Central sensitization of the dorsal horn neurons that is evoked from C fiber activity is responsible for temporal summation of "second pain" (TSSP). This event is called ‘windup’ and relies on a frequency greater or equal to 0.33Hz of the stimulus. Windup is associated with chronic pain and central sensitization. This minimum frequency was determined experimentally by comparing healthy patient fMRI’s when subjected to varying frequencies of heat pulses. The fMRI maps show common areas activated by the TSSP responses which include contralateral thalamus (THAL), S1, bilateral S2, anterior and posterior insula (INS), mid-anterior cingulate cortex (ACC), and supplemental motor areas (SMA). TSSP events are also associated with other regions of the brain that process functions such as somatosensory processing, pain perception and modulation, cognition, pre-motor activity in the cortex.