Figure 5: The time course of changes in the head withdrawal threshold to bilateral stimulation of masseter ((a), (b)) and temporal muscles ((c), (d)) during pre-psychological stress period, psychological stress, and psychological stress removal period. During the pre-psychological stress period, the head withdrawal threshold gradually increased then stabilized on the 7th day (yellow-shadowed part in Figures 5(a)5(d)). During the psychological stress period, the mechanical nociceptive threshold started to decrease on day 1 after psychological stress, and reached lowest on days 5–7 then started to increase again in the following days till the 14th day (pink-shadowed part in Figures 5(a)5(d)). After removal of psychological stress, the index of each group gradually diminished and the pain threshold returned to normal on the 28th day. No significant difference was found between CON group and DT group (cyan-shadowed part in Figures 5(a)5(d)). (* , PS group versus CON group; # , PS group versus DT group; the letter “P” in the horizontal axis meant pre-psychological stress stage.)