Donald Hebb (1949) proposed a rule for synaptic strengthening that can be summarised as "neurons that fire together, wire together." What is the precise Hebbian learning rule?

A: A synapse is strengthened whenever the postsynaptic neuron fires, regardless of presynaptic activity

B: When a presynaptic neuron repeatedly and persistently activates a postsynaptic neuron, the efficiency of the synapse between them is increased through some growth or metabolic change in one or both cells

C: Synaptic strength is determined solely by the frequency of presynaptic firing — higher frequency always produces stronger synapses

D: Synapses are strengthened by inhibitory signals from adjacent neurons that reduce competition between inputs

Hebb's rule, from The Organization of Behaviour (1949), states: "When an axon of cell A is near enough to excite cell B and repeatedly or 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." The key principle is coincidence detection: both the presynaptic and postsynaptic cells must be active simultaneously for the synapse to strengthen. The NMDA receptor implements this biologically — it requires both presynaptic glutamate release and postsynaptic depolarisation to open (a molecular "coincidence detector"). Hebb's rule provides the conceptual foundation for LTP, associative learning, and most artificial neural network learning algorithms.

Long-term potentiation (LTP) was first demonstrated by Bliss and Lømo (1973) in the hippocampus. Which receptor is the key molecular "coincidence detector" that triggers LTP?

A: AMPA receptor — opens when glutamate binds, regardless of membrane voltage

B: GABA-A receptor — opens in response to inhibitory signals to prevent runaway excitation

C: NMDA receptor — requires simultaneous glutamate binding AND postsynaptic depolarisation to open, implementing the Hebbian coincidence requirement

D: Dopamine D1 receptor — binds dopamine to modulate cAMP and gate plasticity signals

The NMDA (N-methyl-D-aspartate) receptor is the molecular gate of Hebbian plasticity. At resting membrane potential, a magnesium ion (Mg²⁺) blocks the NMDA channel even when glutamate is bound. Only when the postsynaptic membrane is sufficiently depolarised (by AMPA receptor activation from prior activity) is the Mg²⁺ block relieved. At that point, glutamate binding opens the NMDA channel, allowing calcium (Ca²⁺) to enter. The Ca²⁺ influx triggers a cascade — phosphorylation of AMPA receptors, insertion of new AMPA receptors, and ultimately structural changes — that collectively constitute LTP. This dual requirement (presynaptic glutamate + postsynaptic depolarisation) is the molecular implementation of Hebb's rule.

Long-term depression (LTD) — a persistent decrease in synaptic strength — is simply the reversal of LTP and plays an equally important role in learning by eliminating weak or incorrect synaptic connections.

Svar: Sant

LTD is induced by low-frequency stimulation (e.g., 1 Hz for 15 minutes) of a synapse, in contrast to LTP's high-frequency induction. The key difference from LTP is the amount of postsynaptic calcium entry through NMDA receptors: LTP requires a large, brief Ca²⁺ influx that activates kinases (CaMKII, PKC); LTD requires a modest, prolonged Ca²⁺ influx that instead activates phosphatases, leading to AMPA receptor internalisation and synaptic weakening. LTD is not merely the "undo" of LTP — it is a distinct and functionally important process. It underlies extinction learning (weakening of learned fear associations), cerebellum-dependent motor learning, and the developmental pruning of excess synaptic connections during critical periods.

Hubel and Wiesel's monocular deprivation experiments in kittens demonstrated the concept of a critical period. What did these experiments show?

A: Visual experience during a specific developmental window is required for binocular cortical neurons to develop; deprivation during this window produces permanent loss of binocular depth perception

B: The visual cortex can fully recover its binocular organisation after monocular deprivation at any age, demonstrating lifelong plasticity

C: Monocular deprivation during the critical period selectively impairs colour vision while leaving spatial acuity intact

D: Kittens deprived of visual input develop compensatory improvements in auditory and tactile acuity through cross-modal plasticity

Hubel and Wiesel (1963, 1970; Nobel Prize 1981) sutured one eye closed in kittens during the first weeks of life. After opening the eye at various ages, they found that deprivation during a sensitive window (roughly 3–8 weeks in cats) caused the deprived eye to lose its cortical representation — V1 neurons became unresponsive to it, driven almost entirely by the non-deprived eye — and this loss was permanent. Deprivation before or after this window had little lasting effect. The work established the concept of critical (or sensitive) periods: windows during which experience is necessary for normal development and during which the cortex is especially susceptible to deprivation. The analogous human condition is amblyopia (lazy eye), which must be treated during childhood.

Cortical remapping after amputation — in which the somatosensory cortex representation of the missing limb is taken over by adjacent body regions — is most associated with which phenomenon?

A: The cortex degenerates in the regions that lost peripheral input, leading to a smaller total cortical surface area

B: Phantom limb sensations, in which stimulation of the face or shoulder can be felt as sensations in the missing hand, reflecting cortical reorganisation

C: The somatosensory cortex becomes exclusively devoted to proprioception from remaining limbs, eliminating all tactile processing

D: Remapping only occurs in children — the adult somatosensory cortex is too rigid to reorganise after amputation

Michael Merzenich's work in monkeys and V. S. Ramachandran's studies in human amputees demonstrated that after amputation, the cortical territory previously representing the missing limb is invaded by representations of adjacent body parts. In the somatosensory homunculus, the hand area is bordered by the face (above) and the upper arm (below). After hand amputation, the face and upper arm representations expand into the former hand territory. Ramachandran showed that this explains phantom limb sensations: touching the face of an amputee can trigger sensations felt as if in the missing hand, because face-touch signals now activate the reorganised "hand" cortex. This is a striking example of use-dependent plasticity and cortical map reorganisation.

Eleanor Maguire's study of London taxi drivers found that experienced navigators had a larger posterior hippocampus than controls. What does this finding illustrate?

A: Spatial memory ability is genetically determined and larger hippocampi select for careers requiring navigation

B: Use-dependent structural plasticity: extensive navigational experience produces measurable growth in the hippocampal region specialised for spatial memory, and the degree of enlargement correlates with years of experience

C: The hippocampus continuously grows throughout life regardless of experience, reflecting age-related neurogenesis

D: Taxi drivers have larger hippocampi because their profession requires fewer declarative memory demands, freeing hippocampal resources for spatial use

Maguire et al. (2000) used structural MRI to compare London taxi drivers (who must learn "The Knowledge" — every street in London, a 2–4 year training process) to controls. Taxi drivers had significantly greater grey matter volume in the posterior hippocampus, which is specialised for spatial representation and navigation. Crucially, the degree of posterior hippocampal enlargement correlated with years of experience as a taxi driver, and those who retired from driving showed some reversal of the effect. This demonstrated that experience — even in adulthood — can produce structural brain changes, not merely functional ones. It is a landmark example of use-dependent structural plasticity in the human adult brain.

Adult neurogenesis — the generation of new neurons in the mature brain — occurs in the hippocampal dentate gyrus and is increased by aerobic exercise and reduced by chronic stress.

Svar: Sant

Adult hippocampal neurogenesis (AHN) is one of the most established forms of structural plasticity in the adult mammalian brain, with new granule cells born in the subgranular zone of the dentate gyrus continuously throughout life. In rodents, AHN is robustly modulated by experience: aerobic exercise (van Praag et al., 1999) is among the strongest known inducers; environmental enrichment, learning, and antidepressants also increase it. Chronic stress and cortisol reduce AHN. The functional significance of AHN for learning and memory in humans is debated — some post-mortem studies in humans have found substantially less evidence for AHN than in rodents — but the regulatory principles (exercise up, stress down) are well-established and have practical implications for brain health.

The phenomenon of "spike-timing-dependent plasticity" (STDP) refines the classical Hebbian rule by showing that:

A: Only the postsynaptic cell's firing rate determines whether a synapse strengthens or weakens, regardless of presynaptic timing

B: The precise temporal order of pre- and postsynaptic firing determines the direction of plasticity: presynaptic firing just before postsynaptic firing strengthens the synapse (LTP); postsynaptic firing before presynaptic firing weakens it (LTD)

C: Synaptic plasticity can only be induced during sleep, when spike-timing irregularities activate NMDA receptors

D: Neural plasticity requires neuromodulatory signals (dopamine or acetylcholine) and cannot occur through glutamatergic activity alone

Spike-timing-dependent plasticity (STDP), established through in vitro and in vivo recordings by Markram, Bi, Poo, and others in the late 1990s, showed that the precise millisecond-order timing of pre- and postsynaptic spikes determines plasticity direction. If the presynaptic neuron fires ~20 ms before the postsynaptic neuron (the causal order predicted by Hebb), the synapse strengthens (LTP). If the order is reversed — postsynaptic before presynaptic — the synapse weakens (LTD). The temporal window is remarkably narrow (typically ±50 ms). STDP provides a precise biophysical implementation of Hebbian causality and has major implications for theories of learning, coding, and the development of neural circuits.