Temporal judgments vary across sensory modalities due to differences in information processing. Auditory intervals, for instance, are typically judged to be longer than visual intervals of the same duration. However, comparisons between tactile and visual duration judgments are scarce, particularly in VR where unique characteristics of the environment can affect the temporal judgements. Here, we compared visual and tactile duration judgments in a virtual reality (VR) environment. Participants performed a cross-modal comparison task, judging whether tactile stimuli delivered to their hands were longer than visual stimuli presented on their matched hand representations in VR. We found that tactile stimuli were consistently judged to be longer than visual stimuli of the same duration. Furthermore, the behavioral data conformed well to a model from the Scalar Expectancy Theory (SET) of timing- with higher variability in visual as compared to tactile judgments. Our findings suggest that duration judgements in VR may depend on factors such as saliency contrast and unique conditions of VR. These results can also help in the design of VR applications.

Perception through touch relies on active exploration strategies that adapt to perceptual goals and object properties. We investigated how information gathering through exploratory procedures (EPs) is organised in haptic shape perception, in particular with respect to the selection of EPs and the prioritisation of shape features - and how these processes are influenced by material properties. In two experiments, participants used a single finger to explore rigid or deformable shapes and judged shape similarity. In Experiment 1, analysis of EPs showed that while contour-following was the dominant strategy for acquiring shape information, participants flexibly supplemented it with additional EPs such as tapping and scanning when exploring deformable shapes. In Experiment 2, trajectory and force data showed a mostly consistent prioritisation of concave regions across exploration parameters (dwell time, velocity, distance, force), regardless of material. Force was modulated by material properties, with participants applying more force to rigid shapes and less to deformable ones. Analysis of the temporal sequence of movements further showed comparable contour following patterns across materials, but slightly more fragmented movements for deformable shape and smoother, more complete contour tracing for rigid ones, and some shape-specific effects. Together, the findings demonstrate that haptic shape perception is both adaptive and structured: exploratory strategies adjust to material contexts, yet perception and exploration remain anchored by stable geometrical cues, particular concavities, supporting reliable shape perception across contexts.

In vision, subliminal stimuli have been shown to effectively influence human behavior. While sub-threshold vibrotactile stimulation has been studied in stochastic resonance research [1], calibrated and frequency-specific subliminal thresholds remain uncharacterized. There are fundamental challenges in determining effective subliminal primes. Here, we aim to establish a procedure for rendering tactile stimuli subthreshold for each individual. We adapted the logic of STEP calibration method [2], which has been rigorously investigated in vision, to estimate individual upper subliminal thresholds. In the present study, we use unilateral vibrotactile stimuli (40 Hz and 240 Hz) as subliminal primes that are bilaterally masked. Participants judge the location at which the prime appears, and prime intensity is adaptively adjusted to keep it just subliminal. Across frequencies, average prime thresholds ranged from 0.005 to 0.080 RMS and are highly correlated within individuals across frequencies and calibration sessions over time. In post-calibration tests, stimuli presented at threshold are identified at near-chance levels, but the estimated thresholds decrease over time with practice. We also found that, in the tactile modality, the calibration method tends to slightly overestimate the stimulus intensity required to remain subliminal, particularly in light of practice effects. Our findings present guidelines for how to design, calibrate, and validate subliminal tactile stimuli. At the same time, these procedures may also inform applications that build on potential mechanisms of subliminal processing.

Individuals with more elastic, more hydrated or smaller fingers usually show better performance in several passive touch tasks. In active touch, people use different exploratory procedures when evaluating object properties, and tune their exploratory parameters. For example, they indent stimuli to assess softness and optimize their peak forces to get relevant information. In this study, we aim to understand whether finger pad size, elasticity and hydration affect individuals' force-tuning and discrimination performance in active softness perception. Participants performed two softness tasks in two different sessions. In one session, hyaluronic acid was applied to their finger pads to soften it, in the other they received no treatment. We assessed individual elasticity and hydration values with cutometer and corneometer in each session, and measured finger pad size in three dimension by caliper. In each task, two pairs of stimuli were presented to the participants (Young's Modulus: 41.5 vs. 45.0; 28.7 vs. 31.3 kPa) who chose the softer stimulus. In the restricted task, they could apply force only up to 2 Newton, whereas there was no force limit in the unconstrained task. We found that participants with smaller finger pad size exerted less force in the restricted task and participants with more hydrated and elastic fingers exerted less force in the unconstrained task. The force-tuning disappeared in the unconstrained task when treatment was applied. These results indicate that people employ strategies according to their finger parameters and to the availability of cues whereas adaptation to treatment is likely to need longer practice.

Humans use distinct exploratory procedures (EPs) in active touch, which are typically specialized for materials with particular properties: for example, pressing for deformable objects such as cushions, or stroking to test a fabric's smoothness . Further, humans can use abstract visual priors for fine-tuning of exploratory movement parameters such as exploration direction. We here test the usage of visual priors in the planning of material-specific EPs, using real-life materials and a naturalistic visual virtual reality environment. We show that humans are better at selecting specialized EPs at initial touch when they have access to valid prior visual information on the material: They used specialized EP earlier, with higher probability, and explored materials for a shorter time. We conclude that visual prior information increases the efficiency of haptic explorations by anticipatory planning of appropriate movement schemes.

The extent to which spatial tactile properties share neural pathways remains unclear, yet it is the key to understanding how the brain constructs coherent object representations from distributed spatial inputs. One basic spatial property is the perceived tactile distance between two simultaneous touches on the skin. It exhibits adaptation aftereffects: when body areas are repeatedly touched at two points, subsequently presented smaller distances are perceived as smaller than on unadapted areas. We investigated whether tactile distance adaptation influences the perception of other spatial properties, macro-scale roughness and curvature, indicating shared neural mechanisms. In experiment 1, adapting the skin to a fixed tactile distance reduced perceived roughness of subsequent gratings with smaller groove widths, as assessed through passive touch at the finger pad. This aftereffect likely originates from early cortical processing, as it is orientation-specific and independent of peripheral receptor desensitization. Experiment 2 demonstrated that curvature perception increases after adaptation to a two-point distance larger than the curve, suggesting overlap in processing pathways. Experiment 3 further supported early processing involvement, as the distance-to-roughness aftereffect did not transfer to adjacent skin regions of the same finger. Experiment 4 revealed bidirectional aftereffects: roughness adaptation also influenced distance perception. However, within-property aftereffects were stronger than cross-property effects. By revealing the existence of cross-property adaptation aftereffects with low-level characteristics, our findings provide evidence that tactile distance, roughness, and curvature share early somatosensory processing. This suggests that spatially defined properties undergo a common initial processing stage, sharing initial steps rather than existing in a hierarchical processing arrangement.

Time perception is a fundamental aspect of human life, and is influenced and regulated by cognitive and sensory processes. For instance, spatial attention is found to modulate temporal judgments when resources are allocated to a specific stimulus location in vision and audition. However, it is unclear to what extent the attentional effects observed in vision and audition can be generalized to the tactile modality. Here, we study the effects of attentional cues on the time perception of tactile stimuli presented on the human torso. Across four experiments, we examined (1) the impact of visual versus tactile spatial cues, (2) the modulation of time perception by dynamic versus static tactile cues, (3) the role of spatial congruency between cue and target locations (front vs. back of the torso), and (4) the influence of cue-target intervals. Participants performed temporal bisection tasks, judging whether the vibrations following the cues were closer to short or long anchor durations. Tactile cues expanded the perceived duration of subsequent stimuli, with dynamic cues having a greater effect than static ones. While no congruency effects were observed for left and right torso locations, front-back congruency enhanced time expansion. The attentional effect peaked at a 100-ms cue-target interval. We conclude that the time-expanding effects of spatial attention extend to tactile stimuli on the human torso given that time expansion follows principles known from spatial attention.

Previous studies have successfully elicited a wide range of emotional responses by stimulating the hand region. The purpose of the current study was to test whether tactile stimuli applied to the torso could elicit similar emotional responses. To this end, we created 45 custom vibrotactile patterns that were presented through a vibrotactile vest to the front, back, and both sides of the torso. The patterns covered a wide range of physical variables such as amplitude, trajectory, and continuity. In an exploratory experiment, participants rated the arousal and valence of these patterns. Emotional responses differed between the patterns, and detailed analyses suggested that vibration amplitude and intensity where these vibrations were applied influenced both valence and arousal judgments. In a follow-up experiment, we systematically varied the amplitude and location of the vibrations. Our results showed that lower amplitudes were less arousing and more pleasant than higher amplitudes. Similarly, vibrations to the back torso were less arousing and more pleasant than those applied to the front or both sides of the torso, which can be explained by the lower sensitivity on the back. Taken together, we suggest that perceived intensity partially explains the relationship between the emotionality of vibration patterns on the torso.

Friction was studied for the human finger pad during the spreading of viscous liquid samples in circular motion on a solid substrate. The samples included both Newtonian and shear-thinning liquids with a range of viscosity between 0.83 mPa · s and 150 Pa · s. During active touch, participants applied varying normal forces and sliding speeds depending on the sample and individual behavior. Friction coefficients vary greatly between participants, but fall on one Stribeck curve when shear-thinning effects were accounted for full-film lubrication. A comparison with the measured height variations during spreading demonstrates that the logarithm of the Hersey number is an instantaneous indicator of the film thickness in the full-film lubrication regime. Comparison of the measured friction coefficients with reported values of the perceived slipperiness for the same samples shows a close correspondence along the Stribeck curve.

Happy and sad moods promote global and local visual processing, respectively. However, it is unclear whether mood also affects the processing level in haptics. Here, we used classical music to induce happy and sad moods in blindfolded participants before they scanned printed, flat 2D embossed configurations with their fingers. We also included a neutral group that did not listen to any music. Global shapes were triangles, circles, or squares (33 mm) composed of smaller local relief shapes (3 mm): either triangles, circles, or squares. Participants explored a probe stimulus with identical local and global shapes, and two comparison stimuli, matching the probe in local or global shape. They reported which comparison stimulus appeared more similar to the probe. In the “sad” group, participants chose the locally matching comparison more frequently than in the “happy” and “neutral” groups, suggesting that unpleasant mood can influence spatial preferences in haptic shape matching. Overall, participants tended to prefer global matches, indicating that under these specific conditions, global-level information may be relatively more prominent in touch.

The human role in human-swarm interaction (HSI) shifts from controller to supervisor, as robots become more autonomous and require efficient search strategies in complex visual environments. Previous research has shown that spatially uninformative brief cues enhance search performance in laboratory environments (namely, "pip-and-pop" effect). Here we examined if these effects can be effectively applicable in HSI. To this end, we conducted two experiments using small mobile robots (Thymio II) to investigate the impact of auditory, tactile, and audiotactile cues on visual search performance and timing judgments. In the first experiment, 20 participants identified a stopped robot among moving robots. The results showed that all cue conditions significantly reduced reaction times (RTs) compared to the no-cue condition, suggesting that brief spatially non-informative signals improve search performance by increasing sensory information accumulation speed. The second experiment involved 12 participants judging the duration of a robot's stop after a tactile cue was presented or not. The findings indicate that tactile cues improve temporal sensitivity without affecting subjective duration judgments. These results highlight the potential of uni-and multisensory cues to enhance HSI performance by facilitating quicker and more accurate human responses, particularly in dynamic environments. The study extends the "pip-and-pop" effect to real-world scenarios, offering insights for designing HSI systems that allow users to interact with robotic swarms more naturally and efficiently.

Research has shown that affective visual and auditory events (e.g., a crying baby) are perceived as lasting longer compared to neutral ones. However, the impact of affective haptic experiences on time perception has hardly been studied. This study investigates the influence of interacting with affective materials on time perception. We selected three materials that are known to evoke pleasant (velvet), unpleasant (sandpaper), and neutral (paper) affective responses. Participants completed a temporal bisection task to assess how each material influenced their perception of time. The task involved presenting the materials in time intervals from 1000 to 2200 ms in 200 ms increments. In each trial, a participant stroked one of the materials, with the duration being limited by two vibrotactile feedback, and judged whether the duration felt closer to a previously learned short or long interval. Expectedly, velvet yielded lower bisection points than paper. Contrary to expectations, bisection points for sandpaper - despite being an unpleasant material - did not significantly differ from that for the control material, paper. These findings suggest that while pleasant haptic material experiences can extend perceived time, unpleasant materials may not have an effect. This effect is partially consistent with the observed time lengthening during affective auditory and visual events.

There is a growing scientific interest in material unpleasantness, yet the role of distinct physical parameters in perceptual and affective haptic experiences with liquids remains to be fully understood. To address this, we investigated how perceptual qualities of liquids relate to measurable physical properties and unpleasantness during active touch. We prepared 15 custom liquid samples using everyday materials. Rheological measurements showed that samples varied between physical viscosity [Formula: see text] and [Formula: see text]. Participants explored each sample using circular rubbing motions with their index fingers. A camera system tracked finger movements, and a force sensor revealed applied normal forces, pull-off force (PoF) and the coefficient of friction (CoF). We compared these physical properties with the perceptual dimensions from our earlier work: perceived viscosity and slipperiness. Perceived viscosity correlated strongly with both physical viscosity and PoF, but not with CoF. Conversely, perceived slipperiness was associated with CoF, but not PoF or physical viscosity, demonstrating distinct links between physics and perception of liquids. Interestingly, PoF but not CoF was significantly linked to unpleasantness, suggesting that PoF but not CoF is crucial for liquid unpleasantness. These findings advance our understanding of how distinct physical properties relate to perceptual and affective experiences of liquids.

Shape perception through touch allows object recognition without visual input. While humans can reliably perceive and discriminate geometric features like curvature, less is known about how perceivers explore and integrate these features to form coherent shape representations when perceiving complex shapes through touch. Here, we investigated whether individuals prioritise certain shape features when gathering information about complex shapes. We compared touch patterns for convex, concave and flat regions, when exploring shapes varying in material properties (rigid-plastic vs. deformable-silicone) and shape features (convex, concave and flat). Participants explored one reference and two test objects using a finger and selected the test object that was most similar in shape to the reference. We assessed dwell time, distance travelled, velocity, and force of touch. Across the two materials, exploration parameters were generally consistent, except that participants applied less force to deformable shapes. For both materials, participants showed a clear preference for exploring concave over convex or flat regions indicated by all parameters, suggesting that these regions were considered as more informative and formed an important basis of their shape judgments.

One of the most important object properties that humans and robots perceive through touch is hardness. This paper investigates information-theoretic active sampling strategies for sample-efficient hardness classification with vision-based tactile sensors. We evaluate three probabilistic classifier models and two model-uncertainty-based sampling strategies on a robotic setup as well as on a previously published dataset of samples collected by human testers. Our findings indicate that the active sampling approaches, driven by uncertainty metrics, surpass a random sampling baseline in terms of accuracy and stability. Additionally, while in our human study, the participants achieve an average accuracy of 48.00 the same set of objects, demonstrating the effectiveness of vision-based tactile sensors for object hardness classification.

Image motion contributes to the perception of visual material properties, and motion signals are generated during active exploration. However, little is known about how specific perceptual tasks influence the actions that generate these cues. In an experiment using virtual reality and real-time hand tracking, we investigated how the demands of perceptual tasks (e.g., judging gloss or lightness) shape exploratory behaviors. Participants either observed or actively explored objects varying in gloss and lightness while performing a matching task. We analyzed how their exploration patterns varied based on the tasks. Using the same stimuli in both tasks, we found that participants explored objects more extensively when judging gloss than when judging lightness. These findings suggest a strategic prioritization of relevant cues for gloss judgments, with participants using larger movements and object rotation to enhance viewing perspectives and highlight detection. Our findings show that exploration behaviors are task dependent, with actions adapted to the demands of the perceptual task at hand.

The perceived time can shrink or expand for emotional stimuli. Converging evidence suggests that emotional time distortions are rooted in the emotional states of the timing agents because emotional stimuli can influence the timing of simultaneous neutral events. As emotional states are transitory, we investigated if time modulating emotional states also influence timing of subsequent neutral events. In each trial, we induced different valence and arousal levels by using affective vibrotactile patterns before participants judged the duration of neutral auditory tones. Compared to neutral patterns, affective patterns modulated participants' time perception of the subsequent tones. We observed an interaction between arousal and valence: Pleasant-Low arousal patterns expanded the timing of subsequent neutral events more than Unpleasant-Low arousal patterns while Pleasant and Unpleasant-High arousal led to a similar temporal expansion. Our results indicate time modulating effects of emotional stimuli are due to changed emotional states and influence time perception likely until the underlying state decays.

Friction between fingertip and surface is a key contribution to tactile perception during active exploration of materials. We explore the role of skin factors such as stratum corneum thickness and hydration, deformability, elasticity, or density of sweat glands and of Meissner corpuscles in friction and tactile perception. The skin parameters were determined non-invasively for the glabrous skin at the index finger pad of 60 participants. Sets of randomly rough plastic surfaces and of micro-structured fibrillar rubber surfaces were explored as model materials with well-defined parameterized textures. Friction varies greatly between participants, and this variation can be explained to 70% by skin factors for the randomly rough plastic surfaces. The predictability of friction by skin factors is much lower for micro-structured rubber surfaces with bendable fibrils, where 50% of variance is explained for the stiffest fibrils but only 20% for the most bendable fibrils. The participants’ age is the key predictor for their tactile sensitivity to perceive the fibrils, where age is negatively correlated to the density of Meissner corpuscles. The results suggest that stratum corneum hydration, skin deformability, and age are important factors for friction and perception in active tactile exploration of materials.

Interindividual differences in biophysical properties such as skin hydration and elasticity have been demonstrated to play a critical role in influencing various aspects of tactile perception. Here, we assess their role for interindividual variation of basic tactile abilities and the tactile distance adaptation aftereffect in a young adult sample. Tactile abilities were defined by tactile sensitivity in a monofilament detection task and spatial acuity in a grating orientation task. In the distance aftereffect, when a body area is repeatedly touched at two points separated by a given distance, subsequently presented smaller distances are perceived as smaller than on unadapted areas. Aftereffect magnitude describes the perceptual shift in a distance discrimination task following adaptation. We examine whether differences in skin hydration and elasticity at the finger pad are related to tactile abilities which in turn affect the magnitude of distance aftereffects. Results revealed that higher hydration and elasticity were related to increased tactile sensitivity and spatial acuity, but magnitude of distance aftereffects was independent from both skin properties and tactile abilities. While these results reemphasize the importance of healthy skin for tactile perception, they suggest individual differences in the magnitude of the distance aftereffect to be independent from peripheral skin properties.