Fig. 1

Method of stroking stimulation under vertical holding condition. a Vertical holding. The experimenter gently squeezed the rat’s upper back skin, pulling backward without stroking (sham stimulation). b Stroking of the ventral region. The stimulation was applied towards the lower part from the upper part of the body (as indicated by the arrow) with the right hand at a frequency of approximately 1 Hz. c, d Responses of 50-kHz USVs to sham (c) or stroking (d) stimulation. Ordinates, number of 50-kHz USVs per 10 s; abscissa, 0 indicates the onset of stimulation. Horizontal bar indicates the 60-s stimulus or sham stimulus period. *p < 0.05, **p < 0.01, vs pre-stimulus control value, using ANOVA followed by Dunnett’s multiple comparison test. n = 6