Sensitive Periods for Cognitive Change
Mechanisms of change during sensitive periods
At the behavioral, cognitive, emotional and social levels, sensitive periods are associated with enhanced learning capabilities. At the neural level sensitive periods are epochs during which the brain is particularly susceptible to experience. Sensitive periods are a property of neural networks and, thus, refer to phases of enhanced neuroplasticity (neural level).
Traditionally, sensitive periods have been studied in ontogenetic development; sensitive periods have been defined as epochs in life during which adequate experience must be available in order to allow for the typical expression of neuro-cognitive and possibly social functions. On the one hand, functional acquisition is incomplete after the end of a sensitive period; on the other hand, knowledge acquired during the sensitive period shows a higher stability. Non-human animal research has identified several mechanisms important for opening and closing of sensitive periods, including synaptic pruning, the elaboration and stabilization of inhibitory neural networks and myelin expression. At the behavioral level, sensitive periods can allow for high learning rates in different domains including perception, cognition, emotional-motivational and social. In case of missing or inadequate experience, however, sensitive periods might result in severe deficits in the very same functional domains. Therefore, sensitive periods act as a double-edged sword: While sensitive period plasticity allows for the rapid acquisition and stabilization of adaptive behavior, this comes with the risk of acquiring maladaptive behavior such as developmental disorders and psychopathologies.
For example, analogous to non-human animal work, the mechanisms of sensitive periods in humans have recently been studied in people with sensory defects such as blind and sight-recovery individuals. Sensitive periods for emotional and social processes are less understood. For example, the robustness and stability of ingroup favoritism has been linked to its early formation, whereas outgroup attitudes and stereotypes appear to be formed in later developmental phases and experiences. Moreover, early trauma has often been observed to be more strongly linked to later psychopathology than later trauma. In fact, adaptive or maladaptive schemas related to self, others and the future (e.g., cognitive triad), skills relevant to mental functioning and wellbeing (e.g., emotion-regulation) or achievement-related successes (communication skills), are generally assumed to be acquired in early childhood. Today it is not known whether sensitive periods for the acquisition of these functions exist in humans.
Recent research on adult neuroplasticity and memory suggests that sensitive periods might exist at different time scales throughout life, that is, there might be short windows of enhanced neuroplasticity following certain types of experience: For example, it has been shown that the reactivation of conditioned fear opens a transient phase for a highly durable extinction of fear memories. Other research has shown that physical exercise opens short periods of neuroplasticity which allows reversing maladaptive early plasticity. Moreover, the exposure to a stressful encounter is thought to induce a time-limited period of enhanced memory formation for ongoing events.
Yet it is unknown whether neural mechanisms are shared by ontogenetic and event-driven sensitive periods, how sensitive periods in response to significant events can be opened and what closes them, what promotes and what interferes with learning during either developmental or event-driven sensitive phases and how newly acquired information or capacities during such phases alter the representation of information acquired before and after these transient periods.
The understanding of the mechanisms of sensitive periods is important for optimizing learning environments throughout the life span and for preventing and treating maladaptive changes (sensori-motor deficits, psychopathology, maladaptive social behavior) and, thus, supports sustainable development.