Figure 3. Active touch encoding via sensitivity to bending moment and its rate of change.

(A) Example rasters showing spiking of a Merkel afferent for 100 randomly selected protraction (top raster) and retraction (bottom raster) contacts. Lavender shading indicates contact (2 ms resolution). Shown at bottom are mean spike rates aligned to contact onset for all protraction (solid, ± SEM, n = 4,392 total) and retraction (dashed ± SEM; n = 1,556 total) contacts. Spike rate differences prior to contact in the two rasters are due to differences in tuning to protraction and retraction self-motion. (B) Mean spike rate (indicated by colors) during contact at each pole location (top) for an example SA afferent, and predicted spike rate from the “full” GAM statistical model (STAR Methods) fitted to predict instantaneous spikes from this neuron (bottom). The color scale for both panels is identical and ranges from 51 to 367 Hz. (C) Actual versus predicted mean spike rates during contact, pooled across neurons and pole locations (data for each neuron as in [B]). (D) Heatmap showing the Pearson correlation coefficient, r, between recorded spikes (smoothed by Gaussian kernel with σ = 4 ms) and predicted spike rates from GAM models (columns) fitted for each neuron (rows; blue circles: Merkel afferents) based on different combinations of mechanical variables. (E) Tuning surface for example Merkel afferent (same as in [A]) showing mean spike rate (color scale) binned by moment (M₀) and its rate of change (M₀′). Trajectories (colored curves) for example contacts are plotted on top of the surface. Each contact begins near the origin and proceeds counter-clockwise across either the top (for protraction) or bottom (for retraction) half of the tuning surface. Dashed lines indicate axis origins. Bins with fewer than 25 observations are white. (F) Schematic depicting the four quadrants of the M₀ − M₀′ tuning surface shown in (E). The whisker can be moving in the protraction or retraction direction, and be in contact with a pole either in front of or behind the whisker. (G) Spike times shown individually (ticks) and smoothed (colored curves, Gaussian kernel with σ = 2 ms) for the example trajectories in (E), overlaid with spike rate “read off” from the tuning surface (black dashed traces). (H) Example M₀ − M₀′ tuning surfaces for three neurons that preferred protraction contacts (leftmost neuron from [E]). (I) Same as (H) but for three neurons that preferred retraction touches. (H–I) Dashed lines indicate the origin of each axis and are colored by afferent type (blue: Merkel; gray: SA). The color scale for each surface ranges from 0 Hz to a maximum spike rate indicated above the surface (blue text: Merkel). Scale bars (red) indicate 2 × 10⁻⁷ N-m and 2 × 10⁻⁸ N-m ms⁻¹ for M₀ and M₀′, respectively. White bins as in (E). See also Figure S2.