The relationship between acute stress and EEG repetition suppression in infants.

Over activation of the hypothalamo-pituitary-adrenal (HPA) axis in stress situations is known to influence learning and memory. In adults, an inverted-U shape relationship between acute stress, and learning and memory has been demonstrated. Whether this model fits learning performances in infants is unknown. In this study, we used EEG repetition suppression as physiological measure of learning and salivary cortisol in response to a stressor to investigate the relationship between acute stress and learning in infants. We hypothesized that EEG repetition suppression would be modulated by acute stress following an inverted-U shape relationship. Saliva samples were collected during an EEG experiment before, during and after EEG net installation in 37 healthy infants (18 males) aged between 6 and 26 months. The effect of variation in stress hormones on repetition suppression were modeled using a linear mixed model, with cortisol, age and sex as predictors. Results indicated that in healthy infants, elevations in stress hormones within the normal range are associated with a higher repetition suppression response and an increased response to the first presentation of the stimulus. The later increase could be related to vigilance. Considering that early childhood is a critical period of development, future studies should keep investigating the influence of stress on learning processes in infants.