Weight illusions and weight discrimination-a revised hypothesis

F = ma. Is the macaque brain Newtonian?

Intuitive Physics, the ability to anticipate how the physical events involving mass objects unfold in time and space, is a central component of intelligent systems. Intuitive physics is a promising tool for gaining insight into mechanisms that generalize across species because both humans and non-human primates are subject to the same physical constraints when engaging with the environment. Physical reasoning abilities are widely present within the animal kingdom, but monkeys, with acute 3D vision and a high level of dexterity, appreciate and manipulate the physical world in much the same way humans do.

The size-weight illusion comes along with improved weight discrimination

When people judge the weight of two objects of equal mass but different size, they perceive the smaller one as being heavier. Up to date, there is no consensus about the mechanisms which give rise to this size-weight illusion. We recently suggested a model that describes heaviness perception as a weighted average of two sensory heaviness estimates with correlated noise: one estimate derived from mass, the other one derived from density. The density estimate is first derived from mass and size, but at the final perceptual level, perceived heaviness is biased by an object's density, not by its size. Here, we tested the models' prediction that weight discrimination of equal-size objects is better in lifting conditions which are prone to the size-weight illusion as compared to conditions lacking (the essentially uninformative) size information. This is predicted because in these objects density covaries with mass, and according to the model density serves as an additional sensory cue. Participants performed a two-interval forced-choice weight discrimination task. We manipulated the quality of either haptic (Experiment 1) or visual (Experiment 2) size information and measured just-noticeable differences (JNDs). Both for the haptic and the visual illusion, JNDs were lower in lifting conditions in which size information was available. Thus, when heaviness perception can be influenced by an object's density, it is more reliable. This discrimination benefit under conditions that provide the additional information that objects are of equal size is further support for the role of density and the integration of sensory estimates in the size-weight illusion.

A meta-analysis of the size-weight and material-weight illusions

The current study comprises the first systematic meta-analysis of weight illusions. We obtained descriptive data from studies in which subjective heaviness estimates were made for pairs or groups of objects that had the same mass and different volumes (size-weight illusion; SWI) or different apparent material properties (material-weight illusion; MWI). Using these data, we calculated mean effect sizes to represent illusion strength. Other study details, including stimulus mass, volume, density, and degree of visual and somatosensory access to the stimuli were also recorded to quantify the contribution of these variables to effect sizes for the SWI. The results indicate that the SWI has a larger mean effect size than the MWI and that the former is consistent in strength when information about stimulus size is gained through somatosensory channels, regardless of visual access. The SWI is weaker when only the visual system provides size information. Effect sizes for the SWI were larger when there was a greater difference in volume across the stimuli. There was also a positive correlation between SWI strength and the difference in physical density across the different experimental stimuli, even after controlling for volume differences. Together, we argue that these findings provide support for theories of weight illusions that are based on conceptual expectancies as well as those that are based on bottom-up processing of physical density. We further propose that these processes, which have been considered dichotomously in the past, may not be mutually exclusive from each other and could both contribute to our perception of weight when we handle objects in everyday life.

The development of the size-weight illusion in children coincides with the development of nonverbal cognition rather than motor skills

We examined how the strength of the size-weight illusion develops with age in typically developing children. To this end, we recruited children aged 5-12 years and quantified the degree to which they experienced the illusion. We hypothesized that the strength of the illusion would increase with age. The results supported this hypothesis. We also measured abilities in manual dexterity, receptive language, and abstract reasoning to determine whether changes in illusion strength were associated with these factors. Manual dexterity and receptive language did not correlate with illusion strength. Conversely, illusion strength and abstract reasoning were tightly coupled with each other. Multiple regression further revealed that age, manual dexterity, and receptive language did not contribute more to the variance in illusion strength beyond children's abilities in abstract reasoning. Taken together, the effects of age on the size-weight illusion appear to be explained by the development of nonverbal cognition. These findings not only inform the literature on child development but also have implications for theoretical explanations on the size-weight illusion. We suggest that the illusion has a strong acquired component to it and that it is strengthened by children's reasoning skills and perhaps an understanding of the world that develops with age.

The influence of size in weight illusions is unique relative to other object features

Research into weight illusions has provided valuable insight into the functioning of the human perceptual system. Associations between the weight of an object and its other features, such as its size, material, density, conceptual information, or identity, influence our expectations and perceptions of weight. Earlier accounts of weight illusions underscored the importance of previous interactions with objects in the formation of these associations. In this review, we propose a theory that the influence of size on weight perception could be driven by innate and phylogenetically older mechanisms, and that it is therefore more deep-seated than the effects of other features that influence our perception of an object's weight. To do so, we first consider the different associations that exist between the weight of an object and its other features and discuss how different object features influence weight perception in different weight illusions. After this, we consider the cognitive, neurological, and developmental evidence, highlighting the uniqueness of size-weight associations and how they might be reinforced rather than driven by experience alone. In the process, we propose a novel neuroanatomical account of how size might influence weight perception differently than other object features do.

Contribution of surface material and size to the expected versus the perceived weight of objects

Because the perceived weight of objects may be affected by various nonweight properties, such as their size and the density of their surface material, relative weight is sometimes misperceived (the size-weight illusion and the material-weight illusion, respectively). A widely accepted explanation for weight illusions is provided by the so-called expectation model, according to which the perceived weight stems from the contrast between the actual and expected weights. In the present study, we varied both the surface material and the size of stimuli, while keeping constant their physical weights. In Experiment 1, the participants lifted the stimuli by grasping them on opposite sides, whereas in Experiment 2 they lifted them by using a string that was attached to their top surface. We used a variant of the random conjoint measurement paradigm to obtain subjective interval scales of the contributions of surface material and size to the expected and the perceived weight of the stimuli. Inconsistently with the predictions from the expectation model, we found, in both experiments, that the surface material contributed more than the size to the expected weight, whereas the size contributed more than the surface material to the perceived weight. The results support the hypothesis that perceived weight may depend on implicit, rather than explicit, weight expectations.

Individualistic weight perception from motion on a slope

Perception of an object’s weight is linked to its form and motion. Studies have shown the relationship between weight perception and motion in horizontal and vertical environments to be universally identical across subjects during passive observation. Here we show a contradicting finding in that not all humans share the same motion-weight pairing. A virtual environment where participants control the steepness of a slope was used to investigate the relationship between sliding motion and weight perception. Our findings showed that distinct, albeit subjective, motion-weight relationships in perception could be identified for slope environments. These individualistic perceptions were found when changes in environmental parameters governing motion were introduced, specifically inclination and surface texture. Differences in environmental parameters, combined with individual factors such as experience, affected participants’ weight perception. This phenomenon may offer evidence of the central nervous system’s ability to choose and combine internal models based on information from the sensory system. The results also point toward the possibility of controlling human perception by presenting strong sensory cues to manipulate the mechanisms managing internal models.

The role of expectancies in the size-weight illusion: a review of theoretical and empirical arguments and a new explanation

The size-weight illusion (SWI) refers to the phenomenon that objects that are objectively equal in weight but different in size or volume are perceived to differ in weight, such that smaller objects feel heavier than larger ones. This article reviews studies trying to support three different viewpoints with respect to the role of expectancies in causing the SWI. The first viewpoint argues for a crucial role; the second admits a role, yet without seeing consequences for sensorimotor processes; and the third denies any causal role for expectancies at all. A new explanation of the SWI is proposed that can integrate the different arguments. A distinctive feature of the new explanation is that it recognizes the causal influence of expectancies, yet combines this with certain reactive and direct behavioral consequences of perceiving size differences that are independent of experience-based expectancies, and that normally result in the adaptive application of forces to lift or handle differently sized objects. The new account explains why the illusion is associated with the repeated generation of inappropriate lifting forces (which can, however, be modified through extensive training), as well as why it depends on continuous visual exposure to size cues, appears at an early age, and is cognitively impenetrable.

Anisotropy in the haptic perception of force direction and magnitude

Although force-feedback devices are already being used, the human ability to perceive forces has not been documented thoroughly. The haptic perception of force direction and magnitude has mostly been studied in discrimination tasks in the direction of gravity. In our study, the influence of physical force direction on haptic perception of force magnitude and direction was studied in the horizontal plane. Subjects estimated the direction and magnitude of a force exerted on their stationary hand. A significant anisotropy in perception of force magnitude and direction was found. Force direction data showed significant subject-dependent distortions at various physical directions. Normalized force magnitude data showed a consistent elliptical pattern, with its minor axis pointing roughly from the subject's hand to his/her shoulder. This pattern could be related to arm stiffness or manipulability patterns, which are also ellipse-shaped. These ellipses have an orientation consistent with the distortion measured in our study. So, forces in the direction of highest stiffness and lowest manipulability are perceived as being smaller. It therefore seems that humans possess a "sense of effort" rather than a "sense of force," which may be more useful in everyday life. These results could be useful in the design of haptic devices.

Charpentier's Papers of 1886 and 1891 on Weight Perception and the Size-Weight Illusion

The French physiologist Augustin Charpentier (1852–1916) published the first accounts of the size-weight illusion—the observation that if two objects differ in size but have equal mass, the smaller will feel heavier when lifted. In the current paper, translations are presented of Charpentier's much-cited 1891 paper on weight perception and the size-weight illusion, and his little-known brief 1886 paper which contains the earliest experimental data on the illusion. Charpentier explained weight illusions in terms of the sense of effort involved in lifting the object and the contrast with the expected effort. Modern research shows that people quickly adapt and use the appropriate force to pick up objects, but the illusion persists even when appropriate force is used; expectations therefore affect the perceptual system more strongly than the motor system.

Superior size-weight illusion performance in patients with schizophrenia: evidence for deficits in forward models

When non-psychiatric individuals compare the weights of two similar objects of identical mass, but of different sizes, the smaller object is often perceived as substantially heavier. This size-weight illusion (SWI) is thought to be generated by a violation of the common expectation that the large object will be heavier, possibly via a mismatch between an efference copy of the movement and the actual sensory feedback received. As previous research suggests that patients with schizophrenia have deficits in forward model/efference copy mechanisms, we hypothesized that schizophrenic patients would show a reduced SWI. The current study compared the strength of the SWI in schizophrenic patients to matched non-psychiatric participants; weight discrimination for same-sized objects was also assessed. We found a reduced SWI for schizophrenic patients, which resulted in better (more veridical) weight discrimination performance on illusion trials compared to non-psychiatric individuals. This difference in the strength of the SWI persisted when groups were matched for weight discrimination performance. The current findings are consistent with a dysfunctional forward model mechanism in this population. Future studies to elucidate the locus of this impairment using variations on the current study are also proposed.

Impact of order and load knowledge on trunk kinematics during repeated lifting tasks

OBJECTIVE

To determine if lifting random unknown weights is more detrimental than lifting sequences of unknown weights and to investigate whether load knowledge impacts the effect of lifting random box weights.

BACKGROUND

Much research has investigated lifting under known load conditions, but few studies have investigated unknown loads, especially when presented in random order. There has been some documentation of alteration in trunk mechanics when there is an overestimation of the unknown load.

METHOD

Ten men and 10 women performed three lifting tasks: random unknown, random known, and same weight. A lumbar motion monitor was used to collect kinematic data, and Borg's Rating of Perceived Exertion (RPE) and a task risk rating were also assessed.

RESULTS

Both presentation order and load knowledge impacted trunk kinematics during repeated lifting tasks. However, these differences were relatively low in magnitude. Furthermore, kinematic response and perceived risk and exertion for these conditions varied between genders.

CONCLUSION

Lifting random unknown loads appears to alter kinematic responses, particularly for men. Women attempt to modify the effect of random unknown loads by changing the lifting style through alterations in upper limb motions (e.g., drag box toward them prior to lifting). However, a need remains for a more comprehensive biomechanical investigation (e.g., spine loading) into the effects of random unknown loads because many of the effect sizes were small.

APPLICATION

Small kinematic adaptations resulting from tasks involving unknown and random loads may be mediated by the use of visual cues, order of presentation, or a change in lifting style.

Rotational kinematics influence multimodal perception of heaviness

Perceived heaviness has been shown to be specific to an object's rotational inertia (I), its resistance to rotational acceleration. According to the kinematic specification of dynamics (KSD) principle, we hypothesized that I is optically specified by rotational kinematics. Using virtual depictions of wielded objects, we investigated whether the visually detected rotational kinematics of wielded objects would influence perceived heaviness in a manner consistent with the inertial model of heaviness perception. We scaled the virtual object's movement so that it rotated more or less than its wielded counterpart, specifying lower and higher I, respectively. Perceived heaviness was inversely related to the rotational scaling factor, consistent with a KSD interpretation of the inertial model.

Metamers in the haptic perception of heaviness and moveableness

It is hypothesized that heaviness perception for a freely wielded nonvisible object can be mapped to a point in a three-dimensional heaviness space. The three dimensions are mass, the volume of the inertia ellipsoid, and the symmetry of the inertia ellipsoid. Within this space, particular combinations yield heaviness metamers (objects of different mass that feel equally heavy), whereas other combinations yield analogues to the size-weight illusion (objects of the same mass that feel unequally heavy). Evidence for the two types of combinations was provided by experiments in which participants wielded occluded hand-held objects and estimated the heaviness of the objects relative to a standard. Further experiments with similar procedures showed that metamers of heaviness were metamers of moveableness but not metamers of length. A promising conjecture is that the haptic perceptual system maps the combination of an object's inertia for translation and inertia for rotation to a perception of the object's maneuverability.

Independence and separability of volume and mass in the size-weight illusion

Numerous size-weight illusion models were classified in the present article according to general recognition theory (Ashby & Townsend, 1986), wherein the illusion results from a lack of perceptual separability, perceptual independence, decisional separability, or a combination of the three. These options were tested in two experiments in which a feature-complete factorial design and multidimensional signal detection analysis were used (Kadlec & Townsend, 1992a, 1992b). With haptic touch alone, the illusion was associated with a lack of perceptual and decisional separability. When the participant viewed the stimulus in his or her hand, the illusion was associated only with a lack of decisional separability. Visual input appeared to improve the discrimination of mass, leaving only the response bias due to expectation.

The size-weight illusion in 2-D nonlinear psychophysics

An extension of unidimensional nonlinear psychophysics is postulated by using forms of cross-coupling between the parameters of the two single-channel recursions, which have already been shown to model some perceptual phenomena. The size-weight illusion is shown to be reproducible in the topology of its relations, and it is suggested that some so-called illusions are in fact the natural consequences of nonlinear cross-coupling. The conditions that produce the illusion involve partially compensating the cross-coupling of sensory dimensions, and a second equilibrium with no cross-coupling, resembling simpler veridical perception, also exists in the behavior of some subjects.

Stroop interference based on the multimodal correlates of haptic size and auditory pitch

In a preliminary experiment subjects were asked to explore three wooden knobs of different sizes and to rate each one on a series of 7-point scales. The results confirmed that an object may possess a number of multimodal qualities that are contingent on its haptic size. The pattern of intercorrelations between the qualities was consistent with the pattern that is observed when subjects respond to pure auditory tones varying in pitch. For example, small (high-pitched) sounds, like small objects, are judged to be sharp, thin, light, weak, fast, tense, and bright. The main experiments used a paradigm based on the Stroop interference effect. Subjects were required to press one of two keys as quickly as possible depending on which of four possible words appeared in the centre of the screen. A 50 Hz or a 5500 Hz tone accompanied each test word, and subjects responded on two keys that differed in size. Subjects were found to respond more slowly when either the pitch of the incidental sound or the size of the key on which they responded was incongruent with the multimodal features represented by the test word. The results confirm that people are automatically and immediately sensitive to the multimodal features of a stimulus when direct sensory evidence for the features is absent.

How important are changes in body weight for mass perception?

It is often assumed that weight judgements depend primarily on the effort experienced in lifting an object against a 1-G force. Changes in effort and in other weight-cues certainly alter apparent heaviness; but there is a tendency towards mass-constancy when such changes are unrelated to mass. Under water or altered G, both the observer's body and other objects change their effective weight: the change in the former probably provides a cue to the latter. Mass-constancy increases with opportunity for adaptation to the change, leaving a negative aftereffect on return to normal circumstances. The discrimination of weight or mass also deteriorates with sudden changes in arm weight, just as it does with other types of maladaptation and with a reduction in sensory cues. The relative importance of arm weight and other factors has not been precisely measured, but experiments in prolonged spaceflight should help to elucidate the issue.

Wundt's views on sensations of innervation: a reevaluation

It is sometimes stated that Wundt believed in the primacy of 'sensations of innervation' in the control of eye and limb position, to the exclusion of afferent feedback. Wundt's own statements on the subject are traced through the six editions of the Grundzüge: he believed that sensations of innervation were a contributory factor along with peripheral feedback; but in the fourth edition he dropped the term and subsumed them under 'central sensations of senses', because increased neurophysiological knowledge made a literal interpretation of the original term impossible. In the fifth edition he developed a more precise model of central processes, described as 'sensory costimulation', and an additional idea of 'reproduced' or conditioned sensations. He mentions clinical evidence on the perception of eye and limb position of weight by paralysed and other subjects. This evidence is discussed in relation to modern theories. It is concluded that Wundt's later theories have some similarities with modern "hybrid' theories of efference-contingent afference.