Galaxies, Glow-worms and Chicken Eyes

I just came across a news item based on a research article in Physical Review E by Jiao et al. with the abstract:

Optimal spatial sampling of light rigorously requires that identical photoreceptors be arranged in perfectly regular arrays in two dimensions. Examples of such perfect arrays in nature include the compound eyes of insects and the nearly crystalline photoreceptor patterns of some fish and reptiles. Birds are highly visual animals with five different cone photoreceptor subtypes, yet their photoreceptor patterns are not perfectly regular. By analyzing the chicken cone photoreceptor system consisting of five different cell types using a variety of sensitive microstructural descriptors, we find that the disordered photoreceptor patterns are “hyperuniform” (exhibiting vanishing infinite-wavelength density fluctuations), a property that had heretofore been identified in a unique subset of physical systems, but had never been observed in any living organism. Remarkably, the patterns of both the total population and the individual cell types are simultaneously hyperuniform. We term such patterns “multihyperuniform” because multiple distinct subsets of the overall point pattern are themselves hyperuniform. We have devised a unique multiscale cell packing model in two dimensions that suggests that photoreceptor types interact with both short- and long-ranged repulsive forces and that the resultant competition between the types gives rise to the aforementioned singular spatial features characterizing the system, including multihyperuniformity. These findings suggest that a disordered hyperuniform pattern may represent the most uniform sampling arrangement attainable in the avian system, given intrinsic packing constraints within the photoreceptor epithelium. In addition, they show how fundamental physical constraints can change the course of a biological optimization process. Our results suggest that multihyperuniform disordered structures have implications for the design of materials with novel physical properties and therefore may represent a fruitful area for future research.

The point made in the paper is that the photoreceptors found in the eyes of chickens possess a property called disordered hyperuniformity which means that the appear disordered on small scales but exhibit order over large distances. Here’s an illustration:

It’s an interesting paper, but I’d like to quibble about something it says in the accompanying news story. The caption with the above diagram states

Left: visual cell distribution in chickens; right: a computer-simulation model showing pretty much the exact same thing. The colored dots represent the centers of the chicken’s eye cells.

Well, as someone who has spent much of his research career trying to discern and quantify patterns in collections of points – in my case they tend to be galaxies rather than photoreceptors – I find it difficult to defend the use of the phrase “pretty much the exact same thing”. It’s notoriously difficult to look at realizations of stochastic point processes and decided whether they are statistically similar or not. For that you generally need quite sophisticated mathematical analysis.  In fact, to my eye, the two images above don’t look at all like “pretty much the exact same thing”. I’m not at all sure that the model works as well as it is claimed, as the statistical analysis presented in the paper is relatively simple: I’d need to see some more quantitative measures of pattern morphology and clustering, especially higher-order correlation functions, before I’m convinced.

Anyway, all this reminded me of a very old post of mine about the difficulty of discerning patterns in distributions of points. Take the two (not very well scanned)  images here as examples:

You will have to take my word for it that one of these is a realization of a two-dimensional Poisson point process (which is, in a well-defined sense completely “random”) and the other contains spatial correlations between the points. One therefore has a real pattern to it, and one is a realization of a completely unstructured random process.

I sometimes show this example in popular talks and get the audience to vote on which one is the random one. The vast majority usually think that the one on the right is the one that is random and the left one is the one with structure to it. It is not hard to see why. The right-hand pattern is very smooth (what one would naively expect for a constant probability of finding a point at any position in the two-dimensional space) , whereas the  left one seems to offer a profusion of linear, filamentary features and densely concentrated clusters.

In fact, it’s the left picture that was generated by a Poisson process using a Monte Carlo random number generator. All the structure that is visually apparent is imposed by our own sensory apparatus, which has evolved to be so good at discerning patterns that it finds them when they’re not even there!

The right process is also generated by a Monte Carlo technique, but the algorithm is more complicated. In this case the presence of a point at some location suppresses the probability of having other points in the vicinity. Each event has a zone of avoidance around it; the points are therefore anticorrelated. The result of this is that the pattern is much smoother than a truly random process should be. In fact, this simulation has nothing to do with galaxy clustering really. The algorithm used to generate it was meant to mimic the behaviour of glow-worms (a kind of beetle) which tend to eat each other if they get too close. That’s why they spread themselves out in space more uniformly than in the random pattern. In fact, the tendency displayed in this image of the points to spread themselves out more smoothly than a random distribution is in in some ways reminiscent of the chicken eye problem.

The moral of all this is that people are actually pretty hopeless at understanding what “really” random processes look like, probably because the word random is used so often in very imprecise ways and they don’t know what it means in a specific context like this. The point about random processes, even simpler ones like repeated tossing of a coin, is that coincidences happen much more frequently than one might suppose. By the same token, people are also pretty hopeless at figuring out whether two distributions of points resemble each other in some kind of statistical sense, because that can only be made precise if one defines some specific quantitative measure of clustering pattern, which is not easy to do.

Follow @telescoper