Exploratory Movements Determine Cue Weighting in Haptic Length Perception of Handheld Rods

Biases in hand perception are driven by somatosensory computations, not a distorted hand model

To sense and interact with objects in the environment, we effortlessly configure our fingertips at desired locations. It is therefore reasonable to assume that the underlying control mechanisms rely on accurate knowledge about the structure and spatial dimensions of our hand and fingers. This intuition, however, is challenged by years of research showing drastic biases in the perception of finger geometry.1,2,3,4,5 This perceptual bias has been taken as evidence that the brain's internal representation of the body's geometry is distorted,6 leading to an apparent paradox regarding the skillfulness of our actions.7 Here, we propose an alternative explanation of the biases in hand perception-they are the result of the Bayesian integration of noisy, but unbiased, somatosensory signals about finger geometry and posture. To address this hypothesis, we combined Bayesian reverse engineering with behavioral experimentation on joint and fingertip localization of the index finger. We modeled the Bayesian integration either in sensory or in space-based coordinates, showing that the latter model variant led to biases in finger perception despite accurate representation of finger length. Behavioral measures of joint and fingertip localization responses showed similar biases, which were well fitted by the space-based, but not the sensory-based, model variant. The space-based model variant also outperformed a distorted hand model with built-in geometric biases. In total, our results suggest that perceptual distortions of finger geometry do not reflect a distorted hand model but originate from near-optimal Bayesian inference on somatosensory signals.

Gone Fishin': Perceiving the length of one object that is non-rigidly attached to a wielded object

We performed four experiments to investigate whether people can perceive the length of a target object (a "fish") that is attached to a freely wielded object (the "fishing pole") by a length of string, and if so, whether this ability is grounded in the sensitivity of the touch system to invariant mechanical parameters that describe the forces and torques required to move the target object. In particular, we investigated sensitivity to mass, static moment, and rotational inertia-the forces required to keep an object from falling due to gravity, the torque required to keep an object from rotating due to gravity, and the torques required to actively rotate an object in different directions, respectively. We manipulated the length of the target object (Experiment 1), the mass of the target object (Experiment 2), and the mass distribution of the target object (Experiments 3 and 4). Overall, the results of the four experiments showed that participants can perform this task. Moreover, when the task is configured such that it more closely approximates a wielding at a distance task, the ability to do so is grounded in sensitivity to such forces and torques.

Perceiving multiple properties of a single person-probe-surface system

We performed three experiments to investigate the extent to which people can selectively perceive different properties of a person-probe-surface system, and the extent to which such abilities are supported by sensitivities to different (components of) invariant mechanical parameters. Participants probed a surface with a wielded object and attempted to perceive an exteroceptive property of the probe (whole length), an exproprioceptive property of the probe (partial length), and an exteroceptive property of the surface (distance). After controlling for potential confounds, we found that participants were able to selectively perceive each of these properties, and that in each case, such abilities were supported by sensitivity to distinct components of mechanical invariants. The results suggest that people can selectively attend to properties both across and within components of a person-probe-surface system and are discussed in the context of the invariant mechanical parameters that support this ability.

Size, weight, and expectations

The size-weight illusion is well-known: if two equally heavy objects differ in size, the large one feels lighter than the small one. Most explanations for this illusion assume that because the information about the relevant attribute (weight itself) is unreliable, information about an irrelevant but correlated attribute (size) is used as well. If such reasoning is correct, one would expect that the illusion can be inverted: if size information is unreliable, weight information will be used to judge size. We explored whether such a weight-size illusion exists by asking participants to lift Styrofoam balls that were coated with glow in the dark paint. The balls (2 sizes, 3 weights) were lifted using a pulley system in complete darkness at 2 distances. Participants reported the size using free magnitude estimation. The visual size information was indeed unreliable: balls that were presented at a 20% larger distance were judged 15% smaller. Nevertheless, the judgments of size were not systematically affected by the 20% weight change (differences < 0.5%). We conclude that because the weight-size illusion does not exist, the mechanism behind the size-weight illusion is specific for judging heaviness.

Assume a Spherical Chicken: Analytic Constraints, Inertia Tensor Information, and Wielded Rod Length Perception

Information associated with the inertia tensor is the preeminent explanation for haptic perception of object properties, notably wielded rod length. Critics counter that tensorial-based information requires non-tensorial supplementation (mass, torque). However, those critiques omit important constraints. With relevant constraints included, the inertia tensor alone completely specifies rod length. I list constraints inherent (but tacit) in haptic rod length perception, and show that object properties associated with the inertia tensor are invariant, even with constraints removed, by involving (a) longitudinal moment equivalents for rod mass and (b) derivatives of moments with respect to varying rotation axes. Analytic outcomes show tensorial-based information is a robust basis for wielded rod length perception, and suggest open questions for empirical exploration.

Effects of Grip-Force, Contact, and Acceleration Feedback on a Teleoperated Pick-and-Place Task

The multifaceted human sense of touch is fundamental to direct manipulation, but technical challenges prevent most teleoperation systems from providing even a single modality of haptic feedback, such as force feedback. This paper postulates that ungrounded grip-force, fingertip-contact-and-pressure, and high-frequency acceleration haptic feedback will improve human performance of a teleoperated pick-and-place task. Thirty subjects used a teleoperation system consisting of a haptic device worn on the subject's right hand, a remote PR2 humanoid robot, and a Vicon motion capture system to move an object to a target location. Each subject completed the pick-and-place task 10 times under each of the eight haptic conditions obtained by turning on and off grip-force feedback, contact feedback, and acceleration feedback. To understand how object stiffness affects the utility of the feedback, half of the subjects completed the task with a flexible plastic cup, and the others used a rigid plastic block. The results indicate that the addition of grip-force feedback with gain switching enables subjects to hold both the flexible and rigid objects more stably, and it also allowed subjects who manipulated the rigid block to hold the object more delicately and to better control the motion of the remote robot's hand. Contact feedback improved the ability of subjects who manipulated the flexible cup to move the robot's arm in space, but it deteriorated this ability for subjects who manipulated the rigid block. Contact feedback also caused subjects to hold the flexible cup less stably, but the rigid block more securely. Finally, adding acceleration feedback slightly improved the subject's performance when setting the object down, as originally hypothesized; interestingly, it also allowed subjects to feel vibrations produced by the robot's motion, causing them to be more careful when completing the task. This study supports the utility of grip-force and high-frequency acceleration feedback in teleoperation systems and motivates further improvements to fingertip-contact-and-pressure feedback.

After experience with the task humans actively optimize shape discrimination in touch by utilizing effects of exploratory movement direction

The active control of exploratory movements is an integral part of active touch. We investigated and manipulated the relationship between the haptic discrimination performance for small bumps and the direction of exploratory movements relative to the body. Shape discrimination performance varied with the direction of stimulus exploration. Experimental manipulations successfully changed the normative relation between exploratory direction and discrimination performance. If participants were rewarded for "good perceptual performance" and had the choice, they displayed clear strategic preferences for exploratory directions that yield optimal performance-but only after having extensive experience with the changed perceptual conditions. Overall, the findings suggest that participants can actively adapt their exploratory movements in order to optimize haptic discrimination performance.

Moving the weber fraction: the perceptual precision for moment of inertia increases with exploration force

How does the magnitude of the exploration force influence the precision of haptic perceptual estimates? To address this question, we examined the perceptual precision for moment of inertia (i.e., an object's "angular mass") under different force conditions, using the Weber fraction to quantify perceptual precision. Participants rotated a rod around a fixed axis and judged its moment of inertia in a two-alternative forced-choice task. We instructed different levels of exploration force, thereby manipulating the magnitude of both the exploration force and the angular acceleration. These are the two signals that are needed by the nervous system to estimate moment of inertia. Importantly, one can assume that the absolute noise on both signals increases with an increase in the signals' magnitudes, while the relative noise (i.e., noise/signal) decreases with an increase in signal magnitude. We examined how the perceptual precision for moment of inertia was affected by this neural noise. In a first experiment we found that a low exploration force caused a higher Weber fraction (22%) than a high exploration force (13%), which suggested that the perceptual precision was constrained by the relative noise. This hypothesis was supported by the result of a second experiment, in which we found that the relationship between exploration force and Weber fraction had a similar shape as the theoretical relationship between signal magnitude and relative noise. The present study thus demonstrated that the amount of force used to explore an object can profoundly influence the precision by which its properties are perceived.