Reading focus
Syllabus: How Does the Nervous System Develop and Change?
Follow how the circuit's current state affects what a particular experience can change.
Why you’re reading it. This turns the developmental overview into a question about susceptibility, persistence, and the conditions for later change.
A question to consider. What observations at several ages would distinguish a gradual decline in sensitivity from a sharply bounded critical period?
Why this reading
The same experience can have different effects at different times. Knudsen develops an account of how a circuit becomes particularly susceptible to experience, how prior changes constrain later ones, and why residual adult plasticity need not recreate an earlier developmental state.
Before you read
Plasticity is a capacity for change under specified conditions; it is not a synonym for improvement. In Knudsen's terminology, a sensitive period involves heightened susceptibility to experience, while a critical period is a subset whose consequences persist under ordinary later experience. A circuit's state depends on connections, cellular properties, and its history. The landscape diagrams represent a proposed framework for those constraints, not a literal map of measured brain energy.
Read for
- Compare the different preparations and the outcome used to identify a sensitive period in each. Notice that timing belongs to a process and measure, not to an entire organism at once.
- Follow the conditions that allow experience to change a circuit, including the availability of informative input and the circuit's capacity to respond.
- Distinguish restoring ordinary experience, increasing adult plasticity, and recreating the full range of earlier plasticity.
Consider afterwards
- What evidence would show heightened sensitivity at one age without establishing an irreversible cutoff?
- How could similar adult performance conceal different developmental histories and neural organizations?
Editorial context
Sensitive and critical period terminology is not uniform across the literature. Reh and colleagues' later synthesis develops molecular regulation across timescales and uses critical period more broadly. Findings about changing plasticity in particular animal circuits do not establish a general way to reopen human development. This review remains useful for asking which experience, circuit, outcome, and later conditions a timing claim actually concerns.