Haptic Judgments Research Articles

Neural representations of haptic object size in the human brain revealed by multivoxel fMRI patterns.

The brain must interpret sensory input from diverse receptor systems to estimate object properties. Much has been learned about the brain mechanisms behind these processes in vision, but our understanding of haptic perception remains less clear. Here we examined haptic judgments of object size, which require integrating multiple cutaneous and proprioceptive afferent signals, as a model problem. To identify candidate human brain regions that support this process, participants (n = 16) in an event-related functional MRI experiment grasped objects to categorize them as one of four sizes. Object sizes were calibrated psychophysically to be equally distinct for each participant. We applied representational similarity logic to whole brain, multivoxel searchlight analyses to identify brain regions that exhibit size-relevant voxelwise activity patterns. Of particular interest was to identify regions for which more similar sizes produce more similar patterns of activity, which constitutes evidence of a metric size code. Regions of the intraparietal sulcus and the lateral prefrontal cortex met this criterion, both within hands and across hands. We suggest that these regions compute representations of haptic size that abstract over the specific peripheral afferent signals generated in a grasp. Results of a matched visual size task, performed by the same participants and analyzed in the same fashion, identified similar regions, indicating that these representations may be partly modality general. We consider these results with respect to perspectives on magnitude estimation in general and to computational views on perceptual signal integration.NEW & NOTEWORTHY Our understanding of the neural basis of haptics (perceiving the world through touch) remains incomplete. We used functional MRI to study human haptic judgments of object size, which require integrating multiple afferent signals. Multivoxel pattern analyses identified intraparietal and prefrontal regions that encode size haptically in a metric and hand-invariant fashion. Effector-independent haptic size estimates are useful on their own and in combination with other sensory estimates for a variety of perceptual and motor tasks.

Accuracy of Intercepting Moving Tactile Targets.

When intercepting a moving target, we typically rely on vision to determine where the target is and where it will soon be. The accuracy of visually guided interception can be represented by a model that combines the perceived position and velocity of the target to estimate when and where to hit it and guides the finger accordingly with a short delay. We might expect the accuracy of interception to similarly depend on haptic judgments of position and velocity. To test this, we conducted separate experiments to measure the precision and any biases in tactile perception of position and velocity and used our findings to predict the precision and biases that would be present in an interception task if it were performed according to the principle described earlier. We then performed a tactile interception task to test our predictions. We found that interception of tactile targets is guided by similar principles as interception of visual targets.

Final Sampling Bias in Haptic Judgments: How Final Touch Affects Decision-Making.

When people make a choice between multiple items, they usually evaluate each item one after the other repeatedly. The effect of the order and number of evaluating items on one's choices is essential to understanding the decision-making process. Previous studies have shown that when people choose a favorable item from two items, they tend to choose the item that they evaluated last. This tendency has been observed regardless of sensory modalities. This study investigated the origin of this bias by using three experiments involving two-alternative forced-choice tasks using handkerchiefs. First, the bias appeared in a smoothness discrimination task, which indicates that the bias was not based on judgments of preference. Second, the handkerchief that was touched more often tended to be chosen more frequently in the preference task, but not in the smoothness discrimination task, indicating that a mere exposure effect enhanced the bias. Third, in the condition where the number of touches did not differ between handkerchiefs, the bias appeared when people touched a handkerchief they wanted to touch last, but not when people touched the handkerchief that was predetermined. This finding suggests a direct coupling between final voluntary touching and judgment.

Self-trained perception need not be veridical: striking can exaggerate judgment by wielding and can transfer exaggeration to new stimuli.

Previous literature on self-training dynamic touch suggested that haptic judgments of length following wielding might benefit from new information through participants' own striking actions with the same stimuli. However, the conclusion that this self-training tended towards a veridical outcome of zero discrepancy between actual length and judged length was premature. In this replication, we allowed adult participants (n = 15) to strike on each trial and changed the stimuli in mid-experiment to determine whether striking helped participants build more accurate perceptions of length transferrable from one stimulus scale to another. We predicted that, if self-training led to better length judgments, the repeated striking would improve judgments and that, in turn, judgments following the switch of stimuli would show a good transfer of what participants had learned. On the other hand, self-training may simply exaggerate inertial properties of stimuli and may be sensitive to sudden changes in the scale of stimuli. Mixed-effect modeling of discrepancies show that striking only accentuated effects of inertial moment, producing exaggerated length judgments. Correlation between perceived length and actual length increased only for participants who experienced a switch in individual stimuli but not stimulus scale. We discuss the implications of these findings for any theoretical relationship between self-organization and veridicality.

A comparison of haptic material perception in blind and sighted individuals

We investigated material perception in blind participants to explore the influence of visual experience on material representations and the relationship between visual and haptic material perception. In a previous study with sighted participants, we had found participants’ visual and haptic judgments of material properties to be very similar (Baumgartner, Wiebel, & Gegenfurtner, 2013). In a categorization task, however, visual exploration had led to higher categorization accuracy than haptic exploration. Here, we asked congenitally blind participants to explore different materials haptically and rate several material properties in order to assess the role of the visual sense for the emergence of haptic material perception. Principal components analyses combined with a procrustes superimposition showed that the material representations of blind and blindfolded sighted participants were highly similar. We also measured haptic categorization performance, which was equal for the two groups. We conclude that haptic material representations can emerge independently of visual experience, and that there are no advantages for either group of observers in haptic categorization.

Visual Influence on Haptic Torque Perception

The brain receives input from multiple sensory modalities simultaneously, yet we experience the outside world as a single integrated percept. This integration process must overcome instances where perceptual information conflicts across sensory modalities. Under such conflicts, the relative weighting of information from each modality typically depends on the given task. For conflicts between visual and haptic modalities, visual information has been shown to influence haptic judgments of object identity, spatial features (e.g., location, size), texture, and heaviness. Here we test a novel instance of haptic-visual conflict in the perception of torque. We asked participants to hold a left-right unbalanced object while viewing a potentially left-right mirror-reversed image of the object. Despite the intuition that the more proximal haptic information should dominate the perception of torque, we find that visual information exerts substantial influences on torque perception even when participants know that visual information is unreliable.

Grabbing subitizing with both hands: bimanual number processing

Visual judgment of small numerosities (<4) is generally assumed to be done through subitizing, which is a faster process than counting. Subitizing has also been shown to occur in haptic judgment of the number of spheres in the hand. Furthermore, interactions have been shown to exist between visually perceived numbers and hand motor action. In this study, we compare enumeration of a set of spheres presented to one hand (unimanual) and enumeration of the same total number of spheres presented divided over the two hands (bimanual). Our results show that, like in vision, a combination of subitizing and counting is used to process numbers in active touch. This shows that numbers are processed in a modality-independent way. This suggests that there are not only interactions between perception of numbers and hand motor action, but rather that number representation is modality-independent.

The role of fractality in perceptual learning: Exploration in dynamic touch.

Perceptual systems must learn to explore and to use the resulting information to hone performance. Optimal performance depends on using information available at many time scales, from the near instantaneous values of variables underlying perception (i.e., detection), to longer term information about appropriate scaling (i.e., calibration), to yet longer term information guiding variable use (i.e., attunement). Fractal fluctuations in explorations would entail fluctuation at all time scales, allowing perceptual systems a flexible way to detect information at all time scales. We tested whether perceptual learning in dynamic touch is related to the fractality of wielding behaviors. A reanalysis of wielding behaviors from Arzamarski, Isenhower, Kay, Turvey, and Michaels (2010) revealed that exploratory movements were fractal and that a fractal-scaling exponent predicts individual differences in haptic judgments.

Young Children Do Not Integrate Visual and Haptic Form Information

Several studies have shown that adults integrate visual and haptic information (and information from other modalities) in a statistically optimal fashion, weighting each sense according to its reliability [1, 2]. When does this capacity for crossmodal integration develop? Here, we show that prior to 8 years of age, integration of visual and haptic spatial information is far from optimal, with either vision or touch dominating totally, even in conditions in which the dominant sense is far less precise than the other (assessed by discrimination thresholds). For size discrimination, haptic information dominates in determining both perceived size and discrimination thresholds, whereas for orientation discrimination, vision dominates. By 8-10 years, the integration becomes statistically optimal, like adults. We suggest that during development, perceptual systems require constant recalibration, for which cross-sensory comparison is important. Using one sense to calibrate the other precludes useful combination of the two sources.

The Effects of Curvature on Haptic Judgments of Extent in Sighted and Blind People

A series of experiments was carried out to examine the effect of curvature on haptic judgments of extent in sighted and blind individuals. Experiment 1 showed that diameters connecting the endpoints of semicircular lines were underestimated with respect to straight lines, but failed to show an effect of visual experience on length judgments. In experiment 2 we tested are lengths. The effects of curvature on perceived path length were weaker, but were still present in this experiment. Visual experience had no effect on path length judgments. Another experiment was performed to examine the effect of repeated tracing (1, 5, 9, or unlimited number of traces) on judgments of the lengths of straight lines and diameters of semicircles. Judgments of extent were more accurate when subjects engaged in larger numbers of traces. There was no effect of number of traces on curve-height judgments, suggesting that subjects were not using height estimates to judge diameters of semicircles. In a further experiment we tested the effect of number of traces on curves that varied in height. Restricting subjects to a single trace magnified the effect of path length on judgments of the distance between the endpoints of curves. Additional experiments showed that curvature effects on diameter judgments were not eliminated when stimuli were in the frontal plane or when the curves were explored with the use of two hands. Arm support had no effect on judged length in experiment 7. A final experiment showed a robust horizontal vertical illusion in haptic perception of convex curves, with overestimation of the heights of the curves compared with their widths. The practical and theoretical implications of the results are discussed.

The Influence of Exploration Mode, Orientation, and Configuration on the Haptic Müller-Lyer Illusion

We studied the impact of manner of exploration, orientation, spatial position, and configuration on the haptic Müller-Lyer illusion. Blindfolded sighted subjects felt raised-line Müller-Lyer and control stimuli. The stimuli were felt by tracing with the index finger, free exploration, grasping with the index finger and thumb, or by measuring with the use of any two or more fingers. For haptic judgments of extent a sliding tangible ruler was used. The illusion was present in all exploration conditions, with overestimation of the wings-out compared to wings-in stimuli. Tracing with the index finger reduced the magnitude of the illusion. However, tracing and grasping induced an overall underestimation of size. The illusion was greatly attenuated when stimuli were felt with the index fingers of both hands. Illusory misperception was not altered by the position in space of the Müller-Lyer stimuli. No effects of changes in the thickness of the line shaft were found, but there were effects of the length of the wing endings for the smaller, 5.1 cm stimuli. The theoretical and practical implications of the results are discussed.

Euclidean Metric Representations of Haptically Explored Triangles

This study explored whether people create Euclidean representations of 2-dimensional right triangles from touch and use them to make spatial inferences in accord with Euclidean distance axioms. Blindfolded participants who were instructed to form visual images of triangles felt the vertical and horizontal sides of right triangles, then estimated the lengths (but not the angles) of the 3 triangle sides. In these 3 experiments, length estimates conformed closely to the Euclidean metric when evaluated on application of the Pythagorean theorem. Participants who used a visual imaging strategy were accurate more often than those who used visual imagery less often. In Experiments 2 and 3, a hypotenuse inference was as accurate as a direct haptic judgment of the hypotenuse. These results demonstrated similar accuracy of the hypotenuse judgments when participants made verbal rather than haptic estimates. The findings indicate that participants can form Euclidean representations under certain conditions from felt 2-dimensional right triangles based on visual images.

Haptic and Visual Perception of an Affordance for Upright Posture

Perception of whether a slanted surface supports upright stance was evaluated visually and haptically. Observers stood approximately 1 m from a 1 m by 1 m plywood board whose angle of inclination varied randomly among seven settings (12, 17, 22, 27, 33, 39, and 45 deg.). For visual trials, observers looked at the surface. For haptic trials, the observers were blindfolded and explored the surface with a hand-held wooden dowel. The perceived behavioral category boundary (i.e., the slope that received 50% yes responses) did not differ for visual and haptic judgments. Although the two perceptual systems did not differ in the mean, participants took longer to make haptic judgments and were less confident in doing so. For both perceptual systems, response time increased and confidence decreased at the category boundary. A second experiment used ascending and descending methods of limits and found both enhanced contrast and hysteresis effects for both perceptual systems. Differences between perceptual systems we...

Effect of modality-specific experience on visual and haptic judgment of orientation.

Although the 'oblique effect' (poorer performance on oblique orientations as compared to performance on vertical and horizontal orientations) is generally understood as a strictly visual phenomenon, a haptic oblique effect occurs for blindfolded subjects required to set a stimulus rod by hand. Because oblique effects are often attributed to the observer's experience with a predominantly horizontal and vertical environment, we assessed the effect of visual and haptic experience by providing subjects with modality-specific inspection periods to familiarize them with the more poorly judged obliques. Oblique error was significantly reduced in magnitude for judgments made by the modality of experience, and for judgments made across modalities. Rate of improvement, consistency of transfer, and the subjective reports of subjects indicate that this haptic oblique effect is more strongly influenced by visual experience and imagery than by haptic experience. It need not be interpreted as an effect based on factors intrinsic to the haptic modality.

Spatial anisotropy in intramodal and cross-modal judgments of stimulus orientation: the stability of the oblique effect.

Visual and haptic judgments of stimulus orientation were examined intramodally and crossmodally by having subjects reproduced standard stimulus orientations simultaneously with their inspection or after a delay. For all conditions, an oblique effect was obtained, i.e. vertical and horizontal orientations were reproduced reliably more accurately than oblique orientations. Although intramodal differences were large, with haptic errors being greater than visual errors, cross-model differences were small. Furthermore, while for intramodal conditions simultaneous visual reproductions were reliably more accurate than delayed reproductions but haptic reproductions were more accurate when delayed, cross-modal errors were reliable greater with simultaneous reproductions, regardless of whether the standard orientation was visual or haptic. The modality differences reflect basic differences in stimulus information processing and the stability of the oblique effect across the experimental conditions suggests that perceptual spatial anisotropic effects are strongly influenced by experiential factors.

Developmental study of a haptic illusion in relation to Piaget's centration theory

Age differences have previously been found in the magnitude of the visual Horizontal-vertical illusion with an L figure. The present study examined the haptic judgments of an L figure among subjects across the ages of 6, 7, 9, 11, 12, 13, and 20 years. Significant differences in amount of haptic illusion were found. The developmental function was curvilinear, decreasing from the 6-year-olds to 7-year-olds, and then increasing from 7-year-olds to a peak among 11-year-olds, followed by a decline to adulthood. Variation in the size of the illusion was also found to be related to the rate at which subjects of a particular age group typically move the limb in scanning the figure. The similarity of the haptic function to that obtained previously for the visual L figure was discussed in relation to Piaget's Centration theory.

Dynamic properties of radial and tangential movements as determinants of the haptic horizontal--vertical illusion with an L figure.

In four experiments involving blindfolded subjects, constant errors in the haptic judgment of extent in the horizontal plane were found to relate consistently to the time and velocity of limb movement. Radial movements, executed at a slower speed and for a longer time, are judged longer than tangential movements of equal extent. The data were considered in relation to certain physiological and kinematic properties of the actively moving limb. Taken together with additional information on judgments of movement duration, the results suggest that the illusion of extent is modulated by the perception of differential time cues. In these terms, it was noted that the haptic horizontal-vertical illusion with the L figure is another instance of the interaction of apparent space and time commonly found in studies of psychological relativity.

Haptic judgments of curvature by blind and sighted humans.

Haptic judgments of curvature by blind and sighted humans.

The effect of instructions on visual and haptic judgment of the müller-lyer illusion

Abstract In an experiment with the Muller-Lyer figure, instructions to judge ‘apparent’ size equality resulted in a larger visual illusion than instructions to judge ‘objective’ size-equality. The two sets of instructions did not, however, have a differential effect on the amount of haptic illusion. One reason for this difference may be that visual inspection permitted S to perceive the illusion figure in relation to its background while haptic inspection did not. The data may also reflect differences in the way spatial information is sampled within the two modalities. Haptic inspection entails widespread sampling from the figure under both instruction conditions. With visual inspection it is possible that widespread sampling occurs with one set of instructions but not the other.

Haptic judgment of the Müller-Lyer illusion by subjects of different ages

Age differences have been commonly found in the magnitude of the visual illusion obtained with the Müller-Lyer illusion. The present experiment has examined haptic (tactualkinesthetic) judgments of this figure by Ss of different ages. Young children, older children, and adults did not differ in amount of haptic illusion. The bearing of this finding on Piaget’s analysis of age differences in visual illusion is discussed.