Fig. 4

The stimulus–response functions (SRFs) of noxious-excitatory units to graded pinching. a Raster plots of the responses of two units (upper two panels) and the corresponding applied forces (lower panel) by a hand-held pincher. The light-gray unit in the upper panel had coding property, i.e., its firing frequency increased with an increasing pinching force. In contrast, the black unit in the lower panel showed that its firing frequency was the strongest at a certain threshold of the pinching force, but firing frequency did not increase any further with an increasing force. The two examples are from two different rats. b (a) Averaged SRF (mean ± SE) of 25 units with hat-function-like features reached threshold at 141 gw, peaked at 247 gw, and diminished with an increasing pinching force. (b) Averaged SRF of 17 units with step-function-like features reached a peak at 565 gw. (c) Averaged SRF of 10 units with coding property significantly increased at 727 gw. *p < 0.05, compared to the baseline data