The Excitable Membrane

Having dispatched with the phrenologists, James next considers the question of localization of brain function — one of the most enduring and important in neuroscience. This section of the Principles is a tour of Neurology in the nineteenth century, and reading it, I’m impressed by how much of it holds up. It can be easy to take a caricature view that scientists of an earlier era were seduced by all kinds of naive ideas that we, in our great modern wisdom, are much more careful to avoid. James lacked our slick contemporary tools for pulling apart brain circuits, but he basically articulates all the important issues, and calls out all the questions that the field is still grappling with. In some readings of James by humanists, all this neurology stuff early in The Principles is taken as a dull, detail-oriented prelude that’s orthogonal to his much more probing and substantive reflections on the Self, the Will, the Stream of Consciousness, etc. I think this gets James and his project wrong, though. He’s trying to ground his Psychology in action, and knows that understanding how action is organized in the brain provides very important constraints on his bolder, more metaphysical theories. Here, I’ll focus mostly on James’ discussion on the localization of motor function in the brain. I’ll take up his similar arguments for the senses in the next post.

“no actions but such as are done for an end, and show a choice of means, can be called indubitable expressions of Mind”.

James, The Principles (1890)

Question numero uno for James is: is the brain more like a sponge or more like a circuitboard? If you rip off a small bit of a sponge and toss it away, you’re not going to observe a catastrophic failure in its ability to sop up liquids. It may sop up less, but its performance will suffer gracefully, by degrees, in direct proportion to the amount of damage. If you fry some random transistor on a circuitboard, on the other hand, the computer it’s slotted into may well never start again. Whereas the sponge works on the principle of mass action, function in the circuitboard is much more localized. Moreover, the particular failures observed in the damaged circuitboard will depend much more strongly on exactly which piece was damaged.

Prior to 1870, James tells us, the prevailing view was that of team sponge, which was championed by the neurologist Jean Pierre Flourens. Flourens ascribed certain brain functions very broadly to specific lobes, but was unconvinced that there was a more granular organization than that.

The year 1870 was a watershed because that was when the German neurologist Eduard Hitzig demonstrated the existence of the “motor strip” in cortex. Applying electrical shocks to very small, contiguous areas of the brain’s surface in dogs, he found he could elicit very specific movements. Moreover, specific acts were mapped onto this strip in an orderly way. If you look under this motor strip on the brain’s surface, you find a massive bundle of fibers that plunge deep and make their way toward the spinal cord, carrying commands to mobilize the muscles. It’s easy to see why these results are so significant for James. Recall that for him, “no actions but such as are done for an end, and show a choice of means, can be called indubitable expressions of Mind”. In fact, he called this “the criterion by which to circumscribe the subject matter of this work [i.e. The Principles].” The motor cortex is the part of the brain that allows mind to make its mark on the world. It’s ‘the prime doer’ in a psychology that is concerned, above all, with doing.

To firm up the idea of the “motor zone”, James gives us a few lines of evidence for it. The argument is very modern, and the basic experimental strategies described will be very familiar to any contemporary neuroscientist. Your three strategies for doing neuroscience detective work are basically to tickle, cut, or correlate. Or, unpacking this a bit more, you can: 1) stimulate (‘tickle’) the brain directly and see what behavior or reportable sensation is elicited; 2) cut out a piece of brain, and see what behavior/aspect of reportable sensation is compromised; and 3) make careful records of which brain processes/features tend to co-occur with which mental or behavioral acts. James summarizes a range of experimental results on motor cortex that I won’t bore you with here, but suffice it to say, he definitely thought like a scientist, and had a superb understanding of the neurology of his day.

James is clearly very impressed with the motor system’s plasticity after injury, and plasticity is another one of the deep concepts he’ll hang his whole Psychology on. He makes the observation that in experiments in dogs, even when the whole motor zone is destroyed, one doesn’t observe complete or permanent paralysis but rather “this curious sort of inertia” in which the animal seems “less easily moved to do anything with the affected parts”. There’s certainly a loss of function after injury to the motor cortex, but it appears to be transient, even in cases where the initial loss is severe. Within eight to ten weeks, James tells us, the lesioned dogs are virtually indistinguishable from the unaltered ones.

Closing this section on (what we’d now call) primary motor cortex, James encourages us to not fall into a kind of easy phrenological thinking here. Having found a “motor center” in the brain that influences the body when cut or tickled doesn’t mean we actually understand its role in movement. To paraphrase some of James’ important questions here: Do volitional acts really arise uncaused here — or is it downstream from higher order motor centers that do the organizing, planning, and ‘willing’? And what “phase of psychic activity” (James’ phrase) is represented in the primary motor center? Does activity here represent something abstract like the consciously felt impulse to move, or is it more like a set of simple unconscious commands to the muscles that actually do the doing? Etc, Etc.