Where does plasticity collide?

The nervous system's capacity to change its structure and function with experience, from the strengthening of a single synapse to the remapping of a cortical area after injury. Hebb proposed in 1949 that when one cell repeatedly takes part in firing another, the connection between them grows more efficient, a postulate later compressed to "cells that fire together wire together." Bliss and Lomo gave it a physiological handle in 1973 with long-term potentiation: a brief high-frequency train to the perforant path left the dentate synapses more responsive for hours or days. The change is use-dependent and, importantly, not one-way, since the same machinery that potentiates a synapse can depress it. Plasticity here names a capacity to re-encode, not a single fixed alteration.[1][2]

Unit

A synapse, a circuit, or a cortical map

Recovery means

Reversible: potentiation can be depressed and re-established

Measured by

Change in evoked response amplitude after stimulation; long-term potentiation and depression

The permanent, non-recoverable deformation a solid undergoes when stressed beyond its yield point, as distinct from the elastic strain that springs back once the load is removed. Below the yield point a metal behaves elastically; above it the material flows and keeps the new shape. Taylor argued in 1934 that this flow is carried by dislocations, line defects in the crystal lattice that move under stress and let planes of atoms slip past one another a step at a time, which is why real crystals yield at a fraction of the stress a perfect lattice would require. Plasticity here is defined by what does not come back: the strain that remains after unloading. It is the exact opposite of the neuroscientist's meaning, where reversibility is the whole point.[3]

Unit

A material specimen under load

Recovery means

None: the deformation is permanent by definition

Measured by

Strain remaining after the load is removed; the stress-strain curve past the yield point

The capacity of a single genotype to produce different phenotypes depending on the environment it develops in, treated as a property of the genotype rather than of any one individual. Bradshaw set out its evolutionary significance in 1965, arguing that plasticity is itself a character under genetic control and specific to particular traits, so that one feature of a plant may be highly plastic while another is buffered against the same variation. The pattern of phenotypes a genotype yields across a range of environments is its reaction norm. West-Eberhard's 2003 synthesis placed developmental plasticity at the centre of evolution, arguing that environmentally induced variants can precede and bias the genetic changes that follow. The referent is a genotype's response across environments, not experience written into nervous tissue and not stress written into a lattice.[4][5]

Unit

A genotype, across environments

Recovery means

Varies: some responses reverse within a life, some fix at a developmental switch

Measured by

The reaction norm: phenotype plotted against environment for a genotype or clone

The within-person modifiability of psychological functioning across the life course, treated as a defining feature of human development rather than a fixed endowment. Baltes made plasticity a central proposition of life-span developmental psychology in 1987: at any age a person's course of development is one of several that were possible, and the range that intervention or training can recover is itself the object of study. Evidence comes from showing that measured abilities move under altered conditions, for instance that older adults regain fluid-ability performance after practice. The claim is about intraindividual change and its limits, so the unit is one developing person over time. That separates it from the genotype of the phenotypic meaning and from the tissue of the neural meaning, even where the underlying biology overlaps.[6]

Unit

One person's development over time

Recovery means

Not a fixed quantity: the point is that functioning can be moved

Measured by

Intraindividual change under intervention or training; the recoverable range of a performance