The (Un)ideal Physicist: How Humans Rely on Object Interaction for Friction Estimates.

To estimate object properties such as mass or friction, our brain relies on visual information to efficiently compute approximations. The role of sensorimotor feedback, however, is not well understood. Here we tested healthy adults (N = 79) in an inclined-plane problem, that is, how much a plane can be tilted before an object starts to slide, and contrasted the interaction group with observation groups who accessed involved forces by watching objects being manipulated. We created objects of different masses and levels of friction and asked participants to estimate the critical tilt angle after pushing an object, lifting it, or both. Estimates correlated with applied forces and were biased toward object mass, with higher estimates for heavier objects. Our findings highlight that inferences about physical object properties are tightly linked to the human sensorimotor system and that humans integrate sensorimotor information even at the risk of nonveridical perceptual estimates.

New evidence for the sensorimotor mismatch theory of weight perception and the size-weight illusion.

The size-weight illusion is a phenomenon where a smaller object is perceived heavier than an equally weighted larger object. The sensorimotor mismatch theory proposed that this illusion occurs because of a mismatch between efferent motor commands and afferent sensory feedback received when lifting large and small objects (i.e., the application of too little and too much lifting force, respectively). This explanation has been undermined by studies demonstrating a separation between the perceived weight of objects and the lifting forces that are applied on them. However, this research suffers from inconsistencies in the choice of lifting force measures reported. Therefore, we examined the contribution of sensorimotor mismatch in the perception of weight in the size-weight illusion and in non-size-weight illusion stimuli and evaluated the use of a lifting force aggregate measure comprising the four most common lifting force measures used in previous research. In doing so, the sensorimotor mismatch theory was mostly supported. In a size-weight illusion experiment, the lifting forces correlated with weight perception and, contrary to some earlier research, did not adapt over time. In a non-size-weight illusion experiment, switches between lifting light and heavy objects resulted in perceiving the weight of these objects differently compared to no switch trials, which mirrored differences in the manner participants applied forces on the objects. Additionally, we reveal that our force aggregate measure can allow for a more sensitive and objective examination of the effects of lifting forces on objects.

Unveiling the invisible: receivers use object weight cues for grip force planning in handover actions.

Handover actions are part of our daily lives. Whether it is the milk carton at the breakfast table or tickets at the box office, we usually perform these joint actions without much conscious attention. The individual actions involved in handovers, that have already been studied intensively at the level of individual actions, are grasping, lifting, and transporting objects. Depending on the object's properties, actors must plan their execution in order to ensure smooth and efficient object transfer. Therefore, anticipatory grip force scaling is crucial. Grip forces are planned in anticipation using weight estimates based on experience or visual cues. This study aimed to investigate whether receivers are able to correctly estimate object weight by observing the giver's kinematics. For this purpose, handover actions were performed with 20 dyads, manipulating the participant role (giver/receiver) and varying the size and weight of the object. Due to the random presentation of the object weight and the absence of visual cues, the participants were unaware of the object weight from trial to trial. Kinematics were recorded with a motion tracking system and grip forces were recorded with customized test objects. Peak grip force rates were used as a measure of anticipated object weight. Results showed that receiver kinematics are significantly affected by object weight. The peak grip force rates showed that receivers anticipate object weight, but givers not. This supports the hypothesis that receivers obtain information about the object weight by observing giver's kinematics and integrating this information into their own action execution.

Object weight can be rapidly predicted, with low cognitive load, by exploiting learned associations between the weights and locations of objects.

Weight prediction is critical for dexterous object manipulation. Previous work has focused on lifting objects presented in isolation and has examined how the visual appearance of an object is used to predict its weight. Here we tested the novel hypothesis that when interacting with multiple objects, as is common in everyday tasks, people exploit the locations of objects to directly predict their weights, bypassing slower and more demanding processing of visual properties to predict weight. Using a three-dimensional robotic and virtual reality system, we developed a task in which participants were presented with a set of objects. In each trial a randomly chosen object translated onto the participant's hand and they had to anticipate the object's weight by generating an equivalent upward force. Across conditions we could control whether the visual appearance and/or location of the objects were informative as to their weight. Using this task, and a set of analogous web-based experiments, we show that when location information was predictive of the objects' weights participants used this information to achieve faster prediction than observed when prediction is based on visual appearance. We suggest that by "caching" associations between locations and weights, the sensorimotor system can speed prediction while also lowering working memory demands involved in predicting weight from object visual properties.NEW & NOTEWORTHY We use a novel object support task using a three-dimensional robotic interface and virtual reality system to provide evidence that the locations of objects are used to predict their weights. Using location information, rather than the visual appearance of the objects, supports fast prediction, thereby avoiding processes that can be demanding on working memory.

Association between accuracy of weight perception and life satisfaction among adults with and without anxiety and mood disorders: a cross-sectional study of the 2015-2018 Canadian Community Health Survey.

BACKGROUND: Weight status and weight perception have a significant impact on life satisfaction. As overweight prevalence increases in Canada, it is important to understand how accuracy of weight perception (AWP) is associated with life satisfaction. This study explored the association between AWP and life satisfaction among Canadian adults with and without anxiety and/or mood disorders. METHODS: Using data from the 2015-2018 cycles of the Canadian Community Health Survey, an indicator of AWP was created to capture concordance between perceived weight and actual weight status. Univariate and multivariate Gaussian generalized linear models were assessed while stratifying by sex and presence of anxiety and/or mood disorders. RESULTS: Our sample included 88 814 males and 106 717 females. For both sexes, perceiving oneself as overweight or underweight, regardless of actual weight status, was associated with lower life satisfaction (ß = -0.93 to -0.30), compared to those who accurately perceived their weight as 'just about right'. Perceiving oneself as overweight or underweight was associated with more pronounced differences in life satisfaction scores in those with anxiety and/or mood disorders (ß = -1.49 to -0.26) than in those without these disorders (ß = -0.76 to -0.25). CONCLUSION: Weight perception is more indicative of life satisfaction than actual weight status, especially in those with anxiety and/or mood disorders.

The effects of TMS over the anterior intraparietal area on anticipatory fingertip force scaling and the size-weight illusion.

When lifting an object skillfully, fingertip forces need to be carefully scaled to the object's weight, which can be inferred from its apparent size and material. This anticipatory force scaling ensures smooth and efficient lifting movements. However, even with accurate motor plans, weight perception can still be biased. In the size-weight illusion, objects of different size but equal weight are perceived to differ in heaviness, with the small object perceived to be heavier than the large object. The neural underpinnings of anticipatory force scaling to object size and the size-weight illusion are largely unknown. In this study, we tested the role of anterior intraparietal cortex (aIPS) in predictive force scaling and the size-weight illusion, by applying continuous theta burst stimulation (cTBS) prior to participants lifting objects of different sizes. Participants received cTBS over aIPS, the primary motor cortex (control area), or Sham stimulation. We found no evidence that aIPS stimulation affected the size-weight illusion. Effects were, however, found on anticipatory force scaling, where grip force was less tuned to object size during initial lifts. These findings suggest that aIPS is not involved in the perception of object weight but plays a transient role in the sensorimotor predictions related to object size. NEW & NOTEWORTHY Skilled object manipulation requires forming anticipatory motor plans according to the object's properties. Here, we demonstrate the role of anterior intraparietal sulcus (aIPS) in anticipatory grip force scaling to object size, particularly during initial lifting experience. Interestingly, this role was not maintained after continued practice and was not related to perceptual judgments measured with the size-weight illusion.

Weight perception and self-rated health: are there differences between cancer survivors and non-cancer survivors?

PURPOSE: To compare relationships between self-perceived weight and self-rated health (SRH) between cancer survivors and non-cancer survivors. METHODS: A cross-sectional study based on the 2014-2019 Korea National Health and Nutrition Examination Survey was conducted for the associations of self-perceived weight status, measured BMI categories, and combined categories by self-perceived weight and measured BMI with SRH in 1622 cancer survivors and 29,903 non-cancer survivors using complex samples ordinal regression analysis after adjusting for sociodemographic factors, health behaviors, and concurrent diseases. RESULTS: Compared to the comparison group (those who were normal weight and self-perceived as average weight in non-cancer survivors), adjusted odds for poorer SRH were higher in cancer survivors and non-cancer survivors who were underweight and self-perceived as underweight (adjusted odds ratios[aORs], 2.69 for non-cancer survivors; 2.62 for cancer survivors), those who were normal weight but self-perceived as underweight or overweight (aORs, 1.69-2.38 for non-cancer survivors; 2.34-3.78 for cancer survivors), and those who were overweight and self-perceived as overweight (aORs, 1.52 for non-cancer survivors; 2.42 for cancer survivors). In cancer survivors, the odds for poorer SRH were greater for those who were normal weight and self-perceived as average weight (aOR = 1.84), whereas the odds were not significant for those who were underweight or overweight and self-perceived as average weight. In non-cancer survivors who self-perceived as average weight, the odds for poorer SRH were significant in those with underweight (aOR = 1.35) or overweight (aOR = 0.84). CONCLUSION: According to actual weight status, the association between self-perceived weight and SRH may differ between cancer survivors and non-cancer survivors.

Prospective BMI changes in preschool children are associated with parental characteristics and body weight perceptions: the ToyBox-study.

OBJECTIVE: To examine the effect of the intervention implemented in the ToyBox-study on changes observed in age- and sex-specific BMI percentile and investigate the role of perinatal factors, parental perceptions and characteristics on this change. DESIGN: A multicomponent, kindergarten-based, family-involved intervention with a cluster-randomised design. A standardised protocol was used to measure children's body weight and height. Information was also collected from parents/caregivers via the use of validated questionnaires. Linear mixed effect models with random intercept for country, socio-economic status and school were used. SETTING: Selected preschools within the provinces of Oost-Flanders and West-Flanders (Belgium), Varna (Bulgaria), Bavaria (Germany), Attica (Greece), Mazowieckie (Poland) and Zaragoza (Spain). PARTICIPANTS: A sample of 6268 preschoolers aged 3·5-5·5 years (51·9 % boys). RESULTS: There was no intervention effect on the change in children's BMI percentile. However, parents' underestimation of their children's actual weight status, parental overweight and mothers' pre-pregnancy overweight/obesity were found to be significantly and independently associated with increases in children's BMI percentile in multivariate modelling. CONCLUSIONS: As part of a wide public health initiative or as part of a counseling intervention programme, it is important to assist parents/caregivers to correctly perceive their own and their children's weight status. Recognition of excessive weight by parents/caregivers can increase their readiness to change and as such facilitate higher adherence to favourable behavioural changes within the family.

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.

Examining the role that weight perception and social influences have on mental health among youth in the COMPASS study.

PURPOSE: We investigated whether social health mitigates the association between weight perception and anxiety and depression 1 year later in a large sample of Canadian youth in a prospective, gender-specific analysis. METHODS: We used 2 years of linked survey data from 20,485 grade 9-11 students who participated in wave 6 (2017/18) and 7 (2018/19) of the COMPASS study. Mental health outcomes included the Generalized Anxiety Disorder-7 item (GAD-7) scale and the 10-item Centre for Epidemiological Studies Depression Scale Revised (CESD-10-R). Social health encompassed students' perceived relationships with friends, family, teachers, and within schools. Multilevel, prospective, linear models regressed mental health (at wave 7) on social health (at wave 6) and weight perception (at wave 6) while controlling for weight status, ethnicity, and grade (at wave 6). Interaction terms were used to test social health factors as moderators in the association between weight perception and mental health. RESULTS: Overweight perceptions were associated with higher anxiety and depression scores among youth; this was more pronounced among females. Social health was associated with lower anxiety and depression scores. Among females only, an overweight perception had the highest predicted scores for significant depressive symptoms. Among males only, underweight perceptions were associated with higher anxiety scores. No social health factors had moderating effects in females, and only two interactions were significant among males: feeling safe at school had protective associations with anxiety scores among those with underweight perceptions while those with overweight perceptions had higher depression scores when they reported rewarding social relationships. CONCLUSION: Overweight perceptions in all youth, and underweight perceptions in males, predicted anxiety and depression symptoms 1 year later. The role of social health should not be discounted as a means of preventing anxiety and depression in youth, although this study suggests it is not sufficient to protect against adverse associations with overweight perceptions for all youth, and underweight perceptions for males.

Body Weight Perception and Health-Related Behaviors Among U.S. Adolescents: Mediating Effects of Body Weight Control Behaviors.

This study examined mediating effects of body weight control behaviors in the relationship between body weight perception and health-related behaviors among 11,458 U.S. adolescents from the 2010 National Youth Physical Activity and Nutrition Survey. Parallel multiple mediation analysis was performed for the secondary data analysis. Nearly one third of adolescents (32.5%) had overweight or obesity; one quarter (25.0%) perceived themselves as slightly overweight and 5.1% thought they were very overweight. More girls (58.6%) had tried to lose weight than boys (32.3%), while boys were more physically active than girls. Healthy and unhealthy weight control behaviors significantly mediated the relationship between adolescents' body weight perception and health-related behaviors (physical activity and screen time). Teachers and parents should help adolescents have accurate weight perception and utilize reliable and healthy weight control strategies. Future studies should consider the intercorrelated relationships among adolescents' perceptions and behaviors regarding weight to provide successful weight control intervention programs.

Sensorimotor Expectations Bias Motor Resonance during Observation of Object Lifting: The Causal Role of pSTS.

Transcranial magnetic stimulation studies have highlighted that corticospinal excitability is increased during observation of object lifting, an effect termed "motor resonance." This facilitation is driven by movement features indicative of object weight, such as object size or observed movement kinematics. Here, we investigated in 35 humans (23 females) how motor resonance is altered when the observer's weight expectations, based on visual information, do not match the actual object weight as revealed by the observed movement kinematics. Our results highlight that motor resonance is not robustly driven by object weight but easily masked by a suppressive mechanism reflecting the correctness of weight expectations. Subsequently, we investigated in 24 humans (14 females) whether this suppressive mechanism was driven by higher-order cortical areas. For this, we induced "virtual lesions" to either the posterior superior temporal sulcus (pSTS) or dorsolateral prefrontal cortex (DLPFC) before having participants perform the task. Importantly, virtual lesion of pSTS eradicated this suppressive mechanism and restored object weight-driven motor resonance. In addition, DLPFC virtual lesion eradicated any modulation of motor resonance. This indicates that motor resonance is heavily mediated by top-down inputs from both pSTS and DLPFC. Together, these findings shed new light on the theorized cortical network driving motor resonance. That is, our findings highlight that motor resonance is not only driven by the putative human mirror neuron network consisting of the primary motor and premotor cortices as well as the anterior intraparietal sulcus, but also by top-down input from pSTS and DLPFC.SIGNIFICANCE STATEMENT Observation of object lifting activates the observer's motor system in a weight-specific fashion: Corticospinal excitability is larger when observing lifts of heavy objects compared with light ones. Interestingly, here we demonstrate that this weight-driven modulation of corticospinal excitability is easily suppressed by the observer's expectations about object weight and that this suppression is mediated by the posterior superior temporal sulcus. Thus, our findings show that modulation of corticospinal excitability during observed object lifting is not robust but easily altered by top-down cognitive processes. Finally, our results also indicate how cortical inputs, originating remotely from motor pathways and processing action observation, overlap with bottom-up motor resonance effects.

Lift observation conveys object weight distribution but partly enhances predictive lift planning.

Observation of object lifting allows updating of internal object representations for object weight, in turn enabling accurate scaling of fingertip forces when lifting the same object. Here, we investigated whether lift observation also enables updating of internal representations for an object's weight distribution. We asked participants to lift an inverted T-shaped manipulandum, of which the weight distribution could be changed, in turns with an actor. Participants were required to minimize object roll (i.e., "lift performance") during lifting and were allowed to place their fingertips at self-chosen locations. The center of mass changed unpredictably every third to sixth trial performed by the actor, and participants were informed that they would always lift the same weight distribution as the actor. Participants observed either erroneous (i.e., object rolling toward its heavy side) or skilled (i.e., minimized object roll) lifts. Lifting performance after observation was compared with lifts without prior observation and with lifts after active lifting, which provided haptic feedback about the weight distribution. Our results show that observing both skilled and erroneous lifts convey an object's weight distribution similar to active lifting, resulting in altered digit positioning strategies. However, minimizing object roll on novel weight distributions was only improved after observing error lifts and not after observing skilled lifts. In sum, these findings suggest that although observing motor errors and skilled motor performance enables updating of digit positioning strategy, only observing error lifts enables changes in predictive motor control when lifting objects with unexpected weight distributions.NEW & NOTEWORTHY Individuals are able to extract an object's size and weight by observing interactions with objects and subsequently integrate this information in their own motor repertoire. Here, we show that this ability extrapolates to weight distributions. Specifically, we highlighted that individuals can perceive an object's weight distribution during lift observation but can only partially embody this information when planning their own actions.

Proprioceptive afferents differentially contribute to effortful perception of object heaviness and length.

When humans handle a tool, such as a tennis racket or hammer, for the first time, they often wield it to determine its inertial properties. The mechanisms that contribute to perception of inertial properties are not fully understood. The present study's goal was to investigate how proprioceptive afferents contribute to effortful perception of heaviness and length of a manually wielded object in the absence of vision. Blindfolded participants manually wielded specially designed objects with different mass, the static moment, and the moment of inertia at different wrist angles and angular kinematics. These manipulations elicited different tonic and rhythmic activity levels in the muscle spindles of the wrist, allowing us to relate differences in muscle activity to perceptual judgments of heaviness and length. Perception of heaviness and length depended on an object's static moment and the moment of inertia, respectively. Manipulations of wrist angle and angular kinematics affected perceived heaviness and length in distinct ways. Ulnar deviation resulted in an object being perceived heavier but shorter. Compared to static holding, wielding the object resulted in it being perceived heavier but wielding did not affect perceived length. These results suggest that proprioceptive afferents differentially contribute to effortful perception of object heaviness and length. Critically, the role of afferent is specific to the mechanical variable used to derive a given object property. These findings open a new possibility of studies on the link between physiology, and different mechanical variables picked up by the perceptual system.

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.

Do weight perception and bullying victimization account for links between weight status and mental health among adolescents?

BACKGROUND: The purpose of this study was to explore whether the way youth perceive their weight and their experiences of bullying victimization account for the increased risk of depression and anxiety symptoms, and poor psychosocial well-being, associated with overweight/obesity in a large sample of Canadian secondary school students. We also explored if associations differed by gender. METHODS: We used cross-sectional survey data from year 7 (2018-19) of the COMPASS study. The sample included 57,059 students in grades 9-12 (Secondary III-V in Quebec) at 134 Canadian secondary schools (Alberta, British Columbia, Ontario, Quebec). First, multiple regression models tested associations between body mass index (BMI) classification and mental health outcomes (anxiety [GAD-7] and depression [CESD-10] symptoms, and psychosocial well-being [Diener's Flourishing Scale]). Second, weight perception and bullying victimization were added to the models. Models were stratified by gender and controlled for sociodemographic covariates and school clustering. RESULTS: When weight perception and bullying victimization were added to the models, obesity BMI status no longer predicted internalizing symptoms and flourishing scores relative to normal-weight BMIs. Students with 'overweight' or 'underweight' perceptions, and experiences of bullying victimization in the past month, reported higher anxiety and depressive symptomatology, and lower flourishing levels, in comparison to students with 'about right' weight perceptions and without experiences of bullying victimization, respectively, controlling for BMI status. Results were largely consistent across boys and girls. CONCLUSIONS: Results suggest perceptions of weight and experiences of bullying independently contribute to differences in mental health outcomes by weight status among youth. Continued efforts targeting weight-based bullying and weight bias, and the promotion of body size acceptance and positive body image, may help reduce the risk of mental illness and poor mental health among adolescents.

Associations Between Weight Perception, Unhealthy Weight Control Behavior, and Suicidal Ideation and Planning Among Korean Adolescents: A National Cross-Sectional Secondary Analysis.

PURPOSE: This study aimed to investigate the associations between weight perception, unhealthy weight control behavior, and suicidal ideation and planning and identify the role of unhealthy weight control behavior as a mediator in these associations among Korean adolescents. DESIGN AND METHODS: This national cross-sectional secondary study used data from the 2015-2017 Korea Youth Risk Behavior Web-Based Surveys, with a final sample of 96,285 adolescents who tried to manage their weight in the 30 days prior to the surveys. A multivariate logistic regression was conducted. RESULTS: Both adolescent groups who perceived themselves to be underweight or overweight were significantly linked to a greater likelihood of suicidal ideation and planning (COR, 95% CI = 1.249, 1.141-1.367; COR, 95% CI = 1.339, 1.282-1.399) respectively. While adolescents with perceptions of being underweight are associated with suicidal ideation and planning through the suppression effect of unhealthy weight control behavior (Pathway A: OR, 95% CI = 0.628, 0.587-0.671), adolescents with perceptions of being overweight are linked with the same through the mediation effect (Pathway A: OR, 95% CI = 1.195, 1.141-1.251). CONCLUSIONS: Adolescents with perceptions of being overweight are highly likely to have unhealthy weight control behavior that can be linked to the possibility of suicidal ideation and planning, while adolescents with perceptions of being underweight are comparatively less likely to engage in such behavior. PRACTICE IMPLICATIONS: It is necessary to identify the status of weight perception among adolescents and closely monitor the unhealthy weight control behavior to prevent suicidal ideation and planning.

Weight perception and symptoms of depression in rural Australian adolescents.

OBJECTIVE: To investigate associations between measured and perceived weight, and symptoms of depression in rural Australian adolescents. METHOD: At baseline a prospective rural adolescent cohort study collected demographic data, measured weight and height, weight self-perception, and presence of depression (Short Mood and Feelings Questionnaire). Using World Health Organisation's (WHO) age and gender body mass index (BMI) standardisations, participants were classified into four perceptual groups: PG1 healthy/perceived healthy; PG2 overweight/perceived overweight; PG3 healthy/perceived overweight; and PG4 overweight/perceived healthy. Logistic regression analyses explored relationships between these groups and symptoms of depression. RESULTS: Data on adolescents (n = 339) aged 9-14. PG1 contained 63% of participants, PG2 18%, PG3 4% and PG4 14%. Across the cohort, 32% were overweight and 13% had symptoms of depression. PG2 (overweight/perceived overweight) were more likely to experience symptoms of depression than PG1 (healthy/perceived healthy; Adjusted Odds Ratio [AOR] 3.1, 95% CI 1.5-6.7). Females in PG3 (healthy/perceived overweight) were more likely to experience symptoms of depression (38%) than males (14%) and females in PG1 (10%, AOR 5.4, 95% CI 1.1-28.2). CONCLUSIONS: Results suggest that perceptions of being overweight may be a greater predictor for symptoms of depression than actual weight. This has public health implications for youth mental health screening and illness prevention.

The role of the anterior intraparietal sulcus and the lateral occipital cortex in fingertip force scaling and weight perception during object lifting.

Skillful object lifting relies on scaling fingertip forces according to the object's weight. When no visual cues about weight are available, force planning relies on previous lifting experience. Recently, we showed that previously lifted objects also affect weight estimation, as objects are perceived to be lighter when lifted after heavy objects compared with after light ones. Here, we investigated the underlying neural mechanisms mediating these effects. We asked participants to lift objects and estimate their weight. Simultaneously, we applied transcranial magnetic stimulation (TMS) during the dynamic loading or static holding phase. Two subject groups received TMS over either the anterior intraparietal sulcus (aIPS) or the lateral occipital area (LO), known to be important nodes in object grasping and perception. We hypothesized that TMS over aIPS and LO during object lifting would alter force scaling and weight perception. Contrary to our hypothesis, we did not find effects of aIPS or LO stimulation on force planning or weight estimation caused by previous lifting experience. However, we found that TMS over both areas increased grip forces, but only when applied during dynamic loading, and decreased weight estimation, but only when applied during static holding, suggesting time-specific effects. Interestingly, our results also indicate that TMS over LO, but not aIPS, affected load force scaling specifically for heavy objects, which further indicates that load and grip forces might be controlled differently. These findings provide new insights on the interactions between brain networks mediating action and perception during object manipulation.NEW & NOTEWORTHY This article provides new insights into the neural mechanisms underlying object lifting and perception. Using transcranial magnetic stimulation during object lifting, we show that effects of previous experience on force scaling and weight perception are not mediated by the anterior intraparietal sulcus or the lateral occipital cortex (LO). In contrast, we highlight a unique role for LO in load force scaling, suggesting different brain processes for grip and load force scaling in object manipulation.

Cold and heavy: grasping the temperature-weight illusion.

The apparent heaviness of weights placed on the skin depends on their temperature. We studied the effects of such a temperature-weight illusion (TWI) on perception and action in 21 healthy volunteers. Cold (18 °C), thermal-neutral (32 °C, skin temperature) and warm (41 °C) test objects were placed onto the palm of the non-dominant hand. Their veridical mass was 350 g (light) or 700 g (heavy). Perception of heaviness was assessed with two psychophysical experiments (magnitude estimation, cross modal matching). Cold heavy objects felt about 20% heavier than thermal-neutral objects of the same mass, shape and material. In a subsequent grip-lift experiment, the test objects were grasped with a precision grip of the dominant hand and lifted off the palm of the non-dominant hand. The grip and lift forces exerted by the fingertips were recorded. The temperature of the objects had significant effects (ANOVA, p < 0.05) on the peak grip and lift forces and on the peak grip force rate (i.e., the initial force incline). The peak grip force was about 10% higher when cold heavy objects were grasped and lifted, compared to lifts of otherwise identical thermal-neutral objects. The TWI was less pronounced when light objects or warm objects were handled. In conclusion, cooling of an object increases its apparent heaviness (perception) and influences scaling of the fingertip forces during grasping and lifting (action).