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.”