Hollow Face illusion tricked me twice

My parents came over to visit me in London for the first time and we went to see some touristy stuff. It's been 2 months since I moved here and I still have not seen the Big Ben.

Before going to the British Museum we went for a walk in the area. We ended up next to Salisbury Welcome Centre, UCL neuroscience department, that has a bunch of interesting illusions on the its walls.

I spotted a Hollow-Face illusion that I heard about on my computational neuroscience course. I begun explaining my parents what this is all about and how we perceive this face as being convex although it is actually concave. This is because our brains have a strong bias in perceiving faces as being convex because of how importat reading faces was because of evolutionary reasons.

Standard arguments there. I was assured I knew what I was looking at and what it was there to be excited about.

Then my mum said the face is following her whenever she moves. I tried doing the same for a while and the feeling she had didn't click in my brain for another 10 seconds. I was looking at the face, moving around it and all I could see is a concave face.

Then, after a while my brain turned a switch and I was able feel exactly what she did.

I was too confident in my knowledge and didn't even read the description which talked about exactly the feeling she was having.

In other words, my brain had another bias: I know what this illusion is about, and I know what I should be seeing there. It took a good few seconds to adjust my perception.

That was pretty cool.