A refractory period for the heart rate response in infant visual attention

A refractory period for the heart rate (HR) response of 14-, 20-, and 26-week-old infants to second stimulus onset and primary stimulus offset during termination of visual attention was investigated. A stimulus that elicited a significant HR deceleration was presented. Following the return of HR to prestimulus level, a second stimulus occurred or the first stimulus ended at time delays of 0, 3, 6, and 9 s. If the second stimulus occurred at 0 or 3 s after HR returned to its prestimulus level, the HR response was smaller than the initial HR orienting response (OR). A usual HR OR resulted by waiting 6-9 s after the return of HR to prestimulus level. A significant offset response occurred only for the 6-sec delay. Active inhibition of vagal cardiac activity during attention termination may cause the observed refractory period in the HR response, which appears to last from 3-6 s following sustained attention.