The Adaptable Mind: What Neuroplasticity and Neural Reuse Tell Us about Language and Cognition - John Zerilli 2021

Can Composite Systems Be Dissociable?
Modules Reconsidered: Whither Modularity?

Up to this point in the discussion, I have simply assumed that a cognitive system consisting of shared domain-general parts cannot ipso facto be separately modifiable. Some, however, have maintained that neural overlaps need not undermine the functional independence of high-level cognitive functions (Carruthers 2006, pp. 23—24). This is just to raise the possibility that high-level cognitive functions could persist as functional modules (as distinct from anatomical modules).

It is undoubtedly true that, of any two cognitive systems considered in isolation, the extent of neural overlap may be only partial, or even negligible. This would render the two systems dissociable vis-à-vis each other to the extent that a modification not affecting shared components would disrupt or improve the affected system independently. But this kind of dissociability—the dissociability of a cognitive system relative to another—comes cheap. Imagine that function A is implemented in the left lateral hemisphere, while function B is implemented in the right lateral hemisphere. The two functions are spatially far apart, and (let us say) may be sufficiently far apart that interventions affecting A will not directly affect B. In this case, relative to each other, it will be true that functions A and B are dissociable (indeed, doubly so). But this is compatible with function A’s being fully nondissociable relative to functions C and D (in the left cerebral hemisphere), and function B’s being fully nondissociable relative to functions E and F (in the right cerebral hemisphere). In short, any two cognitive systems that are functionally independent vis-à-vis each other may yet lack functional independence vis-à-vis any number of other cognitive systems. Taking high-level cognitive systems two at a time is an easy way to demonstrate functional independence. But in the brain, functions do not come two at a time.

A stronger consideration that argues in favor of functional independence for overlapping systems is this:

at the limit, two modules could share all of their processing parts while still remaining dissociable and separately modifiable. For the differences might lie entirely in the patterns of connectivity among the parts, in such a way that those connections could be separately disrupted or improved. (Ritchie & Carruthers 2010, p. 289)

If—going back to functions A and B as discussed here—it could be shown that any of the connection pathways utilized in function A are exclusive to A, such that no other cognitive system makes use of them, it would seem that, to the extent that A’s pathways are unique to A, A would to that extent be functionally independent. (If the same could be said for the pathways utilized in function B, B would likewise—and to that extent—be functionally independent.)

While the consideration is a powerful one, its standing can only be settled empirically. And the evidence we have gives us no reason to suspect that reuse is limited to columns. Neurons, too, can be reused, which—as far as we know—means that all of their parts (cell bodies, dendrites, axons, and, importantly, synapses) will be extensively reused throughout the brain. Thus, once the true scale of reuse dawns, the claim that high-level cognitive functions may persist as functional modules appears less convincing.