Figure 2. Experimental procedures and typical traces.

(A) Evolution of the global variables, the plate position (vertical in gray and horizontal in black) and the contact force (normal in gray and tangential in black), together with the afferent instantaneous firing rate (and the spikes) as a function of time for a typical fast-adapting type I (FA-I) unit in the high friction condition. The plate was pressed against the fingerpad (‘contact’), moved tangentially forward until the occurrence of a full slip, then moved backward, and finally retracted (‘release’). The partial slip phase is highlighted by the gray vertical boxes. Five repetitions are overlaid. The tangential movement was split into three phases: onset (lasting 100 ms), partial slip, and plateau. (B) Heatmaps of the evolution of the local surface strain rates in the contact area as a function of time during the tangential loading, for one of the five repetitions. The three strain components, axial along x and y, and shearing, are shown (colored triangles depict the deformation axes). Compression (negative strain) is in red. The location of the unit receptive field center is shown with a blue circle on the raw fingerpad image on the bottom left. (C, D) Same as in (A, B) for a typical slowly-adapting type I (SA-I) unit. For both units, insets show recorded, superimposed action potentials and their average shape represented by a dark line.

Figure 2—figure supplement 1. Instantaneous firing rate as a function of time for all units recorded and all conditions.

The entire trial sequence is shown, from the initial contact (‘contact’) to the end of the contact (‘release’). Each line is the average of all repetitions of a given condition (between 3 and 5 repetitions). The colors are related to the direction (see the legend in the upper-right corner) and friction condition (darker colors refer to high friction and brighter colors refer to low friction). On the right inset, the approximate location of the receptive field on a reference finger is shown as a filled or empty circle if the unit was inside or outside the contact area, respectively. The normalized average firing rate for each direction (in gray) and the vector of the preferred direction (in black) are also shown. For all units, insets show recorded, superimposed action potentials and their average shape represented by a dark line. The receptive field of the only fast-adapting type II (FA-II) unit (U116-03) was in the second phalange.

Figure 2—figure supplement 2. Correlation of firing rates with contact forces.

Heatmap showing the correlation between each component of the 3D force and force rate vectors (in the tangential plane: U: ulnar; R: radial; P: proximal; D: distal; and in the normal plane: up and down) and the afferent firing rates. The force time series were lagged by 12 ms to account for the conduction delay. The units are sorted by type and location on the finger. The black border on the right depicts units inside the contact area, the gray border shows units on the border of the contact area, and no border means outside the contact area.