I wonder is the result owes a lot to visual processing (a highly developed trait in most birds.) Arguably, with the symbol-pairs used here, each pair look a bit

I wonder is the result owes a lot to visual processing (a highly developed trait in most birds.) Arguably, with the symbol-pairs used here, each pair look a bit like a single but partially-obscured convex object, with the inner ones lying on top of the outer ones.