Two action systems in the human brain

Interactions between dorsal and ventral streams for controlling skilled grasp

The two visual systems hypothesis suggests processing of visual information into two distinct routes in the brain: a dorsal stream for the control of actions and a ventral stream for the identification of objects. Recently, increasing evidence has shown that the dorsal and ventral streams are not strictly independent, but do interact with each other. In this paper, we argue that the interactions between dorsal and ventral streams are important for controlling complex object-oriented hand movements, especially skilled grasp. Anatomical studies have reported the existence of direct connections between dorsal and ventral stream areas. These physiological interconnections appear to be gradually more active as the precision demands of the grasp become higher. It is hypothesised that the dorsal stream needs to retrieve detailed information about object identity, stored in ventral stream areas, when the object properties require complex fine-tuning of the grasp. In turn, the ventral stream might receive up to date grasp-related information from dorsal stream areas to refine the object internal representation. Future research will provide direct evidence for which specific areas of the two streams interact, the timing of their interactions and in which behavioural context they occur.

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

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

A Model of Emergent Category-specific Activation in the Posterior Fusiform Gyrus of Sighted and Congenitally Blind Populations

Theories about the neural bases of semantic knowledge tend between two poles, one proposing that distinct brain regions are innately dedicated to different conceptual domains and the other suggesting that all concepts are encoded within a single network. Category-sensitive functional activations in the fusiform cortex of the congenitally blind have been taken to support the former view but also raise several puzzles. We use neural network models to assess a hypothesis that spans the two poles: The interesting functional activation patterns reflect the base connectivity of a domain-general semantic network. Both similarities and differences between sighted and congenitally blind groups can emerge through learning in a neural network, but only in architectures adopting real anatomical constraints. Surprisingly, the same constraints suggest a novel account of a quite different phenomenon: the dyspraxia observed in patients with semantic impairments from anterior temporal pathology. From this work, we suggest that the cortical semantic network is wired not to encode knowledge of distinct conceptual domains but to promote learning about both conceptual and affordance structure in the environment.

Rethinking actions: implementation and association

Action processing allows us to move through and interact with the world, as well as understand the movements performed by other people. In recent years, there has been increasing interest in the semantics of actions as differentiated from the semantics of objects. However, as the understanding of action semantics has evolved, it is evident that the existing literature conflates two senses of the word 'action'-one that stems from studies of tool use and the other from event representation. In this paper, we suggest that this issue can be clarified by closely examining differences in how the human parietal and temporal cortices of the brain process action-related stimuli. By contrasting the posterior parietal cortex to the posterolateral temporal cortex, we characterize two complementary action systems in the human brain, each with its own specialization of function. We suggest that these two systems be referred to as the parietal Action Implementation System, and the posterolateral temporal Action Association System. While the frontoparietal system is concerned primarily with how we perform actions, and simulate others' actions, the temporal action system is more involved with processing actions from a third-person, conceptual standpoint. Recent work in cognitive neuroscience of perception and language, as well as the neuroanatomical organization of these brain regions support this distinction. We will discuss the implications of this work for cognition-, language-, and neuroscience-based action research.

Functional connectivity associated with hand shape generation: Imitating novel hand postures and pantomiming tool grips challenge different nodes of a shared neural network

Clinical research suggests that imitating meaningless hand postures and pantomiming tool-related hand shapes rely on different neuroanatomical substrates. We investigated the BOLD responses to different tasks of hand posture generation in 14 right handed volunteers. Conjunction and contrast analyses were applied to select regions that were either common or sensitive to imitation and/or pantomime tasks. The selection included bilateral areas of medial and lateral extrastriate cortex, superior and inferior regions of the lateral and medial parietal lobe, primary motor and somatosensory cortex, and left dorsolateral prefrontal, and ventral and dorsal premotor cortices. Functional connectivity analysis revealed that during hand shape generation the BOLD-response of every region correlated significantly with every other area regardless of the hand posture task performed, although some regions were more involved in some hand postures tasks than others. Based on between-task differences in functional connectivity we predict that imitation of novel hand postures would suffer most from left superior parietal disruption and that pantomiming hand postures for tools would be impaired following left frontal damage, whereas both tasks would be sensitive to inferior parietal dysfunction. We also unveiled that posterior temporal cortex is committed to pantomiming tool grips, but that the involvement of this region to the execution of hand postures in general appears limited. We conclude that the generation of hand postures is subserved by a highly interconnected task-general neural network. Depending on task requirements some nodes/connections will be more engaged than others and these task-sensitive findings are in general agreement with recent lesion studies.

Co-lateralized bilingual mechanisms for reading in single and dual language contexts: evidence from visual half-field processing of action words in proficient bilinguals

When reading, proficient bilinguals seem to engage the same cognitive circuits regardless of the language in use. Yet, whether or not such “bilingual” mechanisms would be lateralized in the same way in distinct—single or dual—language contexts is a question for debate. To fill this gap, we tested 18 highly proficient Polish (L1) —English (L2) childhood bilinguals whose task was to read aloud one of the two laterally presented action verbs, one stimulus per visual half field. While in the single-language blocks only L1 or L2 words were shown, in the subsequent mixed-language blocks words from both languages were concurrently displayed. All stimuli were presented for 217 ms followed by masks in which letters were replaced with hash marks. Since in non-simultaneous bilinguals the control of language, skilled actions (including reading), and representations of action concepts are typically left lateralized, the vast majority of our participants showed the expected, significant right visual field advantage for L1 and L2, both for accuracy and response times. The observed effects were nevertheless associated with substantial variability in the strength of the lateralization of the mechanisms involved. Moreover, although it could be predicted that participants' performance should be better in a single-language context, accuracy was significantly higher and response times were significantly shorter in a dual-language context, irrespective of the language tested. Finally, for both accuracy and response times, there were significant positive correlations between the laterality indices (LIs) of both languages independent of the context, with a significantly greater left-sided advantage for L1 vs. L2 in the mixed-language blocks, based on LIs calculated for response times. Thus, despite similar representations of the two languages in the bilingual brain, these results also point to the functional separation of L1 and L2 in the dual-language context.

Manipulation gesture effect in visual and auditory presentations: the link between tools in perceptual and motor tasks

There is much behavioral and neurophysiological evidence in support of the idea that seeing a tool activates motor components of action related to the perceived object (e.g., grasping, use manipulation). However, the question remains as to whether the processing of the motor components associated with the tool is automatic or depends on the situation, including the task and the modality of tool presentation. The present study investigated whether the activation of motor components involved in tool use in response to the simple perception of a tool is influenced by the link between prime and target tools, as well as by the modality of presentation, in perceptual or motor tasks. To explore this issue, we manipulated the similarity of gesture involved in the use of the prime and target (identical, similar, different) with two tool presentation modalities of the presentation tool (visual or auditory) in perceptual and motor tasks. Across the experiments, we also manipulated the relevance of the prime (i.e., associated or not with the current task). The participants saw a first tool (or heard the sound it makes), which was immediately followed by a second tool on which they had to perform a perceptual task (i.e., indicate whether the second tool was identical to or different from the first tool) or a motor task (i.e., manipulate the second tool as if it were the first tool). In both tasks, the similarity between the gestures employed for the first and the second tool was manipulated (Identical, Similar or Different gestures). The results showed that responses were faster when the manipulation gestures for the two tools were identical or similar, but only in the motor task. This effect was observed irrespective of the modality of presentation of the first tool, i.e., visual or auditory. We suggest that the influence of manipulation gesture on response time depends on the relevance of the first tool in motor tasks. We discuss these motor activation results in terms of the relevance and demands of the tasks.

Interactions between dorsal and ventral streams for controlling skilled grasp

The two visual systems hypothesis suggests processing of visual information into two distinct routes in the brain: a dorsal stream for the control of actions and a ventral stream for the identification of objects. Recently, increasing evidence has shown that the dorsal and ventral streams are not strictly independent, but do interact with each other. In this paper, we argue that the interactions between dorsal and ventral streams are important for controlling complex object-oriented hand movements, especially skilled grasp. Anatomical studies have reported the existence of direct connections between dorsal and ventral stream areas. These physiological interconnections appear to be gradually more active as the precision demands of the grasp become higher. It is hypothesised that the dorsal stream needs to retrieve detailed information about object identity, stored in ventral stream areas, when the object properties require complex fine-tuning of the grasp. In turn, the ventral stream might receive up to date grasp-related information from dorsal stream areas to refine the object internal representation. Future research will provide direct evidence for which specific areas of the two streams interact, the timing of their interactions and in which behavioural context they occur.

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The dorsal and ventral streams are both involved in skilled grasping movements.

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Ventral areas feed dorsal areas with information about object identity.

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Grasps of increased complexity require gradually higher recruitment of ventral areas.

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Dorsal stream inputs could fine tune object representations stored in ventral areas.

A neural network model of causative actions

A common idea in models of action representation is that actions are represented in terms of their perceptual effects (see e.g., Prinz, 1997; Hommel et al., 2001; Sahin et al., 2007; Umiltà et al., 2008; Hommel, 2013). In this paper we extend existing models of effect-based action representations to account for a novel distinction. Some actions bring about effects that are independent events in their own right: for instance, if John smashes a cup, he brings about the event of the cup smashing. Other actions do not bring about such effects. For instance, if John grabs a cup, this action does not cause the cup to "do" anything: a grab action has well-defined perceptual effects, but these are not registered by the perceptual system that detects independent events involving external objects in the world. In our model, effect-based actions are implemented in several distinct neural circuits, which are organized into a hierarchy based on the complexity of their associated perceptual effects. The circuit at the top of this hierarchy is responsible for actions that bring about independently perceivable events. This circuit receives input from the perceptual module that recognizes arbitrary events taking place in the world, and learns movements that reliably cause such events. We assess our model against existing experimental observations about effect-based motor representations, and make some novel experimental predictions. We also consider the possibility that the "causative actions" circuit in our model can be identified with a motor pathway reported in other work, specializing in "functional" actions on manipulable tools (Bub et al., 2008; Binkofski and Buxbaum, 2013).

Flexible Reference Frames for Grasp Planning in Human Parietofrontal Cortex

Reaching to a location in space is supported by a cortical network that operates in a variety of reference frames. Computational models and recent fMRI evidence suggest that this diversity originates from neuronal populations dynamically shifting between reference frames as a function of task demands and sensory modality. In this human fMRI study, we extend this framework to nonmanipulative grasping movements, an action that depends on multiple properties of a target, not only its spatial location. By presenting targets visually or somaesthetically, and by manipulating gaze direction, we investigate how information about a target is encoded in gaze- and body-centered reference frames in dorsomedial and dorsolateral grasping-related circuits. Data were analyzed using a novel multivariate approach that combines classification and cross-classification measures to explicitly aggregate evidence in favor of and against the presence of gaze- and body-centered reference frames. We used this approach to determine whether reference frames are differentially recruited depending on the availability of sensory information, and where in the cortical networks there is common coding across modalities. Only in the left anterior intraparietal sulcus (aIPS) was coding of the grasping target modality dependent: predominantly gaze-centered for visual targets and body-centered for somaesthetic targets. Left superior parieto-occipital cortex consistently coded targets for grasping in a gaze-centered reference frame. Left anterior precuneus and premotor areas operated in a modality-independent, body-centered frame. These findings reveal how dorsolateral grasping area aIPS could play a role in the transition between modality-independent gaze-centered spatial maps and body-centered motor areas.

Stable and variable affordances are both automatic and flexible

The mere observation of pictures or words referring to manipulable objects is sufficient to evoke their affordances since objects and their nouns elicit components of appropriate motor programs associated with object interaction. While nobody doubts that objects actually evoke motor information, the degree of automaticity of this activation has been recently disputed. Recent evidence has indeed revealed that affordances activation is flexibly modulated by the task and by the physical and social context. It is therefore crucial to understand whether these results challenge previous evidence showing that motor information is activated independently from the task. The context and the task can indeed act as an early or late filter. We will review recent data consistent with the notion that objects automatically elicit multiple affordances and that top-down processes select among them probably inhibiting motor information that is not consistent with behavior goals. We will therefore argue that automaticity and flexibility of affordances are not in conflict. We will also discuss how language can incorporate affordances showing similarities, but also differences, between the motor information elicited by vision and language. Finally we will show how the distinction between stable and variable affordances can accommodate all these effects.

Spatial representations in older adults are not modified by action: Evidence from tool use

Theories of embodied perception hold that the visual system is calibrated by both the body schema and the action system, allowing for adaptive action-perception responses. One example of embodied perception involves the effects of tool use on distance perception, in which wielding a tool with the intention to act upon a target appears to bring that object closer. This tool-based spatial compression (i.e., tool-use effect) has been studied exclusively with younger adults, but it is unknown whether the phenomenon exists with older adults. In this study, we examined the effects of tool use on distance perception in younger and older adults in 2 experiments. In Experiment 1, younger and older adults estimated the distances of targets just beyond peripersonal space while either wielding a tool or pointing with the hand. Younger adults, but not older adults, estimated targets to be closer after reaching with a tool. In Experiment 2, younger and older adults estimated the distance to remote targets while using either a baton or a laser pointer. Younger adults displayed spatial compression with the laser pointer compared to the baton, although older adults did not. Taken together, these findings indicate a generalized absence of the tool-use effect in older adults during distance estimation, suggesting that the visuomotor system of older adults does not remap from peripersonal to extrapersonal spatial representations during tool use.

The lateral occipitotemporal cortex in action

Understanding and responding to other people's actions is fundamental for social interactions. Whereas many studies emphasize the importance of parietal and frontal regions for these abilities, several lines of recent research show that the human lateral occipitotemporal cortex (LOTC) represents varied aspects of action, ranging from perception of tools and bodies and the way they typically move, to understanding the meaning of actions, to performing overt actions. Here, we highlight common themes across these lines of work, which have informed theories related to high-level vision, concepts, social cognition, and apraxia. We propose that patterns of activity in LOTC form representational spaces, the dimensions of which capture perceptual, semantic, and motor knowledge of how actions change the state of the world.

An enhanced experimental procedure to rationalize on the impairment of perception of action capabilities

It is well documented that changes in the physiological states of the perceiver-actor influence the perception of action capabilities. However, because experimental procedures of most studies involved a limitless availability for stimuli visual encoding and perceptual strategies, it remains difficult to adopt a single position among the large range of alternative interpretations for impaired perception. A reaching-to-grasp paradigm under breathing restriction was adapted from Graydon et al. (Cogn Emot 26:1301-1305, 2012) to standardize the time for encoding of stimuli information and narrowed the involvement of perceptual strategies. In the present study, we propose a highly controlled environment where the discrete information is presented during 300 ms, congruently with neurophysiological studies focused on visuomotor transformation. An underestimation of the perception of action capabilities is found under breath restriction, suggesting that 300 ms for stimuli encoding is sufficient to induce altered visuomotor brain transformations when limiting the involvement of perceptual strategies. This result suggests that such behavior could refer to an impaired brain potentiation of the perceptual occurrence, providing strong hypotheses on the brain dynamics of sensorimotor integration that underlie impaired perception of action capabilities in stressful situations.

A distributed network critical for selecting among tool-directed actions

Tools pose a challenge to the need to select actions appropriate for task goals and environmental constraints. For many tools (e.g., calculator), actions for "using" and "grasping-to-move" conflict with each other and may compete during selection. To date, little is known about the mechanisms that enable selection between possible tool actions or their neural substrates. The study of patients with chronic left hemisphere stroke, many of whom are deficient in tool-use action (apraxic), provides an opportunity to elucidate these issues. Here, 31 such patients pantomimed or recognized tool use actions for "conflict" and "non-conflict" tools. Voxel-based lesion-symptom mapping (VLSM), lesion subtraction, and tractographic overlap analyses were used to determine brain regions necessary for selecting among tool-directed actions. Lesions to posterior middle temporal gyrus (pMTG) and anterior intraparietal sulcus (aIPS) tended to impair production of use actions similarly for both conflict and non-conflict tools. By contrast, lesions to the supramarginal gyrus (SMG), inferior frontal gyrus (IFG)/anterior insula, and superior longitudinal fasciculus (SLF) specifically impaired production of use actions for conflict tools. Patients' errors on conflict tools suggested inappropriate selection of grasping actions and difficulty selecting single actions. Use/grasp conflict had no effect on action recognition. We suggest that the SMG/SLF/IFG pathway implements biased competition between possible tool actions, while aIPS and pMTG compute the structure-based and skilled use actions, respectively, that constitute input to this competitive process. This is the first study to demonstrate a reliable link between a characteristic of single tools (i.e., their association with different use and grasp actions) and action selection difficulties. Additionally, the data allow us to posit an SMG-involved subtype of apraxia characterized by an inability to resolve action competition.

The Representation of Object-Directed Action and Function Knowledge in the Human Brain

The appropriate use of everyday objects requires the integration of action and function knowledge. Previous research suggests that action knowledge is represented in frontoparietal areas while function knowledge is represented in temporal lobe regions. Here we used multivoxel pattern analysis to investigate the representation of object-directed action and function knowledge while participants executed pantomimes of familiar tool actions. A novel approach for decoding object knowledge was used in which classifiers were trained on one pair of objects and then tested on a distinct pair; this permitted a measurement of classification accuracy over and above object-specific information. Region of interest (ROI) analyses showed that object-directed actions could be decoded in tool-preferring regions of both parietal and temporal cortex, while no independently defined tool-preferring ROI showed successful decoding of object function. However, a whole-brain searchlight analysis revealed that while frontoparietal motor and peri-motor regions are engaged in the representation of object-directed actions, medial temporal lobe areas in the left hemisphere are involved in the representation of function knowledge. These results indicate that both action and function knowledge are represented in a topographically coherent manner that is amenable to study with multivariate approaches, and that the left medial temporal cortex represents knowledge of object function.

Prospects for direct social perception: a multi-theoretical integration to further the science of social cognition

In this paper we suggest that differing approaches to the science of social cognition mirror the arguments between radical embodied and traditional approaches to cognition. We contrast the use in social cognition of theoretical inference and mental simulation mechanisms with approaches emphasizing a direct perception of others’ mental states. We build from a recent integrative framework unifying these divergent perspectives through the use of dual-process theory and supporting social neuroscience research. Our elaboration considers two complementary notions of direct perception: one primarily stemming from ecological psychology and the other from enactive cognition theory. We use this as the foundation from which to offer an account of the informational basis for social information and assert a set of research propositions to further the science of social cognition. In doing so, we point out how perception of the minds of others can be supported in some cases by lawful information, supporting direct perception of social affordances and perhaps, mental states, and in other cases by cues that support indirect perceptual inference. Our goal is to extend accounts of social cognition by integrating advances across disciplines to provide a multi-level and multi-theoretic description that can advance this field and offer a means through which to reconcile radical embodied and traditional approaches to cognitive neuroscience.

The neural correlates of planning and executing actual tool use

Human tool use is complex, and underlying neural mechanisms seem to be widely distributed across several brain systems; however, neuroimaging studies of actual tool use are rare because of experimental challenges hindering detailed analysis within one acting subject. We developed a "Tool-Carousel" that enabled us to test actual manipulation of different objects during fMRI and investigate the planning and execution of goal-directed actions. Particularly, we focused on the effects of three factors on object manipulations: the type of object manipulated, the type of manipulation, and the hand to be used. The main focus lay on the question of how complex object use compared with unspecific actions are processed and especially how such representations interact with the knowledge about the object in the action-related dorsal stream. We found that object manipulations with both right and left hand recruit a common network strongly lateralized to the left hemisphere especially during planning but also action execution. Specifically, while activity in the ventral stream was involved in processing semantic information and object properties, a dorso-dorsal pathway (i.e., superior occipital gyrus, superior parietal lobule, and dorsal premotor area) was relevant for monitoring the online control of objects and also a ventro-dorsal pathway (i.e., middle occipital gyrus, inferior parietal lobule, and ventral premotor area) was specifically involved in processing known object manipulations, such as tool use. Data further indicate an interaction of ventral stream areas, such as middle temporal gyrus and lateral occipital complex, with both dorsal pathways. These results provide evidence for left-lateralized occipito-temporo-parieto-frontal network of everyday tool use, which may help to characterize specific deficits in patients suffering from apraxia.

Neural correlates of grasping

Prehension, the capacity to reach and grasp objects, comprises two main components: reaching, i.e., moving the hand towards an object, and grasping, i.e., shaping the hand with respect to its properties. Knowledge of this topic has gained a huge advance in recent years, dramatically changing our view on how prehension is represented within the dorsal stream. While our understanding of the various nodes coding the grasp component is rapidly progressing, little is known of the integration between grasping and reaching. With this Mini Review we aim to provide an up-to-date overview of the recent developments on the coding of prehension. We will start with a description of the regions coding various aspects of grasping in humans and monkeys, delineating where it might be integrated with reaching. To gain insights into the causal role of these nodes in the coding of prehension, we will link this functional description to lesion studies. Finally, we will discuss future directions that might be promising to unveil new insights on the coding of prehension movements.

A kinematic analysis of age-related changes in grasping to use and grasping to move common objects

Grasping is a complex action which requires high-level motor control. Although the impact of aging on grasping has been investigated in some studies, to date little is known as to how the aging process interacts with the purpose of the movement. The aims of the present study were (i) to investigate the effect of aging on grasping movements, and to explore on how this effect is modulated by (ii) the goal of the task, and by (iii) the characteristics of the target such as its location in the visual field, its orientation and its size. Young and elderly adults were asked to grasp to move or to grasp to use objects of different sizes and orientations, presented either in the central or the peripheral visual field. Movement duration did not differ between the two groups. However, elderly participants required a longer approach phase and showed a different grasping strategy, characterized by larger grip aperture and smaller percentage of wrist rotation in comparison to young adults. Elderly adults showed a decrease in accuracy when grasping objects presented in the peripheral, but not in the central visual field. A similar modulation of the kinematic parameters consisting in longer planning and execution phases in the grasp to use in comparison to the grasp to move condition was observed in both groups, suggesting that the effect of aging might be minimized and compensated in more goal-directed tasks.

Conceptual- and production-related predictors of pantomimed tool use deficits in apraxia

Apraxia following left hemisphere stroke disrupts pantomimed tool use (PTU), a task that requires the integrity of a number of cognitive and motor processes. Although previous studies have identified that apraxics have deficits in (1) the integrity of/access to stored tool-use gesture representations, (2) deficits in intrinsic (body-based) coordinate control, and (3) abnormal reliance on visual feedback, no study to date has simultaneously tested the relative contribution of these three deficits to poor PTU performance. In this study we assessed 38 chronic left hemisphere stroke survivors on tests of PTU and the 3 component processes. We then attempted to predict PTU with the component scores using hierarchical regression to control for overall stroke severity and the possibility of correlated component scores. Results showed that over half of the variability in PTU was predictable, with the strongest independent predictor being a test of intrinsic coordinate control without visual feedback. A test of the integrity of/access to stored representations also predicted PTU. These results confirm and extend previous claims that conceptual- and production-related factors affect PTU, even after considering that deficits in both factors are commonly observed to varying degrees in apraxic patients.

Parcellation of left parietal tool representations by functional connectivity

Manipulating a tool according to its function requires the integration of visual, conceptual, and motor information, a process subserved in part by left parietal cortex. How these different types of information are integrated and how their integration is reflected in neural responses in the parietal lobule remains an open question. Here, participants viewed images of tools and animals during functional magnetic resonance imaging (fMRI). k-Means clustering over time series data was used to parcellate left parietal cortex into subregions based on functional connectivity to a whole brain network of regions involved in tool processing. One cluster, in the inferior parietal cortex, expressed privileged functional connectivity to the left ventral premotor cortex. A second cluster, in the vicinity of the anterior intraparietal sulcus, expressed privileged functional connectivity with the left medial fusiform gyrus. A third cluster in the superior parietal lobe expressed privileged functional connectivity with dorsal occipital cortex. Control analyses using Monte Carlo style permutation tests demonstrated that the clustering solutions were outside the range of what would be observed based on chance 'lumpiness' in random data, or mere anatomical proximity. Finally, hierarchical clustering analyses were used to formally relate the resulting parcellation scheme of left parietal tool representations to previous work that has parcellated the left parietal lobule on purely anatomical grounds. These findings demonstrate significant heterogeneity in the functional organization of manipulable object representations in left parietal cortex, and outline a framework that generates novel predictions about the causes of some forms of upper limb apraxia.

Objects tell us what action we can expect: dissociating brain areas for retrieval and exploitation of action knowledge during action observation in fMRI

Objects are reminiscent of actions often performed with them: knife and apple remind us on peeling the apple or cutting it. Mnemonic representations of object-related actions (action codes) evoked by the sight of an object may constrain and hence facilitate recognition of unrolling actions. The present fMRI study investigated if and how action codes influence brain activation during action observation. The average number of action codes (NAC) of 51 sets of objects was rated by a group of n = 24 participants. In an fMRI study, different volunteers were asked to recognize actions performed with the same objects presented in short videos. To disentangle areas reflecting the storage of action codes from those exploiting them, we showed object-compatible and object-incompatible (pantomime) actions. Areas storing action codes were considered to positively co-vary with NAC in both object-compatible and object-incompatible action; due to its role in tool-related tasks, we here hypothesized left anterior inferior parietal cortex (aIPL). In contrast, areas exploiting action codes were expected to show this correlation only in object-compatible but not incompatible action, as only object-compatible actions match one of the active action codes. For this interaction, we hypothesized ventrolateral premotor cortex (PMv) to join aIPL due to its role in biasing competition in IPL. We found left anterior intraparietal sulcus (IPS) and left posterior middle temporal gyrus (pMTG) to co-vary with NAC. In addition to these areas, action codes increased activity in object-compatible action in bilateral PMv, right IPS, and lateral occipital cortex (LO). Findings suggest that during action observation, the brain derives possible actions from perceived objects, and uses this information to shape action recognition. In particular, the number of expectable actions quantifies the activity level at PMv, IPL, and pMTG, but only PMv reflects their biased competition while observed action unfolds.

A goal-based mechanism for delayed motor intention: considerations from motor skills, tool use and action memory

Thinking about our behaviors for a future recall like playing a piano sonata during the next weekend (i.e., delayed motor intention) should engage at some level sensorimotor-based representations. Theoretically, such representations can be stored through both an action- and a goal-based mechanism. An action-based mechanism is related to the specific motor sequence of fingers like the key presses on the piano, and a goal-based mechanism is related to the musical tones generated by the key presses. From these considerations, the present article tries to explore whether the cognitive nature of delayed motor intention is more based on an action or goal mechanism. We reviewed empirical evidence and theoretical accounts of different domains such as motor skills, tool use, and action memory supporting the idea that such delayed motor intentions are rather represented through a goal-based mechanism. The specific role of this goal-based mechanism is to envision the future in an implementation-neutral mode to flexibly and efficiently retrieve an adapted action to environmental constraints. This goal-based account offers an interesting alternative to reshape the classical models about the representations of delayed motor intention. We also discuss how this account can be applied to practical activities in daily life situations.

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.

The effects of visual half-field priming on the categorization of familiar intransitive gestures, tool use pantomimes, and meaningless hand movements

Although the control of meaningful gestures is one of the most left-lateralized functions, the relative contribution of the two hemispheres to their processing is still debated. We tested the effects of primes appearing in the left or right visual field in the form of pictures (Experiment 1), and words (Experiment 2) on categorization of movies showing intransitive (“communicative”) gestures, tool use (transitive) pantomimes, and meaningless movements. Fifteen participants (eight women) watched 36 movies (12 from each category) primed for 150 ms with either a congruent or incongruent stimulus followed by a 50-ms mask. On congruent trials, a picture or word was directly related to the presented gesture, including nonsense pictures or non-words for meaningless actions. On incongruent trials, a picture or word belonged to a different category. In Experiment 1, intransitive gestures were categorized significantly faster than the other two types of hand movements. Moreover, whereas the categorization of transitive gestures was significantly facilitated by congruent pictures on the right, the effect was weaker for intransitive, and reversed for meaningless movements. In Experiment 2, intransitive gestures were again categorized significantly faster, but transitive significantly slower than the other two gesture categories. Yet, there was now a significant facilitation of intransitive, and inhibition of transitive gesture categorization following congruent prime words in the right visual field, and significantly faster categorization of intransitive gestures following incongruent words in the left visual field. These outcomes lend support to the complexity account of differences in left-hemisphere representations of meaningful gestures reported in the neuropsychological, behavioral, and neuroimaging literature. Nevertheless, they also indicate that the representations of intransitive gestures show some differential, and sometimes counterintuitive sensitivity to right hemisphere processing.

Tool use disorders after left brain damage

In this paper we review studies that investigated tool use disorders in left-brain damaged (LBD) patients over the last 30 years. Four tasks are classically used in the field of apraxia: Pantomime of tool use, single tool use, real tool use and mechanical problem solving. Our aim was to address two issues, namely, (1) the role of mechanical knowledge in real tool use and (2) the cognitive mechanisms underlying pantomime of tool use, a task widely employed by clinicians and researchers. To do so, we extracted data from 36 papers and computed the difference between healthy subjects and LBD patients. On the whole, pantomime of tool use is the most difficult task and real tool use is the easiest one. Moreover, associations seem to appear between pantomime of tool use, real tool use and mechanical problem solving. These results suggest that the loss of mechanical knowledge is critical in LBD patients, even if all of those tasks (and particularly pantomime of tool use) might put differential demands on semantic memory and working memory.

The affordance-matching hypothesis: how objects guide action understanding and prediction

Action understanding lies at the heart of social interaction. Prior research has often conceptualized this capacity in terms of a motoric matching of observed actions to an action in one's motor repertoire, but has ignored the role of object information. In this manuscript, we set out an alternative conception of intention understanding, which places the role of objects as central to our observation and comprehension of the actions of others. We outline the current understanding of the interconnectedness of action and object knowledge, demonstrating how both rely heavily on the other. We then propose a novel framework, the affordance-matching hypothesis, which incorporates these findings into a simple model of action understanding, in which object knowledge-what an object is for and how it is used-can inform and constrain both action interpretation and prediction. We will review recent empirical evidence that supports such an object-based view of action understanding and we relate the affordance matching hypothesis to recent proposals that have re-conceptualized the role of mirror neurons in action understanding.

Critical brain regions for tool-related and imitative actions: a componential analysis

Numerous functional neuroimaging studies suggest that widespread bilateral parietal, temporal, and frontal regions are involved in tool-related and pantomimed gesture performance, but the role of these regions in specific aspects of gestural tasks remains unclear. In the largest prospective study of apraxia-related lesions to date, we performed voxel-based lesion-symptom mapping with data from 71 left hemisphere stroke participants to assess the critical neural substrates of three types of actions: gestures produced in response to viewed tools, imitation of tool-specific gestures demonstrated by the examiner, and imitation of meaningless gestures. Thus, two of the three gesture types were tool-related, and two of the three were imitative, enabling pairwise comparisons designed to highlight commonalities and differences. Gestures were scored separately for postural (hand/arm positioning) and kinematic (amplitude/timing) accuracy. Lesioned voxels in the left posterior temporal gyrus were significantly associated with lower scores on the posture component for both of the tool-related gesture tasks. Poor performance on the kinematic component of all three gesture tasks was significantly associated with lesions in left inferior parietal and frontal regions. These data enable us to propose a componential neuroanatomic model of action that delineates the specific components required for different gestural action tasks. Thus, visual posture information and kinematic capacities are differentially critical to the three types of actions studied here: the kinematic aspect is particularly critical for imitation of meaningless movement, capacity for tool-action posture representations are particularly necessary for pantomimed gestures to the sight of tools, and both capacities inform imitation of tool-related movements. These distinctions enable us to advance traditional accounts of apraxia.

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.

Methods to explore the influence of top-down visual processes on motor behavior

Kinesthetic awareness is important to successfully navigate the environment. When we interact with our daily surroundings, some aspects of movement are deliberately planned, while others spontaneously occur below conscious awareness. The deliberate component of this dichotomy has been studied extensively in several contexts, while the spontaneous component remains largely under-explored. Moreover, how perceptual processes modulate these movement classes is still unclear. In particular, a currently debated issue is whether the visuomotor system is governed by the spatial percept produced by a visual illusion or whether it is not affected by the illusion and is governed instead by the veridical percept. Bistable percepts such as 3D depth inversion illusions (DIIs) provide an excellent context to study such interactions and balance, particularly when used in combination with reach-to-grasp movements. In this study, a methodology is developed that uses a DII to clarify the role of top-down processes on motor action, particularly exploring how reaches toward a target on a DII are affected in both deliberate and spontaneous movement domains.

The neural basis of human tool use

In this review, we propose that the neural basis for the spontaneous, diversified human tool use is an area devoted to the execution and observation of tool actions, located in the left anterior supramarginal gyrus (aSMG). The aSMG activation elicited by observing tool use is typical of human subjects, as macaques show no similar activation, even after an extensive training to use tools. The execution of tool actions, as well as their observation, requires the convergence upon aSMG of inputs from different parts of the dorsal and ventral visual streams. Non-semantic features of the target object may be provided by the posterior parietal cortex (PPC) for tool-object interaction, paralleling the well-known PPC input to anterior intraparietal (AIP) for hand-object interaction. Semantic information regarding tool identity, and knowledge of the typical manner of handling the tool, could be provided by inferior and middle regions of the temporal lobe. Somatosensory feedback and technical reasoning, as well as motor and intentional constraints also play roles during the planning of tool actions and consequently their signals likewise converge upon aSMG. We further propose that aSMG may have arisen though duplication of monkey AIP and invasion of the duplicate area by afferents from PPC providing distinct signals depending on the kinematics of the manipulative action. This duplication may have occurred when Homo Habilis or Homo Erectus emerged, generating the Oldowan or Acheulean Industrial complexes respectively. Hence tool use may have emerged during hominid evolution between bipedalism and language. We conclude that humans have two parietal systems involved in tool behavior: a biological circuit for grasping objects, including tools, and an artifactual system devoted specifically to tool use. Only the latter allows humans to understand the causal relationship between tool use and obtaining the goal, and is likely to be the basis of all technological developments.

Integration of visual and motor functional streams in the human brain

A long-standing difficulty in brain research has been to disentangle how information flows across circuits composed by multiple local and distant cerebral areas. At the large-scale level, several brain imaging methods have contributed to the understanding of those circuits by capturing the covariance or coupling patterns of blood oxygen level-dependent (BOLD) activity between distributed brain regions. The hypothesis is that underlying information processes are closely associated to synchronized brain activity, and therefore to the functional connectivity structure of the human brain. In this study, we have used a recently developed method called stepwise functional connectivity analysis. Our results show that motor and visual connectivity merge in a multimodal integration network that links together perception, action and cognition in the human functional connectome.

Brain repair after stroke--a novel neurological model

Following stroke, patients are commonly left with debilitating motor and speech impairments. This article reviews the state of the art in neurological repair for stroke and proposes a new model for the future. We suggest that stroke treatment--from the time of the ictus itself to living with the consequences--must be fundamentally neurological, from limiting the extent of injury at the outset, to repairing the consequent damage. Our model links brain and behaviour by targeting brain circuits, and we illustrate the model though action observation treatment, which aims to enhance brain network connectivity. The model is based on the assumptions that the mechanisms of neural repair inherently involve cellular and circuit plasticity, that brain plasticity is a synaptic phenomenon that is largely stimulus-dependent, and that brain repair required both physical and behavioural interventions that are tailored to reorganize specific brain circuits. We review current approaches to brain repair after stroke and present our new model, and discuss the biological foundations, rationales, and data to support our novel approach to upper-extremity and language rehabilitation. We believe that by enhancing plasticity at the level of brain network interactions, this neurological model for brain repair could ultimately lead to a cure for stroke.

Using in vivo probabilistic tractography to reveal two segregated dorsal 'language-cognitive' pathways in the human brain

Primate studies have recently identified the dorsal stream as constituting multiple dissociable pathways associated with a range of specialized cognitive functions. To elucidate the nature and number of dorsal pathways in the human brain, the current study utilized in vivo probabilistic tractography to map the structural connectivity associated with subdivisions of the left supramarginal gyrus (SMG). The left SMG is a prominent region within the dorsal stream, which has recently been parcellated into five structurally-distinct regions which possess a dorsal-ventral (and rostral-caudal) organisation, postulated to reflect areas of functional specialisation. The connectivity patterns reveal a dissociation of the arcuate fasciculus into at least two segregated pathways connecting frontal-parietal-temporal regions. Specifically, the connectivity of the inferior SMG, implicated as an acoustic-motor speech interface, is carried by an inner/ventro-dorsal arc of fibres, whilst the pathways of the posterior superior SMG, implicated in object use and cognitive control, forms a parallel outer/dorso-dorsal crescent.