Thomas Metzinger is a philosopher who pays lots of attention to cognitive science and psychology, and likes to think about consciousness. Most of the interesting ideas that follow come from his books The Ego Tunnel and Being No One. I hope to write a series of posts summarizing some of the evidence and arguments in Being No One, which focuses on consciousness.
Blindsight patients have damage to their primary visual cortex (V1), leading to a scotoma, or area of blindsight in the visual field. Most but not all visual signals go through V1, so they can still influence the brain in very restricted channels. Blindsight patients don't report seeing things in their scotoma, and don't initiate plans based on it. If they're thirsty and there's a bottle of water in their scotoma, they don't pick it up and drink it.
Human subjects and animal subjects are treated differently in psychological experiments regarding what they do and don't know. Humans are generally asked to report on their own experience, while animal actions are observed. We get interesting results when we ask people to report on their experience, while also observing their actions.
If you ask a blindsight patient what they see in their scotoma, they respond to the point that they can't see anything there. However, if you tell them to do things like "grab the thing in your scomata" they can grasp it. If you ask them to guess what's in it, they can perform better than chance. Some blindsight patients can tell if something is moving in their scotoma, but they can't tell you what it is. They often describe this awareness as a hunch.
Most people consider it fair to say that blindsight patients are not conscious of the things in their scotoma.
Attention and Conscious Experience
Patients with blindsight can act on visual information in their scotomas in some ways, but they can't notice it.
Metzinger argues that humans don't have a conscious experience of what we can't pay attention to. Note: There's a difference between can't pay attention to, and not currently paying attention to.
Visual information in the scotoma isn't accessible to the parts of my brain that plan, or the parts that cause me to say "I can see X". My unconscious is able to refer to this information for things in forced choice situations, but the information isn't available to me.
Constraints on Theories of Consciousness
Any theory which says that you need to be conscious in order to do things is probably wrong. Also, robots work. And machine learning exists. See also unconscious goals.
It's possible for your brain to refer to something, but not have it be consciously available to you. It's also possible to change what these things are.
The parts of your brain causing you to say that you notice something can be cut off from the parts that let you do things. This implies that some neural processes lead to you being conscious and others don't, and that those processes can be interrupted without ruining everything.
1"The Case of Blindsight" by Weiskrantz in the Blackwell Companion to Consciousness (you can get it here, though there are other place on the internet that talk about blindsight)
Heavily drawn from The Ego Tunnel and Being No One (both by Metzinger).
Thanks to John Salvatier for reviewing drafts of this post.