Publications
In vision, only the information projected onto the central portion of our retina is perceived with high resolution. Therefore, the visual system needs to process the full visual scene with coarse resolution through peripheral vision and shift the eye in order to analyse a selected portion in detail (foveation). This process allows to reduce the complexity of visual processing by serializing detailed analysis. A haptic process analogous to foveation has been described in the behavior of the blind star-nosed mole, who detects potential prays with any of its tactile appendages but analyzes it with a specific pair, characterized by higher tactile resolution. Here we tested the hypothesis of haptic foveation behavior in humans. Nine participants searched for a particular configuration of symbols on a planar rigid tactile display. We computed the probability for each finger of touching a potential target after it was previously encountered by any of the other fingers, and the exploration speed of each finger while exploring a potential target. Independent of which finger encountered a potential target first, 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, suggesting that these specialized fingers are involved in detailed analysis. In a second experiment we tested the hypothesis that foveation is performed to gain information. Ten participants searched either for an easy target (a rough patch among smooth ones) or a difficult one (a hole in a certain corner of a patch). Overall, we replicated the results of the first experiment. Corraborating our hypothesis, specialized detailed analysis was reduced in easy search, suggesting that foveation behavior was employed less if it provided less information gain. Our results suggest that in haptic search humans employ foveation-like behavior similar as in vision. Metzger, A., Toscani, M., Valsecchi, M., & Drewing, K. (2020). Foveation-like behavior in human haptic search. Journal of Vision, 20(11), 1105-1105.
In studies investigating haptic softness perception, participants are typically instructed to explore soft objects by indenting them with their index finger. In contrast, performance with other fingers has rarely been investigated. We wondered which fingers are used in spontaneous exploration and if performance differences between fingers can explain spontaneous usage. In Experiment 1 participants discriminated the softness of two rubber stimuli with hardly any constraints on finger movements. Results indicate that humans use successive phases of different fingers and finger combinations during an exploration, preferring index, middle, and (to a lesser extent) ring finger. In Experiment 2 we compared discrimination thresholds between conditions, with participants using one of the four fingers of the dominant hand. Participants compared the softness of rubber stimuli in a two-interval forced choice discrimination task. Performance with index and middle finger was better as compared to ring and little finger, the little finger was the worst. In Experiment 3 we again compared discrimination thresholds, but participants were told to use constant peak force. Performance with the little finger was worst, whereas performance for the other fingers did not differ. We conclude that in spontaneous exploration the preference of combinations of index, middle, and partly ring finger seems to be well chosen, as indicated by improved performance with the spontaneously used fingers. Better performance seems to be based on both different motor abilities to produce force, mainly linked to using index and middle finger, and different sensory sensitivities, mainly linked to avoiding the little finger. Zoeller, A. C., & Drewing, K. (2020). A Systematic Comparison of Perceptual Performance in Softness Discrimination with Different Fingers. Attention, Perception, & Psychophysics, 82(7), 3696-3709.
People display systematic affective reactions to specific properties of touched materials. For example, granular materials such as fine sand feel pleasant, while rough materials feel unpleasant. We wondered how far such relationships between sensory material properties and affective responses can be changed by learning. Manipulations in the present experiment aimed at unlearning the previously observed negative relationship between roughness and valence and the positive one between granularity and valence. In the learning phase, participants haptically explored materials that are either very rough or very fine-grained while they simultaneously watched positive or negative stimuli, respectively, from the International Affective Picture System (IAPS). A control group did not interact with granular or rough materials during the learning phase. In the experimental phase, participants rated a representative diverse set of 28 materials according to twelve affective adjectives. We found a significantly weaker relationship between granularity and valence in the experimental group compared to the control group, whereas roughness-valence correlations did not differ between groups. That is, the valence of granular materials was unlearned (i.e., to modify the existing valence of granular materials) but not that of rough materials. These points to differences in the strength of perceptuo-affective relations, which we discuss in terms of hard-wired versus learned connections.
Haptic perception involves active exploration usually consisting of repeated stereotypical movements. The choice of such exploratory movements and their parameters are tuned to achieve high perceptual precision. Information obtained from repeated exploratory movements (e.g. repeated indentations of an object to perceive its softness) is integrated but improvement of discrimination performance is limited by memory if the two objects are explored one after the other in order to compare them. In natural haptic exploration humans tend to switch between the objects multiple times when comparing them. Using the example of softness perception here we test the hypothesis that given the same amount of information, discrimination improves if memory demands are lower. In our experiment participants explored two softness stimuli by indenting each of the stimuli four times. They were allowed to switch between the stimuli after every single indentation (7 switches), after every second indentation (3 switches) or only once after four indentations (1 switch). We found better discrimination performance with seven switches as compared to one switch, indicating that humans naturally apply an exploratory strategy which might reduce memory demands and thus leads to improved performance. Metzger, A., & Drewing, K. (2020, September). Switching Between Objects Improves Precision in Haptic Perception of Softness. In International Conference on Human Haptic Sensing and Touch Enabled Computer Applications (pp. 69-77). Springer, Cham.
When interacting haptically with objects, humans enhance their perception by using prior information to adapt their behavior. When discriminating the softness of objects, humans use higher initial peak forces when expecting harder objects or a smaller difference between the two objects, which increases differential sensitivity. Here we investigated if prior information about constraints in exploration duration yields behavioral adaptation as well. When exploring freely, humans use successive indentations to gather sufficient sensory information about softness. When constraining the number of indentations, also sensory input is limited. We hypothesize that humans compensate limited input in short explorations by using higher initial peak forces. In two experiments, participants performed a 2 Interval Forced Choice task discriminating the softness of two rubber stimuli out of one compliance category (hard, soft). Trials of different compliance categories were presented in blocks containing only trials of one category or in randomly mixed blocks (category expected vs. not expected). Exploration was limited to one vs. five indentations per stimulus (Exp. 1), or to one vs. a freely chosen number of indentations (Exp. 2). Initial peak forces were higher when indenting stimuli only once. We did not find a difference in initial peak forces when expecting hard vs. soft stimuli. We conclude that humans trade off different ways to gather sufficient sensory information for perceptual tasks, integrating prior information to enhance performance. Zoeller, A. C., & Drewing, K. (2020, September). Systematic Adaptation of Exploration Force to Exploration Duration in Softness Discrimination. In International Conference on Human Haptic Sensing and Touch Enabled Computer Applications (pp. 105-112). Springer, Cham.
When people judge the weight of two objects of equal mass but different size, they perceive the smaller one as being heavier. Up to date, there is no consensus about the mechanisms which give rise to this size-weight illusion. We recently suggested a model that describes heaviness perception as a weighted average of two sensory heaviness estimates with correlated noise: one estimate derived from mass, the other one derived from density. The density estimate is first derived from mass and size, but at the final perceptual level, perceived heaviness is biased by an object’s density, not by its size. Here, we tested the models’ prediction that weight discrimination of equal-size objects is better in lifting conditions which are prone to the size-weight illusion as compared to conditions lacking (the essentially uninformative) size information. This is predicted because in these objects density covaries with mass, and according to the model density serves as an additional sensory cue. Participants performed a two-interval forced-choice weight discrimination task. We manipulated the quality of either haptic (Experiment 1) or visual (Experiment 2) size information and measured just-noticeable differences (JNDs). Both for the haptic and the visual illusion, JNDs were lower in lifting conditions in which size information was available. Thus, when heaviness perception can be influenced by an object’s density, it is more reliable. This discrimination benefit under conditions that provide the additional information that objects are of equal size is further support for the role of density and the integration of sensory estimates in the size-weight illusion. Wolf, C., & Drewing, K. (2020). The size-weight illusion comes along with improved weight discrimination. Plos one, 15(7), e0236440.