Some people who have lost their vision find a “second sight” taking over their eyes – an uncanny, subconscious sense that sheds light into the hidden depths of the human mind.

When Daniel first walked into London’s National Hospital, ophthalmologist Michael Sanders could have had little idea that he would permanently alter our view of human consciousness.

Daniel turned up saying that he was half blind. Although he had healthy eyes, a brain operation to cure headaches seemed to have destroyed a region that was crucial for vision. The result was that almost everything to the left of his nose was invisible to him. It was as if he were looking out of a window, with the curtains drawn across half of his world.

Daniel was adamant that he could not see a thing, yet somehow his unconscious mind was guiding him correctly

And yet, as Sanders began testing him, he noticed something very strange: Daniel could reach out and grab Sanders’ hand, even when it must have fallen right behind his blind spot. It was as if some kind of “second sight” was guiding his behaviour, beyond his conscious awareness.

Intrigued, Sanders referred Daniel to the psychologists Elizabeth Warrington and Lawrence Weiskrantz, who confirmed the hunch with a series of clever tests. They placed a screen in front of Daniel’s blind spot, for instance, and asked him to point at a circle, when it appeared in different places. Daniel was adamant that he could not see a thing, but Weiskrantz persuaded him to just “take a guess”. Surprisingly, he was almost always right. Or Weiskrantz and Warrington would present a single line on the screen, and Daniel had to decide whether it was horizontal or vertical. Again, Daniel was adamant that nothing had appeared before his eyes, yet his accuracy was around 80%, much more than if he had been guessing randomly.

How many of our decisions occur out of our awareness, even when we have the illusion of control?

Clearly, despite his blindness, Daniel’s healthy eyes were still watching the world and passing the information to his unconscious, which was guiding his behaviour. Publishing a report in 1974, Weiskrantz coined the term “blindsight” to describe this fractured conscious state. “Some were sceptical, of course, but it has held its own and become an accepted phenomenon,” Weiskrantz says today. And over the following decades, the condition has come to answer some fundamental questions about the human mind.

Just how many of our decisions occur out of our awareness, even when we have the illusion of control? And if the conscious mind is not needed to direct our actions, then what is its purpose? Why did we evolve this vivid internal life, if we are almost “zombies” acting without awareness?

“These cases open a window into parts of the brain that are normally not visible,” says Marco Tamietto, who is based at Tilburg University. “They offer a view to functions that are difficult to observe – that are normally silent.”

Unravelling the mind

Consciousness is so deeply intertwined with everything we do, that many scientists had previously believed it would be impossible to study. How can you pick apart the rich fabric of our minds to find the one thread that gives rise to the vivid sense of awareness, of feeling and “being” and experiencing the world, without unravelling everything else around it?

Daniel, whose name has been changed for this article and is known in the literature simply as DB, offered some of the first clues. “What you want to do is to look at something that is as close to consciousness as possible, but which is lacking that specific quality, that subjective experience,” says Christopher Allen at Cardiff University. “And that’s what blindsight gives you. The participant is still perceiving, but they lack awareness of perception.”

Despite their blindness, these people can somehow sense emotions in a face – and they even start to unconsciously mimic the expressions

One of the first tasks was to test exactly what blindsight patients are capable of without their conscious visual awareness – and the results have been quite remarkable. Of particular interest has been the fact that they can sense emotion: when presented with faces, they can tell whether it is happy or sad, angry or surprised, and they even start to unconsciously mimic the expressions. “Even though they did not report anything at a conscious level, we could show a change in attitude, a synchronisation of emotional expressions to the pictures in their blind field,” says Tamietto, who has worked extensively with Weiskrantz.

Besides mirroring expressions, they also show physiological signs of stress when they see a picture of a frightened face. “The plan for the future is to try to train them to pay attention to bodily reactions,” says Tamietto. It might be helpful to notice if they are in danger, for instance. “They can use the bodily changes to understand what’s going on in the world – as an indication that there is something interesting or problematic.”

In 2008, Tamietto and Weiskrantz’s team put another blindsight patient through the most gruelling test yet. Unlike Daniel, he was blind across the whole of his visual field, and normally walked with a white cane. But the team took away his cane and then loaded a corridor with furniture that might potentially trip him up, before asking him make his way to the other side. “Despite saying he wasn’t able to see, we saw him shooting by on his very first attempt,” says Tamietto. You can watch it for yourself, on the video below.

Importantly, the participant claimed that not only was he not aware of having seen anything; he was not even aware of having moved out of the way of the objects. He insisted he had just walked straight down the hallway. According to Beatrice de Gelder, who led the work, he was “at a loss to explain or even describe his actions”.

It was like seeing a black shadow moving against a completely black background

Only in very rare circumstances do they come close to being aware of what they are seeing. For instance, one subject was able to distinguish movement in fast, high-contrast films; he described it as being like “a black shadow moving against a completely black background” – a “sense of knowing” that there was something beyond. But even then, he could not describe the content itself, meaning that his experience lacked almost everything we would normally associate with vision. “There’s a lot of controversy about whether those reports truly reflect visual experiences,” says Kentridge.

Reversible blindness

Of all the questions these studies have posed, the most pressing has been why? What causes the conscious and unconscious to decouple so spectacularly? Tellingly, all the blindsight subjects had suffered damage to a region known as V1, at the back of the head, suggesting that it is this region that normally projects the stream of images into our awareness.

To test their ideas, scientists can use a form of non-invasive brain stimulation that disrupts different brain regions, in an attempt to induce a reversible form of blindsight in healthy participants. Keen to know how it feels, I recently took part in one of those experiments at Allen’s lab in Cardiff, UK. (You can see a video of the procedure below.)

The technique is called “transcranial magnetic stimulation”, which uses a strong magnetic field to scramble the neural activity underneath the skull. “The advantage is that you don’t have to cut someone’s head open to demonstrate the same behavioural characteristics as clinical blindsight,” Allen told me before the experiment.

Eventually I noticed a fleeting dark line cross my vision, a bit like an old TV monitor just after you pressed the off switch

The experiment began with Allen placing a magnet over the back of my skull, just above V1. Next, he began applying the magnetic field for short intervals at increasing strengths. At first, all I could feel was a slight tapping sensation (the effect of the magnetic field on my skin) but eventually I did notice a fleeting dark line crossing the centre of my vision, a bit like an old TV monitor just after you pressed the off switch. It only lasted less than a second, however, and although it gave me a small shock, I soon became used to the sensation.

After Allen had found the right power, I sat in front of a computer screen, and he flashed up pictures of arrows for a split second: my job was to say whether they pointed left or right. The pictures were sometimes timed with the TMS signals causing the temporary blindness – and like Daniel in those original experiments, I often saw nothing and felt that I was guessing. Nevertheless, once I had finished, Allen told me that I had answered many more correctly than would be expected by chance alone, suggesting the TMS had succeeded in giving me blindsight.

Through studies such as this, Allen has found tentative evidence that the visual information is funnelled through the “lateral geniculate nucleus”, deep in the centre of the brain – a bypass around V1 that allows the information to be processed unconsciously in areas involved in emotion or movement.

Eventually, the researchers may even understand how the brain creates visual consciousness itself – and why V1 is so crucial. One idea is that consciousness relies on communication to and from many areas of the brain – and maybe V1 is working as a hub that helps orchestrate that broadcast.

Picking apart the experience may also reveal further clues about the power of unconscious mind. To understand how, imagine that you are part of a strange puppet show. You have been blindfolded, and your limbs are tied to invisible strings. Every so often, they are tugged here or there by a hidden puppet master, leading you through a complicated dance. To the audience, it looks like you are in full control of your actions, but you don’t have the foggiest idea of what you’ve just done.

The non-conscious mind acts as the puppet master, pulling the strings without their knowledge

That puppet show is essentially what happens when someone with blindsight navigates their way past obstacles – with the non-conscious mind acting as the puppet master. “It shows that awareness isn’t the whole story,” says Tamietto. “Very often we believe we have decided something, but our brain has made the decision for us before that – in many ways, and in many contexts.”

Juha Silvanto at the University of Westminster  agrees: “Consciousness is just a summary of all the information coming in, but the fact the subconscious can guide behaviour suggests that elaborate processing is going on without us being aware of it.” Indeed, some philosophers have gone as far as to wonder whether we could be little more than “zombies” acting on mostly unconscious impulses.

Some philosophers have gone as far as to claim that we could be little more than “zombies” acting on mostly unconscious impulses

This, in turn, begins to cast doubt on some long-held assumptions about the very nature, and purpose, of consciousness. After all, it is by no means certain that other animals have a rich inner life like us, so it must have emerged for some reason. Previously, psychologists had proposed that we have a kind of “spotlight of attention” that sweeps over our vision, and when it lands on an object, the object pops into consciousness. In this way, our heightened awareness helps highlight the most important parts of a scene, giving us the chance to respond.

Except Robert Kentridge at the University of Durham has evidence to suggest this too may be wrong. His insight came when he was talking to a blindsight subject in between some of the basic visual tests, in which he flashed different images at different parts of the blind spot. The subject had said that he thought he would do better if we were told where, in the blind spot, the image would appear. “It seemed very strange,” says Kentridge – since they have no awareness of what is in their blind spots, they shouldn’t be able to focus their attention there. “It’s as if you were trying to direct attention around the back of head – you shouldn’t be able to do it,” he says.

Even so, he was happy to play along and design a separate experiment where he could give the subject a clue about where the image might appear. The results were a kind of paradox: even though the participant was still not able to actually see anything, his subconscious discrimination seemed to be quicker. In other words, the subject really was “paying attention” – but without being conscious of exactly what he was attending.

For this reason, Kentridge thinks we need to rethink our ideas about consciousness and attention. Rather than it acting as a spotlight to boost perception, he instead suspects that consciousness may have evolved to boost memory, drawing together all the different pieces of information into a cohesive picture that is easier to remember. “You need to encode what’s happening in the world in a single package,” he says.

These are just the first of many clues that may eventually solve the riddles of human consciousness. Sadly, Daniel will not be taking part in those further experiments. “He passed away last November, but was a willing subject for many years,” Weiskrantz tells me. By gently reaching into his darkness, however, he has shown the way for others to follow, guiding us through some of the biggest mysteries of the human mind.

David Robson is BBC Future’s feature writer. He is @d_a_robson on Twitter.

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