Dynamic Touch by Hand and Head During Walking: Protective Behavior for the Head?

We investigated perception of the extent of an object held in the hand or attached to the head during walking. Participants wielded an occluded, weighted rod by hand and by head and reported the perceived location of the rod end (relative to a virtual vertical line) while walking and while standing. There was an unexpected effect of body part on perceived location of the rod end. In particular, participants stood or walked so that the rod end was farther away from the vertical line when wielding with the head than wielding with the hand. It is possible that differences between the tasks and the relative costs of a collision influenced the locomotory behavior in each condition.     

Inertia tensor and weight-percept models of length perception by static holding

S. J. Lederman, S. R. Ganeshan, and R. E. Ellis (1996) reported an experiment demonstrating that for occluded rods of equal mass and length but different diameters length perception by static holding was larger for rods of smaller diameter. They concluded that participants inferred length from illusory weight percepts. However, rods of equal mass and length that differ in diameter also differ in the eigenvalues of their respective inertia tensors. In the present experiments, the authors manipulated the diameters (Experiment 1) and the inertial eigenvalues (Experiments 4 and 5) of statically held objects. As has been shown with wielded objects, perceived length was a function of the eigenvalues. Additional experiments failed to confirm the expectation from the weight-percept model that perceived length maps to the estimated weight (Experiments 2 and 3). Physical quantities, not psychological quantities, seem to explain length perception by static holding.     

The distorted hand metric representation serves both perception and action

ABSTRACT

The representation of our body location is achieved by integrating sensorimotor inputs with information about our body size. Previous studies have shown that the metric representation of our hand, also called the body model, is distorted, namely overestimated in width and underestimated in length, although we are able to perform accurate fine movements. Considering the known dissociation between action-oriented and perception-oriented body representations, we asked whether the body model mainly serves body perception or whether it is also involved in movements. Twenty-one healthy adults were administered with the Localization Task (LT) which required the participants to localise the perceived position of their unseen hand by means of a stick held by their other hand, and the Proprioceptive Matching Task (PMT) which required the participants to match their perceived hand position with a visual target. LT and PMT maps were compared with the actual hand sizes. We found that the PMT map exhibited similar body model distortions, confirming that the body model is involved in motor programming. Furthermore, we observed that a partial adjustment of the distortions occurs in a motor condition.     

Judgments of reachability are independent of visuomotor adaptation

The furthest distance that is judged to be reachable can change after participants have used a tool or if they are led to misjudge the position of their hand. Here we investigated how judged reachability changed when visual feedback about the hand was shifted. We hoped to distinguish between various ways in which visuomotor adaptation could influence judged reachability. Participants had to judge whether they could reach a virtual cube without actually doing so. They indicated whether they could reach this virtual cube by moving their hand. During these hand movements, visual feedback about the position of the hand was shifted in depth, either away from or toward the participant. Participants always adapted to the shifted feedback. In a session in which the hand movements in the presence of visual feedback were mainly in depth, perceived reachability shifted in accordance with the feedback (more distant cubes were judged to be reachable when feedback was shifted further away). In a second session in which the hand movements in the presence of visual feedback were mainly sideways, for some participants perceived reachability shifted in the opposite direction than we expected. The shift in perceived reachability was not correlated with the adaptation to the shift in visual feedback. We conclude that reachability judgments are not directly related to visuomotor adaptation.     

Seeing the body distorts tactile size perception

Vision of the body modulates somatosensation, even when entirely non-informative about stimulation. For example, seeing the body increases tactile spatial acuity, but reduces acute pain. While previous results demonstrate that vision of the body modulates somatosensory sensitivity, it is unknown whether vision also affects metric properties of touch, and if so how. This study investigated how non-informative vision of the body modulates tactile size perception. We used the mirror box illusion to induce the illusion that participants were directly seeing their stimulated left hand, though they actually saw their reflected right hand. We manipulated whether participants: (a) had the illusion of directly seeing their stimulated left hand, (b) had the illusion of seeing a non-body object at the same location, or (c) looked directly at their non-stimulated right-hand. Participants made verbal estimates of the perceived distance between two tactile stimuli presented simultaneously to the dorsum of the left hand, either 20, 30, or 40 mm apart. Vision of the body significantly reduced the perceived size of touch, compared to vision of the object or of the contralateral hand. In contrast, no apparent changes of perceived hand size were found. These results show that seeing the body distorts tactile size perception.     

An illusion you can sink your teeth into: haptic cues modulate the perceived freshness and crispness of pretzels

Eating is a multisensory experience involving more than simply the oral sensation of the taste and smell of foods. It has been shown that the way foods look, sound, and feel like in the mouth all affect food perception. The influence of haptic information available when handling food is relatively unknown. In this study, blindfolded participants bit-into fresh or stale pretzels while rating their freshness staleness and crispness-softness. Information provided to the hand was either congruent (whole pretzel fresh or stale) or incongruent (half pretzel fresh, half stale) with what was presented to the mouth. The results demonstrate that the perception of both freshness and crispness was systematically altered when incongruent information was provided: bit-into fresh pretzel tips were perceived as staler and softer when a stale pretzel tip was held in the hand and vice versa. Haptic information available when handling food thus plays a significant role in modulating food perception.     

Action goal selection and motor planning can be dissociated by tool use

The preparation of eye or hand movements enhances visual perception at the upcoming movement end position. The spatial location of this influence of action on perception could be determined either by goal selection or by motor planning. We employed a tool use task to dissociate these two alternatives. The instructed goal location was a visual target to which participants pointed with the tip of a triangular hand-held tool. The motor endpoint was defined by the final fingertip position necessary to bring the tool tip onto the goal. We tested perceptual performance at both locations (tool tip endpoint, motor endpoint) with a visual discrimination task. Discrimination performance was enhanced in parallel at both spatial locations, but not at nearby and intermediate locations, suggesting that both action goal selection and motor planning contribute to visual perception. In addition, our results challenge the widely held view that tools extend the body schema and suggest instead that tool use enhances perception at those precise locations which are most relevant during tool action: the body part used to manipulate the tool, and the active tool tip.     

Attentionally splitting the mass distribution of hand-held rods

Two experiments on the length-perception capabilities of effortful or dynamic touch differed only in terms of what the subject intended to perceive, while experimental conditions and apparatus were held constant. In each trial, a visually occluded rod was held as still as possible by the subject at an intermediate position. For two thirds of the trials, a weight was attached to the rod above or below the hand. In Experiment 1, in which the subject's task was to perceive the distance reachable with the portion of the rod forward of the hand, perceived extent was a function of the first moment of the mass distribution associated with the forward portion of the rod, and indifferent to the first moment of the entire rod. In Experiment 2, in which the task was to perceive the distance reachable with the entire rod if it was held at an end, the pattern of results was reversed. These results indicate the capability of selective sensitivity to different aspects of a hand-held object's mass distribution, without the possibility of differential exploration specific to these two tasks. Results are discussed in relation to possible roles of differential information, intention, and self-organization in the explanations of selective perceptual abilities.     

Changing grasp position on a wielded object provides self-training for the perception of length

Calibration of perception to environmental properties typically requires experiences in addition to the perceptual task, such as feedback about performance. Recently, it has been shown that such experiences need not come from an external source or from a different perceptual modality. Rather, in some cases, a given perceptual modality can train itself. In this study, we sought to expand on the range of experiences in which this can occur for perception of the length of a wielded occluded object. Specifically, in two experiments, we investigated whether the act of perceiving the length of a wielded object from a given grasp position could recalibrate the perception of length from a different grasp position. In both experiments, three groups of participants perceived the lengths of wielded rods in a pretest, practice, and a posttest. The practice included either (a) experimenter feedback, (b) changing the grasp position on the object (and again attempting to perceive length), or (c) no additional experiences. In Experiment 1, participants changed their grasp position from the middle to the end of each rod, and in Experiment 2, they did so from the end to the middle of each rod. In both experiments, the results showed that perceiving length from a different grasp position can recalibrate (i.e., provide self-training for) the perception of length.     

Perception of whether an object can be carried through an aperture depends on anticipated speed

We investigated whether anticipated speed of locomotion through an aperture influences perception of whether an object can be carried through that aperture. Participants reported whether they would be able to carry objects through an aperture (a) if they were to attempt to walk through the aperture and (b) if they were to attempt to run through the aperture. Furthermore, they did so when the object was held but not seen and when the object was seen but not held. In general, perception was influenced by object width and by anticipated speed but not by perceptual modality. Perceptual boundaries occurred at smaller object widths when participants anticipated running through the aperture than when they anticipated walking through the aperture. The results build on work showing that perception of affordances is influenced by kinetic potential as well as geometric properties and that perception may be supported by the detection of modality-neutral stimulation patterns.     

Temperature perception on the hand during static versus dynamic contact with a surface

Innocuous cooling or heating of the forearm can evoke nociceptive sensations, such as burning, stinging, and pricking (low-threshold thermal nociception, LTN), that are inhibited by dynamic contact. In the present study, I investigated whether LTN can also be perceived on the hand, and if so, whether it is normally suppressed by active touching. Innocuous cold (28°, 25°, and 18°C) and warm (38°, 40°, and 43°C) temperatures were delivered to the distal metacarpal pads and intermediate and distal phalanges of the fingers via a handgrip thermode that subjects either statically held or actively grasped. The same temperatures were delivered to the forearm via another thermode that either rested on the arm or was touched to the arm. Subjects rated the intensity of thermal (warmth, cold) and nociceptive (e.g., burning) sensations and indicated the qualities of sensation experienced. The results showed that LTN can be perceived on the hand, although less frequently and less intensely than on the forearm. Dynamic contact inhibited nociceptive and thermal sensations on the hand, although less strongly than on the forearm. These findings indicate that temperature perception on the hand is attenuated and its quality is changed when thermal stimulation is accompanied by dynamic tactile stimulation, as it is during haptic exploration.     

Embodied Information in Cognitive Tasks: Haptic Weight Sensations Affect Task Performance and Processing Style

Research in the field of embodied cognition showed that incidental weight sensations influence peoples’ judgments about a variety of issues and objects. Most studies found that heaviness compared to lightness increases the perception of importance, seriousness, and potency. In two experiments, we broadened this scope by investigating the impact of weight sensations on cognitive performance. In Experiment 1, we found that the performance in an anagram task was reduced when participants held a heavy versus a light clipboard in their hands. Reduced performance was accompanied by an increase in the perceived effort. In Experiment 2, a heavy clipboard elicited a specific response heuristic in a two-alternative forced-choice task. Participants showed a significant right side bias when holding a heavy clipboard in their hands. After the task, participants in the heavy clipboard condition reported to be more frustrated than participants in the light clipboard condition. In both experiments, we did not find evidence for mediated effects that had been proposed by previous literature. Overall, the results indicate that weight effects go beyond judgment formation and highlight new avenues for future research.     

Effects of motor intention on the perception of somatosensory events: a behavioural and functional magnetic resonance imaging study

The intention to execute a movement can modulate our perception of sensory events, and this modulation is observed ahead of both ocular and upper limb movements. However, theoretical accounts of these effects, and also the empirical data, are often contradictory. Accounts of "active touch", and the premotor theory of attention, have emphasized how movement intention leads to enhanced perceptual processing at the target of a movement, or on the to-be-moved effector. By contrast, recent theories of motor control emphasize how internal "forward" model (FM) estimates may be used to cancel or attenuate sensory signals that arise as a result of self-generated movements. We used behavioural and functional brain imaging (functional magnetic resonance imaging, fMRI) to investigate how perception of a somatosensory stimulus differed according to whether it was delivered to a hand that was about to execute a reaching movement or the alternative, nonmoving, hand. The results of our study demonstrate that a somatosensory stimulus delivered to a hand that is being prepared for movement is perceived to have occurred later than when that same stimulus is delivered to a nonmoving hand. This result indicates that it takes longer for a tactile stimulus to be detected when it is delivered to a moving limb and may correspond to a change in perceptual threshold. Our behavioural results are paralleled by the results of our fMRI study that demonstrated that there were significantly reduced blood-oxygen-level-dependent (BOLD) responses within the parietal operculum and insula following somatosensory stimulation of the hand being prepared for movement, compared to when an identical stimulus was delivered to a nonmoving hand. These findings are consistent with the prediction of FM accounts of motor control that postulate that central sensory suppression of somatosensation accompanies self-generated limb movements, and with previous reports indicating that effects of sensory suppression are observed in higher order somatosensory regions.     

Multimodal Perception of Reachability Expressed Through Locomotion

We investigated the information that supports perception of whether an object is within reach using a locomotor task. Participants adjusted their own position relative to a fixed target by stepping or by propelling a wheelchair until they judged it to be within reach. The to-be-reached object was presented in virtual reality. The display of the target was driven in real time as a function of the observer's movement, thus depicting a stationary virtual object at a definite distance only through the relation across optical and nonoptical patterns of stimulation. We asked participants to judge the distance they could reach with their unaided hand or when holding a rod that extended their effective reach. They could see neither their body nor the rod thereby limiting available visual information about “reachability.” As expected, our results showed that despite the limited information that was available, participants' locomotor adjustments were influenced by (a) their simulated distance from the target, (b) their arm length, and (c) the presence or absence of the rod. The type of motion (stepping or wheelchair) had little influence. However, judgment accuracy was influenced by participants' initial simulated distance from the target. We compare the performance obtained in our locomotor judgment task with previous studies that have used different methods for measuring perceived reaching-ability. We discuss perceptual information that could have supported performance within the framework of the global array.     

The influence of embodiment on multisensory integration using the mirror box illusion

We examined the relationship between subcomponents of embodiment and multisensory integration using a mirror box illusion. The participants’ left hand was positioned against the mirror, while their right hidden hand was positioned 12″, 6″, or 0″ from the mirror – creating a conflict between visual and proprioceptive estimates of limb position in some conditions. After synchronous tapping, asynchronous tapping, or no movement of both hands, participants gave position estimates for the hidden limb and filled out a brief embodiment questionnaire. We found a relationship between different subcomponents of embodiment and illusory displacement towards the visual estimate. Illusory visual displacement was positively correlated with feelings of deafference in the asynchronous and no movement conditions, whereas it was positive correlated with ratings of visual capture and limb ownership in the synchronous and no movement conditions. These results provide evidence for dissociable contributions of different aspects of embodiment to multisensory integration.     

The effect of vertical tongue loading on the position perception of the tongue

We investigated the effects of vertical tongue loading on the position perception of the tongue. Five male and 5 female university students served as subjects. Vertical upward and downward loading forces were applied to the tongue of the subjects. Their task was to judge the perceived horizontal position of the tongue after tongue-loading directions. The means of the judgments for the control conditions (no tongue loading) were compared with the judgments for perceived horizontal position after tongue loading. The results showed that vertical tongue loading produced a shift in the perceived horizontal direction opposite to the applied force. These results fully replicated the analogous aftereffect found by Grover and Craske (1991) for horizontal tongue loading. However, the judgments of perceived horizontal position in the present study had lower variability than did those in Grover and Craske's study, suggesting that mapping of the tongue along the vertical axis is more precise than mapping along the horizontal axis.     

Perceiving extents of rods by wielding: haptic diagonalization and decomposition of the inertia tensor

We report two experiments on the length-perception capabilities of the hand-related haptic subsystem. On each trial, a visually occluded rod was wielded by the subject at a position intermediate between its two ends. The position was either 1/2 or 3/4 of the rod's length. On two-thirds of the trials, a weight was attached to the rod at a point either above or below its center of gravity and not coincident with the hand's position. In Experiment 1, the subject's task was to perceive the distance reachable with the portion of the rod extending beyond the position of the grasp. In the second experiment, the subject's task was to perceive the distance reachable with the entire rod if it were held at its proximal end. In Experiment 1, perceived reaching distance was a function of the moment of inertia of the amount of rod forward of the grasp about an axis through the proximal end of the rod segment. In Experiment 2, perceived reaching distance was a function of the moment of inertia of the entire rod about the given axis of rotation intermediate between the rod's ends. The results are discussed in terms of (a) the notion of smart perceptual instruments capitalizing on invariant properties of the inertia tensor and (b) how the haptic decomposition of moments of inertia follows the principle of equivalence of forces.     

About the role of bottom-up and top-down processes on perception–action interaction in sensorimotor transformations

Theories of common-coding propose that feature codes of perceived and to-be-produced events are likely to interact with each other when they overlap. We investigated the impact of bottom-up and top-down processes on cross talk in a motor replication task. Participants moved a pen on a covered digitizer tablet while a gain varied the relation between hand and cursor amplitude. Then, participants replicated the hand amplitude (intra-modal) or the cursor amplitude (intermodal) without visual feedback. We replicated that, when the not-to-be-replicated amplitude was longer (shorter) than the to-be-replicated amplitude, replications significantly overshot (undershot) (= after-effects). Importantly, after-effects were remarkably smaller in the experimental groups which wore gloves (thin and thick rubber) or goggles (clear and tinted lenses) than in a control group. Our results provide evidence that top-down attention modulated perception–action interaction.     

Visual tracking and entrainment to an environmental rhythm

This study investigated the role that visual tracking plays in coupling rhythmic limb movements to an environmental rhythm. Two experiments were conducted in which participants swung a hand-held pendulum while tracking an oscillating stimulus or while keeping their eyes fixed on a stationary location directly above an oscillating stimulus. It was expected that the participants' rhythmic movements would become entrained to the oscillating stimulus in both conditions but that visual tracking would strengthen this entrainment. Experiment 1 investigated the role of visual tracking in establishing unintentional entrainment. Experiment 2 investigated the role of visual tracking in intentional entrainment. As predicted, participants exhibited greater unintentional coordination and more stable intentional coordination when they tracked the stimulus. These findings highlight the importance of understanding the role of eye movements in environmental coordination.     

Proprioceptive space perception after anaesthetization of the mid-interphalangeal joint of the finger

The effect of a digital block on the voluntary positioning of the index finger from a point of extreme extension through 45° flexion was examined in 12 subjects. A xylocaine ring block of the second proximal interphalangeal joint was used to eliminate the joint receptors while weights were attached to the finger. An inverse relationship was found between weight and distance moved when the subject was required to move the index finger of the nonpreferred hand to the required position. The results indicate that the muscular system is ancillary to the joint system in proprioceptive space perception.