It is worth noticing that actual machines, like their theoretical structures, work by splitting information into elementary units, namely, bits or pixels.
By contrast, to detect expressions or to perform identifications it is required to handle each image as a whole. This is a very noticeable difference and perhaps the difficulty arises from this circumstance. It is easy to find literature in which it is pointed out that actual machines are far from being able of this tasks, but what I am searching for is a proof for or against this possibility.