BEHAVIORAL STATE CHANGE IN NORMAL HUMAN FETUSES FOLLOWING A SINGLE VIBROACOUSTIC STIMULUS - EFFECT OF DURATION OF QUIET SLEEP PRIOR TO STIMULATION

We examined 96 normal human fetuses between 37 and 41 weeks of gestati on to determine the relationship between the duration of the active st ate following a single vibroacoustic stimulus (VAS) and the time spent in prestimulus state 1F, i.e. quiet sleep (QS). All fetuses were obse rved to enter state 1F prior to testing. After remaining in this state for 3 min, each fetus was randomized into one of three groups: no sti mulation (Group I, N = 32); a single 1-s VAS delivered after 3-5 min o f QS (Group II, N = 33); and a single 1-s VAS delivered after 18-20 mi n of QS (Group III, N = 31). Fetuses in Groups I, II, and III were oth erwise behaviourally indistinguishable. Heart rate and eye and body mo vements were monitored until the fetus either returned to state 1F and remained in that state for 3 min or did not return to state 1F within 60 min. Of the 64 fetuses who were stimulated, 62 responded, indicati ng that there was no effect of prestimulus state duration on the sensi tivity to VAS. Fetuses stimulated after 18-20 min of QS behaved simila rly to fetuses who entered an active period spontaneously. However, fe tuses who were in QS for 3-5 min remained in a more active state for a significantly shorter length of time than did fetuses who either had been in QS for 18-20 min before VAS (17.9 min vs. 38.0 min, P = 0.0002 ) or had entered an active period spontaneously (17.9 min vs. 41.1 min , P = 0.0001); 10 (30.3%) fetuses in Group II and two (6.5%) fetuses i n Group III were active for less-than-or-equal-to 3 min following VAS (P = 0.015). Even when the 10 fetuses in Group II who did not make a s tate change were excluded from analysis, the mean duration of the acti ve period was significantly less for Group II fetuses than for fetuses in either Groups I (P = 0.0010) or III (P = 0.0058). We conclude that , for fetuses who were otherwise behaviourally indistinguishable at th e time of stimulation, the nature of the behavioural response followin g a single VAS was dependent upon the prestimulus duration of state 1F .