Cognitive Connie
What is neural plasticity?
For much of the 20th century, the adult brain was widely assumed to be largely fixed after childhood. That assumption has been substantially revised. The brain remains capable of changing throughout life — but the form, magnitude, and ease of plasticity depend on the circuit, developmental stage, and type of experience. Adult brains are not infinitely malleable, and adult plasticity is considerably more restricted than developmental plasticity. The central insight of plasticity research is not that "anything is possible" but that experience, learning, and injury all produce lasting changes in neural structure and function — within real biological constraints.
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 an influential conceptual framework for activity-dependent synaptic change. The rule is summarised as 'fire together, wire together' (a phrase coined by Carla Shatz, not Hebb himself). Hebb's proposal predated the discovery of LTP by nearly 25 years and helped inspire experimental work on synaptic plasticity.
David Hubel & Torsten Wiesel
1926–2013; 1924–Nobel Prize (1981). Demonstrated that monocular deprivation in kittens during a restricted developmental window permanently altered the organisation of binocular neurons in primary visual cortex — providing the foundational evidence for experience-dependent critical periods in cortical development. Their work established the conditions under which experience shapes neural circuits, specifically in the visual system.
Eleanor Maguire
1970–Neuroscientist at University College London who reported an association between taxi-driving experience and posterior hippocampal structure in London taxi drivers (2000): those with more years of experience showed larger posterior hippocampal volume. This cross-sectional finding became a widely cited illustration of use-dependent structural plasticity in adulthood, though cross-sectional associations do not establish causation.
Tim Bliss & Terje Lømo
1973Discovered long-term potentiation (LTP) in hippocampal tissue of anaesthetised rabbits — showing that high-frequency stimulation of the perforant path produced a lasting increase in synaptic strength. This finding provided the first experimental demonstration of activity-dependent synaptic strengthening with the properties required for a memory mechanism, and launched five decades of plasticity research.
Key concepts
Levels of plasticity
Plasticity occurs at multiple distinct levels, and these should not be conflated: **Synaptic plasticity**: changes in the strength or efficacy of existing synapses (LTP, LTD, receptor trafficking). Well characterised in animal models and brain slice preparations. **Structural plasticity**: physical changes to neural structures — dendritic spines, synapse formation or elimination, axonal remodelling. Evidence from animal models is strong; measuring subtle structural changes in living humans with neuroimaging is considerably more difficult and methods remain debated. **Functional/network plasticity**: changes in how existing circuits participate in a task, particularly relevant after injury or during rehabilitation. **Developmental plasticity**: experience-dependent changes during sensitive or critical periods when circuits are especially malleable. **Myelin plasticity**: experience can alter oligodendrocytes and the myelination of axons, influencing conduction speed and circuit timing — an important but often overlooked form of plasticity. Evidence for each type differs substantially in strength, species generalisability, and applicability to humans.
Hebbian plasticity ("fire together, wire together")
The principle, proposed by Donald Hebb (1949), that repeated co-activation of a presynaptic and a postsynaptic neuron can lead to lasting strengthening of the connection between them. This is a general theoretical principle, not a description of a single mechanism. NMDA-receptor-dependent mechanisms provide one important biological route through which activity-dependent synaptic strengthening can occur — by acting as "coincidence detectors" requiring simultaneous presynaptic glutamate release and postsynaptic depolarisation — but Hebbian-like plasticity can be implemented through multiple molecular pathways. The rule has been influential in computational models of learning, though modern artificial neural networks use variants that depart substantially from the original biological proposal.
Synaptic plasticity
The ability of synapses to strengthen or weaken over time in response to activity — changes in neurotransmitter release, receptor number, receptor sensitivity, and spine morphology. Synaptic plasticity underlies most current accounts of learning and memory at the cellular level, though the relationship between synaptic changes and the systems-level process of memory formation involves multiple additional mechanisms and levels of organisation.
Long-term potentiation (LTP)
A persistent increase in synaptic strength following high-frequency stimulation of a neural pathway. First demonstrated in hippocampal tissue of anaesthetised rabbits by Bliss and Lømo (1973). LTP depends on NMDA receptors acting as coincidence detectors and subsequent AMPA receptor insertion into the postsynaptic membrane. LTP is a major experimental model of synaptic plasticity and is strongly associated with learning and memory mechanisms. However, memory is not reducible to LTP — there are multiple forms of plasticity and multiple levels of memory organisation. LTP has been difficult to demonstrate during actual learning in vivo (outside slices or artificial stimulation), and the relationship between synaptic LTP and systems-level memory consolidation is still being worked out.
Long-term depression (LTD)
A persistent decrease in synaptic strength following low-frequency 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 a "forgetting" mechanism — it contributes to circuit refinement and some forms of learning, and may help prevent excessive synaptic strengthening and keep networks adaptable. Plasticity requires both strengthening and weakening: without a mechanism to reduce synaptic weights, circuits would become saturated and lose the ability to encode new information.
Sensitive and critical periods
Developmental windows during which specific circuits are especially sensitive to environmental input. In strict usage, a **critical period** is a window during which a particular experience is necessary for normal development and after which the circuit's plasticity is substantially reduced. The classic example is the visual critical period in kittens: monocular deprivation during the first weeks of life (Hubel & Wiesel, 1963) causes permanent loss of binocular vision and cortical columns for the deprived eye — demonstrating experience-dependent plasticity specifically in the visual cortex during a restricted developmental window. **Sensitive periods** are broader windows during which experience has heightened influence but the constraints are less absolute. Language development involves multiple overlapping sensitive periods for different components (phonology, syntax, vocabulary), with considerable individual variability — not a single, clean critical period analogous to visual deprivation. Critical periods are defined by elevated synaptic plasticity regulated by the balance of excitation and inhibition (parvalbumin interneurons and perineuronal nets play key roles).
Cortical reorganisation and cross-modal plasticity
Topographic cortical maps can reorganise in response to altered sensory input or motor demands. Following limb amputation, the somatosensory cortex region representing the missing limb shows altered responses from adjacent body representations. This reorganisation has been associated with phantom limb phenomena, though the relationship between cortical reorganisation and phantom limb pain is considerably more complex than early accounts suggested — peripheral, spinal, and higher-level mechanisms all contribute. In sensory deprivation, **cross-modal plasticity** can occur: cortical regions associated with one sensory modality become responsive to input from another. In profoundly deaf individuals, regions of auditory cortex may show altered responses to visual input; in blind Braille readers, regions of visual cortex are engaged during tactile reading. "Recruited" is a useful shorthand, but the underlying changes are in connectivity and responsiveness rather than a clean switch of function. Remapping can be rapid (hours after nerve block) or gradual (years of practice), suggesting multiple mechanisms at different timescales.
Adult neurogenesis
The generation of new neurons in the adult brain. In rodents and other mammals, neurogenesis is well established in two regions: the subgranular zone of the hippocampal dentate gyrus and the subventricular zone (with new neurons migrating to the olfactory bulb). In rodent hippocampus, neurogenesis is increased by physical exercise, environmental enrichment, and antidepressants; decreased by stress, alcohol, and ageing. The human situation is considerably more contested. Some studies report evidence for adult hippocampal neurogenesis in humans; others, using different methodological approaches, have found little or none at functionally significant levels in adulthood. Whether adult human hippocampal neurogenesis occurs at a scale that influences cognition or mood remains an active controversy. The SVZ-to-olfactory-bulb pathway seen in rodents does not appear to operate in the same way in adult humans. This is a case where a famous neuroscience finding has become substantially more complicated as methodological standards improved.
Use-dependent plasticity and experience
Changes in neural circuitry driven by repeated use of specific circuits. Maguire et al. (2000) found that London taxi drivers — who spend years learning the city's complex street network — showed larger posterior hippocampal volume than controls, with the size difference correlating with years of experience. This is a striking cross-sectional association between experience and brain structure in adulthood, though cross-sectional differences do not by themselves establish that the experience caused the anatomical difference. Draganski et al. (2004) provided longitudinal evidence: grey matter changes were reported in human adults following juggling skill training, with increases during training and partial reversal after practice stopped. Methodological debates continue about what structural MRI measures actually represent biologically, but the overall picture from cross-sectional and longitudinal evidence supports experience-dependent structural differences in adulthood. Distinguishing **cross-sectional** (experienced people differ from controls) from **longitudinal** (brain measures change during training) evidence is important for evaluating these claims.
Maladaptive plasticity
Plasticity is not synonymous with improvement. The same mechanisms that support learning and recovery can also maintain dysfunction. Examples include: - **Chronic pain**: synaptic strengthening in pain pathways can maintain pain long after the original injury has healed (central sensitisation) - **Addiction**: repeated drug exposure produces lasting changes in reward circuits that drive compulsive drug-seeking - **Maladaptive fear**: fear conditioning can become entrenched, as in PTSD, when normal extinction mechanisms fail - **Some forms of tinnitus**: thalamocortical reorganisation following auditory input loss may contribute to persistent phantom sounds Understanding maladaptive plasticity is as important as understanding beneficial plasticity — and treatments for these conditions often work by targeting the same molecular and circuit mechanisms that underlie adaptive learning.
Test your knowledge
Frequently asked questions
Can you grow new neurons as an adult?+
Adult neurogenesis — the generation of new neurons in the adult brain — is well established in several non-human mammals. In rodents, neurogenesis occurs robustly in the hippocampal dentate gyrus and the subventricular zone (with new neurons migrating to the olfactory bulb). In the rodent hippocampus, new granule cells become integrated into circuits within weeks and show elevated plasticity. Several factors increase it: aerobic exercise (the most robust finding), environmental enrichment, and antidepressants. Stress, alcohol, and ageing decrease it. The human situation is much more contested. Some studies using post-mortem tissue or neuroimaging markers have found evidence for adult hippocampal neurogenesis in humans; other studies, using different methods, have found little or none at functionally significant levels. A major methodological problem is that different tissue preparation and immunostaining approaches yield very different results. As of the mid-2020s, whether adult human hippocampal neurogenesis occurs at a scale relevant to cognition or mood remains actively debated. Proposed functional roles (in animals where neurogenesis is better established) include pattern separation (distinguishing similar memories), temporal encoding, and facilitating replacement of old hippocampal representations. These are plausible but still under investigation even in rodent models.
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 September 2026- 1.
Hebb, D. O. (1949). The Organization of Behavior: A Neuropsychological Theory. Wiley.
+About this source
Proposes the Hebbian learning rule — that correlated activity between neurons strengthens their connection — which became an influential theoretical framework for activity-dependent synaptic plasticity.
- 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
+About this source
Discovery of long-term potentiation (LTP) in hippocampal tissue — the first experimental demonstration of lasting, activity-dependent synaptic strengthening with properties relevant to memory.
- 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
+About this source
Reported training-associated grey matter changes in adult humans following juggling skill acquisition — longitudinal evidence for use-dependent structural plasticity, though subsequent methodological debate has highlighted limits of structural MRI interpretations.
- 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
+About this source
Cross-sectional study reporting larger posterior hippocampal volume in London taxi drivers with more years of experience — a widely cited illustration of experience-associated structural differences in adulthood.
- 5.
Hensch, T. K. (2004). Critical period regulation. Annual Review of Neuroscience, 27, 549–579. https://doi.org/10.1146/annurev.neuro.27.070203.144327
+About this source
Comprehensive review of the molecular and circuit mechanisms regulating critical periods, including the role of parvalbumin interneurons, perineuronal nets, and the excitation/inhibition balance.
- 6.
Kempermann, G., Gage, F. H., Aigner, L., Song, H., Curtis, M. A., Thuret, S., … & Bhardwaj, R. D. (2018). Human adult neurogenesis: Evidence and remaining questions. Cell Stem Cell, 23(1), 25–30. https://doi.org/10.1016/j.stem.2018.04.004
+About this source
Balanced review of the contested evidence for adult human hippocampal neurogenesis, distinguishing what is established in animal models from the ongoing debate about humans.
- 7.
Woolf, C. J. (2011). Central sensitization: Implications for the diagnosis and treatment of pain. Pain, 152(3 Suppl), S2–S15. https://doi.org/10.1016/j.pain.2010.09.030
+About this source
Review of central sensitisation as a form of maladaptive synaptic plasticity in pain pathways — illustrating that the same mechanisms underlying learning can maintain dysfunction.
- 8.
Takeuchi, T., Duszkiewicz, A. J., & Morris, R. G. M. (2014). The synaptic plasticity and memory hypothesis: Encoding, storage and persistence. Philosophical Transactions of the Royal Society B, 369(1633), 20130288. https://doi.org/10.1098/rstb.2013.0288
+About this source
Careful review of the evidence for and against the synaptic plasticity–memory hypothesis, distinguishing what LTP demonstrates from what it does not establish about memory.