Motor responsivity during habituation testing of normal human fetuses.

Abstract

Features of fetal motor responsivity include both the magnitude of the startle response elicited by a single stimulus (i.e., basal reactivity) and the ease by which responding to successive stimuli is inhibited (i.e., habituation). We examined basal motor reactivity and habituation of the motor response in 56 normal human fetuses between 34 and 40 weeks of gestation. Testing consisted of 8 trials of a 1-sec vibroacoustic stimulus (VAS) with a 10-sec interstimulus interval. A score of 0-10 was assigned for each trial based on subjective assessment of intensity and duration of the fetal motor response. Measures of habituation included the ratio of responding after a fixed number of trials divided by the initial response, and the rate of change in the behavioral response over trials. No relationship was found between the rate of motor habituation and either basal reactivity, gestational age, or prestimulus fetal heart rate (FHR) variability. In contrast, more mature fetuses responded less intensely to the first stimulus than did their younger counterparts (r = -0.329, p = 0.005), and fetuses who were initially in a quiet state exhibited a more vigorous startle response as compared to fetuses who were initially in a more active state (r = -0.372, p = 0.001). The relationship between basal reactivity and prestimulus FHR variability was statistically significant even after controlling for gestational age (r = 0.295, p = 0.01). These findings may have important clinical implications regarding the appearance in early life of certain behavioral tendencies such as temperament.