Evan Cesanek

Adaptation effects in grasping the Müller-Lyer illusion

&NA; Recent results have shown that effects of pictorial illusions in grasping may decrease over the course of an experiment. This can be explained as an effect of sensorimotor learning if we consider a pictorial size illusion as simply a perturbation of visually perceived size. However, some studies have reported very constant illusion effects over trials. In the present paper, we apply an error‐correction model of adaptation to experimental data of N = 40 participants grasping the Müller‐Lyer illusion. Specifically, participants grasped targets embedded in incremental and decremental Müller‐Lyer illusion displays in (1) the same block in pseudo‐randomised order, and (2) separate blocks of only one type of illusion each. Consistent with predictions of our model, we found an effect of interference between the two types when they were presented intermixed, explaining why adaptation rates may vary depending on the experimental design. We also systematically varied the number of object sizes per block, which turned out to have no effect on the rate of adaptation. This was also in accordance with our model. We discuss implications for the illusion literature, and lay out how error‐correction models can explain perception‐action dissociations in some, but not all grasping‐of‐illusion paradigms in a parsimonious and plausible way, without assuming different illusion effects.

Does visuomotor adaptation contribute to illusion-resistant grasping?

Do illusory distortions of perceived object size influence how wide the hand is opened during a grasping movement? Many studies on this question have reported illusion-resistant grasping, but this finding has been contradicted by other studies showing that grasping movements and perceptual judgments are equally susceptible. One largely unexplored explanation for these contradictions is that illusion effects on grasping can be reduced with repeated movements. Using a visuomotor adaptation paradigm, we investigated whether an adaptation model could predict the time course of Ponzo illusion effects on grasping. Participants performed a series of trials in which they viewed a thin wooden target, manually reported an estimate of the target’s length, then reached to grasp the target. Manual size estimates (MSEs) were clearly biased by the illusion, but maximum grip apertures (MGAs) of grasping movements were consistently accurate. Illusion-resistant MGAs were observed immediately upon presentation of the illusion, so there was no decrement in susceptibility for the adaptation model to explain. To determine whether online corrections based on visual feedback could have produced illusion-resistant MGAs, we performed an exploratory post hoc analysis of movement trajectories. Early portions of the illusion effect profile evolved as if they were biased by the illusion to the same magnitude as the perceptual responses (MSEs), but this bias was attenuated prior to the MGA. Overall, this preregistered study demonstrated that visuomotor adaptation of grasping is not the primary source of illusion resistance in closed-loop grasping.

Error correction and spatial generalization in human grasp control

Abstract The visual processes that support grasp planning are often studied by analyzing averaged kinematics of repeated movements, as in the literature on grasping and visual illusions. However, by recalibrating visuomotor mappings, the sensorimotor system can adjust motor outputs without changing visual processing, which complicates the interpretation of averaged behavior. We developed a dynamic model of grasp planning and adaptation that can explain why some studies find decrements in illusion effects on grasping while others do not. In two experiments, we tested grasping in a standard three‐phase adaptation paradigm and analyzed adaptation aftereffects on the maximum grip aperture as well as the error correction parameters estimated by our model. Experiment 1 demonstrated that the model accounts for adaptive responses to positive and negative visual size perturbations. Experiment 2 supported the novel hypothesis that visuomotor mappings for grasp planning can compensate for opposing size perturbations when these perturbations are experienced in separate regions of space. Our findings serve to illustrate how the surprising flexibility of grasp adaptation can hide (especially in session‐wise averages) the true effects of visual perturbations on the visual processes that drive grasp planning. HighlightsEffects of visual biases in grasp planning are rapidly overcome by adaptation.Spatial separation allows simultaneous adaptation to opposing size perturbations.A model of the trial‐by‐trial dynamics of grasp planning and adaptation is proposed.In principle, illusion‐resistant grasping doesn’t require accurate size estimation.Implications for the role of haptic feedback in visual form agnosia are discussed.

Transfer of adaptation reveals shared mechanism in grasping and manual estimation

ABSTRACT An influential idea in cognitive neuroscience is that perception and action are highly separable brain functions, implemented in distinct neural systems. In particular, this theory predicts that the functional distinction between grasping, a skilled action, and manual estimation, a type of perceptual report, should be mirrored by a split between their respective control systems. This idea has received support from a variety of dissociations, yet many of these findings have been criticized for failing to pinpoint the source of the dissociation. In this study, we devised a novel approach to this question, first targeting specific grasp control mechanisms through visuomotor adaptation, then testing whether adapted mechanisms were also involved in manual estimation – a response widely characterized as perceptual in function. Participants grasped objects in virtual reality that could appear larger or smaller than the actual physical sizes felt at the end of each grasp. After brief exposure to a size perturbation, manual estimates were biased in the same direction as the maximum grip apertures of grasping movements, indicating that the adapted mechanism is active in both tasks, regardless of the perception‐action distinction. Additional experiments showed that the transfer effect generalizes broadly over space (Exp. 1B) and does not appear to arise from a change in visual perception (Exp. 2). We discuss two adaptable mechanisms that could have mediated the observed effect: (a) an afferent proprioceptive mechanism for sensing grip shape; and (b) an efferent visuomotor transformation of size information into a grip‐shaping motor command. HighlightsManual estimation, a purportedly perceptual task, is affected by grasp adaptation.Adaptation‐transfer does not appear to be mediated by a change in visual perception.The control systems for manual estimation and grasping overlap at least partially.The perception‐action distinction previously ascribed to these two tasks is invalid.Transfer may be a result of proprioceptive shift or standard visuomotor adaptation.

Haptic feedback overrides visual size information during repeated grasping

Haptic feedback consists of the tactile and kinesthetic information available during physical interaction with surfaces and objects. When grasping an object, haptic information specifies object size and therefore may influence grasp kinematics over multiple grasps. In real-world grasping movements, the visual size of an object exactly corresponds to its haptically specified size, confounding the relative influences of these two modalities on grasp performance. As a result, accurate grasp performance in repeated grasping tasks may not be attributed entirely to veridical encoding of visual size. In these experiments, we independently controlled the visual and haptic information presented during repeated grasping of vertically-oriented rectangular bars. Visual bar sizes were presented using a computer display and an oblique mirror such that the bars appeared at eye height 40 cm in front of the seated observer. Haptic size information was provided by physical bars presented at the same 3D locations as the visual bars. Vision of the hand and physical arrangement was unavailable, making it impossible to notice size mismatches. In Experiment 1, three physical bars were grasped in a random order in three blocks of 30 grasps. In each block, the visual bars appeared smaller (-4 mm), larger (+4 mm), or equal in size to the physical bars. Maximum Grip Aperture scaling matched the physical size differences across all three blocks while we found no modulations due to visual size variation. To further investigate the time course of visuo-haptic integration, we ran a follow-up experiment looking at how MGA scaling develops over 14 repeated grasps at bars with seven different visuo-haptic combinations. Our results suggest that visual information is initially dominant in determining the size of the maximum grip aperture, but haptic information gradually overrides visual information in less than ten trials and enables physically accurate grasp performance. Meeting abstract presented at VSS 2015.

Online vision of the hand supports accurate grasp performance in illusory contexts

Grasping is a flexible human motor behavior coordinated on the basis of perceptual information about the structure of surfaces in reachable space. In two original experiments, we investigated the perceptual information supporting accurate grasp performance. Participants in these experiments reached-to-grasp target objects that were situated in illusory contexts under two perceptual conditions: a natural closed-loop condition with full visual feedback and a modified closed-loop condition which selectively prevented online vision of the hand. In natural closed-loop grasping in an illusory context, we found that the anticipatory opening between the forefinger and thumb (grip aperture) reflected the illusory perceptual size in early stages and the veridical physical size in late stages. Dynamic analysis of grip aperture scaling revealed a clear mid-flight correction, suggesting that additional information for motor control was made available during grasp execution. Based on this finding, we conducted a follow-up experiment in which we prevented online vision of the hand. In contrast to the natural closed-loop condition where maximum grip aperture (MGA) was tuned to veridical physical size, in the modified closed-loop condition we found that MGA was tuned to illusory perceptual size. This work focuses on the implications of these results for the perceptual control of action, arguing that they cannot be accounted for by explanations that posit specialized vision-for-action processes capable of extracting metrically accurate, Euclidean spatial information akin to a 3D depth map of the local environment. As an alternative, the results suggest that online control processes based on visual comparison of hand and target positions could support accurate grasp performance in illusory contexts. Meeting abstract presented at VSS 2015.