Find the Odd Disk

Toward the end, I think the confounding effect of the afterimages of the discs was bigger than the difference between the discs. (That is, when looking at the three discs in turn, the effect of the afterimage of disc 1 on judging disc 2, etc.)

I noticed this too. I did much better covering up two of the disks and looking at them one at a time, or comparing pairs of disks.

It could rearrange the discs each time to avoid this issue, perhaps just flipping the pattern vertically each time.