Olivier Barbier

Tactile roughness discrimination of the finger pad relies primarily on vibration sensitive afferents not necessarily located in the hand

This study aims to investigate the relative contribution of remote mechanoreceptors to perception of roughness and spatial acuity. We examined two unilateral pathological conditions affecting differently innervation of the index finger: unilateral carpal tunnel syndrome (n=12) and surgically repaired complete traumatic median nerve section at the wrist following surgical repair (n=4). We employed a control condition consisting of ring-block anesthesia of the entire index in 10 healthy subjects to model pathological denervation of the fingertip. Spatial acuity and the ability to discern roughness were assessed using a grating orientation task and a roughness discrimination task, respectively. In patients with carpal tunnel syndrome, we observed a significant reduction of spatial resolution acuity but an intact ability to discriminate roughness with the fingertip. For patients with traumatic median nerve section there was no recovery with the grating orientation task up to 20 months post surgery but a progressive and full recovery with the roughness discrimination task between 6 and 9 months. Finally, in the anesthetic ring bloc group, the nerve block completely disrupted performances in grating orientation task, but unexpectedly left unaffected performances in the roughness discrimination task. Taken together, these lines of evidence support the view that the neural mechanisms underlying tactile roughness discrimination differ from those involved in spatial resolution acuity. Vibrotaction is necessary and sufficient for the perception of fine textures and, when the innervation of the fingerpad is compromised, information about textures can be captured and encoded by remote mechanoreceptors located in more proximal tissues where the innervation is intact.

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.