Alex, thank you for reviving this thread and providing the link to Eric Warrant's paper. I've printed a copy of that, and will be reading it when time permits.
As it happens, I've recently purchased a couple of books on eyes, and I think I can provide an extract that bears on some of the issues that you've raised.
AlxndrBrg wrote:superposition eyes...but I can't imagine how their signal processing circuits actually manage to form a coherent picture from it
It turns out that the coherent picture is formed by strange optics, not signal processing.
Quoting from pages 191-193 of Animal Eyes by Michael F. Land and Dan-Eric Nilsson (2nd Edition,
ISBN 0199581142),
The real surprise is optical. All superposition eyes produce a single deep-lying erect image in the vicinity of the retina. Not only does this distinguish them from apposition eyes, which have multiple inverted images, but also from camera-type eyes where the image is inverted. Clearly we are dealing here with something quite out of the ordinary.
...
The credit for the discovery and elucidation of this remarkable piece of optics is due to Sigmund Exner, who worked on the problem throughout the 1880s and published his complete findings in 1891. Exner showed that the only way an erect image could be formed was for the optical elements to behave in a rather strange way, as shown in fig. 8.3a. Basically what each has to do is not to form an image from a parallel beam as in a conventional lens, but to redirect light back across the element's axis, to form another parallel beam on the same side of the axis (Fig. 8.3b). Exner realized that although a single lens wouldn't do the job, a two-lens telescope would, and he went on to demonstrate (as well as he could with the technology of the time) that such structures were indeed present in the superposition eyes of insects.
So, with a superposition eye the retina actually has a "simple" job, quite analogous to our own in terms of the connectivity of the neurons that are required.
The neural circuits in most apposition eyes also have it "simple", since each ommatidium only produces a single summed output that represents a small neighborhood within a single erect image.
However, in some apposition eyes such as those of true flies (Diptera), the output of each ommatidium is not a single value but instead a mini-image consisting of seven samples from the inverted image formed by the lens. Dealing with an array of inverted images would seem to be a nightmare, but the eyes have a very simple and clever way of dealing with it: the nerve bundle coming out of each ommatidium simply does a 180-degree twist before feeding into the next layer down. That physical twist neatly undoes the optical inversion caused by the lens, so that again, what most of the retina deals with is just a single erect image. There is some discussion of this at
https://en.wikipedia.org/wiki/Ommatidium.
As for the pseudo-pupils, the situation is not as simple as apposition versus superposition. Dragonfly eyes are apposition also, but they often have prominent secondary pseudopupils (
here, for example). For a long time, I too thought that the secondary pseudopupils represented separated areas of the eye that were all seeing the same part of the environment. But it turns out that the darkness of the secondary pseudopupils is actually a magnified view of dark pigments
around the photosensors, rather than the photosensors themselves. So, the main pseudopupil indeed shows which ommatidia can see the camera lens, but the secondary pseudopupils do not. No matter how complicated the pattern of secondary pseudopupils, no additional processing is required at the neural level. Some further discussion and links at
http://www.photomacrography.net/forum/v ... 218#246218 .
Cool stuff!
--Rik
