Temporal cues contribute to tactile perception of roughness

Abstract

Optimal perception of surface roughness requires lateral movement between skin and surface, suggesting the importance of temporal cues. The roughness of periodic gratings is affected by changing either inter-element spacing (groove width, G) or element width (ridge width, R). Peripheral neural responses to gratings depend quantitatively on a spatial variable, G, and a temporal variable, grating temporal frequency (F(t)), with changes in R acting indirectly through concomitant changes in F(t). We investigated, psychophysically, the contribution of temporal cues to human tactile perception of roughness, using gratings varying in either R or G. Gratings were scanned across the immobile fingerpad with controlled movement speed (S) and contact force. In one experiment, we found that roughness magnitude estimates depended on both G and F(t). In a second experiment, discrimination of the roughness of gratings varying in either R or G was affected by manipulating F(t). Overall, the effect of G on roughness judgments was much stronger than that of F(t), probably explaining why many previous studies using surfaces that varied only in inter-element spacing led to the conclusion that temporal factors play no role in roughness perception. However, the perceived roughness of R-varying gratings was determined by F(t) and not spatial variables. Roughness judgments were influenced by G and F(t) in a manner entirely consistent with predicted afferent response rates. Thus perceived roughness, like peripheral afferent responses, depends in part on temporal variables.

Fig. 1.

A, Cross-sectional view of a periodic grating. G, Groove width; R, ridge width; spatial period = G + R.B, C, Trial structures in the magnitude estimation (B) and discrimination (C) experiments. For magnitude estimation, two successive trials are shown. For discrimination, the two scans comprising a single trial are shown. The subject responded verbally after each trial. ME, Magnitude estimate;ISI, interscan interval.

Fig. 2.

Mean normalized roughness magnitude estimates as a function of speed (S) for gratings varying inR (A) and G(B). Values of the variable parameter are given on the right of each graph. Standard errors are too small to be shown here and are given in Table 2.

Fig. 3.

Mean normalized roughness magnitude estimates as a function of temporal frequency (F_t) for gratings varying in R (A) andG (B). Symbols as in Figure 2.Asterisk in A identifies the only estimate for R-varying gratings that differed significantly from other estimates at the nearly identicalF_t.

Fig. 4.

Predicted afferent firing rates (I_s), for the conditions used in the magnitude estimation experiment, as a function of temporal frequency (F_t) for gratings varying inR (A–C) andG (D–F).SA, Slowly adapting type I afferents; RA, rapidly adapting afferents; PC, Pacinian afferents. Symbols as in Figure 2.

Fig. 5.

Scatter-plots of mean roughness magnitude estimates versus predicted afferent firing rates (I_s), for the conditions used in the magnitude estimation experiment, for gratings varying inR (A–C) andG (D–F). Correlation coefficients are indicated above each plot.SA, RA, and PC are as in Figure 4.

Fig. 6.

Mean discrimination accuracy for gratings varying in R (A) and G(B). CS, Constant speed condition;CF, constant F_t condition;EFD, exaggeratedF_t-difference condition.Asterisks identify significant differences from the CS (baseline) condition. Error bars represent SEMs.

Fig. 7.

Predicted differences between afferent firing rates (I_s) to the standard grating and most commonly used comparison grating in the discrimination experiment. A–C,R-varying gratings: I_s for standard (smoother) grating subtracted fromI_s for comparison (rougher) grating, values shown for comparison grating with R = 1.3;D–F, G-varying gratings:I_s for comparison (smoother) grating subtracted from I_s for standard (rougher) grating, values shown for comparison grating with G= 1.82. SA, RA, and PC are as in Figure 4; CS, CF, andEFD are as in Figure 6. Note thatI_s differences in the CF condition forR-varying gratings, which lie between −0.1 and 0, are too small to be discerned.