Biology subjects

Birznieks, I.

Publications and source records attributed to Birznieks, I..

6 recordsLinked to original sources

bioRxiv2023

Role of finger movement kinematics in friction perception at initial contact with smooth surfaces

When manipulating objects, humans adjust grip force to friction remarkably quickly: it may take just 100 ms to see adjustment to friction at the skin-object interface. While the motor commands adapt, subjects become aware of slipperiness of touched surfaces. In this study, we explore the sensory processes underlying such friction perception when no intentional exploratory sliding movements are present. Previously, we have demonstrated that humans cannot perceive frictional differences when surfaces are brought in contact with an immobilized finger (Khamis et al., 2021b) unless there is a submillimeter lateral displacement (Afzal et al., 2022), or subjects made the movement themselves (Willemet et al., 2021). In the current study, subjects actively interacted with a device that can modulate friction using ultrasound, without an exploratory sliding movement, as they would when gripping an object to lift it. Using a two-alternative forced-choice paradigm, subjects had to indicate which of two surfaces felt more slippery. Subjects could correctly identify the more slippery surface in 87 {+/-} 8% of cases (mean{+/-}SD; n=12). Biomechanical analysis of finger pad skin contacting a flat smooth surface indicated that natural movement kinematics (e.g., tangential movement jitter and physiological tremor) may enhance perception of frictional effects. To test whether this is the case, in a second experiment a hand support was introduced to limit fingertip movement deviation from a straight path. Subject performance significantly decreased (66 {+/-} 12% correct, mean{+/-}SD; n=12), indicating that friction perception at the initial contact is enhanced or enabled by natural movement kinematics. Significance statementSensing surface friction is crucial for automatic grip force control to avoid dropping objects. A slipping handhold can lead to loss of balance and falling. In many instances, the required grip force may exceed hands physical ability or an objects breakage point, therefore cognitive selection of a safe and achievable action plan based on friction perception is critical. Little is known about how our awareness of surface slipperiness is obtained under such circumstances without exploratory movement. The current study demonstrates that natural movement kinematics inducing submillimeter lateral movements play a central enabling role, demonstrating interdependence between the motor system and sensory mechanisms. These findings broaden our fundamental understanding of sensorimotor control and could inform the development of advanced sensor technologies.

neuroscience↗

bioRxiv2023

Memory at your fingertips: how viscoelasticity affects tactile neuron signaling

Human skin and its underlying tissues constitute a viscoelastic medium, implying that any deformation depends not only on the currently applied force, but also the recent loading history. The extent to which this physical memory influences the signaling of first-order tactile neurons during natural hand use is not well understood. Here, we examined the effect of past loading on the responses of fast-adapting (FA-1) and slowly-adapting (SA-1 and SA-2) first-order tactile neurons innervating the human fingertip to loadings applied in different directions representative of object manipulation tasks. We found that variation in the preceding loading affected neurons overall signaling of force direction. Some neurons kept signaling the current direction, while others signaled both the current and preceding direction, or even primarily the preceding direction. In addition, ongoing impulse activity in SA-2 neurons between loadings signaled information related to the fingertips viscoelastic deformation state. We conclude that tactile neurons at the population level signal continuous information about the fingertips viscoelastic deformation state, which is shaped by both its recent history and current loading. Such information might be sufficient for the brain to correctly interpret current force loading and help in computing accurate motor commands for interactions with objects in manipulation and haptic tasks.

neuroscience↗

bioRxiv2022

How tactile afferents in the human fingerpad encode tangential torques associated with manipulation: are we better than monkeys?

Dexterous object manipulation depends critically on information about forces normal and tangential to the fingerpads, and also on torque associated with object orientation at grip surfaces. In this study we investigated how torque information is encoded by human tactile afferents, including slowly adapting type-II (SA-II) afferents, in the fingerpads. SA-II afferent properties seemed perfectly suited for torque encoding but could not be previously investigated as they are absent in the glabrous skin of monkeys. Torques of different magnitudes (3.5-7.5 mNm) were applied in clockwise and anticlockwise directions to a standard central site on the fingerpads of 34 participants. Torques were superimposed on a 2 N, 3 N, or 4 N background normal force. Unitary microneurography recordings were made from fast adapting type-I (FA-I, n=39), slowly adapting type-I (SA-I, n=31), and type-II (SA-II, n=13) afferents supplying the fingerpads. All three afferent types encoded torque magnitude and direction, with SA-II afferents showing excitatory and inhibitory modulation depending on torque direction. Most afferents of all three types had higher torque sensitivity with smaller normal force. FA-I afferents showed the best torque magnitude and worst directional discrimination abilities in both species. Human SA-I afferent response to static torque was inferior to dynamic stimuli, while in monkeys the opposite was true. In humans this might be compensated by the addition of a sustained SA-II afferent input. In comparison to monkeys the performance of each afferent type was inferior in humans, likely due to differences in fingertip tissue compliance and skin friction. [currently 246 words; 250 words max] Significance StatementWe investigate how individual human tactile nerve fibres encode rotational forces (torques) and compare them to their monkey counterparts. Human hands, but not monkey hands, are innervated by a tactile neuron type (SA-II afferents) specialised to encode directional skin strain, yet so far, torque encoding has only been studied in monkeys. We find that human SA- I afferents were generally less sensitive and less able to discriminate torque magnitude and direction than their monkey counterparts, especially during the static phase of torque loading. However, this shortfall in humans could be compensated by the addition of SA-II afferent input. This indicates that variation in afferent types might compliment their specialisation for stimulus features, by slight differences between encoding in the two species.

neuroscience↗

bioRxiv2021

Initial contact shapes the perception of friction

Humans efficiently estimate the grip force necessary to lift a variety of objects, including slippery ones. The regulation of grip force starts with the initial contact, and takes into account the surface properties, such as friction. This estimation of the frictional strength has been shown to depend critically on cutaneous information. However, the physical and perceptual mechanism that provides such early tactile information remains elusive. In this study, we developed a friction-modulation apparatus to elucidate the effects of the frictional properties of objects during initial contact. We found a correlation between participants conscious perception of friction and radial strain patterns of skin deformation. The results provide insights into the tactile cues made available by contact mechanics to the sensorimotor regulation of grip, as well as to the conscious perception of the frictional properties of an object.

biophysics↗

bioRxiv2020

Temporal patterns in electrical nerve stimulation: burst gap code shapes tactile frequency perception

We have previously described a novel temporal encoding mechanism in the somatosensory system, where mechanical pulses grouped into periodic bursts create a perceived tactile frequency based on the duration of the silent gap between bursts, rather than the mean rate or the periodicity. This coding strategy may offer new opportunities for transmitting information to the brain using various sensory neural prostheses and haptic interfaces. However, it was not known whether the same coding mechanisms apply when using electrical stimulation, which recruits a different spectrum of afferents. Here, we demonstrate that the predictions of the burst gap coding model for frequency perception apply to burst stimuli delivered with electrical pulses, re-emphasising the importance of the temporal structure of spike patterns in neural processing and perception of tactile stimuli. Reciprocally, the electrical stimulation data confirm that the results observed with mechanical stimulation do indeed depend on neural processing mechanisms in the central nervous system, and are not due to skin mechanical factors and resulting patterns of afferent activation.

neuroscience↗

bioRxiv2019

Tactile sensory channels over-ruled by frequency decoding system that utilizes spike pattern regardless of receptor type

The established view is that vibrotactile stimuli evoke two qualitatively distinctive cutaneous sensations, flutter (frequencies < 60 Hz) and vibratory hum (frequencies > 60 Hz), subserved by two distinct receptor types (Meissners and Pacinian corpuscle, respectively) which may engage different neural processing pathways or channels and fulfill quite different biological roles. In psychological and physiological literature those two systems have been labelled as Pacinian and non-Pacinian channels. However, we present evidence that low-frequency spike trains in Pacinian afferents can readily induce a vibratory percept with the same low frequency attributes as sinusoidal stimuli of the same frequency thus demonstrating a universal frequency decoding system. We achieved this using brief low-amplitude pulsatile mechanical stimuli to selectively activate Pacinian afferents. This indicates that spiking pattern, regardless of receptor type, determines vibrotactile frequency perception. This mechanism may underlie the constancy of vibrotactile frequency perception across different skin regions innervated by distinct afferent types.

neuroscience↗