Stress impairs cognitive flexibility in infants

Abstract

In human adults, learning and memory under acute stress are characterized by an increased use of rigid habitual response strategies at the cost of flexible cognitive strategies. The immediate effects of stress on cognitive functioning early in life are not well understood. Here we show experimentally that acute stress leads human infants to perform habitual behavior rigidly. We found that 15-mo-old infants exposed to stress thereafter kept performing a previously effective action, even after the action suddenly became ineffective. Infants in a no-stress control group flexibly adjusted their behavior by disengaging from the newly ineffective action in favor of exploring an alternative action. This finding demonstrates that stress impairs infants' ability to adjust their behavior to changing circumstances.S tress is pervasive in modern society. It is by definition aversive, but how potentially stressful experiences affect a person depends on a variety of factors, such as the person's age, developmental history, temperament, and the type of stressor (1, 2). Thus, the very same set of challenges that inspires peak performance in some people might leave others drained. In addition to its relevance for mental and physical well-being, stress alters the way human adults think. Learning and memory under acute stress are characterized by increased use of rigid habitual response strategies at the cost of flexible cognitive strategies (3-5). For example, stressed adults continue to select a particular beverage even after they have become satiated with its flavor, whereas satiated nonstressed adults select a new beverage (4). This shift from hippocampus-dependent to dorsal striatum-dependent memory is mediated by an increase of the stress hormone cortisol (6-8).In the first years of life, core characteristics of the human brain are its rapid development and high plasticity (9). Providing infants with a caring and stimulating environment is generally considered ideal for fostering cognitive development (10). One reason is that infants growing up in such an environment are protected from frequent stress exposure. However, the immediate consequences of stress exposure on infant cognition are not well understood. In a number of previous studies, children between 1 and 5 y of age were unlikely to show a physiological response (i.e., an increase in cortisol levels) to mildly threatening situations, such as being in an unfamiliar environment or being approached by an unfamiliar person (11,12). This human equivalent to the so-called hyporesponsive phase reported in rodents is thought to protect the developing brain from potentially harmful effects of acute stress (13). However, this idea is somewhat at odds with a minority of studies in which stress induction did lead to increased cortisol levels in young children (14,15). Furthermore, chronic stress exposure early in life is associated with long-term impairments in cognitive functioning (16). Thus, even if a cortisol response can be difficult to elicit, it seems unlikely that early...

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