Action is immune to the effects of Weber's law throughout the entire grasping trajectory

Grasping tiny objects

In grasping studies, maximum grip aperture (MGA) is commonly used as an indicator of the object size representation within the visuomotor system. However, a number of additional factors, such as movement safety, comfort, and efficiency, might affect the scaling of MGA with object size and potentially mask perceptual effects on actions. While unimanual grasping has been investigated for a wide range of object sizes, so far very small objects (<5 mm) have not been included. Investigating grasping of these tiny objects is particularly interesting because it allows us to evaluate the three most prominent explanatory accounts of grasping (the perception-action model, the digits-in-space hypothesis, and the biomechanical account) by comparing the predictions that they make for these small objects. In the first experiment, participants ( N = 26 ) grasped and manually estimated the height of square cuboids with heights from 0.5 to 5 mm. In the second experiment, a different sample of participants ( N = 24 ) performed the same tasks with square cuboids with heights from 5 to 20 mm. We determined MGAs, manual estimation apertures (MEA), and the corresponding just-noticeable differences (JND). In both experiments, MEAs scaled with object height and adhered to Weber's law. MGAs for grasping scaled with object height in the second experiment but not consistently in the first experiment. JNDs for grasping never scaled with object height. We argue that the digits-in-space hypothesis provides the most plausible account of the data. Furthermore, the findings highlight that the reliability of MGA as an indicator of object size is strongly task-dependent.

Not only perception but also grasping actions can obey Weber's law

Weber's law, the principle that the uncertainty of perceptual estimates increases proportionally with object size, is regularly violated when considering the uncertainty of the grip aperture during grasping movements. The origins of this perception-action dissociation are debated and are attributed to various reasons, including different coding of visual size information for perception and action, biomechanical factors, the use of positional information to guide grasping, or, sensorimotor calibration. Here, we contrasted these accounts and compared perceptual and grasping uncertainties by asking people to indicate the visually perceived center of differently sized objects (Perception condition) or to grasp and lift the same objects with the requirement to achieve a balanced lift (Action condition). We found that the variability (uncertainty) of contact positions increased as a function of object size in both perception and action. The adherence of the Action condition to Weber's law and the consequent absence of a perception-action dissociation contradict the predictions based on different coding of visual size information and sensorimotor calibration. These findings provide clear evidence that human perceptual and visuomotor systems rely on the same visual information and suggest that the previously reported violations of Weber's law in grasping movements should be attributed to other factors.

Motor invariants in action execution and perception

The nervous system is sensitive to statistical regularities of the external world and forms internal models of these regularities to predict environmental dynamics. Given the inherently social nature of human behavior, being capable of building reliable predictive models of others' actions may be essential for successful interaction. While social prediction might seem to be a daunting task, the study of human motor control has accumulated ample evidence that our movements follow a series of kinematic invariants, which can be used by observers to reduce their uncertainty during social exchanges. Here, we provide an overview of the most salient regularities that shape biological motion, examine the role of these invariants in recognizing others' actions, and speculate that anchoring socially-relevant perceptual decisions to such kinematic invariants provides a key computational advantage for inferring conspecifics' goals and intentions.

Bimanual Grasping Adheres to Weber's Law

Weber's law states that our ability to detect changes in stimulus attributes decreases linearly with their magnitude. This principle holds true for many attributes across sensory modalities but appears to be violated in grasping. One explanation for the failure to observe Weber's law in grasping is that its effect is masked by biomechanical constraints of the hand. We tested this hypothesis using a bimanual task that eliminates biomechanical constraints. Participants either grasped differently sized boxes that were comfortably within their arm span (action task) or estimated their width (perceptual task). Within each task, there were two conditions: One where the hands' start positions remained fixed for all object sizes (meaning the distance between the initial and final hand-positions varied with object size), and one in which the hands' start positions adapted with object size (such that the distance between the initial and final hand-position remained constant). We observed adherence to Weber's law in bimanual estimation and grasping across both conditions. Our results conflict with a previous study that reported the absence of Weber's law in bimanual grasping. We discuss potential explanations for these divergent findings and encourage further research on whether Weber's law persists when biomechanical constraints are reduced.

Grasping Weber's Law in a Virtual Environment: The Effect of Haptic Feedback

Recent findings suggest that the functional separation between vision-for-action and vision-for-perception does not generalize to situations in which virtual objects are used as targets. For instance, unlike actions toward real objects that violate Weber's law, a basic law of visual perception, actions toward virtual objects presented on flat-screens, or in remote virtual environments, obey to Weber's law. These results suggest that actions in virtual environments are performed in an inefficient manner and are subjected to perceptual effects. It is unclear, however, whether this inefficiency reflects extensive variation in the way in which visual information is processed in virtual environments or more local aspects related to the settings of the virtual environment. In the current study, we focused on grasping performance in a state-of-the-art virtual reality system that provides an accurate representation of the 3D space. Within this environment, we tested the effect of haptic feedback on grasping trajectories. Participants were asked to perform bimanual grasping movements toward the edges of virtual targets. In the haptic feedback condition, physical stimuli of matching dimensions were embedded in the virtual environment. Haptic feedback was not provided in the no-feedback condition. The results showed that grasping trajectories in the feedback, but not in the no-feedback condition, could be performed more efficiently, and evade the influence of Weber's law. These findings are discussed in relevance to previous literature on 2D and 3D grasping.

When perception intrudes on 2D grasping: evidence from Garner interference

When participants reach out to pick up a real 3-D object, their grip aperture reflects the size of the object well before contact is made. At the same time, the classical psychophysical laws and principles of relative size and shape that govern visual perception do not appear to intrude into the control of such movements, which are instead tuned only to the relevant dimension for grasping. In contrast, accumulating evidence suggests that grasps directed at flat 2D objects are not immune to perceptual effects. Thus, in 2D but not 3D grasping, the aperture of the fingers has been shown to be affected by relative and contextual information about the size and shape of the target object. A notable example of this dissociation comes from studies of Garner interference, which signals holistic processing of shape. Previous research has shown that 3D grasping shows no evidence for Garner interference but 2D grasping does (Freud & Ganel, 2015). In a recent study published in this journal (Löhr-Limpens et al., 2019), participants were presented with 2D objects in a Garner paradigm. The pattern of results closely replicated the previously published results with 2D grasping. Unfortunately, the authors, who appear to be unaware the potential differences between 2D and 3D grasping, used their findings to draw an overgeneralized and unwarranted conclusion about the relation between 3D grasping and perception. In this short methodological commentary, we discuss current literature on aperture shaping during 2D grasping and suggest that researchers should play close attention to the nature of the target stimuli they use before drawing conclusions about visual processing for perception and action.

Active visuomotor interactions with virtual objects on touchscreens adhere to Weber's law

Recent findings suggest that the functional separation between vision-for-action and vision-for-perception does not generalize to situations in which two-dimensional (2D), virtual objects, are used as targets. For example, unlike grasping movements directed at real, three-dimensional (3D) objects, the trajectories of grasping movements directed at 2D objects adhere to the psychophysical principle of Weber's law, indicating relative and less efficient processing of their size. Such inefficiency could be attributed to the fact that everyday interactions with touchscreens do not usually entail grasping movements. It is possible, therefore, that more typical interactions with virtual objects, which involve active manipulation of their size or location on a touchscreen, could be performed efficiently and in an absolute manner, and would violate Weber's law. We examined this hypothesis in three experiments in which participants performed active interactions with virtual objects. In Experiment 1, participants made swiping gestures to move virtual objects across the touchscreen. In Experiment 2, participants touched the edges of virtual objects to enlarge their size. In Experiment 3, participants freely enlarged the size of virtual objects, without being required to touch their edges upon contact. In all experiments, the resolution of grip aperture decreased with the size of the target object, adhering to Weber's law. These results suggest that active interactions with 2D objects on touchscreens are not performed in a natural, absolute manner which characterize visuomotor control of real objects.

Obeying the law: speed-precision tradeoffs and the adherence to Weber's law in 2D grasping

Visually guided actions toward two-dimensional (2D) and three-dimensional (3D) objects show different patterns of adherence to Weber's law. In 3D grasping, Just Noticeable Differences (JNDs) do not scale with object size, violating Weber's law. Conversely, JNDs in 2D grasping increase with size, showing a pattern of scaler variability between aperture and JND, as predicted by Weber's law. In the current study, we tested whether such scaler variability in 2D grasping reflects genuine adherence to Weber's law. Alternatively, it could be potentially accounted for by a speed-precision tradeoff effect due to an increase in aperture velocity with size. In two experiments, we modified the relation between aperture velocity and size in 2D grasping and tested whether movement trajectories still adhere to Weber's law. In Experiment 1, we aimed to equate aperture velocities between different-sized objects by pre-adjusting the initial finger aperture to match the target's size. In Experiment 2, we reversed the relation between size and velocity by asking participants to hold their fingers wide open prior to grasp, resulting in faster velocities for smaller rather than for larger objects. The results of the two experiments showed that although aperture velocities did not increase with size, adherence to Weber's law was still maintained. These results indicate that the adherence to Weber's law during 2D grasping cannot be accounted for by a speed-precision tradeoff effect, but rather represents genuine reliance on relative, perceptually based computations in visuomotor interactions with 2D objects.

Object complexity modulates the association between action and perception in childhood

Vision for action and vision for perception both rely on shape representations derived within the visual system. Whether the same psychological and neural mechanisms underlie both forms of behavior remains hotly contested, and whether this arrangement is equivalent in adults and children is controversial as well. To address these outstanding questions, we used an established psychophysical heuristic, Weber's law, which, in adults, has typically been observed for perceptual judgment tasks but not for actions such as grasping. We examined whether this perception-action dissociation in Weber's law was present in childhood as it is in adulthood and whether it was modulated by stimulus complexity. Two major results emerged. First, although adults evinced visuomotor behavior that violated Weber's law, young children (4.5-6.5 years) adhered to Weber's law when they grasped complex objects ("Efron" blocks), which varied along both the graspable and non-graspable dimensions to maintain a constant surface area, but not when they grasped simple objects, which varied only along the graspable dimension. Second, adherence to Weber's law was found across all ages in the context of a perceptual task. Together, these findings suggest that, in early childhood, visuomotor representations are modulated by perceptual representations, particularly when a refined description of object shape is needed.

The Size Congruity Effect Vanishes in Grasping: Implications for the Processing of Numerical Information

Judgments of the physical size in which a numeral is presented are often affected by the task-irrelevant attribute of its numerical magnitude, the Size Congruity Effect (SCE). The SCE is typically interpreted as a marker of the automatic activation of numerical magnitude. However, a growing literature shows that the SCE is not robust, a possible indication that numerical information is not always activated in an automatic fashion. In the present study, we tested the SCE via grasping by way of resolving the automaticity debate. We found results that challenge the robustness of the SCE and, consequently, the validity of the automaticity assumption. The SCE was absent when participants grasped the physically larger object of a pair of 3D wooden numerals. An SCE was still recorded when the participants perceptually indicated the general location of the larger object, but not when they grasped that object. These results highlight the importance of the sensory domain when considering the generality of a perceptual effect.

Weber's law in 2D and 3D grasping

Visually guided grasping movements directed to real, 3D objects are characterized by a distinguishable trajectory pattern that evades the influence of Weber's law, a basic principle of perception. Conversely, grasping trajectories directed to 2D line drawings of objects adhere to Weber's law. It can be argued, therefore, that during 2D grasping, the visuomotor system fails at operating in analytic mode and is intruded by irrelevant perceptual information. Here, we explored the visual and tactile cues that enable such analytic processing during grasping. In Experiment 1, we compared grasping directed to 3D objects with grasping directed to 2D object photos. Grasping directed to photos adhered to Weber's law, suggesting that richness in visual detail does not contribute to analytic processing. In Experiment 2, we tested whether the visual presentation of 3D objects could support analytic processing even when only partial object-specific tactile information is provided. Surprisingly, grasping could be performed in an analytic fashion, violating Weber's law. In Experiment 3, participants were denied of any haptic feedback at the end of the movement and grasping trajectories again showed adherence to Weber's law. Taken together, the findings suggest that the presentation of real objects combined with indirect haptic information at the end of the movement is sufficient to allow analytic processing during grasp.

Bimanual grasping does not adhere to Weber's law

According to Weber’s law, a fundamental principle of perception, visual resolution decreases in a linear fashion with an increase in object size. Previous studies have shown, however, that unlike for perception, grasping does not adhere to Weber’s law. Yet, this research was limited by the fact that perception and grasping were examined for a restricted range of stimulus sizes bounded by the maximum fingers span. The purpose of the current study was to test the generality of the dissociation between perception and action across a different type of visuomotor task, that of bimanual grasping. Bimanual grasping also allows to effectively measure visual resolution during perception and action across a wide range of stimulus sizes compared to unimanual grasps. Participants grasped or estimated the sizes of large objects using both their hands. The results showed that bimanual grasps violated Weber’s law throughout the entire movement trajectory. In contrast, Just Noticeable Differences (JNDs) for perceptual estimations of the objects increased linearly with size, in agreement with Weber’s law. The findings suggest that visuomotor control, across different types of actions and for a large range of size, is based on absolute rather than on relative representation of object size.

Visual control of action directed toward two-dimensional objects relies on holistic processing of object shape

Visual perception relies on holistic processing of object shape. In contrast to perception, previous studies demonstrated that vision-for-action operates in a fundamentally different manner based on an analytical representation of objects. This notion was mainly supported by the absence of Garner interference for visually guided actions, compared to robust interference effects for perceptual estimations of the same objects. This study examines the nature of the representations that subserve visually guided actions toward two-dimensional (2D) stimuli. Based on recent results suggesting that actions directed toward 2D objects are mediated by different underlying processes compared to normal actions, we predicted that visually guided actions toward 2D stimuli would rely on perceptually driven holistic representations of object shape. To test this idea, we asked participants to grasp 2D rectangular objects presented on a computer monitor along their width while the values of the irrelevant dimension of length were either kept constant (baseline condition) or varied between trials (filtering condition). Worse performance in the filtering blocks is labeled Garner interference, which indicates holistic processing of object shape. Unlike in previous studies that used real objects, the results showed that grasping toward 2D objects produced a significant Garner interference effect, with more variable within-subject performance in the filtering compared to the baseline blocks. This finding suggests that visually guided actions directed toward 2D targets are mediated by different computations compared to visually guided actions directed toward real objects.