Cognitive Connie
What is neural plasticity?
For much of the 20th century, the adult brain was believed to be fixed — a wiring diagram laid down in childhood and largely unchanging thereafter. This view has been decisively overturned. The adult brain retains remarkable capacity for structural and functional reorganisation in response to experience, learning, and damage. This property is neural plasticity.
Key figures
Donald Hebb
1904–1985Canadian psychologist who proposed the Hebbian learning rule in 'The Organization of Behaviour' (1949): synapses between co-active neurons are strengthened. Provided the conceptual framework for synaptic plasticity, long-term potentiation, and most artificial neural network models. The rule is summarised as 'fire together, wire together' (a phrase coined by Carla Shatz).
David Hubel & Torsten Wiesel
1926–2013; 1924–presentNobel Prize (1981). Pioneered research into critical periods by showing that monocular deprivation in kittens during a sensitive period permanently disrupted the development of binocular neurons in V1. Their work defined the concept of the critical period in cortical development and the conditions required for experience to shape neural circuits.
Eleanor Maguire
1970–presentNeuroscientist at University College London who demonstrated structural hippocampal plasticity in London taxi drivers (2000): years of navigating London's complex street network were associated with a larger posterior hippocampus, and the degree of enlargement correlated with years of experience. Provided compelling evidence that experience can physically reshape human brain structure in adulthood.
Key concepts
Synaptic plasticity
The ability of synapses to strengthen or weaken over time in response to activity. The foundational mechanism of learning and memory at the cellular level. Hebbian plasticity (neurons that fire together wire together, proposed by Donald Hebb in 1949) captures the key principle: repeated co-activation of pre- and postsynaptic neurons leads to lasting strengthening of the synapse. Synaptic plasticity occurs through changes in neurotransmitter release, receptor number, receptor sensitivity, and spine morphology.
Long-term potentiation (LTP)
A persistent increase in synaptic strength following high-frequency stimulation of a neural pathway. First demonstrated in the rabbit hippocampus by Bliss and Lømo in 1973. LTP depends on NMDA receptors (which act as "coincidence detectors," requiring both presynaptic glutamate release and postsynaptic depolarisation) and subsequent AMPA receptor insertion into the postsynaptic membrane. LTP is considered the primary cellular mechanism underlying Hebbian plasticity and memory formation. Its counterpart, long-term depression (LTD), weakens synaptic connections and is equally important for learning.
Long-term depression (LTD)
A persistent decrease in synaptic strength following low-frequency stimulation or asynchronous pre- and postsynaptic activity. LTD is mediated by NMDA receptors and mGluR receptors and leads to AMPA receptor internalisation. LTD is not simply the "forgetting" mechanism — it actively prunes weak or incorrect synaptic connections and is essential for motor learning (in the cerebellum), spatial memory, and the refinement of neural circuits during development.
Critical periods
Developmental windows during which the brain is especially sensitive to specific types of environmental input, and during which experience is necessary for normal neural and behavioural development. The classic example is the visual critical period: monocular deprivation in kittens during the first weeks of life (Hubel & Wiesel, 1963) causes permanent loss of binocular vision and cortical columns for the deprived eye. In humans, the critical period for language acquisition extends from infancy to early adolescence. Critical periods are defined by elevated synaptic plasticity and are regulated by the balance of excitation and inhibition (parvalbumin interneurons and perineuronal nets play key roles).
Cortical remapping
The reorganisation of topographic cortical maps in response to altered sensory input or motor demands. Following amputation, the somatosensory cortex representation of the missing limb can be taken over by adjacent body part representations — a potential mechanism of phantom limb pain. In profoundly deaf individuals, auditory cortex is recruited for visual processing. In blind Braille readers, visual cortex is activated during tactile reading. Remapping can be rapid (within hours after nerve block) or gradual (years of practice), suggesting multiple mechanisms.
Adult neurogenesis
The generation of new neurons in the adult brain. In mammals, neurogenesis occurs in two regions with high certainty: the subgranular zone of the hippocampal dentate gyrus (producing granule cells integrated into hippocampal circuits) and the subventricular zone (producing neurons that migrate to the olfactory bulb). Hippocampal neurogenesis is increased by physical exercise, environmental enrichment, and antidepressants; decreased by stress, alcohol, and aging. Its precise functional role is debated, but it is implicated in pattern separation, forgetting (clearance of old memories to prevent interference), and mood regulation.
Hebbian plasticity ("fire together, wire together")
The principle, proposed by Donald Hebb (1949), that "when an axon of cell A is near enough to excite cell B, and repeatedly and persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased." This has become the foundational principle of computational neuroscience and models of memory: synapses between co-active neurons strengthen. The NMDA receptor implements this rule biologically by requiring simultaneous presynaptic glutamate release and postsynaptic depolarisation.
Use-dependent plasticity
Changes in neural circuitry driven by repeated use of specific circuits — the neural basis of skill learning. In London taxi drivers, who spend years learning the detailed street map of the city ("The Knowledge"), the posterior hippocampus is enlarged relative to controls (Maguire et al., 2000). In musicians, the motor cortex representation of the playing hand is expanded. In people who become blind, visual cortex is recruited for auditory and tactile processing. These findings establish that the brain's architecture responds to the demands placed on it across the lifespan.
Test your knowledge
Frequently asked questions
Can you grow new neurons as an adult?+
Adult neurogenesis — the birth of new neurons in the adult brain — was once thought impossible in mammals. We now know it occurs in at least two regions: the hippocampal dentate gyrus and the subventricular zone (with new neurons migrating to the olfactory bulb). Hippocampal neurogenesis is of particular interest because new granule cells become integrated into hippocampal circuits within weeks and show elevated plasticity compared to mature neurons. Several factors increase it: aerobic exercise (the most robust finding), environmental enrichment, caloric restriction, and antidepressant medications (especially SSRIs). Stress, alcohol, and ageing decrease it. The functional significance is debated. Proposed roles include: pattern separation (distinguishing similar memories), temporal encoding (marking when memories were formed), and 'forgetting' (clearing old hippocampal representations to prevent interference with new ones). Whether adult neurogenesis occurs in the human hippocampus at functionally significant levels — particularly in adults — remains an active controversy, with conflicting studies using different methods.
What is the relationship between LTP and memory?+
Long-term potentiation (LTP) is widely considered the cellular mechanism of memory formation, but the relationship is more complex than 'LTP equals memory.' Evidence for the link: (1) LTP is induced by the same patterns of neural activity that form memories (high-frequency burst firing). (2) Drugs that block NMDA receptors (which trigger LTP) impair memory formation in animals. (3) LTP lasts for hours to weeks in the lab — matching the timescale of memory consolidation. (4) Animals genetically engineered to have impaired LTP show learning deficits in spatial tasks. Complications: (1) LTP has been difficult to demonstrate during actual learning in vivo (rather than in slices or with artificial stimulation). (2) Erasing LTP pharmacologically after learning also erases the memory. (3) The relationship between synaptic LTP and the systems-level process of memory consolidation (hippocampus to cortex over days-months) is not fully understood. The current consensus is that LTP is a necessary but not sufficient mechanism for memory, operating within a broader network of consolidation processes.
Sources
Last reviewed July 2025- 1.
Hebb, D. O. (1949). The Organization of Behavior: A Neuropsychological Theory. Wiley.
+About this source
Proposes Hebbian plasticity ('neurons that fire together wire together') — the foundational principle of synaptic plasticity and LTP.
- 2.
Bliss, T. V., & Lømo, T. (1973). Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path. Journal of Physiology, 232(2), 331–356. https://doi.org/10.1113/jphysiol.1973.sp010273
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Discovery of long-term potentiation (LTP) — the primary synaptic mechanism underlying memory formation.
- 3.
Draganski, B., Gaser, C., Busch, V., Schuierer, G., Bogdahn, U., & May, A. (2004). Neuroplasticity: Changes in grey matter induced by training. Nature, 427(6972), 311–312. https://doi.org/10.1038/427311a
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Demonstrates structural grey matter changes in the adult human brain following skill training — evidence for use-dependent plasticity.
- 4.
Maguire, E. A., Gadian, D. G., Johnsrude, I. S., Good, C. D., Ashburner, J., Frackowiak, R. S. J., & Frith, C. D. (2000). Navigation-related structural change in the hippocampi of taxi drivers. Proceedings of the National Academy of Sciences, 97(8), 4398–4403. https://doi.org/10.1073/pnas.070039597
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Shows structural hippocampal enlargement in London taxi drivers proportional to years of experience — key evidence for adult experience-dependent plasticity.