Visual illusion and action

Effect of the Visual Illusion on Stepping-Over Action and Its Association with Gaze Behavior

An adequate foot clearance height while stepping over an obstacle is important for safety in daily life. In the present study, we examined whether visual illusions affect foot clearance during a stepping-over action, and whether this is further influenced by gaze behavior. Twelve participants stepped over an obstacle placed four meters away under conditions of three different obstacle characteristics: white, horizontal, or vertical lines. We measured the participants' foot clearances during the step-over action and their gaze behavior during the approaching phase. Participants stepped significantly higher over the obstacles in the vertical lines (illusion) condition. The duration of gaze fixation on the obstacle positively correlated with increased foot clearance in the vertical condition, suggesting that the effect of the visual illusion on foot clearance was enhanced by prolonged gaze fixation. Conversely, prolonged fixation negatively correlated with foot clearance in the white (control) condition, implying that a cautious perception of an obstacle may contribute to efficient stepping-over action.

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.

Perception of length and orientation in dry immersion

Introduction

How does gravity (or lack thereof) affect sensory-motor processing? We analyze sensorimotor estimation dynamics for line segments with varying direction (orientation) in a 7-day dry immersion (DI), a ground-based model of gravitational unloading.

Methods

The measurements were carried out before the start of the DI, on the first, third, fifth and seventh days of the DI, and after its completion. At the memorization stage, the volunteers led the leading hand along the visible segment on a touchscreen display, and at the reproduction stage they repeated this movement on an empty screen. A control group followed the same procedure without DI.

Results

Both in the DI and control groups, when memorizing, the overall error in estimating the lengths and directions of the segments was small and did not have pronounced dynamics; when reproducing, an oblique effect (higher variability of responses to oblique orientations compared to cardinal ones) was obtained. We then separated biases (systematic error) and uncertainty (random error) in subjects' responses. At the same time, two opposite trends were more pronounced in the DI group during the DI. On the one hand the cardinal bias (a repulsion of orientation estimates away from cardinal axes) and, to a small extent, the variability of direction estimates decreased. On the other hand, the overestimation bias in length estimates increased.

Discussion

Such error pattern strongly supports the hypotheses of the vector encoding, in which the direction and length of the planned movement are encoded independently of each other when the DI disrupts primarily the movement length encoding.

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 influence of locative expressions on context-dependency of endpoint control in aiming

It has been claimed that increased reliance on context, or allocentric information, develops when aiming movements are more consciously monitored and/or controlled. Since verbalizing target features requires strong conscious monitoring, we expected an increased reliance on allocentric information when verbalizing a target label (i.e. target number) during movement execution. We examined swiping actions towards a global array of targets embedded in different local array configurations on a tablet under no-verbalization and verbalization conditions. The global and local array configurations allowed separation of contextual-effects from any possible numerical magnitude biases triggered from calling out specific target numbers.The patterns of constant errors in the target directionwere used to assess differences between conditions. Variation in the target context configuration systematically biased movement endpoints in both the no-verbalization and verbalization conditions. Ultimately, our results do not support the assertion that calling out target numbers during movement execution increases the context-dependency of targeted actions.

The influence of the Sander parallelogram illusion and early, middle and late vision on goal-directed reaching and grasping

Vision is one of the most robust sensory inputs used for the execution of goal-directed actions. Despite a history of extensive visuomotor research, how individuals process visual context for the execution of movements continues to be debated. This experiment examines how early, middle and late visuomotor control is impacted by illusory characteristics in a reaching and grasping task. Participants either manually estimated or reached out and picked up a three-dimensional target bar resting on a two-dimensional picture of the Sander parallelogram illusion. Participants performed their grasps within a predefined time movement window based on their own average grasp time, allowing for the manipulation of visual feedback. On some trials, vision was only available before the response cue (an auditory tone), while on others vision was occluded until the response cue, becoming available for either the full, early, middle or late portions of the movement. While results showed that the effect of the illusion was stronger on manual estimations than on grasping, maximum grip apertures in the occluded vision and early vision grasping conditions were also consistent to a lesser extent with the illusion. The late vision condition showed longer movement time, wrist deceleration period, time to maximum grip aperture and lower maximum velocity. These findings indicate that visual context affects visuomotor control distinctly depending on when vision is available, and supports the notion that human vision is comprised of two functionally and anatomically distinct systems.

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.

Two Neural Circuits to Point Towards Home Position After Passive Body Displacements

A challenge in motor control research is to understand the mechanisms underlying the transformation of sensory information into arm motor commands. Here, we investigated these transformation mechanisms for movements whose targets were defined by information issued from body rotations in the dark (i.e., idiothetic information). Immediately after being rotated, participants reproduced the amplitude of their perceived rotation using their arm (Experiment 1). The cortical activation during movement planning was analyzed using electroencephalography and source analyses. Task-related activities were found in regions of interest (ROIs) located in the prefrontal cortex (PFC), dorsal premotor cortex, dorsal region of the anterior cingulate cortex (ACC) and the sensorimotor cortex. Importantly, critical regions for the cognitive encoding of space did not show significant task-related activities. These results suggest that arm movements were planned using a sensorimotor-type of spatial representation. However, when a 8 s delay was introduced between body rotation and the arm movement (Experiment 2), we found that areas involved in the cognitive encoding of space [e.g., ventral premotor cortex (vPM), rostral ACC, inferior and superior posterior parietal cortex (PPC)] showed task-related activities. Overall, our results suggest that the use of a cognitive-type of representation for planning arm movement after body motion is necessary when relevant spatial information must be stored before triggering the movement.

Allocentric representations for target memory and reaching in human cortex

The use of allocentric cues for movement guidance is complex because it involves the integration of visual targets and independent landmarks and the conversion of this information into egocentric commands for action. Here, we focus on the mechanisms for encoding reach targets relative to visual landmarks in humans. First, we consider the behavioral results suggesting that both of these cues influence target memory, but are then transformed-at the first opportunity-into egocentric commands for action. We then consider the cortical mechanisms for these behaviors. We discuss different allocentric versus egocentric mechanisms for coding of target directional selectivity in memory (inferior temporal gyrus versus superior occipital gyrus) and distinguish these mechanisms from parieto-frontal activation for planning egocentric direction of actual reach movements. Then, we consider where and how the former allocentric representations of remembered reach targets are converted into the latter egocentric plans. In particular, our recent neuroimaging study suggests that four areas in the parietal and frontal cortex (right precuneus, bilateral dorsal premotor cortex, and right presupplementary area) participate in this allo-to-ego conversion. Finally, we provide a functional overview describing how and why egocentric and landmark-centered representations are segregated early in the visual system, but then reintegrated in the parieto-frontal cortex for action.

Memory for retinotopic locations is more accurate than memory for spatiotopic locations, even for visually guided reaching

To interact successfully with objects, we must maintain stable representations of their locations in the world. However, their images on the retina may be displaced several times per second by large, rapid eye movements. A number of studies have demonstrated that visual processing is heavily influenced by gaze-centered (retinotopic) information, including a recent finding that memory for an object's location is more accurate and precise in gaze-centered (retinotopic) than world-centered (spatiotopic) coordinates (Golomb & Kanwisher, 2012b). This effect is somewhat surprising, given our intuition that behavior is successfully guided by spatiotopic representations. In the present experiment, we asked whether the visual system may rely on a more spatiotopic memory store depending on the mode of responding. Specifically, we tested whether reaching toward and tapping directly on an object's location could improve memory for its spatiotopic location. Participants performed a spatial working memory task under four conditions: retinotopic vs. spatiotopic task, and computer mouse click vs. touchscreen reaching response. When participants responded by clicking with a mouse on the screen, we replicated Golomb & Kanwisher's original results, finding that memory was more accurate in retinotopic than spatiotopic coordinates and that the accuracy of spatiotopic memory deteriorated substantially more than retinotopic memory with additional eye movements during the memory delay. Critically, we found the same pattern of results when participants responded by using their finger to reach and tap the remembered location on the monitor. These results further support the hypothesis that spatial memory is natively retinotopic; we found no evidence that engaging the motor system improves spatiotopic memory across saccades.

Visual Illusions Affect Motor Performance, But not Learning in Highly Skilled Shooters

Enhanced expectancies are an important component of OPTIMAL theory and are thought to contribute to motor performance and learning. There is limited information, however, on the generalizability of OPTIMAL theory to highly skilled individuals. The purpose of this study was to examine the effects of visual illusions, specifically an Ebbinghaus illusion, on the performance and learning of an aiming task using highly skilled 10-m rifle and pistol shooters. Two groups of shooters with international experience were recruited and practiced with perceived larger and smaller targets. Our results indicated that participants who perceived the target larger reported higher self-efficacy immediately after practice. In addition, these participants had higher shooting performance during practice. Our retention test (24 hours later), however, did not produce differences in self-efficacy or shooting performance. Our data suggests that visual illusions are beneficial for motor performance in highly skilled shooters, but may not affect learning in those who are in the latter stages of learning. Further studies should continue examining the role of visual illusions for enhancing expectancies in highly skilled and experienced performers.

Exploiting the geometry of the solution space to reduce sensitivity to neuromotor noise

Throwing is a uniquely human skill that requires a high degree of coordination to successfully hit a target. Timing of ball release appears crucial as previous studies report required timing accuracies as short as 1-2ms, which however appear physiologically challenging. This study mathematically and experimentally demonstrates that humans can overcome these seemingly stringent timing requirements by shaping their hand trajectories to create extended timing windows, where ball releases achieve target hits despite temporal imprecision. Subjects practiced four task variations in a virtual environment, each with a distinct geometry of the solution space and different demands for timing. Model-based analyses of arm trajectories revealed that subjects first decreased timing error, followed by lengthening timing windows in their hand trajectories. This pattern was invariant across solution spaces, except for a control case. Hence, the exquisite skill that humans evolved for throwing is achieved by developing strategies that are less sensitive to temporal variability arising from neuromotor noise. This analysis also provides an explanation why coaches emphasize the "follow-through" in many ball sports.

Effects of wrist tendon vibration and eye movements on manual aiming

In the present study, we investigated whether visual information mediates a proprioceptive illusion effect induced by muscle tendon vibration in manual aiming. Visual information was gradually degraded from a situation in which the targets were present and participants (n = 20; 22.3 ± 2.7 years) were permitted to make saccadic eye movements to designated target positions, to a condition in which the targets were not visible and participants were required to perform cyclical aiming while fixating a point between the two target positions. Local tendon vibration applied to the right wrist extensor muscles induced an illusory reduction of 15% in hand movement amplitude. This effect was greater in the fixation than in the saccade condition. Both anticipatory control and proprioceptive feedback are proposed to contribute to the observed effects. The primary saccade amplitude was also reduced by almost 4% when muscle tendon vibration was locally applied to the wrist. These results confirm a tight link between eye movements and manual perception and action. Moreover, the impact of the proprioceptive illusion on the ocular system indicates that the interaction between systems is bidirectional.

Egocentric and Exocentric Spatial Judgements of Visual Displacement

The role of changes in ego- and exocentric spatial relationships on perceptual judgements about visual displacement was investigated in this study. Subjects were asked to indicate whether a dot in a test stimulus was displaced compared to a dot in a reference stimulus. Subjects were given explicit instructions to report displacement relative to themselves (egocentric) or relative to a circle surrounding the dot (exocentric). Four types of test stimuli were used in which object-circle (exocentric) and object-observer (egocentric) relations were systematically varied. It was found that for test stimuli that reveal conflicting ego- and exocentric spatial information, subjects performed poorly in both instruction conditions. This suggests that ego- and exocentric representations cannot be used independently and are probably interconnected.

The Sander parallelogram illusion dissociates action and perception despite control for the litany of past confounds

The two visual systems hypothesis proposes that human vision is supported by an occipito-temporal network for the conscious visual perception of the world and a fronto-parietal network for visually-guided, object-directed actions. Two specific claims about the fronto-parietal network's role in sensorimotor control have generated much data and controversy: (1) the network relies primarily on the absolute metrics of target objects, which it rapidly transforms into effector-specific frames of reference to guide the fingers, hands, and limbs, and (2) the network is largely unaffected by scene-based information extracted by the occipito-temporal network for those same targets. These two claims lead to the counter-intuitive prediction that in-flight anticipatory configuration of the fingers during object-directed grasping will resist the influence of pictorial illusions. The research confirming this prediction has been criticized for confounding the difference between grasping and explicit estimates of object size with differences in attention, sensory feedback, obstacle avoidance, metric sensitivity, and priming. Here, we address and eliminate each of these confounds. We asked participants to reach out and pick up 3D target bars resting on a picture of the Sander Parallelogram illusion and to make explicit estimates of the length of those bars. Participants performed their grasps without visual feedback, and were permitted to grasp the targets after making their size-estimates to afford them an opportunity to reduce illusory error with haptic feedback. The results show unequivocally that the effect of the illusion is stronger on perceptual judgments than on grasping. Our findings from the normally-sighted population provide strong support for the proposal that human vision is comprised of functionally and anatomically dissociable systems.

Ebbinghaus figures that deceive the eye do not necessarily deceive the hand

In support of the visual stream dissociation hypothesis, which states that distinct visual streams serve vision-for-perception and vision-for-action, visual size illusions were reported over 20 years ago to ‘deceive the eye but not the hand’. Ever since, inconclusive results and contradictory interpretations have accumulated. Therefore, we investigated the effects of the Ebbinghaus figure on repetitive aiming movements with distinct dynamics. Participants performed a Fitts’ task in which Ebbinghaus figures served as targets. We systematically varied the three parameters which have been shown to influence the perceived size of the Ebbinghaus figure’s target circle, namely the size of the target, its distance to the context circles and the size of the context circles. This paper shows that movement is significantly affected by the context size, but, in contrast to perception, not by the other two parameters. This is especially prominent in the approach phase of the movement towards the target, regardless of the dynamics. To reconcile the findings, we argue that different informational variables are used for size perception and the visual control of movements irrespective of whether certain variables induce (perceptual) illusions.

Errors in interception can be predicted from errors in perception

It has been hypothesised that our actions are less susceptible to visual illusions than our perceptual judgements because similar information is processed for perception and action in separate pathways. We test this hypothesis for subjects intercepting a moving object that appears to move at a different speed than its true speed due to an illusion. The object was a moving Gabor patch: a sinusoidal grating of which the luminance contrast is modulated by a two-dimensional Gaussian. We manipulated the patch's apparent speed by moving the grating relative to the Gaussian. We used separate two-interval forced choice discrimination tasks to determine how moving the grating influenced ten people's judgements of the object's position and velocity while they were fixating. Based on their perceptual judgements, and knowing that our ability to correct for errors that arise from relying on incorrect judgements are limited by a sensorimotor delay of about 100 msec, we predicted the extent to which subjects would tap ahead of or behind similar targets when trying to intercept them at the fixation location. The predicted errors closely matched the actual errors that subjects made when trying to intercept the targets. This finding does not support the two visual streams hypothesis. The results are consistent with the idea that the extent to which an illusion influences an action tells us something about the extent to which the action relies on the percept in question.

Interaction of Perception and Action in Discrete and Continuous Rapid Aiming Tasks

Previously, we have shown that discrete and continuous rapid aiming tasks are governed by distinct visuomotor control mechanisms by assessing the combined visual illusion effects on the perceived and effective index of difficulty (ID). All participants were perceptually biased by the combined visual illusion before they performed the rapid aiming tasks. In the current study, the authors manipulated the order of performing perceptual and motor tasks to examine whether perceptual or motor experience with the illusory visual target would influence the subsequent perceived and effective ID in discrete and continuous tapping tasks. The results supported our hypothesis showing that perceptual experience with the illusory visual target in the discrete condition reduced the effective ID in the subsequent discrete tapping task, and motor experience with the illusory visual target in the continuous condition reduced the illusion effects on the perceived ID in the subsequent perceptual judgment task. The study demonstrates the coinfluence of perception and action, and suggests that perception and action influence one another with different magnitude depending on the spatial frame of reference used to perform the perceptuomotor task.

Allocentric information is used for memory-guided reaching in depth: A virtual reality study

Previous research has demonstrated that humans use allocentric information when reaching to remembered visual targets, but most of the studies are limited to 2D space. Here, we study allocentric coding of memorized reach targets in 3D virtual reality. In particular, we investigated the use of allocentric information for memory-guided reaching in depth and the role of binocular and monocular (object size) depth cues for coding object locations in 3D space. To this end, we presented a scene with objects on a table which were located at different distances from the observer and served as reach targets or allocentric cues. After free visual exploration of this scene and a short delay the scene reappeared, but with one object missing (=reach target). In addition, the remaining objects were shifted horizontally or in depth. When objects were shifted in depth, we also independently manipulated object size by either magnifying or reducing their size. After the scene vanished, participants reached to the remembered target location on the blank table. Reaching endpoints deviated systematically in the direction of object shifts, similar to our previous results from 2D presentations. This deviation was stronger for object shifts in depth than in the horizontal plane and independent of observer-target-distance. Reaching endpoints systematically varied with changes in object size. Our results suggest that allocentric information is used for coding targets for memory-guided reaching in depth. Thereby, retinal disparity and vergence as well as object size provide important binocular and monocular depth cues.

The Modulation of Haptic Line Bisection by a Visual Illusion and Optokinetic Stimulation

Research has shown that a variety of different sensory manipulations, including visual illusions, transcutaneous nerve stimulation, vestibular caloric stimulation, optokinetic stimulation, and prism adaptation, can all influence people's performance on spatial tasks such as line bisection. It has been suggested that these manipulations may act upon the ‘higher-order’ levels of representation used to code spatial information. We investigated whether we could influence haptic line bisection in normal participants crossmodally by varying the visual background that participants viewed. In experiment 1, participants haptically bisected wooden rods while looking at a variant of the Oppel–Kundt visual illusion. Haptic-bisection judgments were influenced by the orientation of the visual illusion (in line with previous unimodal visual findings). In experiment 2, haptic-bisection judgments were also influenced by the presence of a leftward or rightward moving visual background. In experiments 3 and 4, the position of the to-be-bisected stimuli was varied with respect to the participant's body midline. The results confirmed an effect of optokinetic stimulation, but not of the Oppel–Kundt illusion, on participants' tactile-bisection errors, suggesting that the two manipulations might differentially affect haptic processing. Taken together, these results suggest that the ‘higher-order’ levels of spatial representation upon which perceptual judgments and/or motor responses are made may have multisensory or amodal characteristics.

Insights into Seeing and Grasping: Distinguishing the Neural Correlates of Perception and Action

Vision contributes to both perception and visuomotor control, and it has been suggested that many higher brain structures specialize in one or the other function. An alternative view, presented here, is that most higher brain areas participate in both visuomotor and perceptual functions. In the anterior frontal cortex, for example, the activity of one population of neurons reflects perceptual reports about a visual stimulus, whereas the activity of an intermingled population reflects movements aimed at the same stimulus. Similarly, posterior parietal and inferior temporal areas appear to function in both visual perception and visuomotor control. Visuomotor signals in higher order cortical areas could contribute to the perception of one’s own action. They also might reflect the existence of two systems for visual information processing: one stressing accuracy for the control of movement and the other generating hypotheses about the world.

Adaptation to size affects saccades with long but not short latencies

Maintained exposure to a specific stimulus property-such as size, color, or motion-induces perceptual adaptation aftereffects, usually in the opposite direction to that of the adaptor. Here we studied how adaptation to size affects perceived position and visually guided action (saccadic eye movements) to that position. Subjects saccaded to the border of a diamond-shaped object after adaptation to a smaller diamond shape. For saccades in the normal latency range, amplitudes decreased, consistent with saccading to a larger object. Short-latency saccades, however, tended to be affected less by the adaptation, suggesting that they were only partly triggered by a signal representing the illusory target position. We also tested size perception after adaptation, followed by a mask stimulus at the probe location after various delays. Similar size adaptation magnitudes were found for all probe-mask delays. In agreement with earlier studies, these results suggest that the duration of the saccade latency period determines the reference frame that codes the probe location.

Allocentric coding in ventral and dorsal routes during real-time reaching: Evidence from imaging-guided multi-site brain stimulation

Recent evidence has revealed an advantage for movements to last target positions in a structured visual display, suggesting a mediating role of allocentric, in addition to egocentric, information in goal-directed reaching. This notion is accommodated by the recently updated perception-action model (Milner and Goodale, 2008), which postulates functional roles of ventral and dorsal neural areas in allocentric coding. In the present study, we used imaging-guided multi-site continuous theta burst stimulation (cTBS) over regions of the ventral and dorsal processing streams to unravel their functional contribution on visually guided reaching in two display conditions: the "egocentric" condition where the target appeared in an empty display and the "allocentric" condition where the target appeared in a structured display with placeholders marking possible target locations. Cortical sites for cTBS were identified individually for each participant via coregistration with magnetic resonance scans. Results indicated that cTBS in the egocentric condition did not affect movement time, but cTBS in the allocentric condition modulated movement time contingent on stimulation site and target position. In particular, cTBS over the lateral occipital cortex (part of the ventral stream) and over the angular gyrus (part of the dorsal stream) eliminated the last-target advantage by slowing down reaching to the salient last target position. cTBS over the superior parietal occipital cortex did not affect the last-target advantage. These outcomes provide the first causal evidence for allocentric coding in ventral and dorsal routes during real-time reaching, thereby supporting the updated perception-action model.

The stuff that motor chunks are made of: Spatial instead of motor representations?

In order to determine how participants represent practiced, discrete keying sequences in the discrete sequence production task, we had 24 participants practice two six-key sequences on the basis of two pre-learned six-digit numbers. These sequences were carried out by fingers of the left (L) and right (R) hand with between-hand transitions always occurring between the second and third, and the fifth and sixth responses. This yielded the so-called LLRRRL and RRLLLR sequences. Early and late in practice, the keypad used for the right hand was briefly relocated from the front of the participants to 90° at their right side. The results indicate that after 600 practice trials, executing a keying sequence relies heavily on a spatial cross-hand representation in a trunk- or head-based reference frame that after about only 15 trials is fully adjusted to the changed hand location. The hand location effect was not found with the last sequence element. This is attributed to the application of explicit knowledge. The between-hand transitions appeared to induce initial segmentation in some of the participants, but this did not consolidate into a concatenation point of successive motor chunks.

Keeping a target in memory does not increase the effect of the Müller-Lyer illusion on saccades

The effects of visual contextual illusions on motor behaviour vary largely between experimental conditions. Whereas it has often been reported that the effects of illusions on pointing and grasping are largest when the movement is performed some time after the stimulus has disappeared, the effect of a delay has hardly been studied for saccadic eye movements. In this experiment, participants viewed a briefly presented Müller-Lyer illusion with a target at its endpoint and made a saccade to the remembered position of this target after a delay of 0, 0.6, 1.2 or 1.8 s. We found that horizontal saccade amplitudes were shorter for the perceptually shorter than for the perceptually longer configuration of the illusion. Most importantly, although the delay clearly affected saccade amplitude, resulting in shorter saccades for longer delays, the illusion effect did not depend on the duration of the delay. We argue that visually guided and memory-guided saccades are likely based on a common visual representation.

Quantifying the Ebbinghaus figure effect: target size, context size, and target-context distance determine the presence and direction of the illusion

Over the last 20 years, visual illusions, like the Ebbinghaus figure, have become widespread to investigate functional segregation of the visual system. This segregation reveals itself, so it is claimed, in the insensitivity of movement to optical illusions. This claim, however, faces contradictory results (and interpretations) in the literature. These contradictions may be due to methodological weaknesses in, and differences across studies, some of which may hide a lack of perceptual illusion effects. Indeed, despite the long history of research with the Ebbinghaus figure, standardized configurations to predict the illusion effect are missing. Here, we present a complete geometrical description of the Ebbinghaus figure with three target sizes compatible with Fitts' task. Each trial consisted of a stimulus and an isolated probe. The probe was controlled by the participant's response through a staircase procedure. The participant was asked whether the probe or target appeared bigger. The factors target size, context size, target-context distance, and a control condition resulted in a 3 × 3 × 3+3 factorial design. The results indicate that the illusion magnitude, the perceptual distinctiveness, and the response time depend on the context size, distance, and especially, target size. In 33% of the factor combinations there was no illusion effect. The illusion magnitude ranged from zero to (exceptionally) 10% of the target size. The small (or absent) illusion effects on perception and its possible influence on motor tasks might have been overlooked or misinterpreted in previous studies. Our results provide a basis for the application of the Ebbinghaus figure in psychophysical and motor control studies.

Rapid eye movements to a virtual target are biased by illusory context in the Poggendorff figure

In order to determine the influence of perceptual input upon oculomotor responses, we examined rapid saccadic eye movements made by healthy human observers to a virtual target defined by the extrapolated intersection of a pointer with a distant landing line. While corresponding perceptual judgments showed no evidence of systematic bias, eye movements showed a strong bias, in the direction of assimilation of the saccade trajectory to the shortest path between the end of the pointer and the landing line. Adding an abutting vertical inducing line to make an angle of 45 deg with the pointer led to a larger bias in the same direction as the classical Poggendorff illusion. This additional Poggendorff effect was similar in direction and magnitude for the eye movements and the perceptual responses. Latency and dynamics of the eye movements were closely similar to those recorded for a control task in which observers made a saccade from the start fixation to an explicit target on the landing line. Further experiments with inducing lines presented briefly at various times during the saccade latency period showed that the magnitude of the saccade bias was affected by inducer presentation during the saccade planning process, but not during the saccade itself. We conclude that the neural mechanisms for extrapolation can feed into the control of eye movements without obvious penalties in timing and accuracy and that this information can instantaneously modify motor response throughout the planning phase, suggesting close association between perceptual and motor mechanisms in the process of visuo-spatial extrapolation.

Allocentric coding: spatial range and combination rules

When a visual target is presented with neighboring landmarks, its location can be determined both relative to the self (egocentric coding) and relative to these landmarks (allocentric coding). In the present study, we investigated (1) how allocentric coding depends on the distance between the targets and their surrounding landmarks (i.e. the spatial range) and (2) how allocentric and egocentric coding interact with each other across targets-landmarks distances (i.e. the combination rules). Subjects performed a memory-based pointing task toward previously gazed targets briefly superimposed (200ms) on background images of cluttered city landscapes. A variable portion of the images was occluded in order to control the distance between the targets and the closest potential landmarks within those images. The pointing responses were performed after large saccades and the reappearance of the images at their initial location. However, in some trials, the images' elements were slightly shifted (±3°) in order to introduce a subliminal conflict between the allocentric and egocentric reference frames. The influence of allocentric coding in the pointing responses was found to decrease with increasing target-landmarks distances, although it remained significant even at the largest distances (⩾10°). Interestingly, both the decreasing influence of allocentric coding and the concomitant increase in pointing responses variability were well captured by a Bayesian model in which the weighted combination of allocentric and egocentric cues is governed by a coupling prior.

Anticipatory Control Through Associative Learning of Subliminal Relations: Invisible May Be Better Than Visible

We showed that anticipatory cognitive control could be unconsciously instantiated through subliminal cues that predicted enhanced future control needs. In task-switching experiments, one of three subliminal cues preceded each trial. Participants had no conscious experience or knowledge of these cues, but their performance was significantly improved on switch trials after cues that predicted task switches (but not particular tasks). This utilization of subliminal information was flexible and adapted to a change in cues predicting task switches and occurred only when switch trials were difficult and effortful. When cues were consciously visible, participants were unable to discern their relevance and could not use them to enhance switch performance. Our results show that unconscious cognition can implicitly use subliminal information in a goal-directed manner for anticipatory control, and they also suggest that subliminal representations may be more conducive to certain forms of associative learning.

Combined visual illusion effects on the perceived index of difficulty and movement outcomes in discrete and continuous fitts' tapping

The speed-accuracy trade-off is a fundamental movement problem that has been extensively investigated. It has been established that the speed at which one can move to tap targets depends on how large the targets are and how far they are apart. These spatial properties of the targets can be quantified by the index of difficulty (ID). Two visual illusions are known to affect the perception of target size and movement amplitude: the Ebbinghaus illusion and Muller-Lyer illusion. We created visual images that combined these two visual illusions to manipulate the perceived ID, and then examined people's visual perception of the targets in illusory context as well as their performance in tapping those targets in both discrete and continuous manners. The findings revealed that the combined visual illusions affected the perceived ID similarly in both discrete and continuous judgment conditions. However, the movement outcomes were affected by the combined visual illusions according to the tapping mode. In discrete tapping, the combined visual illusions affected both movement accuracy and movement amplitude such that the effective ID resembled the perceived ID. In continuous tapping, none of the movement outcomes were affected by the combined visual illusions. Participants tapped the targets with higher speed and accuracy in all visual conditions. Based on these findings, we concluded that distinct visual-motor control mechanisms were responsible for execution of discrete and continuous Fitts' tapping. Although discrete tapping relies on allocentric information (object-centered) to plan for action, continuous tapping relies on egocentric information (self-centered) to control for action. The planning-control model for rapid aiming movements is supported.

Lack of motor prediction, rather than perceptual conflict, evokes an odd sensation upon stepping onto a stopped escalator

When stepping onto a stopped escalator, we often perceive an "odd sensation" that is never felt when stepping onto stairs. The sight of an escalator provides a strong contextual cue that, in expectation of the backward acceleration when stepping on, triggers an anticipatory forward postural adjustment driven by a habitual and implicit motor process. Here we contrast two theories about why this postural change leads to an odd sensation. The first theory links the odd sensation to a lack of sensorimotor prediction from all low-level implicit motor processes. The second theory links the odd sensation to the high-level conflict between the conscious awareness that the escalator is stopped and the implicit perception that evokes an endogenous motor program specific to a moving escalator. We show very similar postural changes can also arise from reflexive responses to visual stimuli, such as contracting/expanding optic flow fields, and that these reflexive responses produce similar odd sensations to the stopped escalator. We conclude that the high-level conflict is not necessary for such sensations. In contrast, the implicitly driven behavioral change itself essentially leads to the odd sensation in motor perception since the unintentional change may be less attributable to self-generated action because of a lack of motor predictions.

Manual asymmetries in the kinematics of a reach-to-grasp action

In the present study, we manipulated the perceived demand of an ecologically valid task to investigate the possible presence of manual asymmetries in a reach-to-grasp action. Participants reached, grasped and sipped from a water glass under low (nearly empty) and high (nearly full) demand conditions. Participants reached to grasp in closed-loop, open-loop and delay visual conditions. Manual asymmetries were found in movement time, peak velocity and maximum grip aperture variability. Consistent with reach-to-point literature: (1) right-handed actions were completed in less time than left-handed actions in visually and memory-guided conditions; (2) right-handed movements were more accurate (i.e., produced more consistent maximum grip apertures) than left-handed movements in visually guided conditions. The results support a theory of left-hemisphere specialization for visual control of action.

Context is quick, knowledge is slow: rapid time-course of contextual modulations in the horizontal-vertical illusion

One of the oldest visual illusions consists of observers' tendency to estimate vertical lines as longer than equal horizontal lines. This Horizontal-Vertical Illusion has been studied widely; however, most studies may not generalize to natural contexts because they have not addressed the low-level processes that occur rapidly and in response to context-free lines vs the possibility for high-level processes that arise over time in response to context. These issues were investigated via a novel design: rapid time-sequencing embedded within the context of natural images. Participants saw different levels of context and the Horizontal-Vertical Illusion was measured. In traditional Horizontal-Vertical Illusion displays without natural context, there was a robust invariance of the illusion to stimulus duration. Within context, however, there was strong modulation: the magnitude of the illusion decreased by over 50% or increased over 30%, depending upon placement within context. Knowledge of object properties such as size or line length did not account for this effect. It appears that previous domain-general investigations of the Horizontal-Vertical Illusion did not address the specific higher-order processes that produce the illusion in natural contexts.

Sind motorische Handlungen auf eine präzise Wahrnehmung angewiesen?

Ziel der vorliegenden Studie war es, den Einfluss spezifischer visueller Wahrnehmungseffekte auf die Handlungskontrolle von closed-loop-kontrollierten Zielbewegungen zu untersuchen. Mittels einer simultanen Größen-Kontrast-Illusion (Ebbinghaus-Titchener-Illusion) wurden die Wahrnehmungseffekte manipuliert. Die Handlung und die inhärenten informationellen motorischen Prozesse wurden über das Fitts’sche Gesetz mittels verschiedener Schwierigkeitsindizes (IDs 3, 4.5) systematisch variiert. Die Aufgabe der Versuchspersonen war es, eine reziproke, zyklische, zielmotorische Präzisionsaufgabe über 30 s hinweg mittels einer Flexions- und Extensionsbewegung, zwischen zwei illusionserzeugenden Stimuli und unter zwei unterschiedlichen Schwierigkeitsindizes so genau und so schnell wie möglich auszuführen. Die Ergebnisse zeigen, dass es sowohl durch die visuelle Illusion, als auch durch die Erhöhung der ID zu einer Minderung in der motorischen Ausführungsleistung kam. Der Befund verweist darauf, dass visuelle Illusionseffekte die Handlungskontrolle bei closed-loop-kontrollierten Zielbewegungen beeinträchtigen.

Distinct task-independent visual thresholds for egocentric and allocentric information pick up

The dominant view of the ventral and dorsal visual systems is that they subserve perception and action. De Wit, Van der Kamp, and Masters (2011) suggested that a more fundamental distinction might exist between the nature of information exploited by the systems. The present study distinguished between these accounts by asking participants to perform delayed matching (perception), pointing (action) and perceptual judgment responses to masked Müller-Lyer stimuli of varying length. Matching and pointing responses of participants who could not perceptually judge stimulus length at brief durations remained sensitive to veridical stimulus length (egocentric information), but not the illusion (allocentric, context-dependent information), which was effective at long durations. Distinct thresholds for egocentric and allocentric information pick up were thus evident irrespective of whether perception (matching) or action (pointing) responses were required. It was concluded that the dorsal and ventral systems may be delineated fundamentally by fast egocentric- and slower allocentric information pick up, respectively.

Delayed pointing movements to masked Müller-Lyer figures are affected by target size but not the illusion

There is ongoing debate with respect to interpretation of the finding that, in contrast to perceptual size judgments, actions are relatively unaffected by the Müller-Lyer illusion. In normal unrestricted viewing situations observers cannot perform an action directed at an object without simultaneously perceiving the object - this makes it difficult to unequivocally establish whether observed effects are a function of vision for perception, vision for action, a combination of both, or of a single all-purpose visual system. However, there is evidence that observers are capable of performing actions towards objects of which they are not consciously aware, implying that two distinct visual thresholds may exist; one accompanying vision for action and one accompanying vision for perception. To investigate this possibility we created a situation in which visual information was presented below the perception threshold, but above the purported action threshold, allowing examination of action responses independent of contributions from vision for perception. Following a perceptual categorization task, participants performed delayed pointing movements towards briefly exposed masked Müller-Lyer targets of different sizes. When the targets were presented below the perception threshold, participants were unable to discriminate between them, yet their delayed pointing movements were affected by target size (but not the illusion). The results imply that vision for action is functional even after a delay and/or that the pickup of egocentric information is associated with a lower visual threshold than the pickup of allocentric information.

Visual illusions and the control of ball placement in goal-directed hitting

When hitting kicking, or throwing balls at targets, online control in the target area is impossible. We assumed this lack of late corrections in the target area would induce an effect of a single-winged Müller-Lyer illusion on ball placement. After extensive practice in hitting balls to different landing locations, participants (N = 9) had to hit a ball to a distant target specified by the vertex of a single-winged Müller-Lyer configuration. Impact velocity was not significantly "tricked" by the pictorial illusion, suggesting that, even when late corrections in the target area are absent, some motor behaviors are not susceptible to the influence of the visual environment surrounding the target.

Anticipatory reaching of seven- to eleven-month-old infants in occlusion situations

The present study examined 7- to 11-month-old infants' anticipatory and reactive reaching for temporarily occluded objects. Infants were presented with laterally approaching objects that moved at different velocities (10, 20, and 40 cm/s) in different occlusion situations (no-, 20 cm-, and 40 cm-occlusion), resulting in occlusion durations ranging between 0 and 4s. Results show that except for object velocity and occlusion distance, occlusion duration was a critical constraint for infants' reaching behaviors. We found that the older infants reached more often, but that an increase in occlusion duration resulted in a decline in reaching frequency that was similar across age groups. Anticipatory reaching declined with increasing occlusion duration, but the adverse effects for longer occlusion durations diminished with age. It is concluded that with increasing age infants are able to retain and use information to guide reaching movements over longer periods of non-visibility, providing support for the graded representation hypothesis (Jonsson & von Hofsten, 2003) and the two-visual systems model (Milner & Goodale, 1995).