10Adult plasticity & rehabilitation

TCThe adult brain still changes

The clearest demonstrations come from the somatosensory system. When an adult monkey loses a finger, the cortical territory that served it does not stay silent — within weeks to months it is taken over by the neighbouring fingers, whose representations expand into the vacated zone. The human correlate is the phantom limb: after amputation, the cortex remaps, and touching the face can be felt in the missing hand. The adult cortical map is dynamic, redrawn by the inputs it currently receives.[1984]

The auditory cortex behaves the same way. After a restricted cochlear lesion in an adult, the cortical region that lost its input is, over weeks, recruited to respond to the frequencies of the surviving cochlea next to the lesion — a tonotopic remapping driven entirely by the change in the periphery.[2009]

TCTraining reshapes the map

Plasticity in the adult is not only a response to injury — it is the machinery of learning. Train an adult animal to discriminate a particular sound frequency and the cortical territory devoted to that frequency enlarges, in proportion to how well the animal learns. The map literally allocates more cortex to what matters. Drag the amount of training below.[1993]

What licenses this remapping in the adult is attention, carried by a neuromodulator. When a sound is made behaviourally important, the nucleus basalis releases acetylcholine across the cortex; pairing a tone with that cholinergic signal is enough, on its own, to enlarge the tone's cortical representation in an adult. The acetylcholine briefly tips the local excitatory–inhibitory balance, opening a transient window in which the attended input strengthens before inhibition restores balance and consolidates the change. This is why passive exposure is not enoughand engaged, attentive practice is: rehabilitation works by recruiting the brain's own attention-gated plasticity machinery.[2007]

CThe brakes on adult plasticity

If the adult brain can still change, why is it so much less plastic than the child's? Part of the answer is that development actively installs brakes. As the brain matures, processes such as myelination and the build-up of growth-inhibitory molecules stabilise existing circuits — a necessary trade-off, because a brain that never consolidated its wiring could never hold a stable memory. Intriguingly, lifting some of these brakes experimentally can restore juvenile-like plasticity, raising the long-term prospect of pharmacologically re-opening windows that have closed.[2009]

FTAcclimatisation & rehabilitation

This is why a newly switched-on adult does not hear their best on day one. Over the following weeks and months, performance typically improves as the brain acclimatisesto the implant's signal — learning to map the unfamiliar electrical patterns onto meaning. Structured auditory training accelerates this, and the principle is exactly the map-expansion seen above: practice with the new signal recruits cortex to represent it. Rehabilitation is applied adult plasticity.[2010]

The magnitude is real and measurable. Adult recipients commonly show open-set word recognition climbing substantially over the first post-operative year — in some series from around a third correct at three months to roughly half by six to twelve months — even though the device itself never changes. That gain is the brain learning the code. The same lifelong capacity shows up beyond hearing: in adults intensively learning a second language, imaging reveals increased myelination in language-related cortex tracking their proficiency. The adult brain remains structurally, not just functionally, modifiable — which is the biological permission slip for rehabilitation.[2014]

FTExploiting adult plasticity

The postlingual adult is the clearest beneficiary: with a language system already built, they need only relearn to map sound onto it, a task adult plasticity is well suited to. Understanding the limits and levers of that plasticity — consistent use, active training, and one day perhaps pharmacological enhancement — is how the field hopes to push adult outcomes higher.[2010]

Two threads remain. One sense we have treated only in passing is the comparison of the two ears, which has its own developmental rules and its own implications for bilateral implantation — the next module — and then the chapter closes on the paradox at the heart of it all: back to age at implantation or on to binaural plasticity.