When infants gain the capacity for pain

The evidence suggests that developing brain networks become mature enough to identify pain as distinct from touch fairly late in development.

"Babies can distinguish painful stimuli as different from general touch from around 35 to 37 weeks gestation -- just before an infant would normally be born," said Lorenzo Fabrizi of University College London.

Infants can't actually tell you whether something hurts or not, so the researchers relied on recordings of brain activity by electroencephalography (EEG).

According to the researchers, recent studies have emphasized the importance of bursts of neuronal activity, both spontaneous and evoked, during the formation of functional brain circuitry. That bursting pattern of activity shifts in development to adult-like responses that are more specific to particular sensory inputs.

EEG recordings of infants between the ages of 28 to 45 weeks gestation show that the brain begins to produce distinct responses to a simple touch versus a clinically essential heel lance considered as painful at about 35 to 37 weeks gestation. (Babies' due dates are based on 40 weeks of pregnancy, and babies are generally considered full term at 37 weeks).

The results may have implications for the treatment, care, and development of premature newborns, Fabrizi said, noting that these children can often grow up to be either more or less sensitive to pain than usual.

"Repeated noxious stimulation of the kind used in this study is a feature of neonatal intensive care," the researchers wrote. "Our finding that noxious heel lance increases neuronal bursting activity in the brain from the earliest age raises the possibility that excess noxious input may disrupt the normal formation of cortical circuits, and that this is a mechanism underlying the long-term neurodevelopmental consequences and altered pain behavior in ex preterm children."