In 1982, Frank Jackson published an article entitled "Epiphenomenal Qualia" in Philosophical Quarterly 32:127. Within it, he outlined a thought experiment known as "Mary's Room", or "Mary and the Black-and-White Room". 
Imagine a scientist called Mary. She is born and raised within a black-and-white room, where she experiences no colours. Mary is a talented scientist, and learns an enormous amount about the physics of colour, the mechanism by which the human eye detects colour, and the pathways through which colour is interpreted within the brain.
One day, Mary steps out the door of her black-and-white room and into the world, and experiences colour for the first time. The question posed by the thought experiment is this: what does Mary learn at this moment?
Jackson presents this thought experiment originally as a refutation of Physicalism, and a demonstration of the non-physical nature of mental states.
It seems just obvious that she will learn something about the world and our visual experience of it. But then is it inescapable that her previous knowledge was incomplete. But she had all the physical information. Ergo there is more to have than that, and Physicalism is false.
The thought experiment strikes at the heart of qualia, the subjective conscious experience. What does Mary learn? What is the difference between Mary before she stepped through this door, and afterwards? In thinking about this, I remain unconvinced that this is a refutation of Physicalism.
Let us extend this thought experiment a little. Imagine that we have a device that can implant memories into a brain, perhaps through some kind of interface with the senses. As far as I can tell, such a device (though incredibly complicated) would be possible.
Now let us imagine that we implant a memory into Mary's brain of her leaving the room and experiencing colour, before returning to the room. Let us assume that there is no evidence of that journey other than the memory and no test that Mary can perform to ascertain whether the event actually happened. What then is the difference in qualia between this version of Mary and the version of Mary that did genuinely leave the room?
My immediate thought is that this difference depends on Mary's beliefs and knowledge around that memory. Firstly, consider that if Mary is cognisant of the implantation and the artificiality of the memory, then it seems as if the memory will be just another piece of information amongst many and that - critically - actually experiencing the outside would still generate a new and novel experience. In such a scenario, there remains some qualia that separates the implanted memory from the real experience.
But consider the scenario where Mary believes those implanted memories to be real. What happens to this qualia? It seems to disappear. The nature of that qualia therefore seems to be intimately related to a belief in presence. That it is the fact that Mary believes the memory to be real that gives it a qualia of subjective experience.
Imagine that you wake up from a dream and find yourself in a new body. You have all your current memories and you are explicitly told that those memories have been implanted within your new body. Would you be the same person? Mary’s Room seems to suggest that we would be something not just slightly different, but entirely so - that all of our memories would be stripped of the qualia that is your belief in your real presence within those memories. Would we look back on our lives with a detached, emotionless objectivity, as dreams that happened to someone else? Or would our egos merely snap under the weight of that existential pressure into a complete breakdown?
Maybe, one day, when we backup ourselves into some fresh new container for our egos, we might program in a little lie for ourselves. A little illusion, a little false belief that rewrites our memories into one of continuous presence. A little sugar to make the medicine go down.