Infantile experience and play motivation

Fully-fledged affective systems in mature animals are in part the result of the impact of infantile experience on brain development. The present experimental series examines whether tactile stimulation in infancy (early handling) influences rough-and-tumble play (R&T) throughout the juvenile period, using a testing regime of 17 days divided into five parts where handled (H) and nonhandled (NH) Wistar rats are assessed daily. In Parts 1 and 2 (age range at the start: 30-33 days) the objective is to study the amount of R&T that the rats are capable of exhibiting under varying lengths of social deprivation. In Part 3 (37-40 days) the objective is to determine whether familiarity with the experimental situation has independent or interactive effects with early handling. In Part 4 (40-43 days) the objective is to obtain evidence of the suppressing effects of an unexpected contextual change. In Part 5 (56-59 days) the objective is to study whether the effects of early handling can still be present at an age when R&T has practically vanished in NH rats. Results show that early handling invigorates R&T affecting pins (i.e., the most rewarding component) at the expense of dorsal contacts by enhancing play motivation in a specific manner, and that it is able to dilate the inverted-U developmental curve of this behavior, thereby providing strong evidence for a direct effect on the neuropsychological systems for play motivation.