This is a summary of my PhD thesis using only the 1000 most popular words in the English language. People have been posting similar pieces here and under the hash tag #upgoerfive on Twitter. This was inspired by the explanation of the Apollo 5 rocket (“up goer five”) on xkcd. I used this text editor.

When you touch something with your fingers, you do not always feel that thing exactly as it is. The things that you have felt in the past change what you feel in the present. When humans feel something that is moving, they are pretty good at guessing how fast it moves. But if they first feel something that is moving, and then feel something else that is moving, the second thing feels like it is moving slower than they would usually guess. I study how wrong people’s guesses are when I change what the first moving thing is like - how fast it is or which direction it moves.

I found that if the first moving thing is really fast, then people are even more wrong when they guess how fast the second thing is. To them it feels really slow. But it doesn’t really matter if the first thing and the second thing move in different directions. Say that both the first and the second thing move to the right. People are a bit wrong about how fast the second thing is, but no more or less wrong than if the first thing had moved to the left instead.

Moving things that people felt in the past can also change which direction they think something is moving, but this is harder to do than changing how fast something seems to move. Because of this, I use a different type of moving thing that is actually made up of a lot of little things near each other that move up and down on the skin, but not across. If the little things move up and down one after the other, then it feels as if something is moving across the skin, even though nothing is. I can change what direction it seems to move by changing which little things go up and down first and which ones go up and down later.

I can also make the little things go up and down so that it doesn’t feel like one direction more than any other. If I do that and ask people to guess what direction it goes, sometimes they will guess one way, and sometimes they will guess another. If people first feel the little things that seem to move in one direction, say left, then they feel the little things going up and down without a clear direction, they will usually say that it went the other way to the first set of little things - in this case, to the right. So you see, the things that people felt in the past can change what direction something seems to move.

If I want to make people feel the second lot of things moving to the right as much as possible, then it is best if I make the first lot of things seem to move quite fast to the left. I can change how fast it seems to move across the skin by changing how much time is between one little thing going up and down and the next little thing going up and down. If there is a short time in between, it will seem to move quickly, and if there is a long time in between, it will seem to move slowly. People most often feel the second lot of things as moving to the right when the first lot of things seemed to move quite fast to the left. If instead the first lot seemed to move more slowly to the left, then people still usually feel the second lot moving to the right but not quite as often.

You might be wondering why we sometimes feel things not exactly as they are. No one really knows for sure, but some people think that not feeling things exactly as they are means that we are better at noticing things change, and that noticing things change is more important (and made people in the past better at living and having children). If something keeps moving in the same way, it is not important to keep noticing it, but when it slows down, say, it might be very good to feel it as even slower so it will really get our attention. On the other hand, people don’t always feel different things as more different from the things they felt before, so this can’t be the whole reason that we sometimes feel things not exactly as they are. Don’t worry though, usually humans feel things pretty well and it is only when people like me make them feel strange things that they sometimes get it wrong.