[bl]

True, a rod or cone cell (collectively, "photoreceptors") is the smallest, indivisible detector unit in the retina, but in normal visual processing (i.e., daylight-lit scenes), they probably never operate in isolation. The photoreceptor signals are immediately integrated by accessory cells in the retina (horizontal cells, bipolar cells, amacrine cells) and the major output neurons of the retina, the ganglion cells projecting to the thalamus, exhibit "center-surround" sensitivity. Furthermore, any one photoreceptor can be used in many receptive fields in the downstream visual processing pathway.

Thus, the mapping of "pixels" to the number of retinal ganglion cells is probably less tenuous than a photoreceptor-to-pixel mapping. Now that I think about it, perhaps an even better definition of a physiological pixel would be a functional measure: the number of distinct, electro-physiologically measured center-surround fields in the thalamus. In that case, the effective megapixel rating of a human visual system is only indirectly related to the sheer numbers of photoreceptors, but more closely related to the wiring pattern. This wiring pattern is much more difficult to experimentally measure than simply counting neurons because it would involve flashing tiny, contrast-y dots of light in front of a fixated mammal while poking an electrode around in the thalamus. [This is outside of my main field, so it may have been done, but I don't know the results.]

Two side notes of interest:

1. Photoreceptors are oriented towards the rear of the retina and are embedded in a dark sheet of cells called the pigment epithelium. The upshot of this is that every photon that is involved in our visual perception has traversed a tangle of bipolar cells, ganglion cells, and their associated axons (it is easy to overlook this fact in diagrams, such as at http://en.wikipedia.org/wiki/Retina , because it is usually only mentioned textually in the figure caption). The fovea is relatively free of these light-scattering objects and, in addition to a higher ratio and density of cone cells, is why primate visual acuity is highest in the fovea.

2. In extremely low light-adapted rod cells, the absorption of single photon can trigger photo-transduction. Thus, our visual system has the capacity to operate at the very limit of physics. If I recall correctly (i.e., no citation on hand), this has even been experimentally demonstrated, although the experiment must have been pretty demanding, what with photon shot noise and all. [This last side note #2 is what I had in mind when I referred normal visual processing in the first paragraph. Even though you might think that this validates the photoreceptor:pixel metaphor, at best a human would probably just report a tiny inkling of a flash in some general vicinity with very poor spatial and temporal resolution.] Ah, I now see ristretto mention that.

[ristretto]

In the end, it's wrong to try to determine how many "megapixels" the eye can see, as its not a camera and everything before and after the optic nerve perform a number of enhancements / processing of the signal, so what matters in the end is what is perceptible and discriminable under specific lighting conditions.

[bl]

I quite agree: I don't think megapixels will get us very far, if we could even formulate a consensus definition. I much prefer a perception/discrimination measure, as you said, and as it seems neuroscientists in the visual field have settled on.

Clouding the issue even further is the fact that different areas of central vision processing handle different features. Some areas are tuned to react to points, some to bars, some to grids, some to movement, some to rotation, etc. Perhaps it would be simpler to make the comparison in the other direction: "How intricate do I have to make this visual scene to fully exercise the perceptual abilities of grating-sensitive neurons in the Lateral Geniculate Nucleus of the thalamus." That way, we could put some sort of upper bound on the useful specifications of a pixelated display. We would then have to iterate over all the known feature sensitivities (bars, grids, rotations, etc.).

Display engineers surely must know that below such-and-such pixels per inch, a screen can present any reasonable perceptible pattern. So outside of casual interest, ristretto and I think that eye "megapixels" is relatively meaningless.