal and occipital lobes is involved. These areas are responsible for processing objects and movements and understanding spatial relationships.

One of Dalrymple’s patients had simultanagnosia due to trauma in these areas. As her brain injuries got better, her attentional windows got larger and her simultanagnosia improved.

It is difficult to say for sure whether Agnes had always had the condition, or whether it occurred later in life. Her brain scans were fairly uneventful, said Shenker, although there was a small amount of atrophy in the parietal lobes that may have signified the beginnings of Alzheimer’s and could have contributed towards the simultanagnosia.

Unfortunately, no one knows how her story ended. After a few sessions, Agnes stopped coming back for follow-up tests. “I don’t think there’s anything wrong with me,” she said. “I don’t know why you’re making such a big fuss!”

--

Join 700,000+ Future fans by liking us on Facebook, or follow us on Twitter, Google+, LinkedIn and Instagram.

If you liked this story, sign up for the weekly bbc.com features newsletter, called “If You Only Read 6 Things This Week”. A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.