The functional subdivision of the visual brain: Is there a real illusion effect on action? A multi-lab replication study

The effect of the Uznadze illusion is temporally dynamic in closed-loop but temporally constant in open-loop grasping

Although it is known that the availability of visual feedback modulates grasping kinematics, it is unclear whether this extends to both the early and late stages of the movement. We tackled this issue by exposing participants to the Uznadze illusion (a medium stimulus appears larger or smaller after exposure to smaller or larger inducers). After seeing smaller or larger discs, participants grasped a medium disc with (closed-loop [CL]) or without (open-loop [OL]) visual feedback. Our main aim was to assess whether the time course of the illusion from the movement onset up to the grasp differed between OL and CL. Moreover, we compared OL and CL illusory effects on maximum grip aperture (MGA) and tested whether preparation time, movement time, and time to MGA predicted illusion magnitude. Results revealed that CL illusory effects decreased over movement time, whereas OL ones remained constant. At the time of MGA, OL, and CL effects were, however, of similar size. Although OL grasps were longer to prepare and showed earlier and larger MGAs, such differences had little impact on modulating the illusion. These results suggest that the early stage of grasping is sensitive to the Uznadze illusion both under CL and OL conditions, whereas the late phase is sensitive to it only under OL conditions. We discuss these findings within the framework of theoretical models on the functional properties of the dorsal stream for visually guided actions.

Modal and amodal cognition: an overarching principle in various domains of psychology

Accounting for how the human mind represents the internal and external world is a crucial feature of many theories of human cognition. Central to this question is the distinction between modal as opposed to amodal representational formats. It has often been assumed that one but not both of these two types of representations underlie processing in specific domains of cognition (e.g., perception, mental imagery, and language). However, in this paper, we suggest that both formats play a major role in most cognitive domains. We believe that a comprehensive theory of cognition requires a solid understanding of these representational formats and their functional roles within and across different domains of cognition, the developmental trajectory of these representational formats, and their role in dysfunctional behavior. Here we sketch such an overarching perspective that brings together research from diverse subdisciplines of psychology on modal and amodal representational formats so as to unravel their functional principles and their interactions.

Coming to grips with reality: Real grasps, but not pantomimed grasps, resist a simultaneous tilt illusion

Investigations of grasping real, 3D objects subjected to illusory effects from a pictorial background often choose in-flight grasp aperture as the primary variable to test the hypothesis that the visuomotor system resists the illusion. Here we test an equally important feature of grasps that has received less attention: in-flight grasp orientation. The current study tested a variant of the simultaneous tilt illusion using a mirror-apparatus to manipulate the availability of haptic feedback. Participants performed grasps with haptic feedback (real grasps) and without it (pantomime grasps), reaching for the reflection of a real, 3D bar atop a background grating that induced a 1.1° bias in the perceived orientation of the bar in a separate sample of participants. Analysis of the hand's in-flight grasp orientation at early, late, and end stages of the reach showed that at no point were the real grasps biased by the illusion. In contrast, pantomimed grasps were affected by the illusion at the late and end stages of the reach. At each stage, the effect on the real grasps was significantly weaker than the effect of the illusion as measured by the mean point of subjective equality (PSE) in a two-alternative forced-choice task. In contrast, the effect on the pantomime grasps was statistically indistinguishable from the mean PSE at all three stages of the reach. These findings reinforce the idea that in-flight grasp orientation, like grasp aperture to pictorial illusions of target size, is refractory to pictorial backgrounds that bias perceived orientation.

Pictorial depth cues always influence reaching distance

We report five experiments to test the influence of pictorial depth on reaching. Our core method is to project a wide-field background of linear perspective and/or texture gradient onto a tabletop, and to measure the amplitude of reaches made to targets within it. In 63 healthy participants performing immediate open-loop reaches across Experiments 1-4, we observed a clear effect of pictorial depth. This effect was driven specifically by the convergence of the background pattern at the target position: for each additional degree of pictorial convergence, reaching distance increased by half a millimetre. In the individual experiments, we applied manipulations that might be expected to modify the influence of pictorial depth. We found no evidence that the effect was modified with monocular viewing, or when participants responded with the left hand, or if a memory delay was inserted before the response. Nor did participants become less susceptible to pictorial depth when visual feedback of terminal reaching errors was provided, although visual feedback during the reach did mitigate the influence of pictorial depth. Finally, the visual form agnosic patient DF showed an entirely normal effect of pictorial depth cues, which leads us to question the idea that this effect emanates from visual analyses of size and shape in the ventral stream, rather than from the dorsal stream, or from earlier stages of visual processing.

Looking at the Ebbinghaus illusion: differences in neurocomputational requirements, not gaze-mediated attention, explain a classic perception-action dissociation

Perceiving and grasping an object present an animal with different sets of computational problems. The solution in primates entails the specialization of separate neural networks for visual processing with different object representations. This explains why the Ebbinghaus illusion minimally affects the grasping hand's in-flight aperture, which normally scales with target size, even though the size of the target disc remains misperceived. An attractive alternative account, however, posits that grasps are refractory to the illusion because participants fixate on the target and fail to attend to the surrounding context. To test this account, we tracked both limb and gaze while participants made forced-choice judgments of relative disc size in the Ebbinghaus illusion or did so in combination with grasping or manually estimating the size of one of the discs. We replicated the classic dissociation: grasp aperture was refractory to the measured illusory effect on perceived size, while judgments and manual estimates of disc size were not. Importantly, the number of display-wide saccades per second and the percentage of total fixation time or fixations directed at the selected disc failed to explain the dissociation. Our findings support the contention that object perception and goal-directed action rely on distinct visual representations. This article is part of a discussion meeting issue 'New approaches to 3D vision'.

How prism adaptation reveals the distinct use of size and positions in grasping

The size of an object equals the distance between the positions of its opposite edges. However, human sensory processing for perceiving positions differs from that for perceiving size. Which of these two information sources is used to control grip aperture? In this paper, we answer this question by prism adaptation of single-digit movements of the index finger and thumb. We previously showed that it is possible to adapt the index finger and thumb in opposite directions and that this adaptation induces an aftereffect in grip aperture in grasping. This finding suggests that grasping is based on the perceived positions of the contact points. However, it might be compatible with grasping being controlled based on size provided that the opposing prism adaptation leads to changes in visually perceived size or proprioception of hand opening. In that case, one would predict a similar aftereffect in manually indicating the perceived size. In contrast, if grasping is controlled based on information about the positions of the edges, the aftereffect in grasping is due to altered position information, so one would predict no aftereffect in manually indicating the perceived size. Our present experiment shows that there was no aftereffect in manually indicating perceived size. We conclude that grip aperture during grasping is based on perceived positions rather than on perceived size.

The effects of Müller-Lyer illusion on toe clearance height in stair ascent

BACKGROUND

The appearance of a stair can impact the safe negotiation of stair climbing. Increasing toe clearance height is a common strategy for reducing the risk of tripping when stepping onto a raised surface.

RESEARCH QUESTION

Can the Müller-Lyer illusion cause people to increase their toe clearance height when walking over a stair?

METHODS

Indoors, 15 healthy individuals (9 women and 6 men) walked up a 3-step staircase with each step 60 cm wide, 18 cm tall, and 30.5 cm deep. Outdoors, 253 people walked up a 2-step staircase with each step 6 m wide, 15 cm tall, and 38 cm deep. Four visual conditions were displayed at both the indoor and outdoor locations: fins out, fins in, line, and control.

RESULTS

While the fins out stairs were perceived taller than all the other displayed stair conditions, the toe clearance height was not different between display conditions when tested indoors. However, toe clearance height was greater in the fins out condition when compared to all other conditions for the outdoor stairs location.

SIGNIFICANCE

In the natural environment when walkers were oblivious to themselves being filmed and not having any knowledge of the task demand, the Müller-Lyer illusion significantly impacted their toe clearance height.

Use of remote data collection methodology to test for an illusory effect on visually guided cursor movements

Investigating the influence of perception on the control of visually guided action typically involves controlled experimentation within the laboratory setting. When appropriate, however, behavioral research of this nature may benefit from the use of methods that allow for remote data collection outside of the lab. This study tested the feasibility of using remote data collection methods to explore the influence of perceived target size on visually guided cursor movements using the Ebbinghaus illusion. Participants completed the experiment remotely, using the trackpad of their personal laptop computers. The task required participants to click on a single circular target presented at either the left or right side of their screen as quickly and accurately as possible (Experiment 1), or to emphasize speed (Experiment 2) or accuracy (Experiment 3). On each trial the target was either surrounded by small or large context circles, or no context circles. Participants’ judgments of the targets’ perceived size were influenced by the illusion, however, the illusion failed to produce differences in click-point accuracy or movement time. Interestingly, the illusion appeared to affect participants’ movement of the cursor toward the target; more directional changes were made when clicking the Perceived Large version of the illusion compared to the Perceived Small version. These results suggest the planning of the cursor movement may have been influenced by the illusion, while later stages of the movement were not, and cursor movements directed toward targets perceived as smaller required less correction compared to targets perceived as larger.

Binocular Viewing Facilitates Size Constancy for Grasping and Manual Estimation

A prerequisite for efficient prehension is the ability to estimate an object’s distance and size. While most studies demonstrate that binocular viewing is associated with a more efficient grasp programming and execution compared to monocular viewing, the factors contributing to this advantage are not fully understood. Here, we examined how binocular vision facilitates grasp scaling using two tasks: prehension and manual size estimation. Participants (n = 30) were asked to either reach and grasp an object or to provide an estimate of an object’s size using their thumb and index finger. The objects were cylinders with a diameter of 0.5, 1.0, or 1.5 cm placed at three distances along the midline (40, 42, or 44 cm). Results from a linear regression analysis relating grip aperture to object size revealed that grip scaling during monocular viewing was reduced similarly for both grasping and estimation tasks. Additional analysis revealed that participants adopted a larger safety margin for grasping during monocular compared to binocular viewing, suggesting that monocular depth cues do not provide sufficient information about an object’s properties, which consequently leads to a less efficient grasp execution.

The Semantics of Natural Objects and Tools in the Brain: A Combined Behavioral and MEG Study

Current literature supports the notion that the recognition of objects, when visually presented, is sub-served by neural structures different from those responsible for the semantic processing of their nouns. However, embodiment foresees that processing observed objects and their verbal labels should share similar neural mechanisms. In a combined behavioral and MEG study, we compared the modulation of motor responses and cortical rhythms during the processing of graspable natural objects and tools, either verbally or pictorially presented. Our findings demonstrate that conveying meaning to an observed object or processing its noun similarly modulates both motor responses and cortical rhythms; being natural graspable objects and tools differently represented in the brain, they affect in a different manner both behavioral and MEG findings, independent of presentation modality. These results provide experimental evidence that neural substrates responsible for conveying meaning to objects overlap with those where the object is represented, thus supporting an embodied view of semantic processing.

Influence of colour on object motor representation

Colour conveys specific information about the status/quality of an object; whereas its role in object recognition has been widely studied, little is known about its role in sensorimotor processes. We performed three experiments to assess whether colour influences the motor representation of graspable objects. In Experiment 1, we used a grasp compatibility task, in which participants categorized each object as natural or artifact, by performing reach-to-grasp movements. Response grasps could be compatible or incompatible with the ones normally used to manipulate the objects. Results showed faster reaction times for natural objects displayed in the correct colour compared with both opposite colour and correct colour artifact objects. In Experiment 2, to directly assess the effect of colour on object motor representation, we used an interference task in which an irrelevant object was shown while performing a pre-specified reach-to-grasp movement (i.e., verbal cues: small vs. large). Results highlighted a reversed compatibility effect when objects were shown in their correct colour, but only at the beginning of the movement (10 ms SOA). Finally, we run a third experiment using the same task as in Experiment 2. In this experiment, we compared the grasp compatibility effect driven by natural objects with the grasp compatibility effect driven by dangerous natural objects (e.g., cactus), which are objects that should not elicit a grasping program. The results of Experiment 3 confirm those of Experiment 2, highlighting also specific processes related to dangerous objects. Taken together, these results revealed that colour can be significant for the motor system, highlighting the close link between colour and shape, and also specific processes related to dangerous objects.

Selective interference of hand posture with grasping capability estimation

Previous studies have shown that judgments about how one would perform an action are affected by the current body posture. Hence, judging one's capability to grasp an object between index and thumb is influenced by their aperture at the time of the judgment. This finding can be explained by a modification of the internal representation of one's hand through the effect of sensorimotor input. Alternatively, the influence of grip aperture might be mediated by a response congruency effect, so that a "less" vs. "more" open grip would bias the judgment toward a "less" vs. "more" capable response. To specify the role of sensorimotor input in prospective action judgments, we asked participants to estimate their capability to grasp circles between index and thumb while performing a secondary task that requires them to squeeze a ball with these two fingers (precision grip) or with a different hand configuration (palm grip). Experiment 1 showed that participants underestimated their grasping capability when the squeezing task involved the same grip as the judged action (precision grip) and their estimates were bound to the relative size of objects as revealed by size-contrast illusions (Ebbinghaus). Experiment 2 showed that the grip reduction caused by the squeezing task also interfered with the discrimination of large numbers in magnitude judgments, but this incongruency effect was only observed for the palm grip. The dissociated effects of the two grips in graspability and numerical judgments indicate that sensorimotor input may affect the perceived ability to grasp objects, independently of response congruency, by modifying the representation of the hand in action.

Using a stair horizontal-vertical illusion to increase foot clearance over an inconsistently taller stair-riser

Introduction

Stair falls can be caused by inconsistent stair dimensions. During ascent, inconsistently taller stair risers lead to reduced foot clearances as the inconsistency goes unnoticed. A stair horizontal-vertical illusion increases perceived riser heights and foot clearance and could offset reduced foot clearances over inconsistently taller risers, though this might impact other stair safety measures.

Method

Twelve participants (age: 22 (3) years) ascended a seven-step staircase under three conditions: i) all steps consistent in riser height (consistent), ii) a 1cm increase in step 5 riser height (inconsistent) and iii) a 1cm increase in step 5 riser height, superimposed with a stair horizontal-vertical illusion (illusion). Vertical foot clearance, foot overhang, and margins of stability were assessed over step 4, 5 and 6. Perceived riser height due to the illusion was determined through a computer perception test. A One-Way Repeated Measures ANOVA compared biomechanical variables between conditions. A One Sample t test compared perceived riser height to the true height.

Results

Over the inconsistent step 5, foot clearance reduced by 0.8cm compared to consistent. Illusion increased foot clearance by 1.1cm and decreased foot overhang by 4% compared to inconsistent. On step 4 the illusion led to more anterior instability compared to inconsistent. Illusion and inconsistent led to more mediolateral stability compared to consistent. The illusion increased perceived riser height by 12%.

Discussion

Foot clearance reductions over inconsistently taller risers can be offset by a stair horizontal-vertical illusion. Additional benefits included a safer foot overhang and unaffected stability over the inconsistent riser. Changes to step 4 stability might have resulted from leaning forward to look at the step 5 illusion. The stair horizontal-vertical illusion could be a practical solution for inconsistently taller stair risers, where a rebuild is usually the only solution.

Grasp aperture corrections in reach-to-grasp movements do not reliably alter size perception

When grasping an object, the opening between the fingertips (grip aperture) scales with the size of the object. If an object changes in size, the grip aperture has to be corrected. In this study, it was investigated whether such corrections would influence the perceived size of objects. The grasping plan was manipulated with a preview of the object, after which participants initiated their reaching movement without vision. In a minority of the grasps, the object changed in size after the preview and participants had to adjust their grasping movement. Visual feedback was manipulated in two experiments. In experiment 1, vision was restored during reach and both visual and haptic information was available to correct the grasp and lift the object. In experiment 2, no visual information was provided during the movement and grasps could only be corrected using haptic information. Participants made reach-to-grasp movements towards two objects and compared these in size. Results showed that participants adjusted their grasp to a change in object size from preview to grasped object in both experiments. However, a change in object size did not bias the perception of object size or alter discrimination performance. In experiment 2, a small perceptual bias was found when objects changed from large to small. However, this bias was much smaller than the difference that could be discriminated and could not be considered meaningful. Therefore, it can be concluded that the planning and execution of reach-to-grasp movements do not reliably affect the perception of object size.

Card posting does not rely on visual orientation: A challenge to past neuropsychological dissociations

A common set of tasks frequently employed in the neuropsychological assessment of patients with visuomotor or perceptual deficits are the card-posting and the perceptual orientation matching tasks. In the posting task, patients have to post a card (or their hand) through a slot of varying orientations while the matching task requires them to indicate the slot's orientation as accurately as possible. Observations that damage to different areas of the brain (dorsal vs. ventral stream) is associated with selective impairment in one of the tasks - but not the other - has led to the suggestion that different cortical pathways process visual orientation information for perception versus action. In three experiments, we show that this conclusion may be premature as posting does not seem to rely on the processing of visual orientation information but is instead performed using obstacle avoidance strategies that require an accurate judgement of egocentric distances between the card's and the slot's edges. Specifically, we found that while matching is susceptible to the oblique effect (i.e., common perceptual orientation bias with higher accuracy for cardinal than oblique orientations), this was not the case for posting, neither in immediate nor in memory-guided conditions. In contrast to matching, posting errors primarily depended on biomechanical demands and reflected a preference for performing efficient and comfortable movements. Thus, we suggest that previous dissociations between perceptual and visuomotor performance in letter posting tasks are better explained by impairments in egocentric and allocentric spatial processing than by independent visual processing systems.

Priming of the Sander Parallelogram illusion separates perception from action

The two-visual stream hypothesis posits that the dorsal stream is less susceptible than the ventral stream to the effects of illusions and visual priming. While previous studies have separately examined these perceptual manipulations, the present study combined the effects of a visual illusion and priming to examine the possibility of dorsally guided actions being susceptible to the perceptual stimuli due to interactions between the two streams. Thirty-four participants were primed with a 'long' or 'short' version of the Sander Parallelogram illusion and were asked to either reach out and grasp or manually estimate the length of a rod placed on a version of the illusion that was on some trials the same as the prime (congruent) and on other trials was the inverse (incongruent). Due to the context-focused nature of ventral processing, we predicted that estimations would be more susceptible to the effects of the illusion and priming than grasps. Results showed that while participants' manual estimations were susceptible to both priming and the illusion, the grasps were only affected by the illusion, not by priming. The influence of the illusion on grip aperture was greater during manual estimations than it was during grasping. These findings support the notion that the functionally distinct dorsal and ventral streams interact under the current experimental paradigm. Outcomes of the study help better understand the nature of stimuli that promote interactions between the dorsal and ventral streams.

Grasping of Real-World Objects Is Not Biased by Ensemble Perception

The visual system is known to extract summary representations of visually similar objects which bias the perception of individual objects toward the ensemble average. Although vision plays a large role in guiding action, less is known about whether ensemble representation is informative for action. Motor behavior is tuned to the veridical dimensions of objects and generally considered resistant to perceptual biases. However, when the relevant grasp dimension is not available or is unconstrained, ensemble perception may be informative to behavior by providing gist information about surrounding objects. In the present study, we examined if summary representations of a surrounding ensemble display influenced grip aperture and orientation when participants reached-to-grasp a central circular target which had an explicit size but importantly no explicit orientation that the visuomotor system could selectively attend to. Maximum grip aperture and grip orientation were not biased by ensemble statistics during grasping, although participants were able to perceive and provide manual estimations of the average size and orientation of the ensemble display. Support vector machine classification of ensemble statistics achieved above-chance classification accuracy when trained on kinematic and electromyography data of the perceptual but not grasping conditions, supporting our univariate findings. These results suggest that even along unconstrained grasping dimensions, visually-guided behaviors toward real-world objects are not biased by ensemble processing.

Dissociating the Influence of Perceptual Biases and Contextual Artifacts Within Target Configurations During the Planning and Control of Visually Guided Action

The failure of perceptual illusions to elicit corresponding biases within movement supports the view of two visual pathways separately contributing to perception and action. However, several alternative findings may contest this overarching framework. The present study aimed to examine the influence of perceptual illusions within the planning and control of aiming. To achieve this, we manipulated and measured the planning/control phases by respectively perturbing the target illusion (relative size-contrast illusion; Ebbinghaus/Titchener circles) following movement onset and detecting the spatiotemporal characteristics of the movement trajectory. The perceptual bias that was indicated by the perceived target size estimates failed to correspondingly manifest within the effective target size. While movement time (specifically, time after peak velocity) was affected by the target configuration, this outcome was not consistent with the direction of the perceptual illusions. These findings advocate an influence of the surrounding contextual information (e.g., annuli) on movement control that is independent of the direction predicted by the illusion.

The next step in optimising the stair horizontal-vertical illusion: Does a perception-action link exist in older adults?

INTRODUCTION

Tripping on stairs results from insufficient foot to step edge clearance and can often lead to a fall in older adults. A stair horizontal-vertical illusion is suggested to increase the perceived riser height of a step and increase foot clearance when stepping up. However, this perception-action link has not been empirically determined in older adults. Previous findings suggesting a perception-action effect have also been limited to a single step or a three-step staircase. On larger staircases, somatosensory learning of step heights may be greater which could override the illusory effect on the top step. Furthermore, the striped nature of the existing stair horizontal-vertical illusion is associated with visual stress and may not be aesthetically suitable for use on public stairs. These issues need resolving before potential future implementation on public stairs.

METHODS

Experiment 1. A series of four computer-based perception tests were conducted in older (N = 14: 70 ± 6 years) and young adults (N = 42: 24 ± 3 years) to test the influence of different illusion designs on stair riser height estimation. Participants compared images of stairs, with horizontal-vertical illusions or arbitrary designs on the bottom step, to a plain stair with different bottom step riser heights and selected the stair they perceived to have the tallest bottom riser. Horizontal-vertical illusions included a previously developed design and versions with modified spatial frequencies and mark space ratios. Perceived riser height differences were assessed between designs and between age groups. Experiment 2. To assess the perception-action link, sixteen older (70 ± 7 years) and fifteen young (24 ± 3 years) adults ascended a seven-step staircase with and without horizontal-vertical illusions tested in experiment 1 placed onto steps one and seven. Foot clearances were measured over each step. To determine whether changes in perception were linked to changes in foot clearance, perceived riser heights for each horizontal-vertical illusion were assessed using the perception test from experiment 1 before and after stair ascent. Additional measures to characterise stair safety included vertical foot clearance, margins of stability, foot overhang, stair speed, and gaze duration, which were assessed over all seven steps.

RESULTS

Experiment 1. All horizontal-vertical illusion designs led to significant increases in the perceived riser height in both young and older adults (12-19% increase) with no differences between age groups. Experiment 2. On step 7, each horizontal-vertical illusion led to an increase in vertical foot clearance for young (up to 0.8 cm) and older adults (up to 2.1 cm). On step 1 significant increases in vertical foot clearance were found for a single horizontal-vertical illusion when compared to plain (1.19 cm increase). The horizontal-vertical illusions caused significant increases in the perceived riser height (young; 13% increase, older; 11% increase) with no differences between illusion design, group or before and after stair ascent. No further differences were found for the remaining variables and steps.

CONCLUSION

Results indicate a perception-action link between perceived riser height and vertical foot clearance in response to modified versions of the horizontal-vertical illusion in both young and older adults. This was shown with no detriment to additional stair safety measures. Further evaluating these illusions on private/public stairs, especially those with inconsistently taller steps, may be beneficial to help improve stair safety for older adults.

The Role of Haptic Expectations in Reaching to Grasp: From Pantomime to Natural Grasps and Back Again

When we reach to pick up an object, our actions are effortlessly informed by the object's spatial information, the position of our limbs, stored knowledge of the object's material properties, and what we want to do with the object. A substantial body of evidence suggests that grasps are under the control of "automatic, unconscious" sensorimotor modules housed in the "dorsal stream" of the posterior parietal cortex. Visual online feedback has a strong effect on the hand's in-flight grasp aperture. Previous work of ours exploited this effect to show that grasps are refractory to cued expectations for visual feedback. Nonetheless, when we reach out to pretend to grasp an object (pantomime grasp), our actions are performed with greater cognitive effort and they engage structures outside of the dorsal stream, including the ventral stream. Here we ask whether our previous finding would extend to cued expectations for haptic feedback. Our method involved a mirror apparatus that allowed participants to see a "virtual" target cylinder as a reflection in the mirror at the start of all trials. On "haptic feedback" trials, participants reached behind the mirror to grasp a size-matched cylinder, spatially coincident with the virtual one. On "no-haptic feedback" trials, participants reached behind the mirror and grasped into "thin air" because no cylinder was present. To manipulate haptic expectation, we organized the haptic conditions into blocked, alternating, and randomized schedules with and without verbal cues about the availability of haptic feedback. Replicating earlier work, we found the strongest haptic effects with the blocked schedules and the weakest effects in the randomized uncued schedule. Crucially, the haptic effects in the cued randomized schedule was intermediate. An analysis of the influence of the upcoming and immediately preceding haptic feedback condition in the cued and uncued random schedules showed that cuing the upcoming haptic condition shifted the haptic influence on grip aperture from the immediately preceding trial to the upcoming trial. These findings indicate that, unlike cues to the availability of visual feedback, participants take advantage of cues to the availability of haptic feedback, flexibly engaging pantomime, and natural modes of grasping to optimize the movement.

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.

Double dissociation between perception and action in children

Previous research has demonstrated a functional dissociation between vision for perception and vision for action. However, the developmental trajectory of this functional dissociation is not well understood. We directly compared the sensitivity of grasping and perceptual estimations within the same experimental design to the real and illusory sizes of objects positioned in the Ponzo illusion display. Two different-sized objects were placed such that the differences between their real sizes and their perceived sizes were pitted against each other. Children aged 5-8 years and adults made perceptual size discriminations and then grasped (action) or estimated (perception) one of the objects based on its perceived size. Consistent with previous results, for the action task, grasping apertures of adults were scaled with the physical differences in the objects' sizes, even in trials where their overt perceptual decisions were deceived by the illusion. In contrast, perceptual estimations were robustly modulated by the illusion. Interestingly, children outperformed adults in their perceptual discriminations but exhibited adult-like behavior in grasping and in perceptual estimations of the objects, demonstrating a dissociation between perception and action. These results suggest that although the two visual functions are not operating at fully mature levels during childhood, some key mechanisms that support a dissociation between these functions are already in place.

A double dissociation between action and perception in bimanual grasping: evidence from the Ponzo and the Wundt-Jastrow illusions

Research on visuomotor control suggests that visually guided actions toward objects rely on functionally distinct computations with respect to perception. For example, a double dissociation between grasping and between perceptual estimates was reported in previous experiments that pit real against illusory object size differences in the context of the Ponzo illusion. While most previous research on the relation between action and perception focused on one-handed grasping, everyday visuomotor interactions also entail the simultaneous use of both hands to grasp objects that are larger in size. Here, we examined whether this double dissociation extends to bimanual movement control. In Experiment 1, participants were presented with different-sized objects embedded in the Ponzo Illusion. In Experiment 2, we tested whether the dissociation between perception and action extends to a different illusion, the Wundt–Jastrow illusion, which has not been previously used in grasping experiments. In both experiments, bimanual grasping trajectories reflected the differences in physical size between the objects; At the same time, perceptual estimates reflected the differences in illusory size between the objects. These results suggest that the double dissociation between action and perception generalizes to bimanual movement control. Unlike conscious perception, bimanual grasping movements are tuned to real-world metrics, and can potentially resist irrelevant information on relative size and depth.

The effect of the Müller-Lyer configuration on saccadic eye movements is not fully due to illusory perception

Previous research has shown that both perception and oculomotor control are affected by visual illusions. While these findings appear to suggest a common code of visual processing for perception and oculomotor control, there remains the possibility that the perceptual and the oculomotor effects emerge through partially different processes. In three experiments, we replicated the previous finding that perception and saccades were both biased by the typical Müller-Lyer configurations. However, using a non-Müller-Lyer setup in which the perceptual illusion effect was much restrained, we did not observe a comparable reduction in the saccadic effect. Instead, the saccadic effect by Müller-Lyer configuration could be partially due to the center-of-gravity (CoG) effect (i.e., the tendency for saccades to land at the center of gravity of the stimuli). These results indicate that the influence of the Müller-Lyer configuration on saccadic eye movements is a mixed effect of perceptual representation and CoG, rather than exclusively due to the illusory perception. We further found that the saccadic and perceptual effects were not correlated at the trial-by-trial level, which suggest that there could be largely independent sources of noise for perception and saccadic control.NEW & NOTEWORTHY The Müller-Lyer illusion affects both perception and oculomotor control, but it is unknown whether these effects arise from the same or different underlying mechanisms. We developed a modified version of the Müller-Lyer configuration, which largely reduced the perceptual illusion effect compared with the typical configuration but reduced the saccadic effect to a much less extent. Such difference indicates that influence of the Müller-Lyer configuration on saccadic eye movements is not fully mediated by illusory perception.

Why some size illusions affect grip aperture

There is extensive literature debating whether perceived size is used to guide grasping. A possible reason for not using judged size is that using judged positions might lead to more precise movements. As this argument does not hold for small objects and all studies showing an effect of the Ebbinghaus illusion on grasping used small objects, we hypothesized that size information is used for small objects but not for large ones. Using a modified diagonal illusion, we obtained an effect of about 10% on perceptual judgements, without an effect on grasping, irrespective of object size. We therefore reject our precision hypothesis. We discuss the results in the framework of grasping as moving digits to positions on an object. We conclude that the reported disagreement on the effect of illusions is because the Ebbinghaus illusion not only affects size, but—unlike most size illusions—also affects perceived positions.

Gaze Direction Changes the Vertical-Horizontal Illusory Effects on Manual Length Estimations

We examined potentially deceptive influences of the vertical-horizontal (V-H) illusion on manual length estimations. When viewing V-H illusory configurations, people perceive that the bisecting segment length exceeds the bisected segment length when segments are actually equal. Participants used downward or rightward pointing movements to manually estimate the length of a short bisecting segment of the V-H illusion in upright or rotated configurations. Participants directed their gaze freely, on the configuration, or on the movement space. Manual length estimations for upright and rotated configurations depended on gaze direction, revealing bisection influences only for restricted viewing. People produced illusory influences on perceptuomotor control only when gaze was directed toward V-H configurations or their movement. Exploitation of deceptive visual cues can direct upper limb control for sensorimotor coordination.

Ebbinghaus visual illusion: no robust influence on novice golf-putting performance

Do visual illusions reliably improve sports performance? To address this issue, we used procedures inspired by Witt et al. (Psychol Sci 23:397-399, 2012) seminal study, which reported that putting on a miniature golf course was positively influenced by an increase in apparent hole size induced by the Ebbinghaus visual illusion. Because Witt et al.'s motor task-putting golf balls toward a hole from the distance of 3.5 m-was impossible for participants who were novices in golf (Experiment 1a), we decided to shorten the putting distance (i.e., 2 m instead of 3.5 m) in Experiment 1b. Otherwise, this second experiment closely followed every other aspects of Witt et al.'s procedure (i.e., one small or one standard golf hole surrounded by a ring of small or large circles). However, this attempt to replicate Witt et al.'s findings failed: the Ebbinghaus illusion significantly influenced neither hole perception nor putting performance. In two subsequent experiments, we encouraged the emergence of the effect of the illusion by simultaneously presenting both versions of the illusion on the mat. This major adaptation successfully modified the perceived size of the hole but had no impact on putting performance (Experiment 2), even when the putting task was made easier by shortening the putting distance to only 1 m (Experiment 3). In the absence of detectable effects of the illusion on putting performance, we conclude that the effects of visual illusions on novice sports performance do not represent a robust phenomenon.

A review of grasping as the movements of digits in space

It is tempting to describe human reach-to-grasp movements in terms of two, more or less independent visuomotor channels, one relating hand transport to the object’s location and the other relating grip aperture to the object’s size. Our review of experimental work questions this framework for reasons that go beyond noting the dependence between the two channels. Both the lack of effect of size illusions on grip aperture and the finding that the variability in grip aperture does not depend on the object’s size indicate that size information is not used to control grip aperture. An alternative is to describe grip formation as emerging from controlling the movements of the digits in space. Each digit’s trajectory when grasping an object is remarkably similar to its trajectory when moving to tap the same position on its own. The similarity is also evident in the fast responses when the object is displaced. This review develops a new description of the speed-accuracy trade-off for multiple effectors that is applied to grasping. The most direct support for the digit-in-space framework is that prism-induced adaptation of each digit’s tapping movements transfers to that digit’s movements when grasping, leading to changes in grip aperture for adaptation in opposite directions for the two digits. We conclude that although grip aperture and hand transport are convenient variables to describe grasping, treating grasping as movements of the digits in space is a more suitable basis for understanding the neural control of grasping.

Grasping Discriminates between Object Sizes Less Not More Accurately than the Perceptual System

Ganel, Freud, Chajut, and Algom (2012) demonstrated that maximum grip apertures (MGAs) differ significantly when grasping perceptually identical objects. From this finding they concluded that the visual size information used by the motor system is more accurate than the visual size information available to the perceptual system. A direct comparison between the accuracy in the perception and the action system is, however, problematic, given that accuracy in the perceptual task is measured using a dichotomous variable, while accuracy in the visuomotor task is determined using a continuous variable. We addressed this problem by dichotomizing the visuomotor measures. Using this approach, our results show that size discrimination in grasping is in fact inferior to perceptual discrimination therefore contradicting the original suggestion put forward by Ganel and colleagues.

"Seeing Rain": Integrating phenomenological and Bayesian predictive coding approaches to visual hallucinations and self-disturbances (Ichstörungen) in schizophrenia

We present a schizophrenia patient who reports "seeing rain" with attendant somatosensory features which separate him from his surroundings. Because visual/multimodal hallucinations are understudied in schizophrenia, we examine a case history to determine the role of these hallucinations in self-disturbances (Ichstörungen). Developed by the early Heidelberg School, self-disturbances comprise two components: 1. The self experiences its own automatic processing as alien to self in a split-off, "doubled-I." 2. In "I-paralysis," the disruption to automatic processing is now outside the self in omnipotent agents. Self-disturbances (as indicated by visual/multimodal hallucinations) involve impairment in the ability to predict moment-to-moment experiences in the ongoing perception-action cycle. The phenomenological approach to subjective experience of self-disturbances complements efforts to model psychosis using the computational framework of hierarchical predictive coding. We conclude that self-disturbances play an adaptive, compensatory role following the uncoupling of perception and action, and possibly, other low-level perceptual anomalies.

Thirst for Intention? Grasping a Glass Is a Thirst-Controlled Action

Every day and every hour, we feel we perform numerous voluntary actions, i.e., actions under the control of our will. Individual’s ability to initiate goal-directed movement is classically described as a hierarchical motor organization, from an intentional module, mostly considered as a black box, to muscular activity supporting action execution. The general focus is usually set on the triggering of action by intention, which is assumed to be the only entry to the action cascade, rather than on the preceding formation of intentions. If intentions play a key role in the specification of movement kinematic parameters, it remains largely unknown whether unconscious cognitive processes may also affect action preparation and unfolding. Recently, a seemingly irrelevant variable, thirst, was shown to modulate a simple arbitrary action such as key-pressing. Thirsty individuals were shown to produce stronger motor inhibition in no-go trials when a glass of water was present. In the present experiment, we intended to explore whether motor inhibition operates not only upstream from the action cascade but may also affect the unfolding of reaching movements, i.e., at a lower-level control. Thirsty vs. non-thirsty control subjects were asked to reach and grasp green (go trial) or red glasses (no-go trial) filled with either water or transparent gel wax with a central candlewick. Thirsty subjects were faster to initiate actions toward the water glasses. They also exhibited an earlier maximal grip aperture and a global reduction of movement time which was mostly explained by a shortening of deceleration time. The deceleration phase was correlated with individual’s thirst rating. In addition, no-go trial toward a glass of water tended to inhibit the next movement toward a glass filled with gel wax. Thus, our results show that an unintentional influence of an internal state can reorganize voluntary action structure not only at the decision-making level but also at the level of motor control. Although subjects explicitly paid more attention and were more cautious to glasses filled with water, they reported no explicit sensation of an increased urge to grasp it, further suggesting that these effects are controlled by covert mechanisms.

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.

Grasping and perception are both affected by irrelevant information and secondary tasks: new evidence from the Garner paradigm

In their Perception-Action Model (PAM), Goodale and Milner (1992) proposed functionally independent and encapsulated processing of visual information for action and perception. In this context, they postulated that visual input for action is processed in an automatized and analytic manner, which renders visuomotor behaviour immune to perceptual interferences or multitasking costs due to sharing of cognitive resources. Here, we investigate the well-known Garner Interference effect under dual- and single-task conditions in its classic perceptual form as well as in grasping. Garner Interference arises when stimuli are classified along a relevant dimension (e.g., their length), while another irrelevant dimension (e.g., their width) has to be ignored. In the present study, participants were presented with differently sized rectangular objects and either grasped them or classified them as long or short via button presses. We found classical Garner Interference effects in perception as expressed in prolonged reaction times when variations occurred also in the irrelevant object dimension. While reaction times during grasping were not susceptible to Garner Interference, effects were observed in a number of measures that reflect grasping accuracy (i.e., poorer adjustment of grip aperture to object size, prolonged adjustment times, and increased variability of the maximum hand opening when irrelevant object dimensions were varied). In addition, multitasking costs occurred in both perception and action tasks. Thus, our findings challenge the assumption of automaticity in visuomotor behaviour as proposed by the PAM.

A visual illusion that influences perception and action through the dorsal pathway

There are two main anatomically and physiologically defined visual pathways connecting the primary visual cortex with higher visual areas: the ventral and the dorsal pathway. The influential two-visual-systems hypothesis postulates that visual attributes are analyzed differently for different functions: in the dorsal pathway visual information is analyzed to guide actions, whereas in the ventral pathway visual information is analyzed for perceptual judgments. We here show that a person who cannot identify objects due to an extensive bilateral ventral brain lesion is able to judge the velocity at which an object moves. Moreover, both his velocity judgements and his interceptive actions are as susceptible to a motion illusion as those of people without brain lesions. These findings speak in favor of the idea that dorsal structures process information about attributes such as velocity, irrespective of whether such information is used for perceptual judgments or to guide actions.

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.

Intentions: The dynamic hierarchical model revisited

Ten years ago, one of us proposed a dynamic hierarchical model of intentions that brought together philosophical work on intentions and empirical work on motor representations and motor control (Pacherie, 2008). The model distinguished among Distal intentions, Proximal intentions, and Motor intentions operating at different levels of action control (hence the name DPM model). This model specified the representational and functional profiles of each type of intention, as well their local and global dynamics, and the ways in which they interact. A core insight of the model was that action control is the result of integrated, coordinated activity across these levels of intention. Since the proposal of the model, empirical and theoretical works in philosophy and cognitive science have emerged that would seem to support and expand on this central insight. In particular, an updated understanding of the nature of sensorimotor processing and motor representations, as well as of how the different levels of intention and control interface and interact, allows for the further specification and precisification of the original DPM model. This article is categorized under: Philosophy > Psychological Capacities Psychology > Motor Skill and Performance Philosophy > Action.

The visual guidance of action is not insulated from cognitive interference: A multitasking study on obstacle-avoidance and bisection

The Perception-Action Model (PAM) considers the visual system to be divided into two streams defined by their specific functions, a ventral stream for vision and a dorsal stream for action. In this study we investigated two behavioral paradigms which according to PAM represent the two contrasting functions of the ventral and dorsal stream, namely bisection and obstacle-avoidance, respectively. It is an assumption of PAM that while ventral stream processing is ultimately linked with processing in other cognitive systems, dorsal stream processing is insulated from cognition. Accordingly it can be expected that a secondary task will interfere with bisection but not with obstacle-avoidance. We tested this prediction using a rapid serial visual presentation task as our secondary task (RSVP). Contrary to expectations we found significant interference for both bisection and obstacle-avoidance. Our findings suggest that dorsal-stream processing is not insulated from cognitive processes.

Taking into consideration explanations of perception-action interactions that may be "less dramatic, but more reflective of what happens in the real world"

Bruce Bridgeman and colleagues reported the first experiments providing evidence of two functionally distinct visual-processing systems. We summarize that work and subsequent research that resulted in modifications of this view. Then, we describe studies of stimulus-response correspondence effects that provide evidence for distinct representations of responses. More recently, Bridgeman and colleagues examined whether "action affects perception", concluding that the phenomena can be more accurately construed as "information affects memory". Although unconvinced about claims of action-affects-perception and embodied cognition, Bridgeman and colleagues concluded that processing of visual information in hand-space is facilitated and cited a phenomenon as supporting evidence. We discuss findings indicating that this phenomenon is due to general spatial coding principles. We think that all researchers should proceed in the manner of Bridgeman of developing novel explanations, devising critical tests between them and alternative possible explanations, and accepting the explanation that best conforms to the results, even if that explanation is a "less dramatic" option.

Is the Ebbinghaus illusion a size contrast illusion?

The Ebbinghaus illusion, in which a central target surrounded by larger context figures looks smaller than when surrounded by smaller context figures, is usually classified as a size contrast illusion. Thus "size contrast" is the dominant account of this effect. However, according to an alternative "contour interaction" account this phenomenon has little to do with size contrast but is rather caused by distance-dependent attractive and repulsive interactions between neural representation of contours. Here evidence is presented against the size contrast account and consistent with the contour interaction account. Experiment 1 was a control study confirming that the illusion can be obtained using displays consisting only of squares, which are more convenient to manipulate than the standardly used circles. In Experiment 2, the standard configuration involving small context figures surrounding the target was compared to a novel configuration, which involved many "spread" small context figures. The illusory effect of the standard context was stronger than the illusory effect of the spread context, in accord with the prediction of the contour interaction account, and contrary to the prediction of the size contrast account. In Experiment 3 two novel configurations were used, based on standard and spread contexts. The results were in accord with the prediction of the contour interaction account, whereas the size contrast account had no prediction because the stimuli did not involve conventional size contrast. Additional aspects of the stimuli and an account of the illusion based on a perspective interpretation are also discussed.

Vision: Non-illusory Evidence for Distinct Visual Pathways for Perception and Action

When visual information about an object's distance is obscured, but its retinal size visible, the object's physical size is ambiguous to vision; however, additional proprioceptive distance information permits physical size to be estimated when grasping the object, but perceptual size estimates remain inaccurate, adding to that evidence for distinct visual pathways for perception and action.

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.