Spatial summation in thermosensation refers to the integration of multiple sensory signals, resulting in the perception of a unique sensation. The relationship between stimulus size and perceived intensity is sub-additive, suggesting an influence of lateral inhibition, a process in which adjacent neurons inhibit each other. We investigated the role of spatial summation and lateral inhibition in thermosensation using computational modeling. We devised a perceptual decision-making task where thermal stimuli were applied to 16 participants' forearms, with temperatures increasing or decreasing from 31°C. Participants were instructed to determine whether the stimuli felt warm or cold. Following this, participants rated the stimuli intensity. The stimuli varied spatially in two dimensions: area size (with 5 possible areas) and distance size (comprising 2 areas with 4 possible distances. We employed generalized mixed-effects models to assess response times and ratings and utilized a Hierarchical Drift Diffusion Model to analyze both response times and accuracy of the warm/cold responses. Participants exhibited faster response times to stimuli with increasing distance and area size. Evidence accumulation rates increased more with area size for cold compared to warm. Our findings indicate that spatial summation operates differently in cold and warm perception and that this difference is due to a difference in the speed of evidence accumulation. In future studies, we aim to investigate the dynamics of spatial summation and lateral inhibition for different stimuli intensities and how these processes are affected by aging or in the context of nerve damage.