But this hint from blindsight proved hard to interpret. Does the primary visual cortex somehow generate awareness? If so, what exactly is being generated and how does it get from the back of the brain into our speech circuitry so that we can say that we have it? Maybe the primary visual cortex doesn’t create awareness itself, but instead sends visual information to a different system in the brain that is more closely related to consciousness. But if that’s so, what is this brain system that causes consciousness, and how does it work?

For a while, it seemed as though blindsight would remain only a tantalizing hint, but in 1999, Robert Kentridge, Charles Heywood, and Larry Weiskrantz stumbled on a new quirk of blindsight. It’s easy to mistake their discovery for a minor detail, but it turned out to be one of the most important insights into consciousness in decades.

Imagine you’re looking at a screen. A distracting dot flashes on the right side. A fraction of a second later, a number appears at exactly the same location. Your job is to report the number as quickly as possible. Your response time is probably pretty fast because the initial dot automatically drew your attention to that location. On the other hand, suppose the dot flashes on the left side of the screen. A fraction of a second later, the number appears on the right side. Now you’re probably slower to read the number. The dot automatically drew your attention to the wrong side and it takes a moment for your attention to readjust. This simple experiment allows researchers to measure how much attention was snagged by that initial dot.

It turns out that in people with blindsight, the dot can snag attention even when it doesn’t snag conscious experience. Bizarrely, attention and awareness can be separated.

This finding was so startling that researchers were curious whether it might be true in anyone, not just people with brain damage. Imagine the same experiment I just described, but the initial dot is very dim and hidden in a distracting grid of colors and lines. Even if you don’t have clinical blindsight, you’ll swear you’ve seen no dot at all. And yet the dot can still snag your attention, sharpening your ability to process anything else that happens at the same location. You can attend to the dot even if you’re not aware of it.

For decades scientists used the terms “awareness” and “attention” more or less interchangeably, as though both referred to what happens when your mind takes hold of something. Blindsight has helped to pry the two concepts apart. We now know that we need a better theory of what they are and how they relate to each other.

One such theory is the Attention Schema Theory (AST), first proposed by my lab in 2011. In that theory, attention and awareness have a precise relationship to each other. Attention is a data-handling trick. It’s the brain’s way of focusing resources on some signals, boosting them and processing them at the expense of other signals. It’s a mechanistic process. Awareness is different. It’s more like the brain’s explicit knowledge about what it’s doing. The brain doesn’t have information about the microscopic details of attention, the neurons and the electrochemical signals, but it can give you a general account. It can say, “Yeah, I’ve got hold of that dot. I’m processing it. I have a kind of mental possession of it.” Awareness is the brain’s schematic description of attention.