In the original online version of this paper, there was a problem in the reference list. Thanks to Elsevier for quickly fixing this as soon as I alerted them to it. Hopefully not too many people downloaded the version with the error.

For anyone who did: the references originally contained a non-existent paper, “Morgan MJ, Melmoth D, Solomon JA (2007) The wallpaper illusion explained. J Vis 7. pp. 10.1–10.11.”
This should of course be “McKee SP, Verghese P, Ma-Wyatt A, Petrov Y (2007) The wallpaper illusion explained. J Vis 7:10 11-11”
The intended reference erroneously appeared with the authors from a paper slightly later in the list. It was cited correctly in the text as “McKee et al 2007”. Apologies to Suzanne, Preeti, Anna and Yuri for the misattribution in the list of references. It was correct in the final submitted version but had been scrambled in the proofs, and I didn’t spot it. Thanks to the wrongly-credited Michael Morgan for pointing it out to me.

Additionally on p. 3, col 2, of the paper, the text reads:
“[staircases] can fail catastrophically if task difficulty is a non-monotonic parameter of interest.” This is the text I submitted, but it should of course read “a non-monotonic function of the parameter of interest”.