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Criticality in Biological Systems

In this WID Pick, the Institute's C4 researchers share the ideas currently driving their work, specifically the concept of criticality in collective behavior.

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WID Picks, sourced from our researchers, share new topics and ideas from the field to keep your brain sharp.

This WID Pick comes from the Institute’s Center for Complexity and Collective Computation (C4):

When we think of some of the marvelous solutions that biological systems have discovered to thrive in an environment, it is hard not to believe that the organisms around today are special. Take their visual sensory organs, for example. These are structures that exploit geometric optics to focus light on the fovea that harnesses quantum mechanical efficiencies — they absorb light and are capable of detecting even single photons in a background of thermodynamic noise. We know that this biological tool designed by the wayward adjustments of evolution just touches the boundary between physical possibility and impossibility; it detects events that happen at the level of a handful of elementary particles!

So, when we start looking at other parts of the biological world, we believe the same: there is something special about the “design.” One notion that has recently come into vogue is that biological systems might be poised at criticality.

“Criticality” is a big term, but what we mean here is that biological systems may be poised at some special region of behavior that maximizes the flow of information from individuals to the group. This could be useful in the context of a large flock of birds where a few individuals on one side move to evade a predator. Roughly speaking, the flock should stay together and respond as one unit. So one would hope that the birds on the other end respond as well. At criticality, behavior on one side can ripple through the entire group, and so it matches our intuition for how well-designed birds should behave.

But in empirical Science, there are also Nagging Doubts. After all, our theories are based on what we measure and how we measure. In the brain, for example, it is currently impossible to measure the action of more than very small fraction of neurons: hundreds out of hundreds of billions of neurons, or about 0.0000001 percent. If we think criticality is important to neuroscience, how can we really make a meaningful statement about the brain with a theory built off of a minuscule subset?

Indeed, that is the very issue pointed out by Schwab et al. The group builds a quantitative argument showing that sampling a subset of a larger system in many cases leads to observations of critical behavior. To be more precise, what they show is that a signature strongly tied to criticality can be produced simply by hiding the rest of the system and measuring a small part.

This is an important point as we as scientists try to understand the Book of Nature with just scattered fragments. Can we make any real claims yet about these complex biological structures that are not severely limited by our methods? And what “discoveries” have we made that are consequences of our limited perception?

Our WID Pick manuscript

Schwab, D. J., Nemenman, I. & Mehta, P. Zipf’s law and criticality in multivariate data without fine-tuning. arXiv q-bio.NC, 1–5 (2013). 

References to explore

Are biological systems poised at criticality?

Social interactions dominate speed control in poising natural flocks near criticality

How advances in neural recording affect data analysis

Energy, quanta and vision

Can a human see a single photon?


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