While looking at Metro UI styles I thought about one of the unique aspects of the icons. Their placement inside circles. E.g.
I can imagine that this provides a target like effect, emphasising the touch interaction. It could also be a way to identify the button within a style that doesn't allow for traditional 3D affordance.
However, what impact will this have on recognition of the button functions themselves? Could this add to the effort required and therefore affect usability?
Has there been any related research to show whether this approach can be effective?