2.2. Tactile stimuli and experimental protocol
During MRI acquisition, a trained experimenter (author JJT) manually applied gentle brush strokes to the infants’ right anterior shin region (along the tibia) in a proximal to distal direction. This site was chosen due to ease of access, as the babies were wrapped in a vacuum mattress that blocked upper extremities. Also, the investigator leaned on the scanner bed, leaning slightly toward the scanner bore, without touching the infant, whilst delivering the stimuli. The length of the stimulated area was measured to cover 15 cm, and brush strokes were applied at a velocity of 3 cm/s for 15s, with randomized inter-stimulus intervals of 10–15 s (resulting in 3 strokes in one 15 s block). The experimenter was guided by auditory cues delivered through the scanner head phones. A total of 11 stimuli were applied.