Abstract
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.
Highlights
IntroductionTo lift an object skillfully, one has to precisely adjust fingertip forces to the weight of the object
Every day we skillfully manipulate various objects with our hands
We investigated the role of anterior intraparietal sulcus (aIPS) and lateral occipital cortex (LO) in force scaling and weight perception when lifting objects
Summary
To lift an object skillfully, one has to precisely adjust fingertip forces to the weight of the object. Sufficient grip force, i.e., the force perpendicular to the object surface, has to be applied to avoid slipping of the object. Object weight can be predicted from visual object properties, such as size or material, to form an anticipatory motor plan and ensure a smooth lifting motion. Because feedback processes are slow, anticipatory scaling of fingertip forces results in more fluent lifts compared with feedback-driven movements. If the motor plan is incorrect, feedback processes during execution of the hand-object interaction, such as haptic perception of object movement or slip at the fingertips, can be used to quickly adapt the motor plan and apply the correct forces (Johansson and Westling 1988)
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