Deep neural network model of haptic saliency

Haptic exploration usually involves stereotypical systematic movements that are adapted to the task. Here we tested whether exploration movements are also driven by physical stimulus features. We designed haptic stimuli, whose surface relief varied locally in spatial frequency, height, orientation, and anisotropy. In Experiment 1, participants subsequently explored two stimuli in order to decide whether they were same or different. We trained a variational autoencoder to predict the spatial distribution of touch duration from the surface relief of the haptic stimuli. The model successfully predicted where participants touched the stimuli. It could also predict participants' touch distribution from the stimulus' surface relief when tested with two new groups of participants, who performed a different task (Exp. 2) or explored different stimuli (Exp. 3). We further generated a large number of virtual surface reliefs (uniformly expressing a certain combination of features) and correlated the model's responses with stimulus properties to understand the model's preferences in order to infer which stimulus features were preferentially touched by participants. Our results indicate that haptic exploratory behavior is to some extent driven by the physical features of the stimuli, with e.g. edge-like structures, vertical and horizontal patterns, and rough regions being explored in more detail

Low-severity fire increases tree defense against bark beetle attacks

Induced defense is a common plant strategy in response to herbivory. Although abiotic damage, such as physical wounding, pruning, and heating, can induce plant defense, the effect of such damage by large-scale abiotic disturbances on induced defenses has not been explored and could have important consequences for plant survival facing future biotic disturbances. Historically, low-severity wildfire was a widespread, frequent abiotic disturbance in many temperate coniferous forests. Native Dendroctonus and Ips bark beetles are also a common biotic disturbance agent in these forest types and can influence tree mortality patterns after wildfire. Therefore, species living in these disturbance-prone environments with strategies to survive both frequent fire and bark beetle attack should be favored. One such example is Pinus ponderosa forests of western North America. These forests are susceptible to bark beetle attack and frequent, low-severity fire was common prior to European settlement. However, since the late 1800s, frequent, low-severity fires have greatly decreased in these forests. We hypothesized that non-lethal, low-severity, wildfire induces resin duct defense in P. ponderosa and that lack of low-severity fire relaxes resin duct defense in forests dependent on frequent, low-severity fire. We first compared axial resin duct traits between trees that either survived or died from bark beetle attacks. Next, we studied axial ducts using tree cores with crossdated chronologies in several natural P. ponderosa stands before and after an individual wildfire and, also, before and after an abrupt change in fire frequency in the 20th century. We show that trees killed by bark beetles invested less in resin ducts relative to trees that survived attack, suggesting that resin duct-related traits provide resistance against bark beetles. We then show low-severity fire induces resin duct production, and finally, that resin duct production declines when fire ceases. Our results demonstrate that low-severity fire can trigger a long-lasting induced defense that may increase tree survival from subsequent herbivory

Multifunctionality of a peri-urban landscape: exploring the diversity of residents’ perceptions and preferences

Developing successful policies for sustainable land use requires understanding the perspectives of different actors. This study explored how residents–an often under-represented and un-organized group–vary in their valuation of ecosystem services (ES) and perception of multifunctionality in a peri-urban setting. We conducted 127 interviews in the Kromme Rijn region of the Netherlands guided by an interactive, visual canvas tool (STREAMLINE). We addressed four research questions: (1) Is there variation among residents regarding preferences for ES? (2) Which competing interests do residents see in this landscape? (3) Where are hotspots of perceived multifunctionality? and (4) Can the level of perceived multifunctionality be explained by its location on the rural–urban gradient? Our findings demonstrate that while the majority of ES are important to residents of a peri-urban landscape, there is variation in relative preference towards a subset of ES (mainly provisioning services). A typology of preferences distinguishes three groups: (A)‘I want it all’–all ES (very) important; (B)‘I want most of it’–majority of ES important; and (C)‘I want some’–several ES not important at all. The majority of competing interests identified by respondents were between biodiversity and either a provisioning or cultural service. Universal hotspots of perceived multi-functionality overlapped with the area around residential areas, whereas natural (grassland) areas and water were considered multifunctional by only a small share of respondents. These perceptions and preferences do not necessarily align with current policy and management efforts, it is advised that residents’ perceptions and values are better accounted for in landscape governance

Memory influences haptic perception of softness

The memory of an object’s property (e.g. its typical colour) can affect its visual perception. We investigated whether memory of the softness of every-day objects influences their haptic perception. We produced bipartite silicone rubber stimuli: one half of the stimuli was covered with a layer of an object (sponge, wood, tennis ball, foam ball); the other half was uncovered silicone. Participants were not aware of the partition. They first used their bare finger to stroke laterally over the covering layer to recognize the well-known object and then indented the other half of the stimulus with a probe to compare its softness to that of an uncovered silicone stimulus. Across four experiments with different methods we showed that silicon stimuli covered with a layer of rather hard objects (tennis ball and wood) were perceived harder than the same silicon stimuli when being covered with a layer of rather soft objects (sponge and foam ball), indicating that haptic perception of softness is affected by memory

Effects of Stimulus Exploration Length and Time on the Integration of Information in Haptic Softness Discrimination

In haptic perception, information is often sampled serially (e.g., a stimulus is repeatedly indented to estimate its softness), requiring that sensory information is retained and integrated over time. Hence, integration of sequential information is likely affected by memory. Particularly, when two sequentially explored stimuli are compared, integration of information on the second stimulus might be determined by the fading representation of the first stimulus. We investigated how the exploration length of the first stimulus and a temporal delay affect contributions of sequentially gathered estimates of the second stimulus in haptic softness discrimination. Participants subsequently explored two silicon rubber stimuli by indenting the first stimulus one or five times and the second stimulus always three times. In an additional experiment, we introduced a 5-s delay after the first stimulus was indented five times. We show that the longer the first stimulus is explored, the more estimates of the second stimulus' softness contribute to the discrimination of the two stimuli, independent of the delay. This suggests that the exploration length of the first stimulus influences the strength of its representation, persisting at least for 5 s, and determines how much information about the second stimulus is exploited for the comparison

Unsupervised learning of haptic material properties.

When touching the surface of an object, its spatial structure translates into a vibration on the skin. The perceptual system evolved to translate this pattern into a representation that allows to distinguish between different materials. Here, we show that perceptual haptic representation of materials emerges from efficient encoding of vibratory patterns elicited by the interaction with materials. We trained a deep neural network with unsupervised learning (Autoencoder) to reconstruct vibratory patterns elicited by human haptic exploration of different materials. The learned compressed representation (i.e., latent space) allows for classification of material categories (i.e., plastic, stone, wood, fabric, leather/wool, paper, and metal). More importantly, classification performance is higher with perceptual category labels as compared to ground truth ones, and distances between categories in the latent space resemble perceptual distances, suggesting a similar coding. Crucially, the classification performance and the similarity between the perceptual and the latent space decrease with decreasing compression level. We could further show that the temporal tuning of the emergent latent dimensions is similar to properties of human tactile receptors

Integration of serial sensory information in haptic perception of softness.

Redundant estimates of an environmental property derived simultaneously from different senses or cues are typically integrated according to the maximum likelihood estimation model (MLE): Sensory estimates are weighted according to their reliabilities, maximizing the percept’s reliability. Mechanisms underlying the integration of sequentially derived estimates from one sense are less clear. Here we investigate the integration of serially sampled redundant information in softness perception. We developed a method to manipulate haptically perceived softness of silicone rubber stimuli during bare-finger exploration. We then manipulated softness estimates derived from single movement segments (indentations) in a multisegmented exploration to assess their contributions to the overall percept. Participants explored two stimuli in sequence, using 2–5 indentations, and reported which stimulus felt softer. Estimates of the first stimulus’s softness contributed to the judgments similarly, whereas for the second stimulus estimates from later compared to earlier indentations contributed less. In line with unequal weighting, the percept’s reliability increased with increasing exploration length less than was predicted by the MLE model. This pattern of results is well explained by assuming that the representation of the first stimulus fades when the second stimulus is explored, which fits with a neurophysiological model of perceptual decisions (Deco, Rolls, & Romo, 2010)

Active Haptic Exploration of Softness: Indentation Force Is Systematically Related to Prediction, Sensation and Motivation

Active finger movements play a crucial role in natural haptic perception. For the perception of different haptic properties people use different well-chosen movement schemes (Lederman and Klatzky, 1987). The haptic property of softness is stereotypically judged by repeatedly pressing one’s finger against an objects’ surface, actively indenting the object. It has been shown that people adjust the peak indentation forces of their pressing movements to the expected stimulus’ softness in order to improve perception (Kaim and Drewing, 2011). Here, we aim to clarify the mechanisms underlying such adjustments. We disentangle how people modulate executed peak indentation forces depending on predictive vs. sensory signals to softness, and investigate the influence of the participants’ motivational state on movement adjustments. In Experiment 1, participants performed a two alternative forced-choice (2AFC) softness discrimination task for stimulus pairs from one of four softness categories. We manipulated the predictability of the softness category. Either all stimuli of the same category were presented in a blocked fashion, which allowed predicting the softness category of the upcoming pair (predictive signals high), or stimuli from different categories were randomly intermixed, which made prediction impossible (predictive signals low). Sensory signals to softness category of the two stimuli in a pair are gathered during exploration. We contrasted the first indentation (sensory signals low) and last indentation (sensory signals high) in order to examine the effect of sensory signals. The results demonstrate that participants systematically apply lower forces when softer objects (as compared to harder objects) are indicated by predictive signals. Notably, sensory signals seemed to be not as relevant as predictive signals. However, in Experiment 2, we manipulated participant motivation by introducing rewards for good performance, and showed that the use of sensory information for movement adjustments can be fostered by high motivation. Overall, the present study demonstrates that exploratory movements are adjusted to the actual perceptual situation and that in the process of fine-tuning, closed- and open-loop mechanisms interact, with varying contributions depending on the observer’s motivation

Top-down modulation of shape and roughness discrimination in active touch by covert attention

Due to limitations in perceptual processing, information relevant to momentary task goals is selected from the vast amount of available sensory information by top-down mechanisms (e.g. attention) that can increase perceptual performance. We investigated how covert attention affects perception of 3D objects in active touch. In our experiment, participants simultaneously explored the shape and roughness of two objects in sequence, and were told afterwards to compare the two objects with regard to one of the two features. To direct the focus of covert attention to the different features we manipulated the expectation of a shape or roughness judgment by varying the frequency of trials for each task (20%, 50%, 80%), then we measured discrimination thresholds. We found higher discrimination thresholds for both shape and roughness perception when the task was unexpected, compared to the conditions in which the task was expected (or both tasks were expected equally). Our results suggest that active touch perception is modulated by expectations about the task. This implies that despite fundamental differences, active and passive touch are affected by feature selective covert attention in a similar way

Target Search and Inspection Strategies in Haptic Search

Haptic search is a common everyday task, usually consisting of two processes: target search and target analysis. During target search we need to know where our fingers are in space, remember the already completed path and the outline of the remaining space. During target analysis we need to understand whether the detected potential target is the desired one. Here we characterized dynamics of exploratory movements in these two processes. In our experiments participants searched for a particular configuration of symbols on a rectangular tactile display. We observed that participants preferentially moved the hand parallel to the edges of the tactile display during target search, which possibly eased orientation within the search space. After a potential target was detected by any of the fingers, there was higher probability that subsequent exploration was performed by the index or the middle finger. At the same time, these fingers dramatically slowed down. Being in contact with the potential target, the index and the middle finger moved within a smaller area than the other fingers, which rather seemed to move away to leave them space. These results suggest that the middle and the index finger are specialized for fine analysis in haptic search