Read - and see - more about the theories here. My only wish was that the Observer had included illustrations for each one, instead of making you click through for links.
As an illustration of the "Unheimlich theory," I especially liked this illustration Jamie Keenan did for a contest for Lolita covers. It’s the corner of a room. No, it’s a girl’s panties and legs. No - it’s both! Very unsettling.
Which led me to that contest. Definitely worth a look.