On the design of eyes

Yes, I know it's a really bad idea to anthropomorphise evolution, and attribute elements of "design" to it, and it's the starting point for all kinds of logical mistakes. I'm going to do it anyway.

The camera-like design of the eye is very neat, but I've never really thought about why it is the way it is. Specifically, what is it with that whole fovea business? Why is there a yellow spot which we use for doing most of the detailed seeing with?

It's not without cost. It means we have to have the eye as an encapsulated spherical unit with a nice, smooth coating that can be swivelled around at high speed, and an optic nerve that can handle that movement. Why would we just have a small area with decent vision which we move around, unlike an actual camera with a uniform input area, which would allow for a whole lot less eye movement?

Previously I'd probably have explained it in mechanical terms - putting all the cones in the centre allows you to put lots of rods around the edge, for seeing movements in the peripherary, in low light etc. It simplifies the optics, as you only need to get good focus in a small area. It allows the presence of a blind spot, by putting the optic nerve connection in part of the low-resolution area.

More recently, I've been thinking about machine vision. Spotting a thing in a scene is an exercise in fancy pattern matching. You want to apply a matching structure over a full scene - convolve it with the image, or otherwise apply the matcher in all the different parts of the image. Doing this with a neural network (like the ones in our heads) is quite a tricky thing. On the other hand, if you limit the recognition to a single area in the input, and then slide the input image under the recogniser, things are simpler. Perhaps our eyes are simply space to time multiplexers for pattern matchers?

I'm pretty sure it's more complex than that. After all, I think I can recognise things even if they're not totally bang in the middle of my vision. However, I think it's a more interesting reason for moving eyes than phyiscal constraints.

Posted 2014-12-13.