Spatial Summation of Warmth: Influence of Duration and Configuration of the Stimulus

It was found that the spatial summation of warmth takes place primarily in the central nervous system and that the rules by which area and intensity trade to preserve a constant liminal or supraliminal sensation of warmth are basically the same for brief (.5 sec) and longer (3 sec) exposures as well as for unitary (on one dermatome) and split (on two dermatomes) fields. Warmth approximated a power function of thermal intensity, but the size of the function's exponent depended systematically on the areal extent and therefore, like spatial summation, on central neural factors. A striking feature of the warmth sense is its capacity to sum the neural effects of thermal stimulation over large areas of skin-up to at least several hundred square centimeters. This spatial summation reveals itself in measurements of absolute threshold. Intensity of stimulation and its areal extent can be traded for each other to preserve a threshold sensation. The rule of trading approximates a reciprocity so that, for example, the effect of halving thermal irradiance can be offset by approximately doubling the stimulated area (Hardy and Oppel, 1937; Kenshalo, Decker, and Hamilton, 1967). Spatial summation reveals itself also in measurements of supraliminal warmth. Irradiance and area can be traded to preserve any constant level of warmth above the absolute threshold. (Often, two combinations of area and irradiance that have equal warmth appear virtually indistinguishable even in their subjective areal dimensions.) The rules for trading area and irradiance vary, however, with level of warmth. With increasing warmth, area makes an ever smaller proportional contribution to the warmth sensation; accordingly, it takes an ever larger proportional change in area to compensate for a given change in irradiance (Stevens and Marks, 1971). In other words, the degree of spatial summation declines with increasing warmth and may fail altogether at a high enough level. Spatial summation reveals itself also in simple reaction time to thermal