Over the past month and a half or so, I ran a user poll on the OpenSesame documentation website. The goal of this poll was to find out who the OpenSesame users are and what they would like to see improved.

I received responses from 250 individual users, who answered on average a bit more than two questions each. Since I estimate the number of active users to be around 1000, I was pleasantly surprised that such a large proportion of users took the time to participate. If you are among those 250, thank you!

The results are shown in full in the graph below, but I will briefly highlight the most noteworthy (to me) results:

Most users are young researchers, who have not been using OpenSesame for very long. Perhaps this is not too surprising, because the project started only a little over a year ago.

Furthermore, most users feel that stability should be the main focus of development, although it's a reasonably close call with the other options. This fits with the fact that most users describe their experience with OpenSesame as 'mostly smooth'. This is not too bad, I think, but the goal is obviously to have the majority of users report a 'smooth' experience. Particularly users who require non-Latin alphabets are likely to run into issues, which is also reflected by the fact that a substantial number of respondents feel that development should focus on this aspect. For those who have had a bumpy ride using …

In the video below you see two lines that are alternately shifted a bit to the left and the right relative to each other. Or are they? When the lines are presented continuously, you can clearly see that they are not shifted at all. They are perfectly aligned!

This is a variation of the Flash Grab illusion, designed by Patrick Cavanagh and Stuart Anstis, the same duo that is responsible for this illusion. Below you can see another animation, which shows the same Flash Grab illusion in a very different way. This one is more like the original, in which the two lines appear to be alternately rotated slightly clockwise and counterclockwise, even though they are perfectly horizontal/ vertical.

So what's going on here? According to Cavanagh and Anstis, there are two phenomena that together result in the illusion. It's a bit of a challenge to follow the logic, but here we go.

The first factor is illusory trajectory shortening: We tend to perceive movement trajectories as being a bit shorter than they really are. In the case of the first video, this means that we perceive the background figure to reverse its direction just before it actually does. In the case of the second video, this means that the disc appears to rotate slightly less than the 90 degrees that it actually rotates.

The second factor is assimilation: The flashed lines are assimilated by the moving background figure. They somehow melt into a single percept. This effect is particularly …

In the video below, you see three rings of coloured dots. In each ring there is one gap (a missing dot), and these gaps rotate like the arms on a clock. So far nothing remarkable. But see what happens if you fixate on the central cross for 15 seconds. Illusory dots of various colours will start to appear where the gaps are!

This illusion is a demonstration of the colour after effect. After effects are very basic phenomena, and most of the video is essentially decoration, not necessary for the illusion occur. In fact, you will even get a colour after effect if you present a single coloured dot, look at it for a while, and then remove it. If the dot is green, like the inner circle in the video, you will observe an after effect in the form of an illusory pinkish dot.

So what's going on here?

Let's start with the fundamentals. Light is electromagnetic radiation, just like radio signals, WiFi, microwave radiation, etc. Different forms of electromagnetic radiation are characterized by different wavelengths, and visible light corresponds to a tiny range from roughly 390 to 750 nanometre (one billionth of a meter). Within the spectrum of visible light, different wavelengths correspond to different colours: Short wavelengths are blue(ish), long wavelengths are red(dish).

Like most people, I learned about the relationship between colour and wavelength during physics class in high school (see [1]). And I distinctly remember that I found this very puzzling. After all …

I enjoy a good puzzle every now and again. Particularly one that seems very easy... Until you try to solve it.

Below are a four classic questions/ puzzles/ problems. If you've read a bit about problem solving before, you might know some of them. But I hope that you will nevertheless find it interesting to revisit them. (And perhaps make the same mistakes as before!) And if you don't know them, you're in for a treat!

The ball and the bat

Let's start with an easy one. As you can tell by the prices, this question is from way back:

A bat and a baseball together cost $1.10. The bat costs one dollar more than the ball. How much does the ball cost?

This is just a warm-up exercise, and you will probably have answered correctly. (If not, shame on you!) But perhaps you noticed that you almost gave the wrong answer. Your intuition (almost) seduced you with an easy, but incorrect answer. Quite possibly, your train of thought was something like this:

Right, obviously the answer is $0.10. But wait... that's too easy. There must be more to it. Oh, I see now. How stupid of me, the correct answer is of course (...).

So the context in which the question was asked (a post about tricky puzzles) made you think twice before answering. And this dramatically increased your chance of answering correctly. But the amazing thing is that, when embedded in a stream of questions, almost everybody …

'Open access' is somewhat of a buzzword within academia. But unfortunately the term is widely misunderstood. All too often, the label 'open access' is used to refer to any content that can be accessed online for free. By this (incorrect) definition, most websites would be open access, because they offer content that you can read (or watch, listen to, etc.) for free. But you are usually not allowed to reproduce that content, or to sell it, modify it, etc. The only thing that you, as a consumer, are allowed to do is view the website right then and there, for as long as the owner of the website permits you.

This may be free of charge. It may even be a nice gesture. But it is not open access.

The definition of 'open access' is still evolving, but by consensus a number of conditions need to be satisfied for it to apply. In PLoS Biology, Michael Carroll gives us the following list:

Full open access content is

• Easily accessible online
• Available to anyone free of charge
• Available for re-use without restriction except that attribution be given to the source.

No one of these alone qualifies content for an open access label.

The paper by Carroll satisfies these criteria. I downloaded it for free. I am allowed to reproduce and redistribute the paper in it's entirety (here it is). And so are you. I didn't need Carroll's permission to upload his paper. And I do not (have to) care if he …