Human perception of shape from touch

Skin-Inspired Multi-Modal Mechanoreceptors for Dynamic Haptic Exploration

Active sensing is a fundamental aspect of human and animal interactions with the environment, providing essential information about the hardness, texture, and tackiness of objects. This ability stems from the presence of diverse mechanoreceptors in the skin, capable of detecting a wide range of stimuli and from the sensorimotor control of biological mechanisms. In contrast, existing tactile sensors for robotic applications typically excel in identifying only limited types of information, lacking the versatility of biological mechanoreceptors and the requisite sensing strategies to extract tactile information proactively. Here, inspired by human haptic perception, a skin-inspired artificial 3D mechanoreceptor (SENS) capable of detecting multiple mechanical stimuli is developed to bridge sensing and action in a closed-loop sensorimotor system for dynamic haptic exploration. A tensor-based non-linear theoretical model is established to characterize the 3D deformation (e.g., tensile, compressive, and shear deformation) of SENS, providing guidance for the design and optimization of multimode sensing properties with high fidelity. Based on SENS, a closed-loop robotic system capable of recognizing objects with improved accuracy (≈96%) is further demonstrated. This dynamic haptic exploration approach shows promise for a wide range of applications such as autonomous learning, healthcare, and space and deep-sea exploration.

Emergence of sense of body ownership but not agency during virtual tool-use training is associated with an altered body schema

In this study we examined if training with a virtual tool in augmented reality (AR) affects the emergence of ownership and agency over the tool and whether this relates to changes in body schema (BS). 34 young adults learned controlling a virtual gripper to grasp a virtual object. In the visuo-tactile (VT) but not the vision-only (V) condition, vibro-tactile feedback was applied to the palm, thumb and index fingers through a CyberTouch II glove when the tool touched the object. Changes in the forearm BS were assessed with a tactile distance judgement task (TDJ) where participants judged distances between two tactile stimuli applied to their right forearm either in proximodistal or mediolateral orientation. Participants further rated their perceived ownership and agency after training. TDJ estimation errors were reduced after training for proximodistal orientations, suggesting that stimuli oriented along the arm axis were perceived as closer together. Higher ratings for ownership were associated with increasing performance level and more BS plasticity, i.e., stronger reduction in TDJ estimation error, and after training in the VT as compared to the V feedback condition, respectively. Agency over the tool was achieved independent of BS plasticity. We conclude that the emergence of a sense of ownership but not agency depends on performance level and the integration of the virtual tool into the arm representation.

Tool-use training in augmented reality: plasticity of forearm body schema does not predict sense of ownership or agency in older adults

In young adults (YA) who practised controlling a virtual tool in augmented reality (AR), the emergence of a sense of body ownership over the tool was associated with the integration of the virtual tool into the body schema (BS). Agency emerged independent of BS plasticity. Here we aimed to replicate these findings in older adults (OA). Although they are still able to learn new motor tasks, brain plasticity and learning capacity are reduced in OA. We predicted that OA would be able to gain control over the virtual tool indicated by the emergence of agency but would show less BS plasticity as compared to YA. Still, an association between BS plasticity and body ownership was expected. OA were trained in AR to control a virtual gripper to enclose and touch a virtual object. In the visuo-tactile (VT) but not the vision-only (V) condition, vibro-tactile feedback was applied through a CyberTouch II glove when the tool touched the object. BS plasticity was assessed with a tactile distance judgement task where participants judged distances between two tactile stimuli applied to their right forearm. Participants further rated their perceived ownership and agency after training. As expected, agency emerged during the use of the tool. However, results did not indicate any changes in the BS of the forearm after virtual tool-use training. Also, an association between BS plasticity and the emergence of body ownership could not be confirmed for OA. Similar to YA, the practice effect was stronger in the visuo-tactile feedback condition compared with the vision-only condition. We conclude that a sense of agency may strongly relate to improvement in tool-use in OA independent of alterations in the BS, while ownership did not emerge due to a lack of BS plasticity.

Comparison of 3D printing model to 3D virtual reconstruction and 2D imaging for the clinical education of interns in hepatocellular carcinoma: a randomized controlled study

Background

The clinical education of interns on hepatocellular carcinoma (HCC) is both crucial and difficult in China, even if the education reform has advanced constantly over the years. The value of specific 3D printing model (3DPM) in clinical education of HCC is uncertain, and relevant literatures are very few. This study aimed to explore the effects of a patient-specific 3D printing liver model on the clinical education of HCC.

Methods

Three laparoscopic hepatectomies were collected. For each case, a 3D virtual reconstruction (3DVR) and 3DPM were created using multi-detector computed tomography (MDCT) data, respectively. A total of 62 interns were randomly assigned to each group (3DPM, 3DVR, and MDCT groups) through a table of random numbers for random grouping. Following lecture-based HCC education, interns in each group selected a corresponding model of HCC. All interns were tested on the hepatic tumor locations, the vessels adjacent to them, surgical planning, and test time using the centesimal system score within 90 min. A questionnaire investigation on the degree of satisfaction, interest, and helpfulness for improving the comprehension ability of liver anatomy and 3D spatial structures was also recorded. The 3DPM group were compared with both 3DVR and MDCT group by theoretical examination scores and questionnaire survey satisfaction to evaluate the effects of 3DPM on the interns' clinical education in HCC.

Results

All the interns completed the test and questionnaire. The 3DPM group gained significantly higher scores on the following test contents: indicating the correct tumor location (3DPM vs. 3DVR, MDCT: 36.7±4.8 vs. 33.2±5.8, 26.8±10.0, P=0.03, P<0.01, respectively), accurately identifying the relationship between the tumor and vessels (3DPM vs. 3DVR, MDCT: 37.1±4.6 vs. 31.6±3.7, 30.0±5.8, P<0.01, P<0.01, respectively), and designing appropriate surgical plans (3DPM vs. 3DVR, MDCT: 8±2.7 vs. 4.9±2.7, 5.9±3.8, P<0.01, P=0.04, respectively). The 3DPM group showed a higher degree of satisfaction (86.2%), interest (92.1%), and helpfulness (80.5%) for improving the comprehension ability of liver anatomy and 3D spatial structures.

Conclusions

The clinical teaching by utilizing 3DPM can significantly improve the professional theoretical level, strengthen clinical thinking and comprehensive ability, and improve the teaching effects of HCC for medical interns.

Interhemispheric communication during haptic self-perception

During the haptic exploration of a planar surface, slight resistances against the hand's movement are illusorily perceived as asperities (bumps) in the surface. If the surface being touched is one's own skin, an actual bump would also produce increased tactile pressure from the moving finger onto the skin. We investigated how kinaesthetic and tactile signals combine to produce haptic perceptions during self-touch. Participants performed two successive movements with the right hand. A haptic force-control robot applied resistances to both movements, and participants judged which movement was felt to contain the larger bump. An additional robot delivered simultaneous but task-irrelevant tactile stroking to the left forearm. These strokes contained either increased or decreased tactile pressure synchronized with the resistance-induced illusory bump encountered by the right hand. We found that the size of bumps perceived by the right hand was enhanced by an increase in left tactile pressure, but also by a decrease. Tactile event detection was thus transferred interhemispherically, but the sign of the tactile information was not respected. Randomizing (rather than blocking) the presentation order of left tactile stimuli abolished these interhemispheric enhancement effects. Thus, interhemispheric transfer during bimanual self-touch requires a stable model of temporally synchronized events, but does not require geometric consistency between hemispheric information, nor between tactile and kinaesthetic representations of a single common object.

Value of 3D printing technology combined with indocyanine green fluorescent navigation in complex laparoscopic hepatectomy

Background

Laparoscopic hepatectomy (LH) has achieved rapid progress over the last decade. However, it is still challenging to apply laparoscopy to lesions located in segments I, VII, VIII, and IVa and the hepatic hilar region due to difficulty operating around complex anatomical structures. In this study, we applied three-dimensional printing (3DP) and indocyanine green (ICG) fluorescence imaging technology to complex laparoscopic hepatectomy (CLH) to explore the effects and value of the modified procedure.

Materials and methods

From January 2019 to January 2021, 54 patients with complex hepatobiliary diseases underwent LH at our center. Clinical data were collected from these patients and retrospectively analyzed.

Results

A total of 30 patients underwent CLH using the conventional approach, whereas 24 cases received CLH with 3DP technology and ICG fluorescent navigation. Preoperative data were compared between the two groups. In the 3DP group, we modified the surgical strategy of four patients (4/24, 16.7%) due to real-time intraoperative navigation with 3DP and ICG fluorescent imaging technology. We did not modify the surgical strategy for any patient in the non-3DP group (P = 0.02). There were no significant differences between the non-3DP and 3DP groups regarding operating time (297.7±104.1 min vs. 328.8±110.9 min, P = 0.15), estimated blood loss (400±263.8 ml vs. 345.8±356.1 ml, P = 0.52), rate of conversion to laparotomy (3/30 vs. 2/24, P = 0.79), or pathological outcomes including the incidence of microscopical R0 margins (28/30 vs. 24/24, P = 0.57). Additionally, there were no significant differences in postoperative complications or recovery conditions between the two groups. No instances of 30- or 90-day mortality were observed.

Conclusion

The optimal surgical strategy for CLH can be chosen with the help of 3DP technology and ICG fluorescent navigation. This modified procedure is both safe and effective, but without improvement of intraoperative and short-term outcomes.

Visual and Tactile Sensory Systems Share Common Features in Object Recognition

Although we use our visual and tactile sensory systems interchangeably for object recognition on a daily basis, little is known about the mechanism underlying this ability. This study examined how 3D shape features of objects form two congruent and interchangeable visual and tactile perceptual spaces in healthy male and female participants. Since active exploration plays an important role in shape processing, a virtual reality environment was used to visually explore 3D objects called digital embryos without using the tactile sense. In addition, during the tactile procedure, blindfolded participants actively palpated a 3D-printed version of the same objects with both hands. We first demonstrated that the visual and tactile perceptual spaces were highly similar. We then extracted a series of 3D shape features to investigate how visual and tactile exploration can lead to the correct identification of the relationships between objects. The results indicate that both modalities share the same shape features to form highly similar veridical spaces. This finding suggests that visual and tactile systems might apply similar cognitive processes to sensory inputs that enable humans to rely merely on one modality in the absence of another to recognize surrounding objects.

Active Haptic Perception in Robots: A Review

In the past few years a new scenario for robot-based applications has emerged. Service and mobile robots have opened new market niches. Also, new frameworks for shop-floor robot applications have been developed. In all these contexts, robots are requested to perform tasks within open-ended conditions, possibly dynamically varying. These new requirements ask also for a change of paradigm in the design of robots: on-line and safe feedback motion control becomes the core of modern robot systems. Future robots will learn autonomously, interact safely and possess qualities like self-maintenance. Attaining these features would have been relatively easy if a complete model of the environment was available, and if the robot actuators could execute motion commands perfectly relative to this model. Unfortunately, a complete world model is not available and robots have to plan and execute the tasks in the presence of environmental uncertainties which makes sensing an important component of new generation robots. For this reason, today's new generation robots are equipped with more and more sensing components, and consequently they are ready to actively deal with the high complexity of the real world. Complex sensorimotor tasks such as exploration require coordination between the motor system and the sensory feedback. For robot control purposes, sensory feedback should be adequately organized in terms of relevant features and the associated data representation. In this paper, we propose an overall functional picture linking sensing to action in closed-loop sensorimotor control of robots for touch (hands, fingers). Basic qualities of haptic perception in humans inspire the models and categories comprising the proposed classification. The objective is to provide a reasoned, principled perspective on the connections between different taxonomies used in the Robotics and human haptic literature. The specific case of active exploration is chosen to ground interesting use cases. Two reasons motivate this choice. First, in the literature on haptics, exploration has been treated only to a limited extent compared to grasping and manipulation. Second, exploration involves specific robot behaviors that exploit distributed and heterogeneous sensory data.

Impact of 3D printing technology on the comprehension of surgical liver anatomy

BACKGROUND AND AIMS

Surgical planning in liver resection depends on the precise understanding of the three-dimensional (3D) relation of tumors to the intrahepatic vascular trees. This study aimed to investigate the impact of 3D printing (3DP) technology on the understanding of surgical liver anatomy.

METHODS

We selected four hepatic tumors that were previously resected. For each tumor, a virtual 3D reconstruction (VIR) model was created from multi-detector computed tomography (MDCT) and was prototyped using a 3D printer. Forty-five surgical residents were evenly assigned to each group (3DP, VIR, and MDCT groups). After evaluation of the MDCT scans, VIR model, or 3DP model of each tumor, surgical residents were asked to assign hepatic tumor locations and state surgical resection proposals. The time used to specify the tumor location was recorded. The correct responses and time spent were compared between the three groups.

RESULTS

The assignment of tumor location improved steadily from MDCT, to VIR, and to 3DP, with a mean score of 34.50, 55.25, and 80.92, respectively. These scores were out of 100 points. The 3DP group had significantly higher scores compared with other groups (p < 0.001). Furthermore, 3DP significantly improved the accuracy of surgical resection proposal (p < 0.001). The mean accuracy of the surgical resection proposal for 3DP, VIR, and MDCT was 57, 25, and 25%, respectively. The 3DP group took significantly less time, compared with other groups (p < 0.005). The mean time spent on assessing the tumor location for 3DP, VIR, and MDCT groups was 93, 223, and 286 s, respectively.

CONCLUSIONS

3D printing improves the understanding of surgical liver anatomy for surgical residents. The improved comprehension of liver anatomy may facilitate laparoscopy or open liver resection.

Shape Localization and Recognition Using a Magnetorheological-Fluid Haptic Display

Smart materials such as magnetorheological fluids (MRF) offer an interesting technology for use in haptic displays as changes in the magnetic field are rapid, reversible, and controllable. These interfaces have been evaluated in a number of medical and surgical simulators where they can provide cues regarding the viscoelastic properties of tissues. The objective of the present set of experiments was first to determine whether a shape embedded in the MRF could be precisely localized and second whether 10 shapes rendered in a MRF haptic display could be accurately identified. It was also of interest to determine how the information transfer associated with this type of haptic display compares to that achieved using other haptic channels of communication. The overall performance of participants at identifying the shapes rendered in the MRF was good with a mean score of 73 percent correct and an Information Transfer (IT) of 2.2 bits. Participants could also localize a rigid object in the display accurately. These findings indicate that this technology has potential for use in training manual palpation skills and in exploring haptic shape perception in dynamic environments.

Tactile Perception of Roughness and Hardness to Discriminate Materials by Friction-Induced Vibration

The human fingertip is an exquisitely powerful bio-tactile sensor in perceiving different materials based on various highly-sensitive mechanoreceptors distributed all over the skin. The tactile perception of surface roughness and material hardness can be estimated by skin vibrations generated during a fingertip stroking of a surface instead of being maintained in a static position. Moreover, reciprocating sliding with increasing velocities and pressures are two common behaviors in humans to discriminate different materials, but the question remains as to what the correlation of the sliding velocity and normal load on the tactile perceptions of surface roughness and hardness is for material discrimination. In order to investigate this correlation, a finger-inspired crossed-I beam structure tactile tester has been designed to mimic the anthropic tactile discrimination behaviors. A novel method of characterizing the fast Fourier transform integral (FFT) slope of the vibration acceleration signal generated from fingertip rubbing on surfaces at increasing sliding velocity and normal load, respectively, are defined as k_v and k_w, and is proposed to discriminate the surface roughness and hardness of different materials. Over eight types of materials were tested, and they proved the capability and advantages of this high tactile-discriminating method. Our study may find applications in investigating humanoid robot perceptual abilities.

Touch uses frictional cues to discriminate flat materials

In a forced-choice task, we asked human participants to discriminate by touch alone glass plates from transparent polymethyl methacrylate (PMMA) plastic plates. While the surfaces were flat and did not exhibit geometric features beyond a few tens of nanometres, the materials differed by their molecular structures. They produced similar coefficients of friction and thermal effects were controlled. Most participants performed well above chance and participants with dry fingers discriminated the materials especially well. Current models of tactile surface perception appeal to surface topography and cannot explain our results. A correlation analysis between detailed measurements of the interfacial forces and discrimination performance suggested that the perceptual task depended on the transitory contact phase leading to full slip. This result demonstrates that differences in interfacial mechanics between the finger and a material can be sensed by touch and that the evanescent mechanics that take place before the onset of steady slip have perceptual value.

From CT scanning to 3-D printing technology for the preoperative planning in laparoscopic splenectomy

BACKGROUND

Three-dimensional printing technology is rapidly changing the way we produce all sort of objects, having also included medical applications. We embarked in a pilot study to assess the value of patient-specific 3-D physical manufacturing of spleno-pancreatic anatomy in helping during patient's counseling and for preoperative planning.

METHODS

Twelve patients scheduled for a laparoscopic splenectomy underwent contrast CT and subsequent post-processing to create virtual 3-D models of the target anatomy, and 3-D printing of the relative solid objects. The printing process, its cost and encountered problems were monitored and recorded. Patients were asked to rate the value of 3-D objects on a 1-5 scale in facilitating their understanding of the proposed procedure. Also 10 surgical residents were required to evaluate the perceived extra value of 3-D printing in the preoperative planning process.

RESULTS

The post-processing analysis required an average of 2; 20 h was needed to physically print each model and 4 additional hours to finalize each object. The cost for the material employed for each object was around 300 euros. Ten patients gave a score of 5, two a score of 4. Six residents gave a score of 5, four a score of 4.

CONCLUSIONS

Three-dimensional printing is helpful in understanding complex anatomy for educational purposes at all levels. Cost and working time to produce good quality objects are still considerable.

Direct coupling of haptic signals between hands

Although motor actions can profoundly affect the perceptual interpretation of sensory inputs, it is not known whether the combination of sensory and movement signals occurs only for sensory surfaces undergoing movement or whether it is a more general phenomenon. In the haptic modality, the independent movement of multiple sensory surfaces poses a challenge to the nervous system when combining the tactile and kinesthetic signals into a coherent percept. When exploring a stationary object, the tactile and kinesthetic signals come from the same hand. Here we probe the internal structure of haptic combination by directing the two signal streams to separate hands: one hand moves but receives no tactile stimulation, while the other hand feels the consequences of the first hand's movement but remains still. We find that both discrete and continuous tactile and kinesthetic signals are combined as if they came from the same hand. This combination proceeds by direct coupling or transfer of the kinesthetic signal from the moving to the feeling hand, rather than assuming the displacement of a mediating object. The combination of signals is due to perception rather than inference, because a small temporal offset between the signals significantly degrades performance. These results suggest that the brain simplifies the complex coordinate transformation task of remapping sensory inputs to take into account the movements of multiple body parts in haptic perception, and they show that the effects of action are not limited to moving sensors.

Proximal-distal, not medial-lateral, movement across an edge increases discrimination of edge sharpness

Edges are fundamental properties of our environment and the objects we interact with. There is a lack of research on the haptic perception of edges, especially the sharpness of an edge. Skinner et al. [2013 PLoS ONE, 8(9): e73283] found that haptic discriminability of sharpness was clearly superior when using a relatively unrestrained, free exploration strategy compared with a static single touch strategy. In the free exploration condition two distinct movement patterns were frequently used by participants: a proximal-distal movement of the fingerpad across the test edge and a medial-lateral movement of the fingerpad along the test edge. Here, using the same stimuli and two-alternative forced-choice method of constant stimuli as Skinner et al. (2013), we demonstrate that a proximal-distal movement results in substantially lower sharpness discrimination thresholds than a medial-lateral movement. The underlying neurophysiology and implications for the design of haptic displays are considered.

Influence of shape on the haptic size aftereffect

Recently, we showed a strong haptic size aftereffect by means of a size bisection task: after adaptation to a large sphere, subsequently grasped smaller test spheres felt even smaller, and vice versa. In the current study, we questioned whether the strength of this aftereffect depends on shape. In four experimental conditions, we determined the aftereffect after adaptation to spheres and tetrahedra and subsequent testing also with spheres and tetrahedra. The results showed a clear influence of shape: the haptic aftereffect was much stronger if adaptation and test stimuli were identical in shape than if their shapes were different. Therefore, it would be more appropriate to term such aftereffects haptic shape-size aftereffects, as size alone could not be the determining factor. This influence of shape suggests that higher cortical areas are involved in this aftereffect and that it cannot be due to adaptation of peripheral receptors. An additional finding is that the geometric property or combination of properties participants use in the haptic size bisection task varies widely over participants, although participants themselves are quite consistent.

Intrinsic horizontal connections process global tactile features in the primary somatosensory cortex: neuroanatomical evidence

To understand manual tactile functions in primates, it is essential to explore the interactions between the finger pad representations in somatosensory cortex. To this end, we used optical imaging and electrophysiological mapping to guide neuroanatomical tracer injections into distal digit tip representations of Brodmann area 3b in the squirrel monkey. Retrogradely labeled cell densities and anterogradely labeled fibers and terminal patches in somatosensory areas were plotted and quantified with respect to tangential distribution. Within area 3b, reciprocal patchy distribution of anterograde and retrograde labeling spanned the representation of the distal pad of multiple digits, indicating strong cross-digit connectivity. Inter-areal connections revealed bundles of long-range fibers projecting anteroposteriorly, connecting area 3b with clusters of labeled neurons and terminal axon arborizations in area 1. Inter-areal linkage appeared to be largely confined to the representation of the injected finger. These findings provide the neuroanatomical basis for the interaction between distal finger pad representations observed by recent electrophysiological studies. We propose that intra-areal connectivity may be heavily involved in interdigit integration such as shape discrimination, whereas long-range inter-areal connections may subserve active touch in a digit-specific manner.

On the edge: haptic discrimination of edge sharpness

The increasing ubiquity of haptic displays (e.g., smart phones and tablets) necessitates a better understanding of the perceptual capabilities of the human haptic system. Haptic displays will soon be capable of locally deforming to create simple 3D shapes. This study investigated the sensitivity of our haptic system to a fundamental component of shapes: edges. A novel set of eight high quality shape stimuli with test edges that varied in sharpness were fabricated in a 3D printer. In a two alternative, forced choice task, blindfolded participants were presented with two of these shapes side by side (one the reference, the other selected randomly from the remaining set of seven) and after actively exploring the test edge of each shape with the tip of their index finger, reported which shape had the sharper edge. We used a model selection approach to fit optimal psychometric functions to performance data, and from these obtained just noticeable differences and Weber fractions. In Experiment 1, participants performed the task with four different references. With sharpness defined as the angle at which one surface meets the horizontal plane, the four JNDs closely followed Weber's Law, giving a Weber fraction of 0.11. Comparisons to previously reported Weber fractions from other haptic manipulations (e.g. amplitude of vibration) suggests we are sufficiently sensitive to changes in edge sharpness for this to be of potential utility in the design of future haptic displays. In Experiment 2, two groups of participants performed the task with a single reference but different exploration strategies; one was limited to a single touch, the other unconstrained and free to explore as they wished. As predicted, the JND in the free exploration condition was lower than that in the single touch condition, indicating exploration strategy affects sensitivity to edge sharpness.

Haptic perception

Fueled by novel applications, interest in haptic perception is growing. This paper provides an overview of the state of the art of a number of important aspects of haptic perception. By means of touch we can not only perceive quite different material properties, such as roughness, compliance, friction, coldness and slipperiness, but we can also perceive spatial properties, such as shape, curvature, size and orientation. Moreover, the number of objects we have in our hand can be determined, either by counting or subitizing. All these aspects will be presented and discussed in this paper. Although our intuition tells us that touch provides us with veridical information about our environment, the existence of prominent haptic illusions will show otherwise. Knowledge about haptic perception is interesting from a fundamental viewpoint, but it also is of eminent importance in the technological development of haptic devices. At the end of this paper, a few recent applications will be presented. WIREs Cogn Sci 2013, 4:357-374. DOI: 10.1002/wcs.1238 CONFLICT OF INTEREST: The authors have declared no conflicts of interest for this article. For further resources related to this article, please visit the WIREs website.

Motor-sensory convergence in object localization: a comparative study in rats and humans

In order to identify basic aspects in the process of tactile perception, we trained rats and humans in similar object localization tasks and compared the strategies used by the two species. We found that rats integrated temporally related sensory inputs ('temporal inputs') from early whisk cycles with spatially related inputs ('spatial inputs') to align their whiskers with the objects; their perceptual reports appeared to be based primarily on this spatial alignment. In a similar manner, human subjects also integrated temporal and spatial inputs, but relied mainly on temporal inputs for object localization. These results suggest that during tactile object localization, an iterative motor-sensory process gradually converges on a stable percept of object location in both species.

Active touch sensing

Active sensing systems are purposive and information-seeking sensory systems. Active sensing usually entails sensor movement, but more fundamentally, it involves control of the sensor apparatus, in whatever manner best suits the task, so as to maximize information gain. In animals, active sensing is perhaps most evident in the modality of touch. In this theme issue, we look at active touch across a broad range of species from insects, terrestrial and marine mammals, through to humans. In addition to analysing natural touch, we also consider how engineering is beginning to exploit physical analogues of these biological systems so as to endow robots with rich tactile sensing capabilities. The different contributions show not only the varieties of active touch--antennae, whiskers and fingertips--but also their commonalities. They explore how active touch sensing has evolved in different animal lineages, how it serves to provide rapid and reliable cues for controlling ongoing behaviour, and even how it can disintegrate when our brains begin to fail. They demonstrate that research on active touch offers a means both to understand this essential and primary sensory modality, and to investigate how animals, including man, combine movement with sensing so as to make sense of, and act effectively in, the world.