Rat fetuses exhibit intrinsic fluctuations in general motor activity and respond to an artificial nipple (AN) with mouthing and oral grasping behavior. The present study examined the relation between the organization of general activity and the expression of these specific responses to an AN on Embryonic Day 21. In Experiment 1, continuous exposure to the AN resulted in nonspecific behavioral activation characterized by an increase in amplitude and high-frequency variability. In Experiment 2, increased amplitude and variability in general activity preceding discrete presentations of the AN resulted in more mouthing and oral grasping responses to the AN. These results suggest that presentation of the AN triggers behavioral reorganization in which the level and variability of overall activity may facilitate expression of well-defined action patterns.

The near-term rat fetus exhibits brief oral grasp responses to discrete presentations of an artificial nipple. In the present experiment, an artificial nipple was presented to individual fetal subjects 10 times. Five of the presentations were timed to occur when spontaneous fetal motor activity was low and five while activity was high, as determined by the baseline activity for the individual fetus. The likelihood of responding to the artificial nipple was increased when the fetus was relatively inactive at the moment of stimulus presentation. Furthermore, stimulus presentations that resulted in oral grasping of the artificial nipple were associated with greater point-to-point variability (2-s intervals) in motor activity during the 30-s period preceding the presentation of the artificial nipple. This pattern of results indicates that the recent history of general motor activity as well as the level of activity at the instant of stimulus presentation may contribute to variation in responding to biologically relevant stimuli early in development.

The near‐term rat fetus exhibits brief oral grasp responses to discrete presentations of an artificial nipple. In the present experiment, an artificial nipple was presented to individual fetal subjects 10 times. Five of the presentations were timed to occur when spontaneous fetal motor activity was low and five while activity was high, as determined by the baseline activity for the individual fetus. The likelihood of responding to the artificial nipple was increased when the fetus was relatively inactive at the moment of stimulus presentation. Furthermore, stimulus presentations that resulted in oral grasping of the artificial nipple were associated with greater point‐to‐point variability (2‐s intervals) in motor activity during the 30‐s period preceding the presentation of the artificial nipple. This pattern of results indicates that the recent history of general motor activity as well as the level of activity at the instant of stimulus presentation may contribute to variation in responding to biologically relevant stimuli early in development. © 1998 John Wiley & Sons, Inc. Dev Psychobiol 33: 125–131, 1998

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