Back in 2009, a Cambridge group – different authors, but led by “SBC”, published a report claiming that people with autism have exceptionally acute vision. Their average visual acuity was claimed to be 2.8
On this scale, 1.0 is defined as normal, and a sharp-eyed young adult with excellent eyesight would get about 1.5. 2.8 means nearly three times as good. Which is, literally, superhuman – a bird of prey would be happy with that. The paper was titled “Eagle-Eyed Visual Acuity In Autism”.
However, what followed was straight out of the Book of Obadiah – “Though you soar like the eagle … from there I will bring you down, sayeth the Lord”. Or in this case, sayeth two experts in visual acuity research, Bach and Dakin, whose qualifications included the fact that they wrote the software used in the original study, which is online here.
They wrote a knock-down critique, arguing that the results were a result of using the wrong settings, which meant that the task was extremely easy. In fact, even perfect performance would only correspond to an acuity of less than 1.
You could never make a test so hard that it would require an acuity of 3.0 on a standard computer. Pixels are just too big. A single pixel is easy to spot, for someone of normal-ish vision. The only way to make it harder would be to use a special, extremely high-res monitor, or to get people to sit a long way from the screen.
So how did a result of nearly 3.0 come out? Because they also turned on data extrapolation, basically saying that if you really aced the easy task, you’d probably do quite well on a harder one. This might be sensible in some situations, but it breaks down when the task was so easy. The autistics seemed to have super vision because they got, say, 99% right, as opposed to 98%.
Yet the present paper represents a happy ending as it’s written by a combined team of Cambridge people, and Bach and Dakin as well, although the lead authors of the original weren’t on it. This time, they used appropriate methods – they got people to sit 4 meters from the screen. To be extra sure, they also gave everyone an eye exam before testing.
And they found no difference at all. The present paper is heartening – rather than grimly sticking to their guns, they admitted their error.
This story should however serve as a cautionary tale; I previously wrote about the fact that in science, a little mistake can cause a lot of problems. This is one of those cases, although arguably there were two seperate mistakes, but one, the extrapolation, was only a problem because of the main mistake, the big pixels.
Tavassoli T, Latham K, Bach M, Dakin SC, & Baron-Cohen S (2011). Psychophysical measures of visual acuity in autism spectrum conditions. Vision research PMID: 21704058