Spatial summation of thermal sensitivity is limited to small areas: Comparisons of the forehead, forearm, abdomen, and foot.

The purpose of the present study was to examine if spatial summation in thermal sensitivity exists when stimulating areas larger than about 1% of body surface area (BSA) (approximately 200 cm). We hypothesized that spatial summation would exist within a limited area and the effect would be insignificant for over the 1%BSA. Fifteen young males participated in this study and we measured their warmth and hot sensation thresholds on the four body regions (the forehead, forearm, abdomen, and instep) using the three sizes of radiant film heaters (10 × 10, 15 × 15, and 20 × 20 cm heating film area). The heating panel was kept at a distance of 10 cm from the skin and the surface temperature of the heating panel increased by 1 °C·s. The results showed that warmth and hot sensation thresholds were higher for the 100 cm condition than the 225 or 400 cm conditions (P < 0.05), but no differences were found between the 225 and 400 cm conditions. Secondly, the instep was most insensitive to the gradual increase of radiant heat among the four body regions for all three stimulating film sizes, even though the hot threshold was lowest for the instep because the initial foot temperature was lower than other skin temperatures. In summary, spatial summation in thermal sensitivity was found for the 100 and 225 cm conditions, but not for the 225 and 400 cm conditions. These results suggest that spatial summation exists but limited to small stimulating areas, smaller than approximately 1% BSA.