When sliding a finger across a bumpy surface, the finger follows the surface geometry (position signal). At the same time the finger is exposed to forces related to the slope of the surface (force signal)[1]. For haptic shape perception the brain uses both signals integrating them by weighted averaging [2]. This is consistent with the Maximum-Likelihood-Estimate (MLE) model on signal integration, previously only applied to passive perception. The model further predicts that signal weight is proportional to signal reliability. Here, we tested this prediction for the integration of force and position signals to perceived curvature by manipulating material properties of the curve. Low as compared to high compliance decreased the reliability and so the weight of the sensorily transduced position signal. High as compared to low friction decreased the reliability and so the weight of the transduced force signal. These results demonstrat that the MLE model extends to situations involving active touch. Drewing, K., Ernst, M. O., & Wiecki, T. (2005, March). Material properties determine how we integrate shape signals in active touch. In 1st Joint Worldhaptic Conference and Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems (WorldHaptics 2005) (pp. 1-6).
We tested whether auditory sequences of beeps can modulate the tactile perception of sequences of taps (two to four taps per sequence) delivered to the index fingertip. In the first experiment, the auditory and tactile sequences were presented simultaneously. The number of beeps delivered in the auditory sequence were either the same as, less than, or more than the number of taps of the simultaneously presented tactile sequence. Though task-irrelevant (subjects were instructed to focus on the tactile stimuli), the auditory stimuli systematically modulated subjects’ tactile perception; in other words subjects’ responses depended significantly on the number of delivered beeps. Such modulation only occurred when the auditory and tactile stimuli were similar enough. In the second experiment, we tested whether the automatic auditory-tactile integration depends on simultaneity or whether a bias can be evoked when the auditory and tactile sequence are presented in temporal asynchrony. Audition significantly modulated tactile perception when the stimuli were presented simultaneously but this effect gradually disappeared when a temporal asynchrony was introduced between auditory and tactile stimuli. These results show that when provided with auditory and tactile sensory signals that are likely to be generated by the same stimulus, the central nervous system (CNS) tends to automatically integrate these signals. Bresciani, J. P., Ernst, M. O., Drewing, K., Bouyer, G., Maury, V., & Kheddar, A. (2005). Feeling what you hear: auditory signals can modulate tactile tap perception. Experimental brain research, 162(2), 172-180.
Based on existing knowledge on human tactile movement perception, we constructed a prototype of a novel tactile multipin display that controls lateral pin displacement and, thus produces shear force. Two experiments focus on the question of whether the prototype display generates tactile stimulation that is appropriate for the sensitivity of human tactile perception. In particular, Experiment I studied human resolution for distinguishing between different directions of pin displacement and Experiment II explored the perceptual integration of information resulting from the displacement of multiple pins. Both experiments demonstrated that humans can discriminate between directions of the displacements, and also that the technically realized resolution of the display exceeds the perceptual resolution (>14°). Experiment II demonstrated that the human brain does not process stimulation from the different pins of the display independent of one another at least concerning direction. The acquired psychophysical knowledge based on this new technology will in return be used to improve the design of the display Drewing, K., Fritschi, M., Zopf, R., Ernst, M. O., & Buss, M. (2005). First evaluation of a novel tactile display exerting shear force via lateral displacement. ACM Transactions on Applied Perception (TAP), 2(2), 118-131.
The successful execution of movements not only requires directing the movement towards the selected goal, but also detecting and compensating for perturbations interfering with the goal of the movement. Here we asked if participants are able to detect external force perturbations, how the executed movement is affected by the perturbation and, how the perturbation interferes with the goal of the task. Participants were instructed to rapidly hit a visual target, which was presented within a three-dimensional visuo-haptic virtual environment. Late responses and failures to hit the target were penalized. Participants were presented with a force pulse, which was applied to their right finger tip during the initial phase of the pointing movement. Force perturbations were applied orthogonally to the movement direction. We determined detection thresholds for perturbations from six different directions (up, down, upper right/left; lower right/left) using a two-interval forced choice paradigm. 5 participants completed the experiment. Surprisingly, detection thresholds for the applied perturbations (threshold about .10 N) were just slightly higher than tactile-kinesthetic detection in a single-task context (about .05N, Lederman & Klatzky, 1999). Detection performance did not depend on the direction of the perturbation, but was better for short perturbations (30 ms presentation time) compared to longer perturbations (50 ms presentation time). Shorter perturbations differed from longer perturbations by a steeper increase in force amplitude (10% of the duration). Locally, perturbations (> about 0.07 N) affected the movement kinematics significantly as compared to trajectories without perturbation. However, the distribution of movement end points at the location of the visual target did not correlate with the direction of the perturbation. These results are a first hint that the brain is able to detect force perturbations during visually guided pointing movements without extra costs. Drewing, K., & Trommershaeuser, J. (2005). Detection and costs of force perturbations during visually-guided pointing movements. Journal of Vision, 5(8), 625-625.
Most models of object recognition and mental rotation are based on the matching of an object’s 2-D view with representations of the object stored in memory. They propose that a time-consuming normalization process compensates for any difference in viewpoint between the 2-D percept and the stored representation. Our experiment shows that such normalization is less time consuming when it has to compensate for disorientations around the vertical than around the horizontal axis of rotation. By decoupling the different possible reference frames, we demonstrate that this anisotropy of the normalization process is defined not with respect to the retinal frame of reference, but, rather, according to the gravitational or the visuocontextual frame of reference. Our results suggest that the visual system may call upon both the gravitational vertical and the visuocontext to serve as the frame of reference with respect to which 3-D objects are gauged in internal object transformations. Waszak, F., Drewing, K., & Mausfeld, R. (2005). Viewer-external frames of reference in the mental transformation of 3-D objects. Perception & psychophysics, 67(7), 1269-1279.
