Deficient sequencing of pantomimes in apraxia

Setting boundaries: Development of neural and behavioral event cognition in early childhood

The ongoing stream of sensory experience is so complex and ever-changing that we tend to parse this experience at "event boundaries," which structures and strengthens memory. Memory processes undergo profound change across early childhood. Whether young children also divide their ongoing processing along event boundaries, and if those boundaries relate to memory, could provide important insight into the development of memory systems. In Study 1, 4-7-year-old children and adults segmented a cartoon, and we tested their memory. Children's event boundaries were more variable than adults' and differed in location and consistency of agreement. Older children's event segmentation was more adult-like than younger children's, and children who segmented events more like adults had better memory for those events. In Study 2, we asked whether these developmental differences in event segmentation had their roots in distinct neural representations. A separate group of 4-8-year-old children watched the same cartoon while undergoing an fMRI scan. In the right hippocampus, greater pattern dissimilarity across event boundaries compared to within events was evident for both child and adult behavioral boundaries, suggesting children and adults share similar event cognition. However, the boundaries identified by a data-driven Hidden Markov Model found that a different brain region-the left and right angular gyrus-aligned only with event boundaries defined by children. Overall, these data suggest that children's event cognition is reasonably well-developed by age 4 but continues to become more adult-like across early childhood. RESEARCH HIGHLIGHTS: Adults naturally break their experience into events, which structures and strengthens memory, but less is known about children's event perception and memory. Study 1 had adults and children segment and remember events from an animated show, and Study 2 compared those segmentations to other children's fMRI data. Children show better recognition and temporal order memory and more adult-like event segmentation with age, and children who segment more like adults have better memory. Children's and adults' behavioral boundaries mapped onto pattern similarity differences in hippocampus, and children's behavioral boundaries matched a data-driven model's boundaries in angular gyrus.

Strong evidence for ideomotor theory: Unwilled manifestation of the conceptual attribute in movement control

Scientific understanding of how the mind generates bodily actions remains opaque. In the early 19th century, the ideomotor theory proposed that humans generate voluntary actions by imagining the sensory consequence of those actions, implying that the idea of an action’s consequence mediates between the intention to act and motor control. Despite its long history and theoretical importance, existing empirical evidence for the ideomotor theory is not strong enough to rule out alternative hypotheses. In this study, we devised a categorization-action task to evaluate ideomotor theory by testing whether an idea, distinguished from a stimulus, can modulate task-irrelevant movements. In Experiment 1, participants categorized a stimulus duration as long or short by pressing an assigned key. The results show that participants pressed the key longer when categorizing the stimulus as long than they did when characterizing it as short. In Experiment 2, we showed that the keypressing durations were not modulated by the decision category when the property of the decision category, the brightness of a stimulus, was not easily transferable to the action. In summary, our results suggest that while the perceived stimulus features have a marginal effect on response duration linearly, the decision category is the main factor affecting the response duration. Our results indicate that an abstract category attribute can strongly modulate action execution, constraining theoretical conjectures about the ideomotor account of how people voluntarily generate action.

Bottom-up and top-down modulation of route selection in imitation

The cognitive system selects the most appropriate action imitative process: a semantic process - relying on long-term memory representations for known actions, and low-level visuomotor transformations for unknown actions. These two processes work in parallel; however, how context regularities and cognitive control modulate them is unclear. In this study, process selection was triggered contextually by presenting mixed known and new actions in predictable or unpredictable lists, while a cue on the forthcoming action triggered top-down control. Known were imitated faster than the new actions in the predictable lists only. Accuracy was higher and reaction times faster in the uncued conditions, and the predictable faster than the unpredictable list in the uncued condition only. In the latter condition, contextual factors modulate process selection, as participants use statistical regularities to perform the task at best. With the cue, the cognitive system tries to control response selection, resulting in more errors and longer reaction times.

Effect of body-part specificity and meaning in gesture imitation in left hemisphere stroke patients

Previous studies showed that imitation of finger and hand/arm gestures could be differentially impaired after brain damage. However, so far, the interaction between gesture meaning and body part in imitation deficits has not been fully assessed. In the present study, we aimed at filling this gap by testing 36 unilateral left brain-damaged patients with and without apraxia (20 apraxics), and 29 healthy controls on an imitation task of either finger or hand/arm meaningful (MF) gestures and meaningless (ML) movements, using a large sample of stimuli and controlling for the composition of the experimental list. Left-brain damaged patients imitated ML finger worse than hand/arm movements, whereas they did not show the same difference in MF gesture imitation. In addition, apraxic patients imitated finger movements worse than hand/arm movements. Furthermore, apraxic patients' imitation performance was equally affected irrespective of the action meaning, whereas non-apraxic patients showed better imitation performance on MF gestures. Results suggest that MF gestures are processed as a whole, as imitation of these gestures relies on the stored motor programs in long-term memory, independently of the body part involved. In contrast, ML movements seem to be processed through direct visuo-motor transformations, with left-brain damage specifically disrupting imitation performance of the more cognitive demanding finger movements.

Action Observation for Neurorehabilitation in Apraxia

Neurorehabilitation and brain stimulation studies of post-stroke patients suggest that action-observation effects can lead to rapid improvements in the recovery of motor functions and long-term motor cortical reorganization. Apraxia is a clinically important disorder characterized by marked impairment in representing and performing skillful movements [gestures], which limits many daily activities and impedes independent functioning. Recent clinical research has revealed errors of visuo-motor integration in patients with apraxia. This paper presents a rehabilitative perspective focusing on the possibility of action observation as a therapeutic treatment for patients with apraxia. This perspective also outlines impacts on neurorehabilitation and brain repair following the reinforcement of the perceptual-motor coupling. To date, interventions based primarily on action observation in apraxia have not been undertaken.

Distortion of Visuo-Motor Temporal Integration in Apraxia: Evidence From Delayed Visual Feedback Detection Tasks and Voxel-Based Lesion-Symptom Mapping

Limb apraxia is a higher brain dysfunction that typically occurs after left hemispheric stroke and its cause cannot be explained by sensory disturbance or motor paralysis. The comparison of motor signals and visual feedback to generate errors, i.e., visuo-motor integration, is important in motor control and motor learning, which may be impaired in apraxia. However, in apraxia after stroke, it is unknown whether there is a specific deficit in visuo-motor temporal integration compared to visuo-tactile and visuo-proprioceptive temporal integration. We examined the precision of visuo-motor temporal integration and sensory-sensory (visuo-tactile and visuo-proprioception) temporal integration in apraxia after stroke by using a delayed visual feedback detection task with three different conditions (tactile, passive movement, and active movement). The delay detection threshold and the probability curve for delay detection obtained in this task were quantitative indicators of the respective temporal integration functions. In addition, we performed subtraction and voxel-based lesion-symptom mapping to identify the brain lesions responsible for apraxia and deficits in visuo-motor temporal integration. The behavioral experiments showed that the delay detection threshold was extended and that the probability curve for delay detection was less steep in apraxic patients compared to controls (pseudo-apraxic patients and unaffected patients), only for the active movement condition, and not for the tactile and passive movement conditions. Furthermore, the severity of apraxia was significantly correlated with the delay detection threshold and the steepness of the probability curve in the active movement condition. These results indicated that multisensory (i.e., visual, tactile, and proprioception) feedback was normally temporally integrated, but motor prediction and visual feedback were not correctly temporally integrated in apraxic patients. That is, apraxic patients had difficulties with visuo-motor temporal integration. Lesion analyses revealed that both apraxia and the distortion of visuo-motor temporal integration were associated with lesions in the fronto-parietal motor network, including the left inferior parietal lobule and left inferior frontal gyrus. We suppose that damage to the left inferior fronto-parietal network could cause deficits in motor prediction for visuo-motor temporal integration, but not for sensory-sensory (visuo-tactile and visuo-proprioception) temporal integration, leading to the distortion of visuo-motor temporal integration in patients with apraxia.

It takes two to pantomime: Communication meets motor cognition

For over a century, pantomime of tool use has been employed to diagnose limb apraxia, a disorder of motor cognition primarily induced by left brain damage. While research consistently implicates damage to a left fronto-temporo-parietal network in limb apraxia, findings are inconsistent regarding the impact of damage to anterior versus posterior nodes within this network on pantomime. Complicating matters is the fact that tool use pantomime can be affected and evaluated at multiple levels. For instance, the production of tool use gestures requires the consideration of semantic characteristics (e.g. how to communicate the action intention) as well as motor features (e.g. forming grip and movement). Together, these factors may contribute substantially to apparent discrepancies in previously reported findings regarding neural correlates of tool use pantomime. In the current study, 67 stroke patients with unilateral left-brain damage performed a classic pantomime task. In order to analyze different error characteristics, we evaluated the proper use of grip and movement for each pantomime. For certain objects, healthy subjects may use body parts as representative for the object, e.g. use of the fingers to indicate scissors blades. To specify the pathological use of body parts as the object (BPO) we only assessed pantomime items that were not prone to this response in healthy participants. We performed modern voxel-based lesion analyses on MRI or CT data to determine associations between brain injury and the frequency of the specific types of pantomime errors. Our results support a model in which anterior and posterior nodes of the left fronto-temporo-parietal network contribute differentially to pantomime of tool use. More precisely, damage in the inferior frontal cortex reaching to the temporal pole is associated with an increased frequency of BPO errors, whereas damage to the inferior parietal lobe is predominantly linked to an increased frequency of movement and/or grip errors. Our work suggests that the validity of attempts to specify the neural correlates of limb apraxia based on tool use pantomime depends on differentiating the specific types of errors committed. We conclude that successful tool use pantomime involves dissociable functions with communicative aspects represented in more anterior (rather ventral) regions and motor-cognitive aspects in more posterior (rather dorsal) nodes of a left fronto-temporo-parietal network.

Deficient supplementary motor area at rest: Neural basis of limb kinetic deficits in Parkinson's disease

Parkinson's disease (PD) patients frequently suffer from limb kinetic apraxia (LKA) affecting quality of life. LKA denotes an impairment of precise and independent finger movements beyond bradykinesia, which is reliably assessed by coin rotation (CR) task. BOLD fMRI detected activation of a left inferior parietal-premotor praxis network in PD during CR. Here, we explored which network site is most critical for LKA using arterial spin labeling (ASL). Based on a hierarchical model, we hypothesized that LKA would predominantly affect the functional integrity of premotor areas including supplementary motor areas (SMA). Furthermore, we suspected that for praxis function with higher demand on temporal-spatial processing such as gesturing, inferior parietal lobule (IPL) upstream to premotor areas would be essential. A total of 21 PD patients and 20 healthy controls underwent ASL acquisition during rest. Behavioral assessment outside the scanner involved the CR, finger tapping task, and the test of upper limb apraxia (TULIA). Whole-brain analysis of activity at rest showed a significant reduction of CR-related perfusion in the left SMA of PD. Furthermore, the positive correlation between SMA perfusion and CR, seen in controls, was lost in patients. By contrast, TULIA was significantly associated with the perfusion of left IPL in both patients and controls. In conclusion, the findings suggest that LKA in PD are linked to an intrinsic disruption of the left SMA function, which may only be overcome by compensatory network activation. In addition, gestural performance relies on IPL which remains available for functional recruitment in early PD.

Efficiency in Rule- vs. Plan-Based Movements Is Modulated by Action-Mode

The rule/plan motor cognition (RPMC) paradigm elicits visually indistinguishable motor outputs, resulting from either plan- or rule-based action-selection, using a combination of essentially interchangeable stimuli. Previous implementations of the RPMC paradigm have used pantomimed movements to compare plan- vs. rule-based action-selection. In the present work we attempt to determine the generalizability of previous RPMC findings to real object interaction by use of a grasp-to-rotate task. In the plan task, participants had to use prospective planning to achieve a comfortable post-handle rotation hand posture. The rule task used implementation intentions (if-then rules) leading to the same comfortable end-state. In Experiment A, we compare RPMC performance of 16 healthy participants in pantomime and real object conditions of the experiment, within-subjects. Higher processing efficiency of rule- vs. plan-based action-selection was supported by diffusion model analysis. Results show a significant response-time increase in the pantomime condition compared to the real object condition and a greater response-time advantage of rule-based vs. plan-based actions in the pantomime compared to the real object condition. In Experiment B, 24 healthy participants performed the real object RPMC task in a task switching vs. a blocked condition. Results indicate that plan-based action-selection leads to longer response-times and less efficient information processing than rule-based action-selection in line with previous RPMC findings derived from the pantomime action-mode. Particularly in the task switching mode, responses were faster in the rule compared to the plan task suggesting a modulating influence of cognitive load. Overall, results suggest an advantage of rule-based action-selection over plan-based action-selection; whereby differential mechanisms appear to be involved depending on the action-mode. We propose that cognitive load is a factor that modulates the advantageous effect of implementation intentions in motor cognition on different levels as illustrated by the varying speed advantages and the variation in diffusion parameters per action-mode or condition, respectively.

Primary somatosensory cortex necessary for the perception of weight from other people's action: A continuous theta-burst TMS experiment

The presence of a network of areas in the parietal and premotor cortices, which are active both during action execution and observation, suggests that we might understand the actions of other people by activating those motor programs for making similar actions. Although neurophysiological and imaging studies show an involvement of the somatosensory cortex (SI) during action observation and execution, it is unclear whether SI is essential for understanding the somatosensory aspects of observed actions. To address this issue, we used off-line transcranial magnetic continuous theta-burst stimulation (cTBS) just before a weight judgment task. Participants observed the right hand of an actor lifting a box and estimated its relative weight. In counterbalanced sessions, we delivered sham and active cTBS over the hand region of the left SI and, to test anatomical specificity, over the left motor cortex (M1) and the left superior parietal lobule (SPL). Active cTBS over SI, but not over M1 or SPL, impaired task performance relative to sham cTBS. Moreover, active cTBS delivered over SI just before participants were asked to evaluate the weight of a bouncing ball did not alter performance compared to sham cTBS. These findings indicate that SI is critical for extracting somatosensory features (heavy/light) from observed action kinematics and suggest a prominent role of SI in action understanding.

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TMS over the somatosensory cortex disrupts performance on a weight judgment task.

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Disruption is specific for judgements based on observed human actions.

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No TMS effect is found for judgements based on observed non-human motion.

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No effect is found when TMS is administered over nearby frontal and parietal region.

Utility of testing for apraxia and associated features in dementia

INTRODUCTION

Existing literature suggests that the presence or absence of apraxia and associated parietal deficits may be clinically relevant in differential diagnosis of dementia syndromes.

AIM

This study investigated the profile of these features in Alzheimer's disease (AD) and frontotemporal dementia (FTD) spectrum disorders, at first presentation.

METHODS

Retrospective case note analysis was undertaken in 111 patients who presented to the Oxford Cognitive Disorders Clinic, Oxford, UK, including 29 amnestic AD, 12 posterior cortical atrophy (PCA), 12 logopenic primary progressive aphasia (lvPPA), 20 behavioural variant FTD (bvFTD), 7 non-fluent variant PPA (nfvPPA), 6 semantic variant PPA (svPPA) and 25 patients with subjective cognitive impairment (SCI). The clinical features of interest were: limb apraxia, apraxia of speech (AOS), and left parietal symptoms of dyslexia, dysgraphia, and dyscalculia.

RESULTS

The prevalence of limb apraxia was highest in PCA, amnestic AD, lvPPA and nfvPPA. AOS was only observed in nfvPPA. Associated parietal features were more prevalent in AD spectrum than FTD spectrum disorders. Group comparisons between key differential diagnostic challenges showed that lvPPA and nfvPPA could be significantly differentiated on the presence of left parietal features and AOS, and amnestic AD could be differentiated from bvFTD, svPPA and SCI by limb apraxia. Regression analysis showed that limb apraxia could successfully differentiate between AD and FTLD spectrum disorders with 83% accuracy.

DISCUSSION

Disease-specific profiles of limb apraxia and associated deficits can be observed. FTD and AD spectrum disorders can be difficult to differentiate due to overlapping cognitive symptoms, and measures of apraxia, in particular, appear to be a promising discriminator.

Apraxia, pantomime and the parietal cortex

Apraxia, a disorder of higher motor cognition, is a frequent and outcome-relevant sequel of left hemispheric stroke. Deficient pantomiming of object use constitutes a key symptom of apraxia and is assessed when testing for apraxia. To date the neural basis of pantomime remains controversial. We here review the literature and perform a meta-analysis of the relevant structural and functional imaging (fMRI/PET) studies. Based on a systematic literature search, 10 structural and 12 functional imaging studies were selected. Structural lesion studies associated pantomiming deficits with left frontal, parietal and temporal lesions. In contrast, functional imaging studies associate pantomimes with left parietal activations, with or without concurrent frontal or temporal activations. Functional imaging studies that selectively activated parietal cortex adopted the most stringent controls. In contrast to previous suggestions, current analyses show that both lesion and functional studies support the notion of a left-hemispheric fronto-(temporal)-parietal network underlying pantomiming object use. Furthermore, our review demonstrates that the left parietal cortex plays a key role in pantomime-related processes. More specifically, stringently controlled fMRI-studies suggest that in addition to storing motor schemas, left parietal cortex is also involved in activating these motor schemas in the context of pantomiming object use. In addition to inherent differences between structural and functional imaging studies and consistent with the dedifferentiation hypothesis, the age difference between young healthy subjects (typically included in functional imaging studies) and elderly neurological patients (typically included in structural lesion studies) may well contribute to the finding of a more distributed representation of pantomiming within the motor-dominant left hemisphere in the elderly.

Neuroanatomical substrates of action perception and understanding: an anatomic likelihood estimation meta-analysis of lesion-symptom mapping studies in brain injured patients

Several neurophysiologic and neuroimaging studies suggested that motor and perceptual systems are tightly linked along a continuum rather than providing segregated mechanisms supporting different functions. Using correlational approaches, these studies demonstrated that action observation activates not only visual but also motor brain regions. On the other hand, brain stimulation and brain lesion evidence allows tackling the critical question of whether our action representations are necessary to perceive and understand others’ actions. In particular, recent neuropsychological studies have shown that patients with temporal, parietal, and frontal lesions exhibit a number of possible deficits in the visual perception and the understanding of others’ actions. The specific anatomical substrates of such neuropsychological deficits however, are still a matter of debate. Here we review the existing literature on this issue and perform an anatomic likelihood estimation meta-analysis of studies using lesion-symptom mapping methods on the causal relation between brain lesions and non-linguistic action perception and understanding deficits. The meta-analysis encompassed data from 361 patients tested in 11 studies and identified regions in the inferior frontal cortex, the inferior parietal cortex and the middle/superior temporal cortex, whose damage is consistently associated with poor performance in action perception and understanding tasks across studies. Interestingly, these areas correspond to the three nodes of the action observation network that are strongly activated in response to visual action perception in neuroimaging research and that have been targeted in previous brain stimulation studies. Thus, brain lesion mapping research provides converging causal evidence that premotor, parietal and temporal regions play a crucial role in action recognition and understanding.

The tool in the brain: apraxia in ADL. Behavioral and neurological correlates of apraxia in daily living

Humans differ from other animals in the way they can skilfully and precisely operate or invent tools to facilitate their everyday life. Tools have dominated our home, travel and work environment, becoming an integral step for our motor skills development. What happens when the part of the brain responsible for tool use is damaged in our adult life due to a cerebrovascular accident? How does daily life change when we lose the previously mastered ability to make use of the objects around us? How do patients suffering from compromised tool use cope with food preparation, personal hygiene, grooming, housework, or use of home appliances? In this literature review we present a state of the art for single and multiple tool use research, with a focus on the impact that apraxia (impaired ability to perform tool-based actions) and action disorganization syndrome (ADS; impaired ability to carry out multi-step actions) have on activities of daily living (ADL). Firstly, we summarize the behavioral studies investigating the impact of apraxia and other comorbidity syndromes, such as neglect or visual extinction, on ADL. We discuss the hallmarks of the compromised tool use in terms of the sequencing of action steps, conceptual errors committed, spatial motor control, and temporal organization of the movement. In addition, we present an up-to-date overview of the neuroimaging and lesion analyses studies that provide an insight into neural correlates of tool use in the human brain and functional changes in the neural organization following a stroke, in the context of ADL. Finally we discuss the current practice in neurorehabilitation of ADL in apraxia and ADS aiming at increasing patients' independence.

The role of action representations in thematic object relations

A number of studies have explored the role of associative/event-based (thematic) and categorical (taxonomic) relations in the organization of object representations. Recent evidence suggests that thematic information may be particularly important in determining relationships between manipulable artifacts. However, although sensorimotor information is on many accounts an important component of manipulable artifact representations, little is known about the role that action may play during the processing of semantic relationships (particularly thematic relationships) between multiple objects. In this study, we assessed healthy and left hemisphere stroke participants to explore three questions relevant to object relationship processing. First, we assessed whether participants tended to favor thematic relations including action (Th+A, e.g., wine bottle-corkscrew), thematic relationships without action (Th-A, e.g., wine bottle-cheese), or taxonomic relationships (Tax, e.g., wine bottle-water bottle) when choosing between them in an association judgment task with manipulable artifacts. Second, we assessed whether the underlying constructs of event relatedness, action relatedness, and categorical relatedness determined the choices that participants made. Third, we assessed the hypothesis that degraded action knowledge and/or damage to temporo-parietal cortex, a region of the brain associated with the representation of action knowledge, would reduce the influence of action on the choice task. Experiment 1 showed that explicit ratings of event, action, and categorical relatedness were differentially predictive of healthy participants' choices, with action relatedness determining choices between Th+A and Th-A associations above and beyond event and categorical ratings. Experiment 2 focused more specifically on these Th+A vs. Th-A choices and demonstrated that participants with left temporo-parietal lesions, a brain region known to be involved in sensorimotor processing, were less likely than controls and tended to be less likely than patients with lesions sparing that region to use action relatedness in determining their choices. These data indicate that action knowledge plays a critical role in processing of thematic relations for manipulable artifacts.

Perturbing the action observation network during perception and categorization of actions' goals and grips: state-dependency and virtual lesion TMS effects

Watching others grasping and using objects activates an action observation network (AON), including inferior frontal (IFC), anterior intraparietal (AIP), and somatosensory cortices (S1). Yet, causal evidence of the differential involvement of such AON sensorimotor nodes in representing high- and low-level action components (i.e., end-goals and grip type) is meager. To address this issue, we used transcranial magnetic stimulation-adaptation (TMS-A) during 2 novel action perception tasks. Participants were shown adapting movies displaying a demonstrator performing goal-directed actions with a tool, using either power or precision grips. They were then asked to match the end-goal (Goal-recognition task) or the grip (Grip-recognition task) of actions shown in test pictures to the adapting movies. TMS was administered over IFC, AIP, or S1 during presentation of test pictures. Virtual lesion-like effects were found in the Grip-recognition task where IFC stimulation induced a general performance decrease, suggesting a critical role of IFC in perceiving grips. In the Goal-recognition task, IFC and S1 stimulation differently affected the processing of "adapted" and "nonadapted" goals. These "state-dependent" effects suggest that the overall goal of seen actions is encoded into functionally distinct and spatially overlapping neural populations in IFC-S1 and such encoding is critical for recognizing and understanding end-goals.

Vicarious motor activation during action perception: beyond correlational evidence

Neurophysiological and imaging studies have shown that seeing the actions of other individuals brings about the vicarious activation of motor regions involved in performing the same actions. While this suggests a simulative mechanism mediating the perception of others' actions, one cannot use such evidence to make inferences about the functional significance of vicarious activations. Indeed, a central aim in social neuroscience is to comprehend how vicarious activations allow the understanding of other people's behavior, and this requires to use stimulation or lesion methods to establish causal links from brain activity to cognitive functions. In the present work, we review studies investigating the effects of transient manipulations of brain activity or stable lesions in the motor system on individuals' ability to perceive and understand the actions of others. We conclude there is now compelling evidence that neural activity in the motor system is critical for such cognitive ability. More research using causal methods, however, is needed in order to disclose the limits and the conditions under which vicarious activations are required to perceive and understand actions of others as well as their emotions and somatic feelings.

Apraxia impairs intentional retrieval of incidentally acquired motor knowledge

Apraxia caused by left hemispheric stroke typically impairs skilled sequential movements. After stroke, apraxic patients need to reacquire motor skills by motor learning. The current study assessed for the first time incidental motor sequence learning in apraxic patients. Forty-eight human subjects (henceforth called "patients") with left hemispheric stroke affecting the middle cerebral artery territory (18 with apraxia and 30 without apraxia) and 17 age-matched healthy controls were tested on a visuomanual serial reaction time task. Subjects performed four blocks consisting of repetitions of a complex six element sequence containing ambiguous pairwise transitions before a new and unfamiliar sequence was introduced in block 5. Reaction time (RT) disadvantages in this fifth block indicated incidental sequence-specific motor learning. The intentional retrieval of the learned motor knowledge was assessed subsequently with a free recall task. Voxel-based lesion-symptom mapping (VLSM) was performed to investigate for the first time the lesion correlates of deficits in learning and retrieving sequential motor knowledge. Despite generally prolonged RTs, apraxic patients showed sequence-specific motor learning as could be observed in nonapraxic patients and healthy controls. However, apraxic patients showed reduced intentional retrieval of the learned sequence. VLSM revealed that impaired intentional retrieval of motor sequence knowledge resulted from dorsal premotor cortex lesions. Apraxic patients showed a dissociation of preserved incidental motor (sequence) learning and deficient intentional retrieval of this incidentally learned motor knowledge. The data suggest that novel approaches for treating apraxia should focus on incidental motor learning, but that automatic rather than intentional retrieval strategies should be enforced.

Neural correlates of cognitive impairment in posterior cortical atrophy

With the prospect of disease-modifying drugs that will target the physiopathological process of Alzheimer's disease, it is now crucial to increase the understanding of the atypical focal presentations of Alzheimer's disease, such as posterior cortical atrophy. This study aimed to (i) characterize the brain perfusion profile in posterior cortical atrophy using regions of interest and a voxel-based approach; (ii) study the influence of the disease duration on the clinical and imaging profiles; and (iii) explore the correlations between brain perfusion and cognitive deficits. Thirty-nine patients with posterior cortical atrophy underwent a specific battery of neuropsychological tests, mainly targeting visuospatial functions, and a brain perfusion scintigraphy with 99mTc-ethyl cysteinate dimer. The imaging analysis included a comparison with a group of 24 patients with Alzheimer's disease, matched for age, disease duration and Mini-Mental State Examination, and 24 healthy controls. The single-photon emission computed tomography profile in patients with posterior cortical atrophy was characterized by extensive and severe hypoperfusion in the occipital, parietal, posterior temporal cortices and in a smaller cortical area corresponding to the frontal eye fields (Brodmann areas 6/8). Compared with patients with Alzheimer's disease, the group with posterior cortical atrophy showed more severe occipitoparietal hypoperfusion and higher perfusion in the frontal, anterior cingulate and mesiotemporal regions. When considering the disease duration, the functional changes began and remained centred on the posterior lobes, even in the late stage. Correlation analyses of brain perfusion and neuropsychological scores in posterior cortical atrophy highlighted the prominent role of left inferior parietal damage in acalculia, Gerstmann's syndrome, left-right indistinction and limb apraxia, whereas damage to the bilateral dorsal occipitoparietal regions appeared to be involved in Bálint's syndrome. Our findings provide new insight into the natural history of functional changes according to disease duration and highlight the role of parietal and occipital cortices in the cognitive syndromes that characterize the posterior cortical atrophy.

Tool use and action planning in apraxia

Apraxia after left inferior parietal lesions has been widely interpreted as evidence of damage or impaired access to representations of tool-use, but most research has investigated pantomime of tool actions, not handling of actual tools. An alternative account is that inferior parietal damage does not affect tool-use representations but impairs cognitive processing about postural and hand-tool spatial relationships which is necessary for planning and controlling any complex action. Four apraxic patients and 10 age-matched controls were asked to reach rapidly for tools or abstract objects of similar dimensions. Under conditions of time pressure and divided attention, the patients frequently failed to invert the hand to grasp inverted tools by the handle, whereas ability to invert the hand to avoid a barrier and grasp abstract objects was largely unimpaired. Frequency of errors in tool grasping correlated with severity of apraxia. When inverted tools were correctly grasped, rotation of the wrist occurred later during the reaching movement than when inverting the hand to grasp an abstract object. These data are consistent with the theory of degraded access to tool-use representations in apraxia, but this theory cannot account for co-occurring deficits in imitating or matching meaningless hand or body postures.

Proximal and distal. Rethinking linguistic form and use for clinical purposes

With clinical purposes in mind, a review of the proximal/distal opposition is carried out in order to define a universal parameter of variability in semiotic procedures. By taking into consideration different-although notionally inter-related-senses of the proximal/distal opposition, a cluster of semiotic properties is proposed, which initially permits one to characterize dimensions of variability in the form and use of gestures. The subsequent and central aim of this paper is, however, to demonstrate that the same, or homologous, properties can also serve to characterize variability in the use of language, by assuming a basic connection between gesturing and linguistic behaviour. The main focus of interest and the starting point for reflections are communicative impairments as manifested in apraxia and aphasia.

[Structural and functional neuroimaging of the pathophysiology of apraxia]

A better understanding of the neural bases of apraxia is an important prerequisite to develop new therapeutic strategies for the disabling apraxic deficits after left-hemisphere stroke, like disturbed imitation of gestures, deficient pantomime, and object use deficits. Recently, functional and structural imaging methods allowed deeper insights into the pathophysiology of apraxia: While apraxic object use deficits result from the dysfunction of an extended fronto-parietal network within the left hemisphere, pantomime deficits are caused by impaired functioning of the left inferior frontal cortex. Further apraxia-related, motor cognitive processes (i.e., gesture imitation, integration of temporal and spatial movement information, and intentional movement planning) depend on the integrity of the left parietal cortex. Newly developed functional and structural imaging methods, like dynamic causal modelling (DCM) and diffusion tensor imaging (DTI), promise to further elucidate the pathophysiology of apraxia at the network level.

Critical brain regions for action recognition: lesion symptom mapping in left hemisphere stroke

A number of conflicting claims have been advanced regarding the role of the left inferior frontal gyrus, inferior parietal lobe and posterior middle temporal gyrus in action recognition, driven in part by an ongoing debate about the capacities of putative mirror systems that match observed and planned actions. We report data from 43 left hemisphere stroke patients in two action recognition tasks in which they heard and saw an action word ('hammering') and selected from two videoclips the one corresponding to the word. In the spatial recognition task, foils contained errors of body posture or movement amplitude/timing. In the semantic recognition task, foils were semantically related (sawing). Participants also performed a comprehension control task requiring matching of the same verbs to objects (hammer). Using regression analyses controlling for both the comprehension control task and lesion volume, we demonstrated that performance in the semantic gesture recognition task was predicted by per cent damage to the posterior temporal lobe, whereas the spatial gesture recognition task was predicted by per cent damage to the inferior parietal lobule. A whole-brain voxel-based lesion symptom-mapping analysis suggested that the semantic and spatial gesture recognition tasks were associated with lesioned voxels in the posterior middle temporal gyrus and inferior parietal lobule, respectively. The posterior middle temporal gyrus appears to serve as a central node in the association of actions and meanings. The inferior parietal lobule, held to be a homologue of the monkey parietal mirror neuron system, is critical for encoding object-related postures and movements, a relatively circumscribed aspect of gesture recognition. The inferior frontal gyrus, on the other hand, was not predictive of performance in any task, suggesting that previous claims regarding its role in action recognition may require refinement.

Action knowledge, visuomotor activation, and embodiment in the two action systems

Scientific interest in the relationship between cognition and action has increased markedly in the past several years, fueled by the discovery of mirror neurons in monkey prefrontal and parietal cortex and by the emergence of a movement in cognitive psychology, termed the embodied cognition framework, which emphasizes the role of simulation in cognitive representations. Guided by a functional neuroanatomic model called the Two Action Systems account, which posits numerous points of differentiation between structure- and function-based actions, we focus on two of the major issues under recent scrutiny: the relationship between representations for action production and recognition, and the role of action in object representations. We suggest that mirror neurons in humans are not critical for full action understanding, and that only function-based (and not structure-based) action is a component of embodied object concepts.

Temporal order processing of syllables in the left parietal lobe

Speech processing requires the temporal parsing of syllable order. Individuals suffering from posterior left hemisphere brain injury often exhibit temporal processing deficits as well as language deficits. Although the right posterior inferior parietal lobe has been implicated in temporal order judgments (TOJs) of visual information, there is limited evidence to support the role of the left inferior parietal lobe (IPL) in processing syllable order. The purpose of this study was to examine whether the left inferior parietal lobe is recruited during temporal order judgments of speech stimuli. Functional magnetic resonance imaging data were collected on 14 normal participants while they completed the following forced-choice tasks: (1) syllable order of multisyllabic pseudowords, (2) syllable identification of single syllables, and (3) gender identification of both multisyllabic and monosyllabic speech stimuli. Results revealed increased neural recruitment in the left inferior parietal lobe when participants made judgments about syllable order compared with both syllable identification and gender identification. These findings suggest that the left inferior parietal lobe plays an important role in processing syllable order and support the hypothesized role of this region as an interface between auditory speech and the articulatory code. Furthermore, a breakdown in this interface may explain some components of the speech deficits observed after posterior damage to the left hemisphere.

Associations of postural knowledge and basic motor skill with dyspraxia in autism: implication for abnormalities in distributed connectivity and motor learning

Children with autism often have difficulty performing skilled movements. Praxis performance requires basic motor skill, knowledge of representations of the movement (mediated by parietal regions), and transcoding of these representations into movement plans (mediated by premotor circuits). The goals of this study were (a) to determine whether dyspraxia in autism is associated with impaired representational ("postural") knowledge and (b) to examine the contributions of postural knowledge and basic motor skill to dyspraxia in autism. Thirty-seven children with autism spectrum disorder (ASD) and 50 typically developing (TD) children, ages 8-13, completed (a) an examination of basic motor skills, (b) a postural knowledge test assessing praxis discrimination, and (c) a praxis examination. Children with ASD showed worse basic motor skill and postural knowledge than did controls. The ASD group continued to show significantly poorer praxis than did controls after accounting for age, IQ, basic motor skill, and postural knowledge. Dyspraxia in autism appears to be associated with impaired formation of spatial representations, as well as transcoding and execution. Distributed abnormality across parietal, premotor, and motor circuitry, as well as anomalous connectivity, may be implicated.