Tactile sensitivity in the rat: a correlation between receptor structure and function.

Single cutaneous fibers were recorded in the median nerve of the deeply anesthetized rat and the receptor morphology in the forelimb glabrous skin was analyzed to establish a probable correlation between receptor anatomy and physiology. Receptor complexes in the glabrous skin of the rat forelimb were stained immunologically with antibodies NF-200 and PGP-9.5, confirming the presence of Meissner corpuscles and Merkel complexes within the dermal papilla similar to other mammals including primates.

Correlation of fingertip shear force direction with somatosensory cortical activity in monkey.

To examine the activity of somatosensory cortex (S1) neurons to self-generated shear forces on the index and thumb, two monkeys were trained to grasp a stationary metal tab with a key grip and exert forces without the fingers slipping in one of four orthogonal directions for 1 s. A majority (∼85%) of slowly adapting and rapidly adapting (RA) S1 neurons had activity modulated with shear force direction. The cells were recorded mainly in areas 1 and 2 of the S1, although some area 3b neurons also responded to shear direction or magnitude.

Neuronal activity in somatosensory cortex related to tactile exploration.

The very light contact forces (∼0.60 N) applied by the fingertips during tactile exploration reveal a clearly optimized sensorimotor strategy. To investigate the cortical mechanisms involved with this behavior, we recorded 230 neurons in the somatosensory cortex (S1), as two monkeys scanned different surfaces with the fingertips in search of a tactile target without visual feedback.

Tactile sensory system: encoding from the periphery to the cortex.

Specialized mechanoreceptors in the skin respond to mechanical deformation and provide the primary input to the tactile sensory system. Although the morphology of these receptors has been documented, there is still considerable uncertainty as to the relation between cutaneous receptor morphology and the associated physiological responses to stimulation. Labelled-line models of somatosensory processes in which specific mechanoreceptors are associated with particular sensory qualities fail to account for the evidence showing that all types of tactile afferent units respond to a varying extent to most types of natural stimuli.

Roughness of simulated surfaces examined with a haptic tool: effects of spatial period, friction, and resistance amplitude.

A specifically designed force-feedback device accurately simulated textures consisting of lateral forces opposing motion, simulating friction. The textures were either periodic trapezoidal forces, or sinusoidal forces spaced at various intervals from 1.5 mm to 8.

Perception of simulated local shapes using active and passive touch.

This study reexamined the perceptual equivalence of active and passive touch using a computer-controlled force-feedback device. Nine subjects explored a 6 x 10-cm workspace, with the index finger resting on a mobile flat plate, and experienced simulated Gaussian ridges and troughs (width, 15 mm; amplitude, 0.5 to 4.

Duchenne, Charcot and Babinski, three neurologists of La Salpetrière Hospital, and their contribution to concepts of the central organization of motor synergy.

Many currently accepted notions of motor control originate from a few seminal concepts developed in the latter half of the 19th century (see Bennett and Hacker, 2002). The goal of this review is to retrace some current ideas about motor control back to the thought of three French neurologists of Hospital of the Salpetrière hospital in Paris during the latter half of the 19th century and early 20th century (Fig. 1): Guillaume Duchenne de Boulogne (1806-1875), Jean-Martin Charcot (1825-1893), and Joseph Babinski (1857-1932).

Altered gravity highlights central pattern generator mechanisms.

In many nonprimate species, rhythmic patterns of activity such as locomotion or respiration are generated by neural networks at the spinal level. These neural networks are called central pattern generators (CPGs). Under normal gravitational conditions, the energy efficiency and the robustness of human rhythmic movements are due to the ability of CPGs to drive the system at a pace close to its resonant frequency.

Paradoxical effect of digital anaesthesia on force and corticospinal excitability.

The role played by sensory information in maintaining motor cortical representations is still incompletely understood. We investigated the effect of digital anaesthesia of the index finger and thumb on the amplitude of motor evoked potentials to transcranial magnetic stimulation (TMS) recorded from the first dorsal interosseus, F-wave response probability and maximal key pinch force. Whereas digital anaesthesia led to a 29% decrease in maximal force, both motor evoked potential amplitudes and F-wave probability remained unchanged.

The cutaneous contribution to adaptive precision grip.

Only after injury, or perhaps prolonged exposure to cold that is sufficient to numb the fingers, do we suddenly appreciate the complex neural mechanisms that underlie our effortless dexterity in manipulating objects. The nervous system is capable of adapting grip forces to a wide range of object shapes, weights and frictional properties, to provide optimal and secure handling in a variety of potentially perturbing environments. The dynamic interplay between sensory information and motor commands provides the basis for this flexibility, and recent studies supply somewhat unexpected evidence of the essential role played by cutaneous feedback in maintaining and acquiring predictive grip force control.

Effects of muscimol inactivation of the cerebellar nuclei on precision grip.

A single monkey was trained to perform a grasp, lift, and hold task in which a stationary hand- held object was sometimes subjected to brief, predictable force-pulse perturbations. The displacement, grip, and lifting forces were measured as well the three-dimensional forces and torques to quantify specific motor deficits after reversible inactivation of the cerebellar nuclei. A prior single-cell recording study in the same monkey provided the stereotaxic coordinates used to guide intranuclear injections of muscimol.

Responses of cerebellar interpositus neurons to predictable perturbations applied to an object held in a precision grip.

Two monkeys were trained to lift and hold an instrumented object at a fixed height for 2.5 s using a precision grip. The device was equipped with load cells to measure both the grip and lifting or load forces.

The effects of digital anesthesia on force control using a precision grip.

A total of 20 right-handed subjects were asked to perform a grasp-lift-and-hold task using a precision grip. The grasped object was a one-degree-of-freedom manipuladum consisting of a vertically mounted linear motor capable of generating resistive forces to simulate a range of object weights. In the initial study, seven subjects (6 women, 1 man; ages 24-56 yr) were first asked to lift and hold the object stationary for 4 s.

Importance of cutaneous feedback in maintaining a secure grip during manipulation of hand-held objects.

Previous research has shown that grip and load forces are modulated simultaneously during manipulation of a hand-held object. This close temporal coupling suggested that both forces are controlled by an internal model within the CNS that predicts the changes in tangential force on the fingers. The objective of the present study was to examine how the internal model would compensate for the loss of cutaneous sensation through local anesthesia of the index and thumb.

Deployment of fingertip forces in tactile exploration.

The purpose of this study was to examine how contact forces normal to the skin surface and shear forces tangential to the skin surface are deployed during tactile exploration of a smooth surface in search of a tactile target. Six naive subjects participated in two experiments. In the first experiment, the subjects were asked to explore a series of unseen smooth plastic surfaces by using the index finger to search for either a raised or recessed target.

Role of friction and tangential force variation in the subjective scaling of tactile roughness.

The present study examined the contribution of normal (Fz) and tangential (Fx) forces, and their ratio, kinetic friction (Fx/Fz), to the subjective magnitude estimations of roughness. The results suggested that the rate of variation in tangential stroking force is a significant determinant of roughness perception. In the first experiment, six volunteer subjects scaled the roughness of eight surfaces explored with a single, active scan of the middle finger.

Distribution and terminal arborizations of cutaneous mechanoreceptors in the glabrous finger pads of the monkey.

Recent electrophysiological studies demonstrated that neurons in the somatosensory cortex of monkeys respond to tangential forces applied to glabrous skin. To unravel the peripheral basis for this cortical response, we determined the distribution of presumptive low-threshold mechanoreceptors innervating the distal finger pads of monkeys. Endings were reconstructed in immunolabeled serial sections imaged by epifluorescence and confocal microscopy.

Magnitude estimation of tangential force applied to the fingerpad.

Prior research on large-fibre skin mechanoreceptors in humans and monkeys has demonstrated their sensitivity to perpendicular skin indentation and to the rate of force application. Although some studies have examined skin afferent responses to stretch, relatively few investigations have examined the neural and perceptual correlates of shear forces applied tangentially to the skin. The present study assessed the ability of human subjects to scale different levels of tangential force applied to the distal pad of the index finger.