Principles of part-whole selective perception by dynamic touch extend to the torso

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.

Empirical Evidence for Extended Cognitive Systems

We present an empirically supported theoretical and methodological framework for quantifying the system‐level properties of person‐plus‐tool interactions in order to answer the question: “Are person‐plus‐tool‐systems extended cognitive systems?” Nineteen participants provided perceptual judgments regarding their ability to pass through apertures of various widths while using visual information, blindfolded wielding a rod, or blindfolded wielding an Enactive Torch—a vibrotactile sensory‐substitution device for detecting distance. Monofractal, multifractal, and recurrence quantification analyses were conducted to assess features of person‐plus‐tool movement dynamics. Trials where people utilized the rod or Enactive Torch demonstrated stable “self‐similarity,” or indices of healthy and adaptive single systems, regardless of aperture width, trial order, features of the participants’ judgments, and participant characteristics. Enactive Torch trials exhibited a somewhat greater range of dynamic fluctuations than the rod trials, as well as less movement recurrence, suggesting that the Enactive Torch allowed for more exploratory movements. Findings provide support for the notion that person‐plus‐tool systems can be classified as extended cognitive systems and a framework for quantifying system‐level properties of these systems. Implications concerning future research on extended cognition are discussed.

Heavy-tailed distributions in haptic perception of wielded rods

Humans identify properties (e.g., the length or weight) of objects through touch using somatosensory perceptions in the limbs. Humans identify these properties by manipulating an object to access its inertial qualities. However, there is little work evidencing a unifying pattern of movements humans use to access these inertial properties. The current study examined if participants' wielding movements followed a systematic distribution-specifically, a Lévy-like distribution that is characterized by heavy-tails and is often seen in efficient foraging behavior. Participants wielded rods they could not see and were tasked to identify whether the rod they were wielding was the longer or shorter of two rods. While participants wielded the rod, the rod's motion was captured. Results demonstrate that the sampling of angular accelerations produced heavy-tailed distributions. Since angular acceleration has a distinct physical-mathematical relationship with inertia, this finding is consistent with the interpretation that the haptic subsystems are sensitive to the inertial properties of an object. Angular acceleration from wielding motions appear to follow a similar distribution as optimal foraging strategies-perhaps it is the case that humans are foraging for information about the inertia of an object through changes in angular acceleration and wielding movements.

Selective perception in probing by foot: Perceiving the length of a probe and the distance of a probed surface

Perception of properties of object wielded by means of muscular effort exhibits both task specificity and anatomical independence. A person can perceive different properties of an object wielded by a given anatomical component and can perceive a given property of an object wielded by different anatomical components. Task-specificity and anatomical independence are fundamental characteristics of the haptic system described a biotensegrity system embedded in lawfully structured energy arrays. We investigate whether both characteristics are also exhibited when a person attempts to perceive properties by means of a wielded object. Participants used a foot-wielded rod to probe a surface and reported the length of the rod and the distance of the surface probed (on separate sets of trials). The ability to differentiate these properties generalized across anatomical components, and perception of each property by foot was supported by sensitivities to the same invariant mechanical parameters that support perception of each property by hand. The results suggest that the biotensegrity hypothesis applies to perception both of and by means an object attached to the body.

Bodywide fluctuations support manual exploration: Fractal fluctuations in posture predict perception of heaviness and length via effortful touch by the hand

The human haptic perceptual system respects a bodywide organization that responds to local stimulation through full-bodied coordination of nested tensions and compressions across multiple nonoverlapping scales. Under such an organization, the suprapostural task of manually hefting objects to perceive their heaviness and length should depend on roots extending into the postural control for maintaining upright balance on the ground surface. Postural sway of the whole body should thus carry signatures predicting what the hand can extract by hefting an object. We found that fractal fluctuations in Euclidean displacement in the participants' center of pressure (CoP) contributed to perceptual judgments by moderating how the participants' hand picked up the informational variable of the moment of inertia. The role of fractality in CoP displacement in supporting heaviness and length judgments increased across trials, indicating that the participants progressively implicate their fractal scaling in their perception of heaviness and length. Traditionally, we had to measure fractality in hand movements to predict perceptual judgments by manual hefting. However, our findings suggest that we can observe what is happening at hand in the relatively distant-from-hand measure of CoP. Our findings reveal the complex relationship through which posture supports manual exploration, entailing perception of the intended properties of hefted objects (heaviness or length) putatively through the redistribution of forces throughout the body.

Where is your head? Perception of relative position of the head on a wielded object

Perception of exteroceptive properties (e.g., object length) by effortful or dynamic touch is both task-specific and anatomically independent. We investigate whether task-specificity and anatomical independence generalize to perception of proexteroceptive properties of the person-object system (i.e., relative position of the body on a wielded object). Moreover, we do so when objects are wielded by a body part that is unlikely to be well practiced in such tasks-the head. Experiment 1 found that participants can perceive the relative location of the head on a wielded object and that such perception is likely supported by task-specific sensitivity to an invariant mechanical stimulation pattern-rotational inertia. Experiment 2 found that participants have at least some ability to differentiate between this property and a related exteroceptive property (i.e., partial length of a wielded object extending to one side of the head). The results are discussed in terms of information for perception by effortful touch and a description of the haptic system as a biotensegrity structure.

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.

Perception of the length of an object through dynamic touch is invariant across changes in the medium

Rotational inertia-a mechanical quantity that describes the differential resistance of an object to angular acceleration in different directions-has been shown to support perception of the properties of that object through dynamic touch (wielding). The goal of the present study was to examine if perception of the length of an object through dynamic touch depends on its rotational inertia, independent of the medium in which it is wielded. The participants (n = 14) wielded 12 different objects held in air or completely immersed in water and reported perceived lengths of those objects. Each object consisted of a rod of a particular density with a particular number of stacked steel rings attached at a particular location along its length. Perceived length was invariant across medium. In addition, a single-valued function of the major eigenvalue, I ₁, and the minor eigenvalue, I ₃, of the rotational inertia, I, of the 12 objects predicted the perceived lengths of those objects in both air and water, and the perceived lengths were invariant across the two media. These results support the hypothesis that the informational support for perception of the length of an object through dynamic touch is invariant across changes in the medium.

Location of a grasped object's effector influences perception of the length of that object via dynamic touch

Perception of properties of a grasped object via dynamic touch (wielding) contributes to dexterity in tool use (e.g., using a hammer, screwdriver) and sports (e.g., hockey, tennis). These activities differ from simple object manipulation in that they involve making contact with an intended target. In the present study, we examined whether and how making (percussive) contact with a target influences perception of the length of a grasped object via dynamic touch. Making contact with a target by the tip resulted in a more accurate perception of the length than simple wielding. However, making contact with the target at a point along the length did not influence the accuracy of perception. These findings suggest that the location of a grasped object's effector influences perception of properties of that object via dynamic touch. We discuss these findings in terms of time-varying properties of vibrations generated by the percussive contact of the grasped object and target.

Temperature influences perception of the length of a wielded object via effortful touch

Individuals can perceive the properties of an attached or grasped object by wielding it through muscular effort-an ability referred to as dynamic or effortful touch. Sensitivity to the forces required to move such objects and to the resulting global patterns of tissue deformation underlies such perception. Given that perception via dynamic touch is movement-based, we hypothesized that manipulations that affect the ability to produce and control muscular movements might affect perception via dynamic touch. Cooling muscles from 40 to 10 °C impedes the development and transmission of muscular force and diminishes muscle stretch-reflex sensitivity. Accordingly, we anticipated that changes in hand temperature would alter the ability to detect patterns of tissue deformation and thus perception of the properties of wielded objects. In two experiments, participants wielded dowels with different lengths and rotational inertias (Experiment 1) and objects with identical lengths and different rotational inertias (Experiment 2). They reported perceived lengths of these objects, in the absence of vision, in cool (~ 10 °C), neutral (~ 30 °C), and warm temperature conditions (~ 40 °C). Actual length predicted perceived length of the dowels (Experiment 1), and rotational inertia predicted perceived length of the objects (Experiment 2); perceived lengths were longer in the warm condition than in the cool condition. In consideration of known temperature-induced changes in tissue structure and function, our results support the hypothesis that comparable processes underlie the control of movement and perception via dynamic touch.

Heads Up!

Possibilities for behavior (i.e., affordances) can be perceived with units spanning anatomical components and external objects. For example, affordances for standing on an inclined surface can be perceived with an object held in the hand or attached to the head. We investigated whether these two person-plus-object perceptual systems exhibit the same pattern of nonlinear phase transitions in perception of this affordance. Blindfolded participants explored an inclined surface with a rod held in the hand or attached to the head and reported whether they could stand on that surface. Inclinations were presented in ascending or descending sequences. In both conditions, responses exhibited negative hysteresis - perceptual boundaries occurred at steeper angles for descending than for ascending sequences. The generalization of this pattern across components that differ in physiology, sensitivity, and experience is consistent with both the soft assembly of perceptual devices and with a dynamical systems perspective on perception of affordances.

Evaluating the contributions of muscle activity and joint kinematics to weight perception across multiple joints

Perceived heaviness is clearly a function of muscle activity: objects feel heavy, in part because they are lifted with more force than lighter feeling objects. Recent research showed that participants scale their perceptions to the ratio of muscle activity to lift acceleration during elbow lifts (Waddell et al. J Exp Psychol Hum Percept Perform 42:363-374, 2016). The current study sought psychophysiological functions relating perceived heaviness to EMG and peak lift acceleration across multiple lifts employing different muscles as prime movers. Participants lifted objects with three arm lifts-shoulder, elbow, and wrist-and reported perceived heaviness. In each lift, EMG was recorded from the anterior deltoid, biceps brachii, and forearm flexors, and peak angular acceleration was recorded about each joint. The resulting psychophysiological functions revealed the hypothesized ratio of muscle activity to peak lift acceleration in all lifts. Principal component regressions showed that the EMG of the forearm flexors and peak acceleration of the lifting joint were most relevant for perceived heaviness. The special role of forearm flexors in perceiving heaviness across different lifts was interpreted in terms of the invariant structure of the inertia tensor about the wrist.

Turning perception on its head: cephalic perception of whole and partial length of a wielded object

Flexibility is a fundamental hallmark of perceptual systems. In particular, there is a great deal of flexibility in the ability to perceive properties of occluded objects by effortful or dynamic touch-hefting, wielding, or otherwise manipulating those objects by muscular effort. Perception of length of an occluded wielded object is comparable when that object is wielded by anatomical components that differ in sensitivity, dexterity, and functionality. Moreover, perception of this property is supported by an analogous sensitivity to inertial properties across such components. We investigated the ability to perceive whole and partial length of an object wielded by hand or by head. Experiment 1 found that perception of length by these anatomical components is qualitatively and quantitatively indistinguishable. Experiment 2 found that perception of length is supported by the same specific sensitivity to inertial properties in each case. Experiment 3 found that perception of whole length and partial length are each supported by specific sensitivities to inertial properties and that this is the case for both hand and by head. The results are discussed in the context of the nature of the stimulation patterns and the organization of the haptic system that are likely to support such flexibility in perception.

Use your head! Perception of action possibilities by means of an object attached to the head

Perceiving any environmental property requires spontaneously assembling a smart perceptual instrument-a task-specific measurement device assembled across potentially independent anatomical units. Previous research has shown that to a large degree, perception of a given environmental property is anatomically independent. We attempted to provide stronger evidence for this proposal by investigating perception by an organization of anatomical and inert components that likely requires the spontaneous assembly of a novel smart perceptual instrument-a rod attached to the head. Specifically, we compared cephalic and manual perception of whether an inclined surface affords standing on. In both conditions, perception reflected the action capabilities of the perceiver and not the appendage used to wield the rod. Such results provide stronger evidence for anatomical independence of perception within a given perceptual system and highlight that flexible task-specific detection units can be assembled across units that span the body and inert objects.

Recognizing familiar objects by hand and foot: Haptic shape perception generalizes to inputs from unusual locations and untrained body parts

The limits of generalization of our 3-D shape recognition system to identifying objects by touch was investigated by testing exploration at unusual locations and using untrained effectors. In Experiments 1 and 2, people found identification by hand of real objects, plastic 3-D models of objects, and raised line drawings placed in front of themselves no easier than when exploration was behind their back. Experiment 3 compared one-handed, two-handed, one-footed, and two-footed haptic object recognition of familiar objects. Recognition by foot was slower (7 vs. 13 s) and much less accurate (9 % vs. 47 % errors) than recognition by either one or both hands. Nevertheless, item difficulty was similar across hand and foot exploration, and there was a strong correlation between an individual's hand and foot performance. Furthermore, foot recognition was better with the largest 20 of the 80 items (32 % errors), suggesting that physical limitations hampered exploration by foot. Thus, object recognition by hand generalized efficiently across the spatial location of stimuli, while object recognition by foot seemed surprisingly good given that no prior training was provided. Active touch (haptics) thus efficiently extracts 3-D shape information and accesses stored representations of familiar objects from novel modes of input.

Is calibration of the perception of length modality-independent?

Perception becomes increasingly scaled to environmental properties when feedback is provided about the perception of a given property. Understanding the process of calibration requires a description of the stimulation patterns to which perceivers become calibrated as a result of such feedback. Recent investigations using a transfer-of-calibration paradigm have shown that recalibration of the perception of length transfers from audition to touch. Such results have provided preliminary evidence that calibration of this property is modality-independent (i.e., that feedback about length calibrates perceivers to a modality-independent stimulation pattern). The experiment reported here provides a stronger test of this claim by demonstrating that perception transfers in both directions (i.e., from audition to touch, and vice versa). The results provide further evidence that calibration is modality-independent, and they are consistent with the more general proposal that the stimulation patterns that support perception are also modality-independent.

Fractal fluctuations in quiet standing predict the use of mechanical information for haptic perception

Movement science has traditionally understood high-dimensional fluctuations as either antithetical or irrelevant to low-dimensional control. However, fluctuations incident to changeful, sometimes unpredictable stimulation must somehow reshape low-dimensional aspects of control through perception. The movement system's fluctuations may reflect cascade dynamics in which many-sized events interact nonlinearly across many scales. Cascades yield fractal fluctuations, and fractality of fluctuations may provide a window on the interactions across scale supporting perceptual processes. To test these ideas, we asked adult human participants to judge whole or partial length for unseen rods (with and without added masses). The participants' only experience with the objects came from supporting them across their shoulders during quiet standing. First, the degree of fractal temporal correlations in trial-by-trial series of planar Euclidean displacements in center of pressure (COP) significantly improved prediction of subsequent trial-by-trial judgments, above and beyond prediction by traditional predictors of haptic perception and conventional measures of COP variability. Second, comparison with linear surrogate data indicated the presence of nonlinear interactions across scale in these time series. These results demonstrate that high-dimensional fluctuations may serve a crucial role in the cascade dynamics supporting apparently low-dimensional control strategies.

Rule for scaling shoulder rotation angles while walking through apertures

Background

When an individual is trying to fit into a narrow aperture, the amplitude of shoulder rotations in the yaw dimension is well proportioned to the relative aperture width to body width (referred to as the critical ratio value). Based on this fact, it is generally considered that the central nervous system (CNS) determines the amplitudes of shoulder rotations in response to the ratio value. The present study was designed to determine whether the CNS follows another rule in which a minimal spatial margin is created at the aperture passage; this rule is beneficial particularly when spatial requirements for passage (i.e., the minimum passable width) become wider than the body with an external object.

Methodology/Principal Findings

Eight young participants walked through narrow apertures of three widths (ratio value = 0.9, 1.0, and 1.1) while holding one of three horizontal bars (short, 1.5 and 2.5 times the body width). The results showed that the amplitude of rotation angles became smaller for the respective ratio value as the bar increased in length. This was clearly inconsistent with the general hypothesis that predicted the same rotation angles for the same ratio value. Instead, the results were better explained with a new hypothesis which predicted that a smaller rotation angle was sufficient to produce a constant spatial margin as the bar-length increased in length.

Conclusion

The results show that, at least under safe circumstances, the CNS is likely to determine the amplitudes of shoulder rotations to ensure the minimal spatial margin being created at one side of the body at the time of crossing. This was new in that the aperture width subtracted from the width of the body (plus object) was taken into account for the visuomotor control of locomotion through apertures.

Obtaining information by dynamic (effortful) touching

Dynamic touching is effortful touching. It entails deformation of muscles and fascia and activation of the embedded mechanoreceptors, as when an object is supported and moved by the body. It is realized as exploratory activities that can vary widely in spatial and temporal extents (a momentary heft, an extended walk). Research has revealed the potential of dynamic touching for obtaining non-visual information about the body (e.g. limb orientation), attachments to the body (e.g. an object's height and width) and the relation of the body both to attachments (e.g. hand's location on a grasped object) and surrounding surfaces (e.g. places and their distances). Invariants over the exploratory activity (e.g. moments of a wielded object's mass distribution) seem to ground this 'information about'. The conception of a haptic medium as a nested tensegrity structure has been proposed to express the obtained information realized by myofascia deformation, by its invariants and transformations. The tensegrity proposal rationalizes the relative indifference of dynamic touch to the site of mechanical contact (hand, foot, torso or probe) and the overtness of exploratory activity. It also provides a framework for dynamic touching's fractal nature, and the finding that its degree of fractality may matter to its accomplishments.