The Habitual Brain: How Routine Action And Thought Are The Structure Of Life

Habits are a blessing, and a curse.

What is a habit?

It is a skillfully routinized manner of action or thought.

Some philosophers have unwisely supposed that a habit-free existence would be preferable to our own. In this they are mistaken. But the deeper error is the supposition that such an existence might even be possible.

A habit-free existence would be a robotic existence; it would be one in which nothing could be taken for granted. But if nothing can be taken for granted, you can’t get started on anything. How could you talk, if you couldn’t take your own fluency, that is to say, your own habitual mastery of words, meanings, and ways of talking, for granted? How could you read the newspaper? Imagine that you had to think about and decide where to put your feet in the morning! Without habits nothing recognizable as a human or even animal form of life would be possible. To have a mind like ours, you need habits like ours. The fields of robotics and AI should note this well: instead of making clever robots, they should try to design machines with habits! Habits of thought and action form the skeletal structure of life as we know it.

I have been told that Goethe said that architecture is frozen music. Actually, architecture is frozen habit.

The park near my house has two kinds of paths. There are paved paths laid out by the park’s original designers. And there are unpaved paths created by pedestrians who were unwilling to be constrained by those latter. Both are bona fide pathways. One set were planned and executed. The other arose out of the habitual activity of the park’s users.

An environment, natural, or human-made, reflects a compromise between design and habit. Exactly the same is true of our bodies. Indeed, habit is, or ought to be considered as, a fundamental category for biological thought.

Consider this question: What makes a region of cortex visual rather than auditory? What explains the assignment of visual function to a cortical field? Answer: experience. The assignment of function is the result of species-wide habits of dynamic exchange with the environment. This is why when an individual mammal is born blind, its “visual” structures get conscripted to perform non-visual functions; function answers to the animal’s real-world transactions. Although genetic factors — which themselves reflect ancestral environmental pressures and adaptations — largely govern the growth of anatomical structures in the brain, non-genetic, environmental (epigenetic) factors determine their function. The blind mole-rat, for example, has eyes, but they are tiny and disused; despite this, the retina of the developing mole-rat sprouts cellular projections into what would be, in related mammalian species, “visual” areas of the brain. In the mole-rat, however, these morphologically visual areas perform mainly non-visual sensory functions. (On the evolution of cortex and the competition between genetic and epigenetic factors, I recommend Leah Krubitzers’s “The magificent compromise: cortical field evolution in mammals.” Neuron 56, 2007: 201-208. I rely on this paper here.)

A lot of human somatosensory cortical real estate is devoted to the hands; it wouldn’t be if the hands weren’t so well-suited to human needs and predicaments. And of course the hands wouldn’t be so well-suited if not for the correspondingly adequate cortical infrastructure. This is a virtuous circle. We see this sort of circle everywhere. We can digest milk because we drink milk; we drink milk because its nourishing and we can digest it. Mutations in genes governing cell growth and death can affect the anatomy of a region of cortex. But likewise, cortex gets affected by mutations in genes governing the growth of sensory appendages such has hands, limbs and bills. This is why platypus touch-cortex is hugely devoted to supporting the use of the bill as a perceptual organ. (Another example of this sort of circular “cortical magnification” is the enlargement of auditory cortex in echolocating bats.)

All this shows that how an animal habitually lives is a crucial element in understanding the biological processes whereby the animal — taken as an individual or as a species — becomes the sort of animal that it is.

I believe that habit plays an even deeper, more constitutive role in shaping mind and brain. Let’s go back to the question what makes a region of cortex visual as opposed to auditory. I don’t now mean, what causes it to acquire this function. I mean, what is it for it to have acquired this function. Or better even: what is the function it acquires? I have argued — first in a series of papers with the late philosopher Susan Hurley, and later in my book Out of Our Heads: Why You Are Not Your Brain and Other Lessons From the Biology of Consciousness (Hill and Wang, 2009) — that you can’t explain the distinctively visual character of neural activity in a cortical region without looking at the embedding of that neural activity in a behavioral context. Movements of the body produce sensory change. As Paris-based psychologist Kevin O’Regan and I had argued, to each of the distinct sensory modalities (seeing, touching, etc) there corresponds a distinct way in which movement produces characteristic sensory events. Hence the hypothesis: what makes neural activity visual (for example) is not the intrinsic character of the neural activity, but the way the neural activity depends on and varies as a function of the animal’s movement. Or to sum up: the brain is visual when its supports habits that are vision-like, it is auditory when it supports touch-like habits, and so on.

Mrganka Sur, the MIT neuroscientist, surgically altered newborn ferrets, splicing retinal cells so that they sprouted connections into what would normally be auditory fields. What happens as result of this rewiring of the animal’s sensory periphery to the cortical heart of the brain? Does the rewired ferret come to hear with its eyes? No, it comes to see with its auditory brain. And this is exactly what our proposal predicts. “Auditory” cortex does visual work when it is harnessed by and integrated within a vision-like sensorimotor dynamic of exchange with the environment. True, as critics have observed, such rewired cortex actually takes on the sorts of lower-level organizational properties (receptive fields, etc) that one finds in “normal” visual cortex. But this further supports our hypothesis. For after all, what drives this plasticity is the fact that the neural region is entrained in vision-like sensorimotor dynamics, and moreover, the brute fact of the existence of these low-level properties explains nothing on its own.

We like to think that we can explain what we are, and what we can do, in terms of the brain. But it turns out that the brain itself, and its role in consciousness and cognition, is best understand in relation to what we are and what we can do. Either way, habit plays a basic role in making sense of what we are.

Coming soon: Why habits are also a curse.

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