The Excitable Membrane

After summarizing the evidence for localized motor function in the brain (the cortex in particular), James does the same for sensory function. Again, I’m impressed by how much this view from 1890 could be ported over to an Intro Neuro class in 2024. That’s not to say nothing here is wrong or dated. Just that James had a keen eye for what the interesting and durable results were. I wonder, actually, if his conspicuous reflections on Neurology in the early part of his Principles were written with future readers like us in mind. As if to say: “I may be hollering at you from back in 1890 with very modest tools and early results… but I understand enough about the brain that you should take the rest of this work seriously.”

The first big result in sensory localization, according to James, came from Ferrier, who found that stimulating the occipital lobe (the part of cortex all the way at the back of brain) evoked eye and head movements in monkeys, as if they were visually fixating on something. Similarly, damaging the same area led to blindness in the visual field opposite the site of the injury.

In summarizing the deficits that followed these sorts of lesion, James makes a distinction — first advocated for by Munk — between ‘sensorial’ and ‘psychic’ blindness. The former is the complete inability to see, whereas the latter is ‘merely’ the inability to recognize the meaning or utility of what one sees, as if the world is written in an unintelligible language. This gets at the important idea that vision is a fundamentally interpretive act, one where we are constantly layering on schemas, concepts, and affordances to the raw sensory materials of the visual field.

James goes into a somewhat laborious (to me, at least) description of the comparative neurology of vision between amphibians, birds, rabbits, and people. I’ll spare you the details and just say that he’s trying to evaluate the claim that destroying the hemispheres is more likely to lead to permanent blindness in ‘higher’ animals like us, whereas many aspects of basic vision are handled perfectly well without a cortex in the ‘lower’ animals. Recall that the big theoretical vision here is what James referred to as the Meynert scheme, in which higher animals are progressively more ‘cerebral’.

James is a careful thinker about how to interpret lesion studies in general, and offers us the suggestion that observing loss of function vs. preservation of function after brain damage aren’t really ‘opposite’ experimental results, as we might naively suppose. If you damage brain area X and function F is preserved, we can confidently say that F doesn’t depend on X (in the sense that X is not absolutely required for F). On the other hand, if we do a similar experiment where we damage brain area Z, and the same function F is lost, the interpretation is more ambiguous, owing to the brain’s massive plasticity and parallelism and the possibilities these entail for compensating for the damage. If function F can be recovered after damage to Z, we can’t say Z is its necessary seat. Z just happens to be the particular way the function was implemented prior to the damage. James wants to steer us clear of “area X is the seat of function Y” sorts of functional absolutism.

The section ends with James’ grand if quite vague picture of how sensory and motor function hang together in the brain. He postulates that cortical areas represent low-level primitives of sensory impressions and movements that can be coupled together through associative mechanisms. The world “pours in” though the senses, which propagate their currents upward, until there’s an eventual downward discharge from cortical motor centers to prompt action. Although he very much believes in and cherishes peoples’ spontaneous powers of expression, the picture given here (which he will later critique) doesn’t really position us as true originators of action. Rather, it describes us more like conduits for actions that are conditioned on the state of the world. James imagines the brain to be a stream of complex currents, with “[a]ll the currents probably hav[ing] feelings going with them, and sooner or later bringing movements about.” When the stream runs occipitally, we have visual feelings. When it runs temporally, we hear things. And so on.