Fig. 1. The figure shows temporal and spatial summation. A heat stimulus (thermofoil thermode), with an intensity producing a subjective sensation at or slightly more than the pain threshold (PT), is used. Temporal summation: If the stimulus is repeated with a low frequency (e.g.,  every 4 s), the subjective sensation measured using a visual analog scale (VAS) remains the same. If the stimulus is repeated at a faster rate (e.g.,  every 1 s), VAS score increases during the stimulations because of a central summation of the response. Spatial summation: Increasing the stimulation area, by applying the same stimulus to a larger number of probes, increases the VAS score.