No evidence for top-down expertise effects on action perception in sprinters using static images.

Athletes have been found to demonstrate a superior ability to detect subtle variations in dynamic displays (e.g., point-light displays and videos) depicting expert actions compared to non-athletes. The current study aimed to determine whether this advantage also exists when dynamic information is unavailable (i.e., using static images). Using a staircase procedure, two frames from a video depicting an athlete either walking (everyday action) or performing a sprint start (expert action) were presented, and athletes (sprinters) and non-athletes were asked to indicate whether the images were identical or different. We examined whether presenting the images sequentially (temporal task) or simultaneously (spatial task) influenced participants' discrimination performance. We predicted that the sprinters would outperform the non-sprinters in the spatial task as body postures could be compared directly but not in the temporal task due to larger representational momentum effects for athletes. Contrary to our hypotheses, the sprinters and non-sprinters performed similarly in all tasks and conditions. In line with the prediction that representational momentum may impair performance, participants' thresholds were lower for the spatial than the temporal task. However, post-hoc analysis suggested that this effect is likely to be better explained by a task order effect whereby participants who completed the temporal task first exhibited an advantage in the spatial task, while there were no performance differences for participants who completed the opposite task order. In sum, our results provide no evidence for the idea that motor expertise affects action perception (i.e., perceptual resonance) in a simple psychophysical task employing static images.

Grasping tiny objects.

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

The redundant target paradigm and its use as a blindsight-test: A meta-analytic study.

The redundant target effect (RTE) is the well-known effect whereby a single target is detected faster when a second, redundant target is presented simultaneously. The RTE was shown in different experimental designs and applied in various clinical contexts. However, there are also studies showing non-effects or effects in the opposite direction. Our meta-analysis aims to investigate the replicability of the RTE. Herein, we focused on the clinical context within which the RTE has been applied most often and for which it gained particular prominence: The research on blindsight and other forms of residual vision in patients with damage to the neuronal visual system. The application of the RTE in clinical contexts assumes that whenever vision is present, an RTE will be found. Put differently, the RTE as a tool to uncover residual vision presumes that the RTE is a consistent feature of vision in the healthy population. We found a significant summary effect size of the RTE in healthy participants. The effect size depended on certain experimental features: task type, target configuration in the redundant condition, and how reaction times were computed in the single condition. A specific feature combination is typically used in blindsight research. Analyzing studies with this feature combination revealed a significant summary effect size in healthy participants predicting positive RTEs for future studies. A power-analysis revealed a required sample size of 14 participants to obtain an RTE with high reliability. However, the required sample size is rarely reached in blindsight research. Rather, blindsight research is mostly based on single-case studies. In summary, the RTE is a robust effect on group level but does not occur in every single individual. This means failure to obtain an RTE in a single patient should not be interpreted as evidence for the absence of residual vision in this patient.

Action Perception in Athletes: Expertise Facilitates Perceptual Discrimination.

Prior research has demonstrated that athletes outperform non-athletes on action perception tasks involving anticipation of sport-related actions. We conducted two experiments to determine whether this advantage persists on tasks without anticipation and/or transfers to non-sport actions. In Experiment 1, motor experts (sprinters) and non-experts were shown two consecutive videos of an athlete either walking or sprinting. The participants' task was to indicate whether the videos were identical or different. The sprinters were more accurate in these judgments than non-experts, indicating that their athleticism was associated with motor expertise that enhanced their perception of both expert and everyday actions. Further analysis revealed that participants who reported basing their decisions on a specific and informative cue (i.e., the distance between where the athlete's foot landed and a line on the track) outperformed those who did not. However, the sprinters benefitted more from using this cue than the non-sprinters. In Experiment 2, we assessed whether non-experts' performance improved if the number of available cues was reduced to make the informative cue easier to identify. Non-experts completed the same task as in Experiment 1, with half of the participants viewing the upper part of the athletes' body and the other half viewing the lower part containing the informative cue. However, the non-experts still did not reliably identify the cue, and performance did not vary between the two non-expert sub-groups. The results of these experiments suggest that motor expertise indirectly affects action perception by improving experts' ability to identify and use informative cues.

Resource limitations in bimanual pointing.

Performing coordinated bimanual movements, that is, movements with two hands simultaneously, is a requirement in many activities. At the same time, these movements are subject to temporal and spatial constraints. Here, we focus on the constraints that become observable when pointing movements of different (asymmetric) rather than same (symmetric) amplitudes have to be executed ("spatial interference effect"). The respective performance costs are larger when the stimuli used to indicate the movement targets are symbolic compared with when the endpoints of the movements are cued directly. Previous studies have thus concluded that the source of spatial interference is both 'cognitive' and 'motoric', or more precisely occurs during response selection as well as motor programming. We here asked whether the contribution from motor programming is motoric in the sense as envisaged in dual-task models, that is, whether it can run in parallel to, and interference-free with, other processing stages. In two PRP experiments, Task 1 was bimanual pointing and Task 2 was auditory pitch-discrimination. Based on the effect propagation-logic, the results suggest that the motor programming contribution to bimanual interference also taps into capacity-limited resources and cannot be construed as running in parallel as assumed for the motor stage in dual-task models.

Crossmodal Texture Perception Is Illumination-Dependent.

Visually perceived roughness of 3D textures varies with illumination direction. Surfaces appear rougher when the illumination angle is lowered resulting in a lack of roughness constancy. Here we aimed to investigate whether the visual system also relies on illumination-dependent features when judging roughness in a crossmodal matching task or whether it can access illumination-invariant surface features that can also be evaluated by the tactile system. Participants ( N = 32) explored an abrasive paper of medium physical roughness either tactually, or visually under two different illumination conditions (top vs oblique angle). Subsequently, they had to judge if a comparison stimulus (varying in physical roughness) matched the previously explored standard. Matching was either performed using the same modality as during exploration (intramodal) or using a different modality (crossmodal). In the intramodal conditions, participants performed equally well independent of the modality or illumination employed. In the crossmodal conditions, participants selected rougher tactile matches after exploring the standard visually under oblique illumination than under top illumination. Conversely, after tactile exploration, they selected smoother visual matches under oblique than under top illumination. These findings confirm that visual roughness perception depends on illumination direction and show, for the first time, that this failure of roughness constancy also transfers to judgements made crossmodally.

Bimanual Grasping Adheres to Weber's Law.

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

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.

Perceptual uncertainty and action consequences independently affect hand movements in a virtual environment.

When we use virtual and augmented reality (VR/AR) environments to investigate behaviour or train motor skills, we expect that the insights or skills acquired in VR/AR transfer to real-world settings. Motor behaviour is strongly influenced by perceptual uncertainty and the expected consequences of actions. VR/AR differ in both of these aspects from natural environments. Perceptual information in VR/AR is less reliable than in natural environments, and the knowledge of acting in a virtual environment might modulate our expectations of action consequences. Using mirror reflections to create a virtual environment free of perceptual artefacts, we show that hand movements in an obstacle avoidance task systematically differed between real and virtual obstacles and that these behavioural differences occurred independent of the quality of the available perceptual information. This suggests that even when perceptual correspondence between natural and virtual environments is achieved, action correspondence does not necessarily follow due to the disparity in the expected consequences of actions in the two environments.

Decision making in slow and rapid reaching: Sacrificing success to minimize effort.

Current studies on visuomotor decision making come to inconsistent conclusions regarding the optimality with which these decisions are made. When executing rapid reaching movements under uncertainty, humans tend to automatically select optimal movement paths that take into account the position of all potential targets (spatial averaging). In contrast, humans rarely employ optimal strategies when making decisions on whether to pursue two action goals simultaneously or prioritise one goal over another. Here, we manipulated whether spatial averaging or pre-selection of a single target would provide the optimal strategy by varying the spatial separation between two potential movement targets as well as the time available for movement execution. In Experiment 1, we aimed to determine the time needed to reach for targets with small and large separation between them and to measure baseline strategies under low time pressure. Given generous time limits, participants did not employ a pure averaging approach but instead tended to pre-select the target that was easiest to reach and corrected their movement path in-flight if required. In Experiment 2, a strict time limit was set such that the optimal strategy to reach the correct target depended on the separation between the potential targets: for small separations, there was enough time to employ averaging strategies, but higher success for larger separations required pre-selecting the final target instead. While participants varied in the strategies they preferred, none of them flexibly adjusted their movement strategies depending on the spatial separation of the targets. In Experiment 3, we confirm the bias toward targets that are easiest to reach and show that this comes at the expense of overall task success. The results suggest a strong tendency for humans to minimize immediate movement effort and a general failure to adapt movement strategies flexibly with changes in the task parameters.

Temporal-order judgement task suggests chronological action representations in motor experts and non-experts.

Motor priming studies have suggested that human movements are mentally represented in the order in which they usually occur (i.e., chronologically). In this study, we investigated whether we could find evidence for these chronological representations using a paradigm which has frequently been employed to reveal biases in the perceived temporal order of events-the temporal-order judgement task. We used scrambled and unscrambled images of early and late movement phases from an everyday action sequence ("stepping") and an expert action sequence ("sprinting") to examine whether participants' mental representations of actions would bias their temporal-order judgements. In addition, we explored whether motor expertise mediated the size of temporal-order judgement biases by comparing the performances of sprinting experts with those of non-experts. For both action types, we found significant temporal-order judgement biases for all participants, indicating that there was a tendency to perceive images of human action sequences in their natural order, independent of motor expertise. Although there was no clear evidence that sprinting experts showed larger biases for sprinting action sequences than non-experts, considering sports expertise in a broader sense provided some tentative evidence for the idea that temporal-order judgement biases may be mediated by more general motor and/or perceptual familiarity with the running action rather than specific motor expertise.

Fearing the wurst: Robust approach bias towards non-vegetarian food images in a sample of young female vegetarian eaters.

Previous studies have shown that humans show an implicit approach bias toward food related items which is moderated by hunger and properties of the food items displayed (such as their palatability and calorie content). However, little is known about if and how this approach bias is moderated by food preferences and/or diet choices. In this study, we compared approach-avoidance biases in a group of young female omnivore and vegetarian eaters towards images of vegetarian and non-vegetarian food items using a manikin stimulus-response compatibility task. While vegetarian eaters showed a slightly larger approach bias for vegetarian than for non-vegetarian food stimuli, this bias was of similar size to that observed in the omnivorous group. Most interestingly, vegetarian eaters' approach bias towards non-vegetarian food pictures also did not differ from that of the omnivorous group, despite vegetarians rating those pictures as much less pleasant. Our findings suggest that approach biases towards food items are quite robust and do not rapidly change with dietary practice. However, despite approach biases often guiding behaviour, vegetarian eaters successfully withstand these implicit action tendencies and avoid non-vegetarian produce. Potential implications of this finding for the addiction literature are discussed.

Individual differences in processing resources modulate bimanual interference in pointing.

Coordinating both hands during bimanual reaching is a complex task that can generate interference during action preparation as often indicated by prolonged reaction times for movements that require moving the two hands at different amplitudes. Individual processing constraints are thought to contribute to this interference effect. Most importantly, however, the amount of interference seems to depend considerably on overall task demands suggesting that interference increases as the available processing resources decrease. Here, we further investigated this idea by comparing performance in a simple direct cueing and a more difficult symbolic cueing task between three groups of participants that supposedly vary in their processing resources, i.e., musicians, young adults and older adults. We found that the size of interference effects during symbolic cueing varied in the tested groups: musicians showed the smallest and older adults the largest interference effects. More importantly, a regression model, using processing speed and processing capacity as predictor variables, revealed a clear link between the available processing resources and the size of the interference effect during symbolic cueing. In the easier direct cueing task, no reliable interference was observed on a group level. We propose that the susceptibility to bimanual interference is modulated by the task-specific processing requirements in relation with the available processing resources of an individual.

Shared attention for action selection and action monitoring in goal-directed reaching.

Dual-task studies have shown higher sensitivity for stimuli presented at the targets of upcoming actions. We examined whether attention is directed to action targets for the purpose of action selection, or if attention is directed to these locations because they are expected to provide feedback about movement outcomes. In our experiment, endpoint accuracy feedback was spatially separated from the action targets to determine whether attention would be allocated to (a) the action targets, (b) the expected source of feedback, or (c) to both locations. Participants reached towards a location indicated by an arrow while identifying a discrimination target that could appear in any one of eight possible locations. Discrimination target accuracy was used as a measure of attention allocation. Participants were unable to see their hand during reaching and were provided with a small monetary reward for each accurate movement. Discrimination target accuracy was best at action targets but was also enhanced at the spatially separated feedback locations. Separating feedback from the reaching targets did not diminish discrimination accuracy at the movement targets but did result in delayed movement initiation and reduced reaching accuracy, relative to when feedback was presented at the reaching target. The results suggest attention is required for both action planning and monitoring movement outcomes. Dividing attention between these functions negatively impacts action performance.

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.

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.

Visual feedback explains why propointing is better than antipointing in spatial neglect.

Rossit et al. (2011) showed that neglect patients perform normally in a propointing task but not in an antipointing task which requires pointing towards the mirrored position of a target. It is assumed that antipointing relies on information from the perceptual pathway of our visual brain. Therefore, this finding supports the notion that neglect is a disorder that primarily affects perceptual spatial representations within the brain leaving spatial maps used for visuomotor guidance intact. Alternatively, performance of patients might be compromised in both tasks, but only obviously so in tasks in which online corrections are made more difficult. It can be argued that online-corrections via visual feedback are less effective in antipointing because a direct comparison between hand and target is not possible in this condition. Secondly, it is also known that neglect patients have a pronounced egocentric bias which is assumed to be associated with a deviation of the perceived body midline. Since the midline is used to compute the end-position in the antipointing task this could also explain why patients are worse in antipointing. We investigated the influence of visual feedback on pro- and antipointing and the effect of providing a visual reference line for the antipointing task in right-brain damaged patients with neglect (n = 20), right-brain damaged patients without neglect (n = 23) and in a group of healthy participants (n = 22). The withdrawal of visual feedback had a stronger effect on propointing compared to antipointing. This effect was stronger in neglect patients than in patients without neglect or healthy controls. The introduction of a reference line reduced errors in antipointing performance, particularly in neglect patients with a strong egocentric bias. The results support our alternative account and challenge the hypothesis that the spatial disorder in neglect affects primarily perceptual maps within the visual system.

Do we have distinct systems for immediate and delayed actions? A selective review on the role of visual memory in action.

The perception-action model with its assumptions of distinct visual pathways for perception and visuomotor control has been highly influential but also contentious. The controversy largely focused on the evidence from studies on perceptual illusions and this scientific field has been reviewed quite a few times in recent years. In contrast another aspect of the model, namely the role of visual memory in action control, received comparatively little attention. With respect to visual memory the perception-action model proposes that only the perceptual or ventral stream can maintain a sustained representation of the visual world while the visuomotor system or dorsal stream has to rely on currently available visual information. Consequently, visual information from the dorsal system cannot guide actions that are based on memorized visual information. We call this feature of the perception-action model: the dorsal amnesia hypothesis. There are at least two reasons for why this hypothesis is of special relevance. Firstly, it provides a particularly clear criterion to distinguish between functions of the ventral and dorsal stream. Secondly, this hypothesis led to some unexpected discoveries which provided particularly compelling evidence in favour of the model. In this review, we will revisit all relevant empirical areas, ranging from physiological examinations and neuropsychological studies to behavioural experiments in neurologically intact participants. Based on this review, we conclude that the dorsal amnesia hypothesis is in our view no longer tenable.

Avoiding unseen obstacles: Subcortical vision is not sufficient to maintain normal obstacle avoidance behaviour during reaching.

Previous research found that a patient with cortical blindness (homonymous hemianopia) was able to successfully avoid an obstacle placed in his blind field, despite reporting no conscious awareness of it [Striemer, C. L., Chapman, C. S., & Goodale, M. A., 2009, PNAS, 106(37), 15996-16001]. This finding led to the suggestion that dorsal stream areas, that are assumed to mediate obstacle avoidance behaviour, may obtain their visual input primarily from subcortical pathways. Hence, it was suggested that normal obstacle avoidance behaviour can proceed without input from the primary visual cortex. Here we tried to replicate this finding in a group of patients (N = 6) that suffered from highly circumscribed lesions in the occipital lobe (including V1) that spared the subcortical structures that have been associated with action-blindsight. We also tested if obstacle avoidance behaviour differs depending on whether obstacles are placed only in the blind field or in both the blind and intact visual field of the patients simultaneously. As expected, all patients successfully avoided obstacles placed in their intact visual field. However, none of them showed reliable avoidance behaviour - as indicated by adjustments in the hand trajectory in response to obstacle position - for obstacles placed in their blind visual field. The effects were not dependent on whether one or two obstacles were present. These findings suggest that behaviour in complex visuomotor tasks relies on visual input from occipital areas.