Whatever this looks like, it’s really two sheets of dots sliding over each other. It’s all perfectly flat. But the brain interprets it as a revolving cylinder. To do this, it must assume a surface order, like “front-right, back-left.” Occasionally this assumption reverses, and the cylinder appears to change direction. The percept is called bistable, meaning it’s one thing or the opposite, but never ambiguous.
We found neurons in a cortical area called MT that change their activity when the perceived surface order flips. We explained the bistability in terms of simple MT networks. A few years later the London Science Museum erected an exhibit representing our experiment. The original paper is here: bradley and andersen, Nature 98