Involvement of the posterior tool processing network during explicit retrieval of action tool and semantic tool knowledge: an fMRI study.

Our ability to understand how to interact with familiar objects is supported by conceptual tool knowledge. Conceptual tool knowledge includes action tool and semantic tool knowledge which are supported by the ventro-dorsal and the ventral pathways, respectively. This apparent functional segregation has been recently called into question. In a block-design fMRI study, 35 participants were asked to complete manipulation, function, and association judgment tasks about pairs of familiar objects. Our results showed that lateral occipitotemporal cortex in the ventral pathway was more sensitive to manipulation and function judgment tasks compared with association judgment tasks. Functional connectivity analyses revealed distinct coupling patterns between inferior parietal lobule, lateral occipitotemporal cortex, and fusiform gyrus. Taken together, these data indicate that action tool and semantic tool knowledge are both supported by ventral and ventro-dorsal pathways. Moreover, the explicit retrieval of these representations is supported by the functional coupling of common and distinct brain regions of the posterior tool processing network varying according to the kind of relations to be retrieved.

The social cognitive dimension of pantomime.

Pantomime production is commonly interpreted as reflecting tool-use-related cognitive processes. Yet, in everyday life, pantomime deserves a communication function and the exaggeration of amplitude found during pantomime compared to real tool use may reflect the individual's attempt to communicate the intended gesture. Therefore, the question arises about whether pantomime is a communicative behavior that is nevertheless supported only by non-social cognitive processes. We contribute to answering this question by using kinematic analyses. Participants performed the pantomime of using a saw or a hammer from visual presentation in three conditions: Free (no specific instructions), Self (focus on the real tool-use action), and Others (focus on the communicative dimension). Finally, they used the tool with the corresponding object (Actual condition). Participants' social cognition were assessed using gold standard questionnaires. Our results indicated that the manipulation of instructions had a minor effect on the exaggeration of amplitude during pantomime. We reported a link between the social cognition score and the amplitude in the Others condition for the hammer, which suggests that social cognitive processes could take part in pantomime production in some conditions. Nevertheless, this result does not alter our conclusion that social cognitive processes might be far from necessary for pantomime production.

No self-advantage in recognizing photographs of one's own hand: experimental and meta-analytic evidence.

Visually recognising one's own body is important both for controlling movement and for one's sense of self. Twenty previous studies asked healthy adults to make rapid recognition judgements about photographs of their own and other peoples' hands. Some of these judgements involved explicit self-recognition: "Is this your hand or another person's?" while others assessed self-recognition implicitly, comparing performance for self and other hands in tasks unrelated to self-other discrimination (e.g., left-versus-right; match-to-sample). We report five experiments with three groups of participants performing left-versus-right (Experiment 1) and self-versus-other discrimination tasks (Experiments 2 to 5). No evidence was found for better performance with self than with other stimuli, but some evidence was found for a self-disadvantage in the explicit task. Manipulating stimulus duration as a proxy for task difficulty revealed strong response biases in the explicit self-recognition task. Rather than discriminating between self and other stimuli, participants seem to treat self-other discrimination tasks as self-detection tasks, raising their criterion and consistently responding 'not me' when the task is difficult. A meta-analysis of 21 studies revealed no overall self-advantage, and suggested a publication bias for reports showing self-advantages in implicit tasks. Although this may appear counter-intuitive, we suggest that there may be no self-advantage in hand recognition.

Prism Adaptation in M1.

During prism adaptation (PA), active exposure to an optical shift results in sustained modifications of the sensorimotor system, which have been shown to expand to the cognitive level and serve as a rehabilitation technique for spatial cognition disorders. Several models based on evidence from clinical and neuroimaging studies offered a description of the cognitive and the neural correlates of PA. However, recent findings using noninvasive neurostimulation call for a reexamination of the role of the primary motor cortex (M1) in PA. Specifically, recent studies demonstrated that M1 stimulation reactivates previously vanished sensorimotor changes 1 day after PA, induces after-effect strengthening, and boosts therapeutic effects up to the point of reversing treatment-resistant unilateral neglect. Here, we articulate findings from clinical, neuroimaging, and noninvasive brain stimulation studies to show that M1 contributes to acquiring and storing PA, by means of persisting latent changes after the behavioral training is terminated, consistent with studies on other sensorimotor adaptation procedures. Moreover, we describe the hierarchical organization as well as the timing of PA mechanisms and their anatomical correlates, and identify M1 as an anatomo-functional interface between low- and high-order PA-related mechanisms.

Non-invasive brain stimulation shows possible cerebellar contribution in transfer of prism adaptation after-effects from pointing to throwing movements.

Whether sensorimotor adaptation can be generalized from one context to others represents a crucial interest in the field of neurological rehabilitation. Nonetheless, the mechanisms underlying transfer to another task remain unclear. Prism Adaptation (PA) is a useful method employed both to study short-term plasticity and for rehabilitation. Neuro-imaging and neuro-stimulation studies show that the cerebellum plays a substantial role in online control, strategic control (rapid error reduction), and realignment (after-effects) in PA. However, the contribution of the cerebellum to transfer is still unknown. The aim of this study was to test whether interfering with the activity of the cerebellum affected transfer of prism after-effects from a pointing to a throwing task. For this purpose, we delivered cathodal cerebellar transcranial Direct Current Stimulation (tDCS) to healthy participants during PA while a control group received cerebellar Sham Stimulation. We assessed longitudinal evolutions of pointing and throwing errors and pointing trajectories orientations during pre-tests, exposure and post-tests. Results revealed that participants who received active cerebellar stimulation showed (1) altered error reduction and pointing trajectories during the first trials of exposure; (2) increased magnitude but reduced robustness of pointing after-effects; and, crucially, (3) slightly altered transfer of after-effects to the throwing task. Therefore, the present study confirmed that cathodal cerebellar tDCS interferes with processes at work during PA and provides evidence for a possible contribution of the cerebellum in after-effects transfer.

Sympathy for the underdog: people are inclined to adopt the emotional perspective of powerless (versus powerful) others.

Upon learning of the story of Cinderella, most people spontaneously adopt the emotional perspective of this helpless young woman rather than of her older sisters who oppress her. The present research examines whether this pattern reveals a general human tendency to empathise more with the emotions of individuals with low (versus high) power. Six experiments (N = 878) examined how power influences the focus of people's emotional attributions. Participants were presented with situations in which one character exercised power over another one and had to resolve a referential ambiguity by considering the perspective of one or the other character. Results show that participants largely privileged the emotional states of the low-power character over those of the high-power character. This effect was observed with different types of stimuli (comics and video clips), with high- and low-power roles attributed to pairs of different genders (Experiments 1-4) or same gender (Experiments 5-6). Finally, the tendency persisted - though it was reduced - when participants adopted a less passive role with respect to the characters (Experiment 3) and when power occurred in a less despotic way (Experiment 6). Results are discussed with respect to social attention and sensitivity to fairness.

Take a Seat and Get Into Its Shoes! When Humans Spontaneously Represent Visual Scenes From the Point of View of Inanimate Objects.

Human description of the surrounding world may spontaneously rely on others' perspective, which is a crucial component of social cognition. In five studies, participants were asked to describe the spatial relations between objects in visual scenes including, or not, other agents. In Experiment 1, a substantial proportion of participants used an other-centered perspective in the presence of another agent, replicating classical findings. To our own surprise, we also observed that an even greater number of participants used an other-centered perspective when the human agent was replaced by an armchair. In order to explore this phenomenon, Experiments 2 to 5 compared the respective strength of chair-centered and agent-centered perspectives and/or set them into conflict. A significant proportion of participants spontaneously took the seat's perspective even when it may not be sat on (Experiments 3 and 4) and even when the seat was not referred to (Experiments 4 and 5). Altogether, these findings suggest that perspective taking may spontaneously apply to inanimate objects. These results question whether such tendencies originate from social cognitive skills-as classically assumed-or reveal a nonsocial phenomenon. Future works should specifically test the widely assumed social nature of spontaneous perspective-taking.

Effect of object substitution, spontaneous compensation and repetitive training on reaching movements in a patient with optic ataxia.

We report the case of M.B. who demonstrated severe optic ataxia with the right hand following stroke in the left hemisphere. The clinical picture may shed light on both the pathological characteristics of reaching and grasping actions, and potential rehabilitation strategies for optic ataxia. First, M.B. demonstrated a dissociation between severely impaired reaching and relatively spared grasping and tool use skills and knowledge, which confirms that grasping may be more intermingled with non-motoric cognitive mechanisms than reaching. Besides, M.B.'s reaching performance was sensitive to movement repetition. We observed a substitution effect: Reaching time decreased if M.B. repeatedly reached toward the same object but increased when object identity changed. This may imply that not only object localization but also object identity, is integrated into movement programming in reach-to-grasp tasks. Second, studying M.B.'s spontaneous compensation strategies ascertained that the mere repetition of reaching movements had a positive effect, to the point M.B. almost recovered to normal level after an intensive one-day repetitive training session. This case study seems to provide one of the first examples of optic ataxia rehabilitation. Reaching skills can be trained by repetitive training even two years post-stroke and despite the presence of visuo-imitative apraxia.

Paradoxical adaptation of successful movements: The crucial role of internal error signals.

Visuo-motor adaptation has been classically studied using movements aimed at visual targets with visual feedback. In this type of experimental design, the respective roles of the different error signals cannot be fully disentangled. Here, we show that visuo-motor adaptation occurs despite the terminal success of the action and the compensation of the external error by a jump of the visual target. By using three grasping task conditions we manipulated the retinal error signal between the seen hand and the target (external error) and the conflict between the hand's visual reafference and either the proprioceptive or the efference copy signal (internal error), in order to estimate their respective roles in prism adaptation. In all conditions, subjects were asked to rapidly grasp an object. In the classical 'Prism' condition the object was stationary, which provided both external and internal errors. In the 'Prism & Jump' condition, at movement onset the object was suddenly displaced (jump) toward its virtual image location (visually displaced by the prism) which also corresponded to the location where the movement was planned to and executed through prisms. This jump therefore cancelled the external error (between the seen target and the seen hand), whereas the internal error (between the seen hand and the expected visual reafference of the hand, or between the seen hand and the hand felt by proprioception) was unchanged (because it is independent of the presence of the goal). In the 'Jump' condition, the movement was planned and executed without prismatic goggles and consequently with no internal error (no difference between where the hand visual reafference is expected to be and where it actually is), but the object was suddenly displaced at movement onset by a displacement equivalent to a prism shift which provided an external error. The 'Prism' and 'Prism & Jump' conditions exhibited similar aftereffects, whereas no aftereffect was observed in the 'Jump' condition. These results suggest that successful actions can be subjected to adaptation and that internal error is the only signal necessary to elicit true visuomotor adaptation characterized by context-independent generalization.

Paired-Pulse Parietal-Motor Stimulation Differentially Modulates Corticospinal Excitability across Hemispheres When Combined with Prism Adaptation.

Rightward prism adaptation ameliorates neglect symptoms while leftward prism adaptation (LPA) induces neglect-like biases in healthy individuals. Similarly, inhibitory repetitive transcranial magnetic stimulation (rTMS) on the right posterior parietal cortex (PPC) induces neglect-like behavior, whereas on the left PPC it ameliorates neglect symptoms and normalizes hyperexcitability of left hemisphere parietal-motor (PPC-M1) connectivity. Based on this analogy we hypothesized that LPA increases PPC-M1 excitability in the left hemisphere and decreases it in the right one. In an attempt to shed some light on the mechanisms underlying LPA's effects on cognition, we investigated this hypothesis in healthy individuals measuring PPC-M1 excitability with dual-site paired-pulse TMS (ppTMS). We found a left hemisphere increase and a right hemisphere decrease in the amplitude of motor evoked potentials elicited by paired as well as single pulses on M1. While this could indicate that LPA biases interhemispheric connectivity, it contradicts previous evidence that M1-only MEPs are unchanged after LPA. A control experiment showed that input-output curves were not affected by LPA per se. We conclude that LPA combined with ppTMS on PPC-M1 differentially alters the excitability of the left and right M1.

Prisms to Shift Pain Away: Pathophysiological and Therapeutic Exploration of CRPS with Prism Adaptation.

Complex Regional Pain Syndrome (CRPS) is an invalidating chronic condition subsequent to peripheral lesions. There is growing consensus for a central contribution to CRPS. However, the nature of this central body representation disorder is increasingly debated. Although it has been repeatedly argued that CRPS results in motor neglect of the affected side, visual egocentric reference frame was found to be deviated toward the pain, that is, neglect of the healthy side. Accordingly, prism adaptation has been successfully used to normalize this deviation. This study aimed at clarifying whether 7 CRPS patients exhibited neglect as well as exploring the pathophysiological mechanisms of this manifestation and of the therapeutic effects of prism adaptation. Pain and quality of life, egocentric reference frames (visual and proprioceptive straight-ahead), and neglect tests (line bisection, kinematic analyses of motor neglect and motor extinction) were repeatedly assessed prior to, during, and following a one-week intense prism adaptation intervention. First, our results provide no support for visual and motor neglect in CRPS. Second, reference frames for body representations were not systematically deviated. Third, intensive prism adaptation intervention durably ameliorated pain and quality of life. As for spatial neglect, understanding the therapeutic effects of prism adaptation deserves further investigations.

Three timescales in prism adaptation.

It has been proposed that motor adaptation depends on at least two learning systems, one that learns fast but with poor retention and another that learns slowly but with better retention (Smith MA, Ghazizadeh A, Shadmehr R. PLoS Biol 4: e179, 2006). This two-state model has been shown to account for a range of behavior in the force field adaptation task. In the present study, we examined whether such a two-state model could also account for behavior arising from adaptation to a prismatic displacement of the visual field. We first confirmed that an "adaptation rebound," a critical prediction of the two-state model, occurred when visual feedback was deprived after an adaptation-extinction episode. We then examined the speed of decay of the prism aftereffect (without any visual feedback) after repetitions of 30, 150, and 500 trials of prism exposure. The speed of decay decreased with the number of exposure trials, a phenomenon that was best explained by assuming an "ultraslow" system, in addition to the fast and slow systems. Finally, we compared retention of aftereffects 24 h after 150 or 500 trials of exposure: retention was significantly greater after 500 than 150 trials. This difference in retention could not be explained by the two-state model but was well explained by the three-state model as arising from the difference in the amount of adaptation of the "ultraslow process." These results suggest that there are not only fast and slow systems but also an ultraslow learning system in prism adaptation that is activated by prolonged prism exposure of 150-500 trials.

Upper limb kinematics after cervical spinal cord injury: a review.

Although a number of upper limb kinematic studies have been conducted, no review actually addresses the key-features of open-chain upper limb movements after cervical spinal cord injury (SCI). The aim of this literature review is to provide a clear understanding of motor control and kinematic changes during open-chain upper limb reaching, reach-to-grasp, overhead movements, and fast elbow flexion movements after tetraplegia. Using data from MEDLINE between 1966 and December 2014, we examined temporal and spatial kinematic measures and when available electromyographic recordings. We included fifteen control case and three series case studies with a total of 164 SCI participants and 131 healthy control participants. SCI participants efficiently performed a broad range of tasks with their upper limb and movements were planned and executed with strong kinematic invariants like movement endpoint accuracy and minimal cost. Our review revealed that elbow extension without triceps brachii relies on increased scapulothoracic and glenohumeral movements providing a dynamic coupling between shoulder and elbow. Furthermore, contrary to normal grasping patterns where grasping is prepared during the transport phase, reaching and grasping are performed successively after SCI. The prolonged transport phase ensures correct hand placement while the grasping relies on wrist extension eliciting either whole hand or lateral grip. One of the main kinematic characteristics observed after tetraplegia is motor slowing attested by increased movement time. This could be caused by (i) decreased strength, (ii) triceps brachii paralysis which disrupts normal agonist-antagonist co-contractions, (iii) accuracy preservation at movement endpoint, and/or (iv) grasping relying on tenodesis. Another feature is a reduction of maximal superior reaching during overhead movements which could be caused by i) strength deficit in agonist muscles like pectoralis major, ii) strength deficit in proximal synergic muscles responsible for scapulothoracic and glenohumeral joint stability, iii) strength deficit in distal synergic muscles preventing the maintenance of elbow extension by shoulder elbow dynamic coupling, iv) shoulder joint ankyloses, and/or v) shoulder pain. Further studies on open chain movements are needed to identify the contribution of each of these factors in order to tailor upper limb rehabilitation programs for SCI individuals.

Long-lasting reduction in postural asymmetry by prism adaptation after right brain lesion without neglect.

BACKGROUND: Right brain damage (RBD) involves postural asymmetry and spatial frame disorders. In acute RBD patients, postural asymmetry is immediately reduced after one single session of prism adaptation (PA), without assessment of effects on spatial frames. AIM: To assess long-term effects of PA on posture and spatial frames in chronic RBD patients, without neglect. METHOD: Six chronic RBD patients without neglect (mean delay 45 months) were included. Each patient sustained 10 PA sessions of 20 min during 2 weeks. Outcome measures were: (1) posturographic analysis (mediolateral position of centre of pressure (X cop), (2) subjective straight ahead (SSA) and perception of longitudinal body axis (LBA). Each parameter was assessed by three pretests and three post-tests (+2 h, day + 3 and day + 7). RESULTS: In pretests, patients showed a shift of the X cop and SSA. In post-tests, results displayed (1) a significant reduction in mediolateral postural asymmetry at D + 7; (2) a significant left deviation of SSA at D + 3 and enduring at D + 7; and (3) no significant modification of LBA. The mean curves of X cop and SSA between pre- and post-tests were similar. CONCLUSIONS: PA involves persistent reduction in postural asymmetry in RBD patients without neglect. These findings were obtained at a chronic stage. This new effect cannot be explained by reduction in spatial attentional shift. Improvement may be explained by a better calibration of extra personal space frames used for posture, without effect on personal space frame. Findings argue in favour of a bottom-up effect of PA on mechanisms underlying spatial cognition.

Contributions of the cerebellum and the motor cortex to acquisition and retention of motor memories.

We investigated the contributions of the cerebellum and the motor cortex (M1) to acquisition and retention of human motor memories in a force field reaching task. We found that anodal transcranial direct current stimulation (tDCS) of the cerebellum, a technique that is thought to increase neuronal excitability, increased the ability to learn from error and form an internal model of the field, while cathodal cerebellar stimulation reduced this error-dependent learning. In addition, cathodal cerebellar stimulation disrupted the ability to respond to error within a reaching movement, reducing the gain of the sensory-motor feedback loop. By contrast, anodal M1 stimulation had no significant effects on these variables. During sham stimulation, early in training the acquired motor memory exhibited rapid decay in error-clamp trials. With further training the rate of decay decreased, suggesting that with training the motor memory was transformed from a labile to a more stable state. Surprisingly, neither cerebellar nor M1 stimulation altered these decay patterns. Participants returned 24hours later and were re-tested in error-clamp trials without stimulation. The cerebellar group that had learned the task with cathodal stimulation exhibited significantly impaired retention, and retention was not improved by M1 anodal stimulation. In summary, non-invasive cerebellar stimulation resulted in polarity-dependent up- or down-regulation of error-dependent motor learning. In addition, cathodal cerebellar stimulation during acquisition impaired the ability to retain the motor memory overnight. Thus, in the force field task we found a critical role for the cerebellum in both formation of motor memory and its retention.

Effect of juggling expertise on pointing performance in peripheral vision.

Juggling is a very complex activity requiring motor, visual and coordination skills. Expert jugglers experience a "third eye" monitoring leftward and rightward ball zenith positions alternately, in the upper visual fields, while maintaining their gaze straight-ahead. This "third eye" reduces their motor noise (improved body stability and decrease in hand movement variability) as it avoids the numerous head and eye movements that add noise into the system and make trajectories more uncertain. Neuroimaging studies have shown that learning to juggle induces white and grey matter hypertrophy at the posterior intraparietal sulcus. Damage to this brain region leads to optic ataxia, a clinical condition characterised by peripheral pointing bias toward gaze position. We predicted that expert jugglers would, conversely, present better accuracy in a peripheral pointing task. The mean pointing accuracy of expert jugglers was better for peripheral pointing within the upper visual field, compatible with their subjective experience of the "third eye". Further analyses showed that experts exhibited much less between-subject variability than beginners, reinforcing the interpretation of a vertically asymmetrical calibration of peripheral space, characteristic of juggling and homogenous in the expert group. On the contrary, individual pointing variability did not differ between groups neither globally nor in any sector of space, showing that the reduced motor noise of experts in juggling did not transfer to pointing. It is concluded that the plasticity of the posterior intraparietal sulcus related to juggling expertise does not consist of globally improved visual-to-motor ability. It rather consists of peripheral space calibration by practicing horizontal covert shifts of the attentional spotlight within the upper visual field, between left and right ball zenith positions.

No inherent left and right side in human 'mental number line': evidence from right brain damage.

Spatial reasoning has a relevant role in mathematics and helps daily computational activities. It is widely assumed that in cultures with left-to-right reading, numbers are organized along the mental equivalent of a ruler, the mental number line, with small magnitudes located to the left of larger ones. Patients with right brain damage can disregard smaller numbers while mentally setting the midpoint of number intervals. This has been interpreted as a sign of spatial neglect for numbers on the left side of the mental number line and taken as a strong argument for the intrinsic left-to-right organization of the mental number line. Here, we put forward the understanding of this cognitive disability by discovering that patients with right brain damage disregard smaller numbers both when these are mapped on the left side of the mental number line and on the right side of an imagined clock face. This shows that the right hemisphere supports the representation of small numerical magnitudes independently from their mapping on the left or the right side of a spatial-mental layout. In addition, the study of the anatomical correlates through voxel-based lesion-symptom mapping and the mapping of lesion peaks on the diffusion tensor imaging-based reconstruction of white matter pathways showed that the rightward bias in the imagined clock-face was correlated with lesions of high-level middle temporal visual areas that code stimuli in object-centred spatial coordinates, i.e. stimuli that, like a clock face, have an inherent left and right side. In contrast, bias towards higher numbers on the mental number line was linked to white matter damage in the frontal component of the parietal-frontal number network. These anatomical findings show that the human brain does not represent the mental number line as an object with an inherent left and right side. We conclude that the bias towards higher numbers in the mental bisection of number intervals does not depend on left side spatial, imagery or object-centred neglect and that it rather depends on disruption of an abstract non-spatial representation of small numerical magnitudes.

Learning from illusions: From perception studies to perspective-taking interventions.

Visual illusions have always fascinated people but they have often been confined to the field of entertainment. Although philosophers, psychologists and neuroscientists have used them to explore the bases of human perception and to teach about vision, these attractive tools have still remained largely underexploited. The goal of the present paper is to argue that visual illusions can also serve as a powerful medium to question our relation to the world and to others, as they demonstrate that we do not fully perceive reality and that each interpretation of the world may be equally sound. Further, specific 3D visual illusions, such as 3D ambiguous objects that give rise to two specific interpretations, enable the viewer to realize that their perception is tied to their viewing point, and that this may also apply to social cognition and interactions. Specifically, this low-level embodied experience should generalize to other levels and enhance the consideration of others' perspective independently of the type of representations. Therefore, the use of illusions in general, and 3D ambiguous objects in particular, constitutes an avenue for future interventions designed to increase our perspective-taking abilities and the pacification of social relations through mutual understanding, which is particularly relevant in the current era.

Inter-Task Transfer of Prism Adaptation through Motor Imagery.

Prism adaptation (PA) is a useful method to investigate short-term sensorimotor plasticity. Following active exposure to prisms, individuals show consistent after-effects, probing that they have adapted to the perturbation. Whether after-effects are transferable to another task or remain specific to the task performed under exposure, represents a crucial interest to understand the adaptive processes at work. Motor imagery (MI, i.e., the mental representation of an action without any concomitant execution) offers an original opportunity to investigate the role of cognitive aspects of motor command preparation disregarding actual sensory and motor information related to its execution. The aim of the study was to test whether prism adaptation through MI led to transferable after-effects. Forty-four healthy volunteers were exposed to a rightward prismatic deviation while performing actual (Active group) versus imagined (MI group) pointing movements, or while being inactive (inactive group). Upon prisms removal, in the MI group, only participants with the highest MI abilities (MI+ group) showed consistent after-effects on pointing and, crucially, a significant transfer to throwing. This was not observed in participants with lower MI abilities and in the inactive group. However, a direct comparison of pointing after-effects and transfer to throwing between MI+ and the control inactive group did not show any significant difference. Although this interpretation requires caution, these findings suggest that exposure to intersensory conflict might be responsible for sensory realignment during prism adaptation which could be transferred to another task. This study paves the way for further investigations into MI's potential to develop robust sensorimotor adaptation.