Questions: Adult Neurogenesis: Generation of New Neurons in Mature Brain

Question 1

A rodent's hippocampal neurogenesis is experimentally blocked through targeted irradiation of the subgranular zone. Which memory task would you most expect to be impaired?

• A. Recognizing a previously encountered object presented alone in a familiar context
• B. Distinguishing between two spatial contexts that share many overlapping features
• C. Forming any new long-term memories, since hippocampal function is broadly compromised
• D. Retrieving memories formed years before the neurogenesis block, since new neurons replace old ones

Answer: (B)

Blocking neurogenesis specifically impairs pattern separation — the ability to distinguish similar, overlapping memories — which is the dentate gyrus function that newly born neurons contribute to. Simple object recognition in a familiar context (option A) does not heavily depend on fine-grained pattern separation. Option C overstates the effect: research shows general hippocampal memory function remains largely intact when neurogenesis alone is blocked. Adult-born neurons contribute specifically to pattern separation, not to memory in general.

Question 2

During what period are newly generated hippocampal granule neurons thought to make their greatest functional contribution?

• A. Immediately at birth, before any dendritic processes have extended
• B. A window of approximately 4–6 weeks after birth, when they are hyperexcitable with enhanced synaptic plasticity
• C. After full maturation at 3–6 months, when they become indistinguishable from older granule cells
• D. Throughout their entire lifespan, with no particular critical window

Answer: (B)

Newly born hippocampal neurons undergo maturation over several weeks. During approximately weeks 4–6 after birth, these young neurons are hyperexcitable and display enhanced long-term potentiation compared to mature granule cells, making them especially responsive to new experiences. This critical window means their functional contribution is temporally specific — learning during this period promotes the survival of the recently born neurons, directly linking neurogenesis timing to memory encoding.

Question 3

Adult neurogenesis has been demonstrated throughout the adult brain, broadly overturning the classical dogma that neurons cannot be replaced.

• T. True
• F. False

Answer: False

The overturning of the classical dogma is real, but the scope is more limited than 'throughout the brain.' Adult neurogenesis is well-established in two specific regions: the subgranular zone (SGZ) of the hippocampal dentate gyrus and the subventricular zone (SVZ), whose new neurons migrate to the olfactory bulb. Evidence for neurogenesis in other brain regions, including the neocortex, remains debated and is not well-established in humans. The brain's capacity for neurogenesis is real but regionally restricted, not global.

Question 4

Chronic stress and elevated glucocorticoid levels suppress adult hippocampal neurogenesis, which may contribute to stress-related cognitive and mood disturbances.

• T. True
• F. False

Answer: True

Chronic stress dramatically reduces both the proliferation of neural progenitor cells and the survival of newborn neurons in the dentate gyrus — one of the most robust findings in this field. Since adult neurogenesis contributes to pattern separation and contextual memory functions, its suppression under chronic stress may be part of the mechanism linking stress to cognitive deficits and mood disorders. Antidepressants that promote neurogenesis may restore some of these functions.

Question 5

Why are adult-born hippocampal neurons thought to contribute specifically to pattern separation rather than to memory formation in general?

Model answer: Adult-born neurons in the dentate gyrus pass through a phase of hyperplasticity — enhanced excitability and LTP — before maturing. The dentate gyrus performs pattern separation: converting similar inputs from the entorhinal cortex into distinct representations in CA3, reducing interference between overlapping memories. When neurogenesis is experimentally blocked, animals show specific deficits distinguishing similar contexts but retain normal memory for clearly distinct experiences. This double dissociation — impaired pattern separation, spared general memory — indicates that adult-born neurons contribute a specialized computational function, not memory broadly.

The logic is that if newborn neurons contributed to memory broadly, blocking neurogenesis would produce broad amnesia. The selectivity of the deficit is the key evidence for a specific role. The newborn neurons' hyperplasticity during their critical window may be precisely what gives the dentate gyrus a fresh encoding capacity for similar inputs.