Figure 2

Step in friction level. (a) Top panel, superimposed recordings of normal and lateral force for the three amplitudes of ultrasonic vibration tested (0.1 μm, 0.75 μm and 1.2 μm). The action of the device from t = 0 effectively reduced the lateral force on the side of the modulation for the two larger amplitudes. Panels below show PSTHs (10 ms bins) and raster plots of unit firing for four representative units (FAII₁, SAI₁, SAI₂ and SAII₂). Colours illustrate the friction condition, i.e. decrease in friction level over time (red) or increase (blue). Dot filling represents the sliding direction, with filled dots for the ulnar direction and unfilled dots for the radial direction. Note that the sequences of repeats have been grouped per friction condition in the plots to improve their readability; hence they do not reflect the randomization sequences. For each unit, all repeats are shown. Repeats are left empty when no spikes were elicited, which highlights the scarcity of the responses to the stimulation. FAII₁ and SAI₁ showed time-locked responses to the moment of change in friction. SAI₂ and SAII₂ showed the same effect, revealed by the PSTH, as well as a reduction in the number of spikes with friction reduction (stable phase of the stimulation). (b) Top panel, average change lateral force (i.e. including the conditions of increase or decrease in friction) measured separately for the two sliding directions, and for the three amplitudes of ultrasonic vibration tested for the 8 units. Panels below show the average difference in number of spikes (between the baseline vs. stimulation phase) for each unit. The error bars represent the standard error of the mean. (c) Correlations between the change in lateral force vs. the change in the number of spikes are shown at group level, in the ulnar (black dots, black regression line); and radial directions (unfilled dots, dashed regression line).