Publications
Past sensory experience can influence present perception. We studied the effect of adaptation in haptic softness perception. Participants compared two silicon rubber stimuli, a reference and a comparison stimulus, by indenting them simultaneously with the index fingers of their two hands and decided which one felt softer. In adaptation conditions the index finger that explored the reference stimulus had previously been adapted to another rubber stimulus. The adaptation stimulus was indented 5 times with a force of >15 N, thus the two index fingers had a different sensory past. In baseline conditions there was no previous adaptation. We measured the Points of Subjective Equality (PSEs) of one reference stimulus to a set of comparison stimuli. We used four different adaptation stimuli, one was harder, two were softer and one had approximately the same compliance as compared to the reference stimulus. PSEs shifted as a function of the compliance of the adaptation stimulus: the reference was perceived to be softer when the finger had been adapted to a harder stimulus and it was perceived to be harder after adaptation to a softer stimulus. We conclude that recent sensory experience causes a shift of haptically perceived softness away from the softness of the adaptation stimulus. The finding that perceived softness is susceptible to adaptation suggests that there might be neural channels tuned to different softness values and softness is an independent primary perceptual quality. Metzger, A., & Drewing, K. (2016, July). Haptic aftereffect of softness. In International Conference on Human Haptic Sensing and Touch Enabled Computer Applications (pp. 23-32). Springer, Cham.
In haptic perception sensory signals depend on how we actively move our hands. For textures with periodically repeating grooves, movement direction can determine temporal cues to spatial frequency. Moving in line with texture orientation does not generate temporal cues. In contrast, moving orthogonally to texture orientation maximizes the temporal frequency of stimulation, and thus optimizes temporal cues. Participants performed a spatial frequency discrimination task between stimuli of two types. The first type showed the described relationship between movement direction and temporal cues, the second stimulus type did not. We expected that when temporal cues can be optimized by moving in a certain direction, movements will be adjusted to this direction. However, movement adjustments were assumed to be based on sensory information, which accumulates over the exploration process. We analyzed 3 individual segments of the exploration process. As expected, participants only adjusted movement directions in the final exploration segment and only for the stimulus type, in which movement direction influenced temporal cues. We conclude that sensory signals on the texture orientation are used online during exploration in order to adjust subsequent movements. Once sufficient sensory evidence on the texture orientation was accumulated, movements were directed to optimize temporal cues. Lezkan, A., & Drewing, K. (2016, July). Going against the grain–Texture orientation affects direction of exploratory movement. In International Conference on Human Haptic Sensing and Touch Enabled Computer Applications (pp. 430-440). Springer, Cham.
In the flash-lag illusion, a brief visual flash and a moving object presented at the same location appear to be offset with the flash trailing the moving object. A considerable amount of studies investigated the visual flash-lag effect, and flash-lag-like effects have also been observed in audition, and cross-modally between vision and audition. In the present study, we investigate whether a similar effect can also be observed when using only haptic stimuli. A fast vibration (or buzz, lasting less than 20 ms) was applied to the moving finger of the observers and employed as a “haptic flash.” Participants performed a two-alternative forced-choice (2AFC) task where they had to judge whether the moving finger was located to the right or to the left of the stationary finger at the time of the buzz. We used two different movement velocities (Slow and Fast conditions). We found that the moving finger was systematically misperceived to be ahead of the stationary finger when the two were physically aligned. This result can be interpreted as a purely haptic analogue of the flash-lag effect, which we refer to as “buzz-lag effect.” The buzz-lag effect can be well accounted for by the temporal-sampling explanation of flash-lag-like effects. Cellini, C., Scocchia, L., & Drewing, K. (2016). The buzz-lag effect. Experimental brain research, 234(10), 2849-2857.
Roughness is probably the most salient dimension pertaining to the perception of textures by touch and has been widely investigated. There is a controversy on how roughness relates to the texture’s spatial period and which factors influence this relation. Here, roughness during bare finger exploration of coarse textures is studied for different types of textures with elements of low height (0.3 mm). Participants were presented with square-wave gratings that were defined along one dimension and sine-wave gratings that were defined along one or two dimensions. Textures of each type varied in their spatial half period between 0.25 and 5.17 mm. Participants explored the textures by a lateral movement or a stationary finger contact. In all conditions judged roughness increased with spatial period up to a peak roughness and then decreased again. The exact function depended on the texture type, but hardly on exploration mode. We conclude that roughness is an inverted U-shaped function of texture period, if the textures are of low amplitude. The effects are explained by the interplay of two components contributing to the spatial code to roughness: variability in skin deformation due to the finger’s intrusion into the texture, which increases with the textures’ period up to a maximum (when the skin contacts the texture’s ground), and variability associated with the spatial frequency of the deformation, which decreases with spatial period. Drewing, K. (2016, July). Low-amplitude textures explored with the bare finger: roughness judgments follow an inverted U-shaped function of texture period modified by texture type. In International Conference on Human Haptic Sensing and Touch Enabled Computer Applications (pp. 206-217). Springer, Cham.
The Gestalt theory of perception offered principles by which distributed visual sensations are combined into a structured experience (“Gestalt”). We demonstrate conditions whereby haptic sensations at two fingertips are integrated in the perception of a single object. When virtual bumps were presented simultaneously to the right hand's thumb and index finger during lateral arm movements, participants reported perceiving a single bump. A discrimination task measured the bump's perceived location and perceptual reliability (assessed by differential thresholds) for four finger configurations, which varied in their adherence to the Gestalt principles of proximity (small versus large finger separation) and synchrony (virtual spring to link movements of the two fingers versus no spring). According to models of integration, reliability should increase with the degree to which multi-finger cues integrate into a unified percept. Differential thresholds were smaller in the virtual-spring condition (synchrony) than when fingers were unlinked. Additionally, in the condition with reduced synchrony, greater proximity led to lower differential thresholds. Thus, with greater adherence to Gestalt principles, thresholds approached values predicted for optimal integration. We conclude that the Gestalt principles of synchrony and proximity apply to haptic perception of surface properties and that these principles can interact to promote multi-finger integration. Lezkan, A., Manuel, S. G., Colgate, J. E., Klatzky, R. L., Peshkin, M. A., & Drewing, K. (2016). Multiple Fingers–One Gestalt. IEEE transactions on haptics, 9(2), 255-266.