Tactile roughness discrimination threshold is unrelated to tactile spatial acuity

The present study examined the relationship between the tactile roughness discrimination threshold (TRDT) and the tactile spatial resolution threshold (TSRT) at the index fingertip in humans. A new device was built for measuring TRDT, allowing pair-wise presentations of two sets of six different sandpaper grits. The smoothest grits ranged from 18 to 40 microm and the roughest grits ranged from 50 to 195 microm particle size. The reference sandpaper had a 46 microm particle size. A two-alternative forced choice paradigm and a double interlaced adaptive staircase procedure yielding a 75% just noticeable difference (75%jnd) was used according to Zwislocki and Relkin. Contact force and scanning velocity were measured at the fingertip with a built-in sensor. The TSRT was assessed with an extended set of grating domes. Fifty-three male and female subjects, spanning a wide age range participated in this study. The JND75% or TRDT was lower for the smoothest sandpapers (15+/-8.5 microm) compared to the roughest sandpapers (44+/-32.5 microm). TRDT performance was unrelated to age or gender. Additionally, grit size had no effect on the mean forces (normal and tangential) exerted at the fingertip or the mean scan velocities. In contrast, there was a significant degradation of TSRT performance with age. Lastly, there was no significant correlation between TRDT and TRST performance. Results of this study support the theory that the neural mechanisms underlying the perception of tactile roughness discrimination for fine textures differ from those involved in spatial resolution acuity often associated with the SA1 afferents.