Responses of cutaneous mechanoreceptors within fingerpad to stimulus information for tactile softness sensation of materials

Softness sensation is one of primitive tactile textures. While the psychophysical characteristics of softness sensation have been thoroughly studied, it is lack of a deep understanding of the underlying neuromechanical principles. On the stimulus–response processes of human fingerpad touching fabrics and the physiological properties of slowly adapting type I (SAIs) cutaneous mechanoreceptors within fingerpad, a fabric-skin-receptor coupling model was built and validated. By the fabric-skin-receptor model a series of numerical experiments was conducted, and how the evoked neural responses of cutaneous mechanoreceptors change with the composite compliance of both fingerpad skin and the materials in contact was investigated. The results indicated that the evoked neural responses of populations of cutaneous mechanoreceptors by the physical stimulus from fabrics were nearly proportional to the perceived softness magnitude, and nonlinearly increased and then decreased with the effective elastic modulus of fabrics or the relative elastic modulus of fabrics to soft tissues within fingerpad, where the nonlinear inflection point depended on the touching force level. Therefore, it concluded that the tactile judgment of the physical information for softness sensation of objects was an encoding of neural responses of populations of SAIs cutaneous mechanoreceptors, and the physical information depended on the mechanical interaction of fingerpad and objects in contact.     

3-D finite-element models of human and monkey fingertips to investigate the mechanics of tactile sense

The biomechanics of skin and underlying tissues plays a fundamental role in the human sense of touch. It governs the mechanics of contact between the skin and an object, the transmission of the mechanical signals through the skin, and their transduction into neural signals by the mechanoreceptors. To better understand the mechanics of touch, it is necessary to establish quantitative relationships between the loads imposed on the skin by an object, the state of stresses/strains at mechanoreceptor locations, and the resulting neural response. Towards this goal, 3-D finite-element models of human and monkey fingertips with realistic external geometries were developed. By computing fingertip model deformations under line loads, it was shown that a multi-layered model was necessary to match previously obtained in vivo data on skin surface displacements. An optimal ratio of elastic moduli of the layers was determined through numerical experiments whose results were matched with empirical data. Numerical values of the elastic moduli of the skin layers were obtained by matching computed results with empirically determined force-displacement relationships for a variety of indentors. Finally, as an example of the relevance of the model to the study of tactile neural response, the multilayered 3-D finite-element model was shown to be able to predict the responses of the slowly adapting type I (SA-I) mechanoreceptors to indentations by complex object shapes.     

Tactile efficiency of insect antennae with two hinge joints

Antennae are the main organs of the arthropod tactile sense. In contrast to other senses that are capable of retrieving spatial information, e.g. vision, spatial sampling of tactile information requires active movement of the sense organ. For a quantitative analysis of basic principles of active tactile sensing, we use a generic model of arbitrary antennae with two hinge joints (revolute joints). This kind of antenna is typical for Orthoptera and Phasmatodea, i.e. insect orders that contain model species for the study of antennal movements, including cricket, locust and stick insect. First, we analyse the significance of morphological properties on workspace and sampling acuity. It is shown how joint axis orientation determines areas out of reach while affecting acuity in the areas within reach. Second, we assume a parametric set of movement strategies, based on empirical data on the stick insect Carausius morosus, and investigate the role of each strategy parameter on tactile sampling performance. A stochastic environment is used to measure sampling density, and a viscous friction model is assumed to introduce energy consumption and, thus, a measure of tactile efficiency. Up to a saturation level, sampling density is proportional to the range or frequency of joint angle modulation. The effect of phase shift is strong if joint angle modulation frequencies are equal, but diminishes for other frequency ratios. Speed of forward progression influences the optimal choice of movement strategy. Finally, for an analysis of environmental effects on tactile performance, we show how efficiency depends on predominant edge direction. For example, with slanted and non-orthogonal joint axis orientations, as present in the stick insect, the optimal sampling strategy is less sensitive to a change from horizontal to vertical edge predominance than with orthogonal and non-slanted joint axes, as present in a cricket.Electronic Supplementary Material Supplementary material is available in the online version of this article at     

Neural mechanisms of tactile motion integration in somatosensory cortex

How are local motion signals integrated to form a global motion percept? We investigate the neural mechanisms of tactile motion integration by presenting tactile gratings and plaids to the fingertips of monkeys, using the tactile analogue of a visual monitor and recording the responses evoked in somatosensory cortical neurons. The perceived directions of the gratings and plaids are measured in parallel psychophysical experiments. We identify a population of somatosensory neurons that exhibit integration properties comparable to those induced by analogous visual stimuli in area MT and find that these neural responses account for the perceived direction of the stimuli across all stimulus conditions tested. The preferred direction of the neurons and the perceived direction of the stimuli can be predicted from the weighted average of the directions of the individual stimulus features, highlighting that the somatosensory system implements a vector average mechanism to compute tactile motion direction that bears striking similarities to its visual counterpart.     

Human psychophysics: Functional interpretation for contrast sensitivity versus spatial frequency curve

The hypothesis that neural processing in the human visual pathways compensates for both optical degradation as well as noise contamination at the photoreceptor level is introduced and shown to be consistent with the high frequency portion of the contrast sensitivity function for threshold detection of sinusoidal gratings in addition to the suprathreshold phenomenon of matching sinusoidal gratings of different spatial frequencies. This offers a unifying interpretation for why, at threshold conditions, the high spatial frequency portion of the image is blurred as severely by the nervous system as it is by the optics (e.g. Campbell and Green, 1965) while in extreme suprathreshold conditions the nervous system effectively deblurs the image (e.g. Georgeson and sullivan, 1975; Kulikowski, 1976). These conclusions do not necessitate a highly specific form of visual processing such as Fourier channeling.This research was conducted at Yale University, Department of Ophthalmology and Visual Science, New Haven, Connecticut, USA, throughout which period A.W.S. was a John Simon Guggenheim fellow     

A simple computer model of excitable synaptically connected neurons

 The space-lumped two-variable neuron model is studied. Extension of the neural model by adding a simple synaptic current allows the demonstration of neural interactions. The production of synchronous burst activity in this simple two-neuron excitatory loop is modeled, including the influence of random background excitatory input. The ability of the neuron model to integrate inputs spatially and temporally is shown. Two refractory periods after stimuli were identified and their role in burst cessation is demonstrated. Our findings show that simple neural units without long-lasting membrane processes are capable of generating long lasting patterns of activity. The results of simulation of simple background activity suggest that an increase in background activity tends to cause decreased activity of the network. This phenomenon, as well as the existence of two refractory periods, allows for burst cessation without inhibition in this simple model. Received: 6 December 1996/Accepted in revised form: 7 April 1997     

Change in EMG with skin friction at different frequencies during elbow flexion

Modulation of muscle activation in superficial and deeper regions may be induced by tactile stimulation. The purpose of this study was to examine changes in muscle activation with skin friction. Subjects performed an isometric elbow flexion at 30% maximal voluntary cotraction (MVC) with skin friction at different frequencies (0.5–2.7?Hz). Surface electromyography (S-EMG) and intramuscular EMG were obtained from the elbow flexor muscles (BBS: short head of biceps brachii, BBL: long head of biceps brachii, BRA: brachialis). S-EMG activity decreased at a higher frequency of 2.7?Hz and increased linearly with an increase in skin friction frequency (0.5–2.7?Hz) in BBS. A decrease in high-threshold motor unit (HT-MU) firing rate in superficial regions and an increase in low-threshold motor unit (LT-MU) firing rate in deeper regions were observed with skin friction (2.7?Hz) in BBS. The actions of inhibitory interneurons may be influenced by cutaneous afferent input with skin friction. Muscle activation of BBS depended on the intensity of the stimulus. Skin friction over BBS results in an inhibitory response in superficial regions of BBS, most likely due to the increase in firing rate of low-threshold cutaneous mechanoreceptors.     

SA-I mechanoreceptor position in fingertip skin may impact sensitivity to edge stimuli

BACKGROUND: The skin plays a role in conditioning mechanical indentation into distributions of stress/strain that mechanoreceptors convert into neural signals. Solid mechanics methods have modelled the skin to predict the in vivo neural response from mechanoreceptors. Despite their promise, current models cannot explain the role that anatomical positioning and receptor organ morphology play in producing differences in neural response. This work hypothesises that the skin's intermediate ridges may help explain, in part, the sensitivity of slowly adapting type I (SA-I) mechanoreceptors to edge stimuli. METHOD: Two finite-element models of the fingertip were built, validated and used to analyse the functionality of the intermediate ridges. One of the two-dimensional, cross-sectional models included intermediate ridges, while the other did not. The analysis sought to determine if intermediate ridges (1) increase the magnitude of strain energy density (SED) near the SA-I location and (2) help differentiate one 2.0-mm indenter from two 0.5-mm wide indenters with a 1.0-mm gap. RESULTS: Higher concentrations of SED were found near the tips of the intermediate ridges, the anatomical location that coincides with the SA-I receptors. This first result suggested that the location of the SA-Is in the stiffer epidermal tissue helps magnify their response to edge stimuli. The second result was that both models were equally capable of predicting the spatial structure within the in vivo neural responses, and therefore the addition of intermediate ridges did not help in differentiating the indenters. CONCLUSION: The finding, a 15%-35% increase in response when the sampling point lies within the stiffer tissue at the same depth, seeks to inform the positioning of force sensors in robotic skin substrates.     

External sensilla of the locust abdomen provide the central nervous system with an interganglionic network

External mechanoreceptors and contact chemoreceptors on the cuticle of the sixth abdominal segment of locusts have divergent primary projections of their sensory neurons that form arbours in the segmental and anterior abdominal ganglia. Homologous interganglionic projections from adjacent segments converge in the neuropile of each abdominal ganglion. Of the contributing types of sensilla, three were previously unknown for locust pregenital segments: tactile mechanosensory hairs with dual innervation, external proprioceptors of the hairplate type covered by intersegmental membranes and single campaniform sensilla that monitor cuticular strain in sternites and tergites. In general, interdependence of motor coordination in the abdominal segments is based on a neural network that relies heavily on intersegmental primary afferents that cooperate to identify the location, parameters and strength of external stimuli.     

Tactile Discrimination of Gratings: Psychophysical and Neural Correlates in Human and Monkey

Human and monkey performance on discriminating tactile gratings revealed comparable cross-species Weber functions. Neural data obtained while monkeys performed discriminations revealed some matching of neural and psychometric functions. Nearly constant firing rate differences occurred at discrimination threshold for unequal groove widths. Firing rate differences of some cells decreased on trials preceding discrimination errors, and thus predicted performance.     

Tactile discrimination of gratings: psychophysical and neural correlates in human and monkey.

Human and monkey performance on discriminating tactile gratings revealed comparable cross-species Weber functions. Neural data obtained while monkeys performed discriminations revealed some matching of neural and psychometric functions. Nearly constant firing rate differences occurred at discrimination threshold for unequal groove widths. Firing rate differences of some cells decreased on trials preceding discrimination errors, and thus predicted performance.     

Orientation processing by synaptic integration across first-order tactile neurons

Our ability to manipulate objects relies on tactile inputs from first-order tactile neurons that innervate the glabrous skin of the hand. The distal axon of these neurons branches in the skin and innervates many mechanoreceptors, yielding spatially-complex receptive fields. Here we show that synaptic integration across the complex signals from the first-order neuronal population could underlie human ability to accurately (< 3°) and rapidly process the orientation of edges moving across the fingertip. We first derive spiking models of human first-order tactile neurons that fit and predict responses to moving edges with high accuracy. We then use the model neurons in simulating the peripheral neuronal population that innervates a fingertip. We train classifiers performing synaptic integration across the neuronal population activity, and show that synaptic integration across first-order neurons can process edge orientations with high acuity and speed. In particular, our models suggest that integration of fast-decaying (AMPA-like) synaptic inputs within short timescales is critical for discriminating fine orientations, whereas integration of slow-decaying (NMDA-like) synaptic inputs supports discrimination of coarser orientations and maintains robustness over longer timescales. Taken together, our results provide new insight into the computations occurring in the earliest stages of the human tactile processing pathway and how they may be critical for supporting hand function.     

Branchial mechanoreceptor activity during spontaneous ventilation in channel catfish

Extracellular afferent neural activity was recorded in vivo from cranial nerve IX (glossopharyngeal) from mechanoreceptors in the first gill arch of anesthetized, spontaneously breathing channel catfish (Ictalurus punctatus). Single unit and paucifiber recordings show that both phasic and tonic receptors were active during normal ventilation. Phasic receptors were characterized as having a burst of activity during some phase of the ventilatory cycle. Most of these occurred during peak adduction or peak abduction. Phasic receptors were not active during spontaneous apnic periods. Tonic receptors were always active, even during apneas, firing frequency was modulated by breathing movements with peak activity occurring during adduction. Flow-sensitive mechanoreceptors were identified in anesthetized, paralyzed catfish. These receptors decreased activity when the ventilatory water flow was stopped. Hypercapnia (5% CO(2) in air) stimulated ventilatory rate and amplitude but had no effect on mechanoreceptor activity. The discharge characteristics of branchial mechanoreceptors indicate that they could be involved in the timing and coordination of ventilatory movements and maintenance of the 'gill curtain' to minimize ventilatory dead space. Unlike ventilatory mechanoreceptors in the air breathing organs of gar and lungs of lungfish and tetrapods, branchial mechanoreceptors were insensitive to hypercapnia.     

The functions of antennal mechanoreceptors and antennal joints in tactile discrimination of the honeybee (<Emphasis Type="Italic">Apis mellifera</Emphasis> L.)

Honeybees learn and discriminate excellently between different surface structures and different forms of objects, which they scan with their antennae. The sensory plate on the antennal tip plays a key role in the perception of mechanosensory and gustatory information. It is densely covered with small tactile hairs and carries a few large taste hairs. Both types of sensilla contain a mechanoreceptor, which is involved in the antennal scanning of an object. Our experiments test the roles of the mechanoreceptors on the antennal tip in tactile antennal learning and discrimination. Joints between head capsule and scapus and between scapus and pedicellus enable the bee to perform three-dimensional movements when they scan an object. The role of these joints in tactile antennal learning and discrimination is studied in separate experiments. The mechanoreceptors on the antennal tip were decisive for surface discrimination, but not for tactile acquisition or discrimination of shapes. When the scapus–pedicellus joint or the headcapsule–scapus joint was fixed on both antennae, tactile learning was still apparent but surface discrimination was abolished. Fixing both scapi to the head capsule reduced tactile acquisition.     

Neuronal mechanisms underlying behavioral switching in a pteropod mollusc

Summary In the pteropod mollusc Clione limacina, wing retraction takes precedence over spontaneous and continuous swimming, a phenomenon here defined as behavioral switching. The wing retraction system is organized as a simple reflex in which wing mechanoreceptors activate a pair of retraction interneurons which in turn excite at least two pairs of retraction motoneurons.Activation of individual mechanoreceptors does not inhibit swimming or trigger wing retraction. A pair of retraction interneurons can fully suppress swimming when induced to fire at physiological frequencies, and may be both sufficient and necessary for swim inhibition.Retraction interneurons monosynaptically inhibit both swim interneurons and swim motoneurons. Retraction motoneurons inhibit swim motoneurons through a polysynaptic pathway.A model summarizing the neural circuitry underlying behavioral switching in Clione is presented. A comparison of this model with the behavioral choice model in Pleurobranchaea reveals that the overall neural mechanisms for behavioral choice and behavioral switching are similar as both involve dual function interneurons that not only activate their own motor pathway, but also inhibit the competing motor system. While inhibition is biased toward the afferent side of the competing circuit in behavioral choice, it is biased to the efferent side in behavioral switching.     

Perception of Body Ownership Is Driven by Bayesian Sensory Inference

Recent studies have shown that human perception of body ownership is highly malleable. A well-known example is the rubber hand illusion (RHI) wherein ownership over a dummy hand is experienced, and is generally believed to require synchronized stroking of real and dummy hands. Our goal was to elucidate the computational principles governing this phenomenon. We adopted the Bayesian causal inference model of multisensory perception and applied it to visual, proprioceptive, and tactile stimuli. The model reproduced the RHI, predicted that it can occur without tactile stimulation, and that synchronous stroking would enhance it. Various measures of ownership across two experiments confirmed the predictions: a large percentage of individuals experienced the illusion in the absence of any tactile stimulation, and synchronous stroking strengthened the illusion. Altogether, these findings suggest that perception of body ownership is governed by Bayesian causal inference—i.e., the same rule that appears to govern the perception of outside world.     

The cutaneous intrinsic visceral afferent nervous system: A new model for acupuncture analgesia

The mechanism of acupuncture, whilst not known with certainty, has previously been considered to be stimulatory. A novel hypothesis is presented here in which C fiber tactile afferent axons bifurcate at acupuncture points and then diverge, running along acupuncture meridians, to subsequently communicate with Merkel cells. It is proposed that acupuncture disrupts the bifurcation of these axons, preventing neural transmission between Merkel cells as well as central communication with the spinal cord. Making use of the known phenomenon that acupuncture points have lower electrical resistance than adjacent skin, this hypothesis was tested using an electrical circuit model and successfully predicted the observed 10³ reduction in skin resistance at acupuncture points. In addition to explaining acupuncture and the roles of both Merkel cells and C fiber tactile afferents, the model has greater implications for neuroscience, through the postulation of a new division of the autonomic nervous system.     

Electrolocation in the platypus--some speculations

In the platypus, electroreceptors are located in rostro-caudal rows in skin of the bill, while mechanoreceptors are uniformly distributed across the bill. The electrosensory area of the cerebral cortex is contained within the tactile somatosensory area, and some cortical cells receive input from both electroreceptors and mechanoreceptors, suggesting a close association between the tactile and electric senses. Platypus can determine the direction of an electric source, perhaps by comparing differences in signal strength across the sheet of electroreceptors as the animal characteristically moves its head from side to side while hunting. The cortical convergence of electrosensory and tactile inputs suggests a mechanism for determining the distance of prey items which, when they move, emit both electrical signals and mechanical pressure pulses. Distance could be computed from the difference in time of arrival of the two signals. Much of the platypus' feeding is done by digging in the bottom of streams with the bill. Perhaps the electroreceptors could also be used to distinguish animate and inanimate objects in this situation where the mechanoreceptors would be continuously stimulated. Much of this is speculation, and there is still much to be learned about electroreception in the platypus and its fellow monotreme, the echidna.     

From spikers to bursters via coupling: Help from heterogeneity

It has been shown previously that identical spiking cells, incapable of bursting by themselves, may burst under weak diffusive coupling conditions. With stronger coupling, the coupled system reverts to bursting can be recovered by introducing heterogeneity in the model parameters. For a two-cell system, we explain the phenomenon with bifurcation analysis. For larger clusters of coupled cells, we demonstrate by numerical simulation that the phenomenon carries over. In addition, we use a perturbation analysis to deduce the dependence of the heterogeneity parameter used in the bifurcation analysis on the original heterogeneity parameters and the coupling strength. Implications of the phenomenon of emergent bursting are discussed in the context of electrical activity in pancreatic beta cells.