Vibrotactile spatial summation

By means of a two-interval, forced-choice technique, absolute thresholds were obtained at several sites on the skin for short bursts of mechanical vibration. The difference between the energy required for a single vibrator, in dB, to reach threshold and the energy required for two vibrators presented successively to reach threshold was taken as the measure of probability summation. Spatial summation was computed as the difference between the energies required for threshold for the two vibrators presented successively and simultaneously. Both probability summation, 0.52 dB lowering in threshold, and spatial summation, 1.94 dB lowering in threshold, remained constant as a function of the distance between the two vibrators on the thigh. Similar amounts of summation were obtained unilaterally and bilaterally on the fingers. When the frequency of vibration was lowered to 9 Hz, no spatial summation was found. When the frequencies of vibration at the two test sites were different, 160 Hz at one locus, 360 Hz at the other locus, there was no spatial summation.