INTRODUCTION: Spatial summation of pain (SSp) occurs when perceived pain increases with the size of the stimulated area. The growth in pain intensity during SSp is often nonlinear and disproportionate relative to the extent of stimulation. However, the underlying mechanisms remain poorly understood.

AIM(S): This study aimed to investigate SSp in a temporally dynamic context, capturing its progression in real time.

METHOD(S): Electrical noxious stimulation was delivered via five electrodes attached to the foot, arranged in a continuous linear pattern either along or across the dorsum of the foot. Ten healthy participants (N = 10) underwent four 25-second trials, each trial with a different electrode activation pattern: ascending (from one to five electrodes activated sequentially), descending (from five to one), random (random number of electrodes activated), and control (single, randomly selected electrode). Pain intensity was rated continuously using a computerised visual analogue scale (VAS).

RESULTS: Substantial SSp was observed, as indicated by a significant effect of the number of electrodes activated (p < 0.001). However, the pattern of SSp varied depending on the type of trial (p < 0.001). Ascending trials were more painful compared to descending trials (p < 0.001) and control trials (p < 0.01). Conversely, descending trials were less painful than random trials (p < 0.05). Interestingly, random trials were equally painful as control trials, in which only a single electrode was activated.

CONCLUSIONS: This study demonstrates that a dynamic paradigm can effectively evoke spatial summation of pain and capture its temporal modulation. Moreover, the sequence of spatial input appears to influence pain perception independently of stimulus area, suggesting that the order of spatial recruitment may be a key factor in SSp mechanisms.