Xavier Libouton

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.

Particle size influence in an impaction bone grafting model. Comparison of fresh-frozen and freeze-dried allografts

BACKGROUND Impaction bone grafting with large particles is considered as mechanically superior to smaller morsels. Interest of freeze-dried irradiated bone for impaction bone grafting has been observed with small particles. Influence of bone process on other particle sizes still needed to be assessed. MATERIAL AND METHODS Twenty-four osteoarthrotic femoral heads were used to prepare fresh-frozen and freeze-dried irradiated cancellous bone. Each group was divided into four batches of different particle sizes, each batch containing 18 samples. The different particle sizes were obtained with a Retsch Cross Beater Mill SK 100, Noviomagus rotating bone mills with two sizes of rasps and a Luer bone rongeur. Bone grafts were impacted in a contained cylinder. Stiffness was monitored during impaction. RESULTS Freeze-dried irradiated grafts showed higher stiffness than fresh-frozen bone whatever the size of the particles. Large particles obtained with the rongeur and the large rasp from the Noviomagus bone mill were mechanically superior than small particles up to 30 impactions. INTERPRETATION Large particles offer better mechanical performance as a greater magnitude of force would be required to deform and break the particles. Freeze-dried irradiated bone brittleness reduces this advantage after 30 impactions. Large particles embrittlement leads to similar mechanical results as small particles at higher impaction rate. This may account for partial collapse of the graft layer in clinical situation when impaction rate is lower. This model supports the use of small particles obtained with thin rasp bone mill when freeze-dried irradiated bone for impaction bone grafting and large particles obtained with the Rongeur when fresh-frozen bone is available.

Freeze-dried irradiated bone brittleness improves compactness in an impaction bone grafting model.

Background Defatted bone chips with or without freeze-drying and irradiation have mechanical advantages as compared to fresh-frozen controls in in vitro models of impaction. These improved results have been ascribed to replacement of viscous bone marrow by saline and embrittlement of the freeze-dried bone by irradiation.Material and methods To determine which of these hypotheses is correct, we compared the development of stiffness and compactness of morselized bone graft that had been: 1) fat-reduced with saline, and 2) fresh-frozen, solvent-detergent defatted, 3) freeze-dried irradiated and 4) not irradiated. We used 12 osteo-arthrotic femoral heads to prepare these four batches of morselized bone, and impacted 18 samples from each batch in a cylinder. The frozen bone grafts were tested after thawing at room temperature for 2 hours and the freeze-dried grafts were tested after 30 minutes of rehydration. We monitored the development of compactness and stiffness of the material during impaction.Results The stiffness of the freeze-dried irradiated bone was greater than that of the other three series after 10, 50 and 150 impactions. The freeze-dried bone chips that were not irradiated and the chips defatted with saline alone were less stiff than the fresh-frozen control after 150 impactions.Interpretation The brittleness of freeze-dried irradiated bone, caused by loss of the capacity to absorb energy in a plastic way, increases the compactness and stiffness of the morselized grafts. Washing bone with saline alone or treating bone with solvent-detergent but no irradiation had no similar mechanical advantage and the bone did not impact better than fresh-frozen undefatted bone in our model.

Biased visuospatial perception in complex regional pain syndrome

Complex regional pain syndrome (CRPS) is a chronic pain condition associating sensory, motor, trophic and autonomic symptoms in one limb. Cognitive difficulties have also been reported, affecting the patients’ ability to mentally represent, perceive and use their affected limb. However, the nature of these deficits is still a matter of debate. Recent studies suggest that cognitive deficits are limited to body-related information and body perception, while not extending to external space. Here we challenge that statement, by using temporal order judgment (TOJ) tasks with tactile (i.e. body) or visual (i.e. extra-body) stimuli in patients with upper-limb CRPS. TOJ tasks allow characterizing cognitive biases to the advantage of one of the two sides of space. While the tactile TOJ tasks did not show any significant results, significant cognitive biases were observed in the visual TOJ tasks, affecting mostly the perception of visual stimuli occurring in the immediate vicinity of the affected limb. Our results clearly demonstrate the presence of visuospatial deficits in CRPS, corroborating the cortical contribution to the CRPS pathophysiology, and supporting the utility of developing rehabilitation techniques modifying visuospatial abilities to treat chronic pain.

Multisensory components of rapid motor responses to fingertip loading

Tactile and muscle afferents provide critical sensory information for grasp control, yet the contribution of each sensory system during online control has not been clearly identified. More precisely, it is unknown how these two sensory systems participate in online control of digit forces following perturbations to held objects. To address this issue, we investigated motor responses in the context of fingertip loading, which parallels the impact of perturbations to held objects on finger motion and fingerpad deformation, and characterized surface recordings of intrinsic (first dorsal interosseous, FDI) and extrinsic (flexor digitorum superficialis, FDS) hand muscles based on statistical modeling. We designed a series of experiments probing the effects of peripheral stimulation with or without anesthesia of the finger, and of task instructions. Loading of the fingertip generated a motor response in FDI at ~60 ms following the perturbation onset, which was only driven by muscle stretch, as the ring-block anesthesia reduced the gain of the response occurring later than 90 ms, leaving responses occurring before this time unaffected. In contrast, the motor response in FDS was independent of the lateral motion of the finger. This response started at ~90 ms on average and was immediately adjusted to task demands. Altogether these results highlight how a rapid integration of partially distinct sensorimotor circuits supports rapid motor responses to fingertip loading.NEW & NOTEWORTHY To grasp and manipulate objects, the brain uses touch signals related to skin deformation as well as sensory information about motion of the fingers encoded in muscle spindles. Here we investigated how these two sensory systems contribute to feedback responses to perturbation applied to the fingertip. We found distinct response components, suggesting that each sensory system engages separate sensorimotor circuits with distinct functions and latencies.