Moving animals do a funny thing. I call it Command Copy. In Chapter 12 of An Immense World, Ed Yong explains: “When an animal decides to move, its nervous system issues a motor command – a set of neural signals that tell its muscles what to do. But on its way to the muscles, this command is duplicated. The copy heads to the sensory systems, which uses it to simulate the consequences of the intended movement. When the movement actually occurs, the senses have already predicted the self-produced signals that they are about to experience. And by comparing that prediction against reality, they can work out which signals are actually coming from the outside world and react to them appropriately. All of this happens unconsciously, and while it isn’t intuitive, it is central to our experience of the world.”
This is also why you can’t tickle yourself. Unless you have certain types of schizophrenia that may also predispose you to ‘hearing’ voices and ‘seeing’ delusions. It’s possible that in these cases, something goes wrong with the copy command or with the sensory apparatus. There are two types, as Yong explains: exafference refers to signals from the external world, while reafference refers to signals from the organisms’ own actions; “think of them as other-produced and self-produced”. The crux is that “these signals are the same from the point of view of the sense organs”. Apparently, Command Copy is how pretty much all organisms resolve this dilemma, which is a rather interesting fact.
Most of us humans are conscious of the act of making predictions. Our brain seems to have evolved to be an advanced-prediction organ. Consciousness seems to be built, or at least follow analogous rules to Command Copy. One might argue that sensing one’s environment and being able to react in a life-saving way to it (eat food! avoid poison!) is the basis of building a life-form that ‘survives’. To sense one’s environment, it must be able to distinguish self from non-self (the environment). It’s why we think of the simplest organism as a cell with a boundary – a cell membrane at the very least. (These ‘simple’ organisms are biochemically complex when you zoom in and try to elucidate its inner workings.)
While organisms have receptors that can respond to single photons or single molecules, by and large organisms react to analog rather than digital signals. Only when a certain threshold concentration of molecules has been ‘detected’ is a response elicited. While response to photon seems digital in that very first detection step, it is almost always coupled to an analog chemical (molecular) signal. Why is this? I’m not sure exactly, but I think it has to do with scale. To complexify, which inevitably involves a division of labor so that different molecular machines carry out different tasks, one has to increase in size. To maintain a certain semblance of stability (to ‘persist’ and not die or be dissembled), you can’t be so sensitive that a single stray molecule or photon disrupts your entire existence. By becoming multiscale, you buffer yourself against such indignities. But by becoming complex, you run the risk of system failure – hard to diagnose because complexity ties things together in circular Gordian knots.
As an origins of life researcher who studies protometabolism, this idea of Command Copy is fundamental to autocatalysis. There needs to be stoichiometric increase to grow in size and scale, a necessary ingredient for complexity (and survival). Hence, I’ve been examining reactions that do this at a simpler molecular level in a few steps. And what is exciting to me is that baked into these simple autocatalytic cycles is the potential (perhaps inevitable if an energy source and molecular food sources are available) to expand towards complexity. Command Copy in a sense is the heart of it all, although I’m still struggling to map my chemical level ideas to organismal level responses that Yong describes. There’s likely a continuum that connects these, and it’s what keeps me intrigued with research and the quest for answers to difficult questions!
P.S. An Immense World has been a marvelous catalyst for encouraging me to think deeper thoughts. Here are my previous blog posts on Yong’s book.
· Umwelt
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