Individual A-LTMRs (all soft brush sensitive) were separated into RA and SA types on the basis of their adaptive responses to ramp-and-hold indentation of the skin, as per the criteria used in Vallbo et al. (23). Three groups of RA units were identified as follows: Aβ RA1-LTMR (hair unit), responsive to deflection of individual hairs and light air puffs—air puff responses were abolished after hair removal; Aβ field LTMR, comprising multiple spots of high sensitivity with no response to hair displacement or remote tapping of the skin; and Aβ RA2-LTMR (Pacinian unit), comprising a single spot of maximal sensitivity and robust response to remote tapping/vibration. Two groups of SA-LTMRs (types 1 and 2) were identified where several features were examined including spontaneous firing, stretch sensitivity, and receptive field characteristics. In addition, an interspike interval pattern to sustained indentation (100 mN for 30 to 60 s) was tested, where possible, with a skewed and broad pattern for SA1 and a normal distribution for SA2. Coefficients of variation of interspike intervals (23, 54) were 0.80 for SA1 (median; range, 0.32 to 3.53; n = 18) and 0.29 for SA2 (median; range, 0.15 to 0.60; n = 10).

Mechanical threshold and receptive field size of individual units were determined using Semmes-Weinstein monofilaments (nylon fiber; Aesthesio, Bioseb, Pinellas Park, FL, USA). If a unit responded to the same (weakest) monofilament in at least 50% of trials, then it was taken as the mechanical threshold. Measurement of mechanical threshold was the minimum criterion for inclusion in the sample for all LTMR types, except RA1-LTMRs whose preferred stimulus is hair movement, and thus, responsiveness to light air puffs was determined. For receptive field mapping, predetermined monofilament forces of 100 and 600 mN were used for LTMRs and HTMRs, respectively. The receptive field area was estimated by treating it as an ellipse. Latency responses to surface electrical stimulation of the receptive field were captured, which, together with the distance from the stimulation site to the recording electrode, were used to calculate the conduction velocity (in meters per second). Electrically and mechanically evoked spikes were compared on an expanded time scale to ensure that the electrically stimulated unit was the same as the one that was mechanically probed (Fig. 1D). In one instance, the response latency was estimated by rapid mechanical tapping, using an electronic filament, of a receptive field spot.

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