Cutaneous Masking. I. Psychophysical Observations on Interactions of Multipoint Stimuli in Man

1. Psychophysical masking of cutaneous sensation at the locus of punctate test stimulation has been quantitatively examined with phasic mechanical and brief air-pulse stimuli using a conditioning-test stimulus paradigm. 2. Masking was maximal at the minimal interstimulus distance effective with this paradigm, varies inversely with interstimulus distance, and is demonstrable with the conditioning and test stimuli up to 10 cm apart on the forearm. 3. The degree of masking was found to be a direct function of the relative intensity of the conditioning stimulus with respect to the test stimulus. 4. Variations in the interstimulus interval permitted an investigation of the temporal features of cutaneous masking. It was detectable from 10 ms before to 70 ms after conditioning stimulation. Maximum masking occurred when the test stimulus was delivered about 10 ms following conditioning stimulus onset. 5. We also noted the much less marked, but still significant, enhancement phenomenon, in which weak conditioning stimuli, at just-threshold intensity levels, lowered the detection threshold for sensation at the test stimulus locus. We found this enhancement of sensation to have the same spatial distribution as did masking, but a much reduced time course. It began with the test stimulus presented simultaneously with the conditioning stimulus, peaked with 10--15 ms interstimulus intervals, but decayed in less than 40 ms. 6. Since psychophysical experiments often form the framework for the understanding of physiologic processes, it is suggested that these behavioral determinations of enhancement and masking may be correlated with the electrophysiologic properties of excitation and inhibition in neurons of the major primary somatic pathways of the central nervous system.