Relationships between touch sensations and estimated population responses of peripheral afferent mechanoreceptors

Trapezoidal indentations of the skin by a 0.5-mm-diameter probe were presented at different rates and loads (forces) to the human fingertip, in order to compare estimates of population responses of cutaneous mechanoreceptors with the quality and magnitude of tactile sensations. The subjects were first trained to attend to and evaluate variations in the magnitude of touch sensations associated with the onset ramp, the plateau period, and the offset ramp. They examined a series of line drawings that illustrated a variety of temporal profiles for sensation magnitude. The line drawings provided a straight-forward means of describing temporal fluctuations of sensation intensity, which corresponded well to psychophysical ratios that were determined subsequently with a matching procedure. Influences of ramp rate on qualities of touch sensations were evaluated by tabulating verbal descriptions of sensory experiences. Each of three rate conditions generated a different quality of sensation during the dynamic portions of stimulation. Onsets and offsets at 100 g/s were described as "taps". During ramps at 10 g/s the quality was described as "rolling" or "moving". At 1 g/s no sense of motion was detected; instead, a "pressure" sensation was identified. Touch sensations during the plateau were always described as a pressure. The subjective magnitudes of touch sensations associated with the onset, plateau, and offset were equated by comparing different components of paired stimuli. At 100 g/s, when subjects matched the offset sensation from the first of a pair of stimuli with the onset sensation from the second, the force of the stimulus producing the offset sensation was 1.3 times greater than the intensity of the stimulus that produced the onset sensation. Matching of the plateau sensation (evaluated during the last 1.5 s of the 2.5-s plateau period) with the onset sensation required a plateau stimulus that was 1.7 times greater in force than the stimulus which produced the onset. Comparison of stimulus intensities producing a match of plateau and offset sensations with stimulus intensities predicted from the previous matches (onset versus offset and onset versus plateau) demonstrated a mean within-subject error of 4%. The mean ratio of plateau to offset forces that produced a match was 1.8:1.3. In a matching procedure in which subjects compared the subjective magnitudes of plateau sensations following onset ramps of different rates, onset ramp rate significantly influenced the magnitude of pressure sensations. The ratios of plateau forces which produced equal magnitudes of sensation following 1, 10, and 100 g/s ramps were 1.6:1.3:1.0.(ABSTRACT TRUNCATED AT 400 WORDS)