The primate as a model for the human temperature-sensing system: 2. Area of skin receiving thermal stimulation (spatial summation).

The thermal sensitivities of three humans and one monkey were measured using the "yes-no" paradigm based on the Theory of Signal Detection. The aim was to evaluate the monkey's thermal-sensing system as a model for that of humans. Three of the principal variables of human thermal sensations--the temperature to which the skin was adapted, the rate of temperature change, and the site of application of the thermal stimuli--were held constant. The other three variables--area of stimulation, intensity, and direction of the temperature change--were varied systematically. All four subjects displayed spatial summation for both warming and cooling. Isodetectability curves (d'e = 1) to small temperature changes, both for humans and for the monkey, could reasonably be fitted by the function I = kappa A-b, where I is stimulus intensity, A is the area of stimulation, and b is the rate at which spatial summation occurred. The rate of summation, b, to warming stimuli for the humans ranged from 0.60 to 1.14, while that for the monkey was 0.14. The rate of summation to cooling stimuli for the humans ranged from 0.50 to 0.87, while that for the monkey was 0.43. The main species difference was that summation on the monkey palm all but ceased for both warming and cooling stimuli applied to areas larger than 4 cm2. Data from the human subjects did not demonstrate an upper limit of spatial summation. However, there was an indication that the human subjects would show a ceiling for spatial summation near the largest area tested in this study. Thus, when considering the spatial extent of a thermal stimulus and its influence upon thermal sensations, it may be more appropriate to compare areas relative to body size, rather than absolute values.