I recently read this alarming report in Perspectives on Psychological Science (Kievit, 2011):

A group of international Bayesians was arrested today in the Rotterdam harbor. According to Dutch customs, they were attempting to smuggle over 1.5 million priors into the country, hidden between electronic equipment. The arrest represents the largest capture of priors in history.

“This is our biggest catch yet. Uniform priors, Gaussian priors, Dirichlet priors, even informative priors, it’s all here,” says customs officers Benjamin Roosken, responsible for the arrest. (…)

Sources suggest that the shipment of priors was going to be introduced into the Dutch scientific community by “white-washing” them. “They are getting very good at it. They found ghost-journals with fake articles, refer to the papers where the priors are allegedly based on empirical data, and before you know it, they’re out in the open. Of course, when you look up the reference, everything is long gone,” says Roosken.

This fake report is quite possibly the geekiest joke in the history of man, so you're forgiven if you don't get it right away. It's about statistics, so a very brief introduction is in order.

Psychologists typically investigate whether two groups (or one group under two conditions) differ from each other in some respect. For example, they may investigate whether men and women differ in their cleaning habits, by comparing the number of times that men and women vacuum each week. Here's some fake data for 6 participants (3 men and 3 women):

Men: 2 …

In the 19th century, the anthropologist Samuel George Morton set out on the, by today's standards, highly dubious quest to show that cranial capacity differs between racial groups. Essentially, he filled almost a 1000 skulls with seed or leadshot, gave each of them a good shake to make sure that every nook and cranny was filled, and then measured the amount of filling that came out as he emptied the skulls. According to his findings, Caucasians had the largest skulls. As, of course, he had suspected all along.

Morton's experiments were not that famous until, more than a century later, they were rediscovered by the eminent biologist Stephen Jay Gould. And he was having none of it. According to Gould, Morton's findings were driven by his racist expectations. Caucasian should have the largest cranial capacity, so, when measuring the skulls, Morton made sure that they did, perhaps merely by subtle “unconscious or dimly perceived finagling.” Gould used this example to prove his broader point that experimental results are inevitably biased, because researchers are only human and simply cannot help but massage the data just the tiniest bit. And, as they say, if you torture the data it will confess to anything.

Now, I'm in general sympathetic to Gould's views, but in this case he was wrong. In a recent study in PLoS Biology, Lewis and colleagues remeasured almost half of the skulls that had been used by Morton (Gould did not have access to the actual skulls. He derived his …

Lo and behold, the OpenSesame documentation area is online at osdoc.cogsci.nl! The documentation area serves as a central point for everything that is related to OpenSesame, the graphical experiment builder. From tutorials and plug-ins to example experiments, it's all there. This is obviously a big improvement over my previous "system" of having documentation in the form of a bunch of loosely linked blog posts.

The documentation area is part of the preparation for the next version of OpenSesame, 0.24 "Cody Crick", which I hope to release in a month or so. I have tested it quite a bit and things are already working quite nicely. If you are interested in getting a sneak preview, you can find out how to get your hands on the development version through, yes, the documentation area!

Yesterday, the guys at the faculty techical department and I have been playing around with OpenSesame, trying to get a clear picture of how accurate the timing really is. We used a kind of modified buttonbox that simulates a button press when a light sensor is triggered. We attached the sensor to the screen and made an experiment that simply shows a white screen and waits for a button press. Since the sensor should, for all intents and purposes, respond instantaneously to the white display, we can use the "reaction times" as a measure of how accurate the timing is. Low reaction times are good and, even more importantly, a low variation is good.

I'm very pleased to say that the results look excellent, particularly when using the new "psycho" back-end (for now only available as an experimental GitHub code snapshot, but this will be part of 0.24), which uses PsychoPy to handle all display operations. On Windows XP, the reaction times are around 3ms and fairly constant. For Linux users it might be worth to note that, although the average reaction times are about the same (around 5ms), there is substantially more variation (I tested it on Ubuntu 10.04) (Update 1/6: I ran some more tests, and the problem was the compositing layer. With this turned off, the timing on Linux is excellent.) E-Prime users might be interested to know how this compares to E-Prime's performance. Well... there is no real difference. Using the same set-up …

Last week I was at the annual meeting of the Vision Sciences Society. One of the more exciting events of this conference was the illusion contest. And of the more exciting illusions of this contest was the one that you can see below, created by Mark Wexler. (The winner of the contest, incidentally, was the awesome illusion by Suchow and Alvarez that I described earlier.)

You will see a slowly (or at least not very quickly) clockwise rotating ring. You will also see intermittent counterclockwise rotations, which are brief and much faster.

So what's going on here? Basically, and as you might have guessed, the counterclockwise jumps are illusionary. The only thing that happens is that for a few frames the coloured squares out of which the ring is composed are completely randomized. Obviously, this randomized "noise" is not really rotating in any particular direction. And yet we perceive a fairly clear counterclockwise rotation.

This illusion bears some resemblance to the traditional motion after effect, in which prolonged exposure to a motion in one direction results in a small, but clear after effect of perceived motion in the opposite direction. But usually the illusory motion is much slower than the real motion that induces the after effect. In contrast, in Wexler's illusion the illusory motion is much faster than the motion that induced the effect.

It seems to be that the "noise" amplifies the motion after effect. Even if the line-segments were not randomized, you would still perceive a regular …