Single- and group-reared rats were randomly assigned to concomitantly experience (or not experience) experimenter-induced tactile stimulation by repeated exposure to 5 min of handling on postnatal days 21–70. Handling was performed during the light phase of the light:dark cycle between 2:00 to 5:30 PM on each weekday, except weekends. Handling consisted of caressing the fur of the neck and the dorsal surface of the animal with one hand, while leaving a hand in contact with the vibrissae and snout, to facilitate olfactory recognition. During handling, the rat was placed in the lap of the experimenter or on a flat surface. If the rat did not display signs of discomfort (e.g., defecation/urination, tail rigidity, intense grooming, or startle responses), the handler proceeded to pet the rat. Handled and non-handled rats were kept on opposite sides of a shared holding room and, within each side, were further arranged by rearing condition.
