Comparing Fire: Precision of Visual Perception of Fire Intensity

Controlling fire for use as a tool without causing harm was a pivotal advancement in human evolution. An aspect of controlling fire is perceiving changes in intensity. The nature of fires can make this challenging as flames flicker, even for stable fires. For developing fires, this is in addition to growth being non-linear and occurring vertically and sometimes horizontally with the footprint of the fire either remaining the same (e.g., bonfire) or increasing (e.g., brush fire). The present study investigated how precisely observers could detect differences in the intensities of simulated fires. Research with non-symbolic visual quantities indicates that humans can reliably detect small differences in spatial extent with precision of corresponding mental representations calculated as Weber fractions (e.g., w = .10, 11cm2 vs. 10cm2). Using a similar approach, a series of experiments (N=120) compared the precision of judgments about which of two fires was greater in intensity or growing faster in intensity when the footprint was fixed or varied. Videos of fires (8s) were created using fire simulation software and the ratio difference was varied from small (21:20) to large (64:8). Using Weber fractions (smaller values indicating more precise estimates) to characterize performance, precision was significantly worse when comparing fire growth than intensity and better when footprint varied (intensity: footprint fixed, w = 0.09, footprint varied, w = 0.03; growth: footprint fixed, w = 2.24, footprint varied, w = 1.30). This provides initial estimates of the precision of mental representations of fire intensity and growth. The similar precision when comparing fire intensity versus past observations with spatial extent is notable as the flames of the fires varied during the video. This may have been due to having multiple indicators of intensity, including the size of flames and smoke produced, which is supported by the benefit of varying fire footprint.