Tactile motion adaptation reduces perceived speed but shows no evidence of direction sensitivity

Introduction: While the directionality of tactile motion processing has been studied extensively, tactile speed processing and its relationship to direction is little-researched and poorly understood. We investigated this relationship in humans using the ‘tactile speed aftereffect’ (tSAE), in which the speed of motion appears slower following prolonged exposure to a moving surface. Method: We used psychophysical methods to test whether the tSAE is direction sensitive. After adapting to a ridged moving surface with one hand, participants compared the speed of test stimuli on the adapted and unadapted hands. We varied the direction of the adapting stimulus relative to the test stimulus. Results: Perceived speed of the surface moving at 81 mms−1 was reduced by about 30% regardless of the direction of the adapting stimulus (when adapted in the same direction, Mean reduction = 23 mms−1, SD = 11; with opposite direction, Mean reduction = 26 mms−1, SD = 9). In addition to a large reduction in perceived speed due to adaptation, we also report that this effect is not direction sensitive. Conclusions: Tactile motion is susceptible to speed adaptation. This result complements previous reports of reliable direction aftereffects when using a dynamic test stimulus as together they describe how perception of a moving stimulus in touch depends on the immediate history of stimulation. Given that the tSAE is not direction sensitive, we argue that peripheral adaptation does not explain it, because primary afferents are direction sensitive with friction-creating stimuli like ours (thus motion in their preferred direction should result in greater adaptation, and if perceived speed were critically dependent on these afferents’ response intensity, the tSAE should be direction sensitive). The adaptation that reduces perceived speed therefore seems to be of central origin

First spikes in ensembles of human tactile afferents code complex spatial fingertip events

It is generally assumed that primary sensory neurons transmit information by their firing rates. However, during natural object manipulations, tactile information from the fingertips is used faster than can be readily explained by rate codes. Here we show that the relative timing of the first impulses elicited in individual units of ensembles of afferents reliably conveys information about the direction of fingertip force and the shape of the surface contacting the fingertip. The sequence in which different afferents initially discharge in response to mechanical fingertip events provides information about these events faster than the fastest possible rate code and fast enough to account for the use of tactile signals in natural manipulation

Somatotopic mismatch following stroke : a pathophysiological condition escaping detection

Clinical evaluation of somatosensory deficits in stroke patients is very limited and usually does not include testing of somatotopic organisation, which is a prerequisite for meaningful interpretation of sensory input and sensorimotor control. Detailed tactile testing of the left hand of a 54-year-old patient suffering from sensory deficit and central pain after a right-sided stroke revealed severe distortion of somatotopic sensory maps as evidenced by incorrect localisation of the point stimuli. Unlike previously reported gross somatotopic remapping taking place within reduced representational space after lesion, this is the first case report revealing chaotic scrambled somatosensory maps. While the incidence of such scrambled somatotopic representation of tactile input is not yet known in stroke patients, current observations indicate that in-depth investigations of somatotopic organisation of affected area may reveal the underlying cause for various functional deficits including central pain. Thus, new rehabilitation strategies may need to be developed specifically for such patients

(Neuroscience and material science joins efforts to create a new type of artificial tactile sensors)

Brain-Machine-Interfaces and mind controlled manipulators are no longer science fiction. It is time to go even further. The functionality of hand prosthesis and manipulators is limited by the availability of sensory information about features of grasped object and manipulative forces. Artificial sensors, which could match functionality of human tactile receptors, currently are not available. The aims of this study are: 1. Using neurophysiological knowledge about functional properties of human tactile afferents and use of sensory information in sensorimotor transformations during object manipulation acknowledge obstacles which have precluded engineers from developing highly efficient artificial sensors; 2. Define requirements crucial for building artificial tactile sensors mimicking biological prototype; 3. To test mechanical features and evaluate stimulus-response characteristics of unique mechanosensitive conductive rubber material developed by our material science team. Mechanosensitive conductive rubber was manufactured by blending polyisoprene caoutchouc with highly structured nano–size carbon black (Degussa Printex XE2), Cyclohexyl-Benzothiazole-Sulfenamide, zinc oxide and sulphur. Mechanosensitive conductive rubber demonstrated high sensitivity to deformation, feature stability across relevant environmental conditions and ability to respond to fast dynamic stimuli in the frequency range of up to 50 Hz, which matches features of biological type of receptors involved in signalling frictional information. Frictional information is the key parameter required to control grip forces during object manipulation. Our mechanosensitive conductive rubber demonstrated unique combination of features like sensitivity to deformation (bending and stretch) and softness which makes it exceptionally suitable for manufacturing artificial tactile sensors to be used in intelligent hand prosthesis and robotic manipulators

Somatotopic mismatch of hand representation following stroke : is recovery possible?

Well-organized somatotopic representation of the hand is required to interpret input from cutaneous mechanoreceptors. Previous reports have identified patients with various distortions of somatotopic representation after stroke. Importantly, those patients were investigated years after the stroke, indicating that afferent signal regained access to the cortical circuits; however, further plastic changes, which would re-establish somatotopic order and ability to correctly localize tactile stimuli, did not follow. Thus, it was not known whether somatotopic organization could be restored in such patients and whether there is a potential for new rehabilitation strategies. This is the first case report demonstrating normalization of somatotopic representation

Consistent interindividual increases or decreases in muscle sympathetic nerve activity during experimental muscle pain

We recently showed that long-lasting muscle pain, induced by intramuscular infusion of hypertonic saline, evoked two patterns of cardiovascular responses across subjects: one group showed parallel increases in muscle sympathetic nerve activity (MSNA), blood pressure, and heart rate, while the other group showed parallel decreases. Given that MSNA is consistent day to day, we tested the hypothesis that individuals who show increases in MSNA during experimental muscle pain will show consistent responses over time. MSNA was recorded from the peroneal nerve, together with blood pressure and heart rate, during an intramuscular infusion of hypertonic saline causing pain for an hour in 15 subjects on two occasions, 2-27 weeks apart. Pain intensity ratings were not significantly different between the first (5.8 ± 0.4/10) and second (6.1 ± 0.2) recording sessions. While four subjects showed significant decreases in the first session (46.6 ± 9.2 % of baseline) and significant increases in the second (159.6 ± 8.9 %), in 11 subjects, there was consistency in the changes in MSNA over time: either a sustained decrease (55.6 ± 6.8 %, n = 6) or a sustained increase (143.5 ± 6.1 %, n = 5) occurred in both recording sessions. There were no differences in pain ratings between sessions for any subjects. We conclude that the changes in MSNA during long-lasting muscle pain are consistent over time in the majority of individuals, reflecting the importance of studying interindividual differences in physiology

Decoding tactile afferent activity to obtain an estimate of instantaneous force and torque applied to the fingerpad

Dexterous manipulation is not possible without sensory information about object properties and manipulative forces. Fundamental neuroscience has been unable to demonstrate how information about multiple stimulus parameters may be continuously extracted, concurrently, from a population of tactile afferents. This is the first study to demonstrate this, using spike trains recorded from tactile afferents innervating the monkey fingerpad. A multiple-regression model, requiring no a priori knowledge of stimulus-onset times or stimulus combination, was developed to obtain continuous estimates of instantaneous force and torque. The stimuli consisted of a normal-force ramp (to a plateau of 1.8, 2.2, or 2.5 N), on top of which -3.5, -2.0, 0, +2.0, or 3.5 mNm torque was applied about the normal to the skin surface. The model inputs were sliding windows of binned spike counts recorded from each afferent. Models were trained and tested by 15-fold crossvalidation to estimate instantaneous normal force and torque over the entire stimulation period. With the use of the spike trains from 58 slow-adapting type I and 25 fast-adapting type I afferents, the instantaneous normal force and torque could be estimated with small error. This study demonstrated that instantaneous force and torque parameters could be reliably extracted from a small number of tactile afferent responses in a real-time fashion with stimulus combinations that the model had not been exposed to during training. Analysis of the model weights may reveal how interactions between stimulus parameters could be disentangled for complex population responses and could be used to test neurophysiologically relevant hypotheses about encoding mechanisms

A point process approach to encode tactile afferents

In daily activities, humans manipulate objects and do so with great precision. Empirical studies have demonstrated that signals encoded by mechanoreceptors facilitate the precise object manipulation in humans, however, little is known about the underlying mechanisms. Current models range from complex to simple regression fit. These models do not describe the dynamics of neural data well. Because experimental neural data is limited to spike instances, they can be viewed as point processes. We discuss the point process framework and use it to simulate neural data possessing behaviors similar to experimental neural data. The characteristics of neural data were identified via visualization and descriptive statistics based on the experimental data. Then we fit candidate models to the simulated data and perform goodness-of fit to assess how well the models perform. This type of analysis facilitates the mapping of neural data to stimulus. Given this mapping, we can generate a population of spike trains, and infer from them in order to recover the applied stimulus. The knowledge acquired may provide insight into some fundamental sensory mechanisms that are responsible for coordinating force components during object manipulation.We envisage that the knowledge may guide the design of sensory-controlled bio-medical devices and robotic manipulators

(Contemporary Problems and Practice in Physiology : Collection of Scientific Articles)

Understanding of psychological processes underlying teaching and learning in family, at school and university requires very complex knowledge spaning various research disciplines. Physiological knowledge linking pedagogy and psychology explains the mechanisms influencing mental processes in children and adults during different stages of life and in various situations. This collection of the articles shows that Latvian physiologists are productively working in several directions. Many papers are in the field of teaching and learning psycho-physiology. RTTEMA physiologists present evidence and explain how physical activity contributes to the development of the movement skills and movement coordination in pre-school age children (J.Porozovs et al.), while research conducted at the Daugavpils University and at the National Defence Academy shows the effects of physical activity on students’ health and physical condition (I.Kaminska et al., L.Pļaviņa). In our opinion, students of biology will be especially interested in a paper, which compiles the latest findings in skeletal muscle physiology (K.Eglīte). There is no doubt that the reader will notice a comprehensive studies carried out at RTTEMA concerning the integrative teaching techniques and evaluation of efficiency of different teaching approaches (J.Gedrovics; D.Cēdere; A.Kauliņa)

Classifying torque, normal force and direction using monkey afferent nerve spike rates

In this study, tactile afferents in monkey fingertips were mechanically stimulated, using a flat disc shaped probe, with several magnitudes of torque, in clockwise and anticlockwise directions. In order to prevent slip during the stimulation event, a sufficient normal force was also applied, with three different magnitudes tested. Recordings were made from afferents innervating the glabrous skin covering the entire distal segment of the finger. A Parzen window classifier was used to assess the capacity of tactile afferents to discriminate, concurrently and in real-time, the three stimulus parameters; namely, background normal force, torque magnitude and direction. Despite the potentially confounding interactions between stimulus parameters, classification accuracy was very high and was improved even further by selecting subsets of best performing afferents