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 p resented at different rates and loads (forces) to the human fingertip, in order to compare estimates of population responses of cutaneous me chanoreceptors with the quality and magnitude of tactile sensations. T he 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 d rawings that illustrated a variety of temporal profiles for sensation magnitude. The line drawings provided a straight-forward means of desc ribing temporal fluctuations of sensation intensity, which corresponde d well to psychophysical ratios that were determined subsequently with a matching procedure. Influences of ramp rate on qualities of touch s ensations were evaluated by tabulating verbal descriptions of sensory experiences. Each of three rate conditions generated a different quali ty 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 s ense of motion was detected; instead, a ''pressure'' sensation was ide ntified. Touch sensations during the plateau were always described as a pressure. The subjective magnitudes of touch sensations associated w ith the onset, plateau, and offset were equated by comparing different components of paired stimuli. At 100 g/s, when subjects matched the o ffset sensation from the first of a pair of stimuli with the onset sen sation from the second, the force of the stimulus producing the offset sensation was 1.3 times greater than the intensity of the stimulus th at produced the onset sensation. Matching of the plateau sensation (ev aluated during the last 1.5 s of the 2.5-s plateau period) with the on set sensation required a plateau stimulus that was 1.7 times greater i n force than the stimulus which produced the onset. Comparison of stim ulus intensities producing a match of plateau and offset sensations wi th stimulus intensities predicted from the previous matches (onset ver sus offset and onset versus plateau) demonstrated a mean within-subjec t error of 4%. The mean ratio of plateau to offset forces that produce d a match was 1.8:1.3. In a matching procedure in which subjects compa red the subjective magnitudes of plateau sensations following onset ra mps of different rates, onset ramp rate significantly influenced the m agnitude of pressure sensations. The ratios of plateau forces which pr oduced equal magnitudes of sensation following 1, 10, and 100 g/s ramp s were 1.6:1.3:1.0. Psychophysical ratios of dynamic to plateau magnit udes of sensation were considerably discrepant with ratios of dynamic to plateau discharge of afferents supplying the glabrous skill of monk eys. Ratios of onset to plateau to offset sensation magnitude were 1.7 :1.0:1.3 for trapezoidal presentations of 2 g of force at 100 g/s. Usi ng the same stimulus conditions (Cohen and Vierck 1993), the correspon ding values for estimates of the population discharge from rapidly ada pting (RA) and slowly adapting (SA) afferents supplying the fingertip were 40.8:1:19.4. The onset to plateau ratio for the population respon se of type I SA (SAI) afferents (5.6:1) was also substantially larger than the corresponding psychophysical ratio. The influence of ramp rat e on the magnitude of the plateau sensations also did not correspond w ith neural responses. The discharge rate of SAI afferents during the l ast 1.5 s of the 2.5-s plateau periods was independent of ramp rate (C ohen and Vierck 1993). However, psychophysical ratios of 1.6:1.3:1.0 f or the magnitudes of plateau sensations following ramp rates of 100, 1 0, and 1 g/s were discrepant with the neural ratios of 1.0:1.0:1.0. Te mporal filtering from perceptual processing is not sufficient to accou nt for the discrepancies between neural and psychophysical ratios. The refore, the discharge from quickly and SA afferents must not combine l inearly to determine sensation magnitude. The intensity of the tap sen sation appears to be coded in a different way from pressure sensations ; for the tap sensation the intensive code could be the number of acti ve fibers, and the intensive code for pressure could be discharge rate among SA afferents.