Tool Representations in Human Visual Cortex

Tools such as pens, forks, and scissors play an important role in many daily-life activities, an importance underscored by the presence in visual cortex of a set of tool-selective brain regions. This review synthesizes decades of neuroimaging research that investigated the representational spaces in the visual ventral stream for objects, such as tools, that are specifically characterized by action-related properties. Overall, results reveal a dissociation between representational spaces in ventral and lateral occipito-temporal cortex (OTC). While lateral OTC encodes both visual (shape) and action-related properties of objects, distinguishing between objects acting as end-effectors (e.g., tools, hands) versus similar noneffector manipulable objects (e.g., a glass), ventral OTC primarily represents objects' visual features such as their surface properties (e.g., material and texture). These areas act in concert with regions outside of OTC to support object interaction and tool use. The parallel investigation of the dimensions underlying object representations in artificial neural networks reveals both the possibilities and the difficulties in capturing the action-related dimensions that distinguish tools from other objects. Although artificial neural networks offer promise as models of visual cortex computations, challenges persist in replicating the action-related dimensions that go beyond mere visual features. Taken together, we propose that regions in OTC support the representation of tools based on a behaviorally relevant action code and suggest future paths to generate a computational model of this object space.

The neural correlates of limb apraxia: An anatomical likelihood estimation meta-analysis of lesion-symptom mapping studies in brain-damaged patients

Limb apraxia is a motor disorder frequently observed following a stroke. Apraxic deficits are classically assessed with four tasks: tool use, pantomime of tool use, imitation, and gesture understanding. These tasks are supported by several cognitive processes represented in a left-lateralized brain network including inferior frontal gyrus, inferior parietal lobe (IPL), and lateral occipito-temporal cortex (LOTC). For the past twenty years, voxel-wise lesion symptom mapping (VLSM) studies have been used to unravel the neural correlates associated with apraxia, but none of them has proposed a comprehensive view of the topic. In the present work, we proposed to fill this gap by performing a systematic Anatomic Likelihood Estimation meta-analysis of VLSM studies which included tasks traditionally used to assess apraxia. We found that the IPL was crucial for all the tasks. Moreover, lesions within the LOTC were more associated with imitation deficits than tool use or pantomime, confirming its important role in higher visual processing. Our results questioned traditional neurocognitive models on apraxia and may have important clinical implications.

Gesture meaning modulates the neural correlates of effector-specific imitation deficits in left hemisphere stroke

BACKGROUND

Previous studies on left hemisphere (LH) stroke patients reported effector-specific (hand, fingers, bucco-facial) differences in imitation performance. Furthermore, imitation performance differed between meaningless (ML) and meaningful (MF) gestures. Recent work suggests that a gesture's meaning impacts the body-part specificity of gesture imitation.

METHODS

We tested the hypothesis that the gesture's meaning (ML vs MF) affects the lesion correlates of effector-specific imitation deficits (here: bucco-facial vs arm/hand gestures) using behavioural data and support vector regression-based lesion-symptom mapping (SVR-LSM) in a large sample of 194 sub-acute LH stroke patients.

RESULTS

Behavioural data revealed a significant interaction between the effector used for imitation and the meaning of the imitated gesture. SVR-LSM analyses revealed shared lesion correlates for impaired imitation independent of effector or gesture meaning in the left supramarginal (SMG) and superior temporal gyri (STG). Besides, within the territory of the left middle cerebral artery, impaired imitation of bucco-facial gestures was associated with more anterior lesions, while arm/hand imitation deficits were associated with more posterior lesions. MF gestures were specifically associated with lesions in the left inferior frontal gyrus and the left insular region. Notably, an interaction of effector-specificity and gesture meaning was also present at the lesion level: A more pronounced difference in imitation performance between the effectors for ML (versus MF) gestures was associated with left-hemispheric lesions in the STG, SMG, putamen, precentral gyrus and white matter tracts.

CONCLUSION

The current behavioural data show that ML gestures are particularly sensitive in assessing effector-specific imitation deficits in LH stroke patients. Moreover, a gesture's meaning modulated the effector-specific lesion correlates of bucco-facial and arm/hand gesture imitation. Hence, it is crucial to consider gesture meaning in apraxia assessments.

Speech apraxia and oral apraxia: association or dissociation? A multivariate lesion-symptom mapping study in acute stroke patients

The anatomical relationship between speech apraxia (SA) and oral apraxia (OA) is still unclear. To shed light on this matter we studied 137 patients with acute ischaemic left-hemisphere stroke and performed support vector regression-based, multivariate lesion–symptom mapping. Thirty-three patients presented with either SA or OA. These two symptoms mostly co-occurred (n = 28), except for few patients with isolated SA (n = 2) or OA (n = 3). All patient with either SA or OA presented with aphasia (p < 0.001) and these symptoms were highly associated with apraxia (p < 0.001). Co-occurring SA and OA were predominantly associated with insular lesions, while the insula was completely spared in the five patients with isolated SA or OA. Isolated SA occurred in case of frontal lesions (prefrontal gyrus and superior longitudinal fasciculus), while isolated OA occurred in case of either temporoparietal or striatocapsular lesions. Our study supports the notion of a predominant, but not exclusive, role of the insula in verbal and non-verbal oral praxis, and indicates that frontal regions may contribute exclusively to verbal oral praxis, while temporoparietal and striatocapsular regions contribute to non-verbal oral praxis. However, since tests for SA and OA so far intrinsically also investigate aphasia and apraxia, refined tests are warranted.

Post-stroke cognitive deficits rarely come alone: Handling co-morbidity in lesion-behaviour mapping

Post‐stroke behavioural symptoms often correlate and systematically co‐occur with each other, either because they share cognitive processes, or because their neural correlates are often damaged together. Thus, neuropsychological symptoms often share variance. Many previous lesion‐behaviour mapping studies aimed to methodologically consider this shared variance between neuropsychological variables. A first group of studies controlled the behavioural target variable for the variance explained by one or multiple other variables to obtain a more precise mapping of the target variable. A second group of studies focused on the shared variance of multiple variables itself with the aim to map neural correlates of cognitive processes that are shared between the original variables. In the present study, we tested the validity of these methods by using real lesion data and both real and simulated data sets. We show that the variance that is shared between post‐stroke behavioural variables is ambiguous, and that mapping procedures that consider this variance are prone to biases and artefacts. We discuss under which conditions such procedures could still be used and what alternative approaches exist.

Degradation of Praxis Brain Networks and Impaired Comprehension of Manipulable Nouns in Stroke

Distributed brain systems contribute to representation of semantic knowledge. Whether sensory and motor systems of the brain are causally involved in representing conceptual knowledge is an especially controversial question. Here, we tested 57 chronic left-hemisphere stroke patients using a semantic similarity judgment task consisting of manipulable and nonmanipulable nouns. Three complementary methods were used to assess the neuroanatomical correlates of semantic processing: voxel-based lesion-symptom mapping, resting-state functional connectivity, and gray matter fractional anisotropy. The three measures provided converging evidence that injury to the brain networks required for action observation, execution, planning, and visuomotor coordination are associated with specific deficits in manipulable noun comprehension relative to nonmanipulable items. Damage or disrupted connectivity of areas such as the middle posterior temporal gyrus, anterior inferior parietal lobe, and premotor cortex was related specifically to the impairment of manipulable noun comprehension. These results suggest that praxis brain networks contribute especially to the comprehension of manipulable object nouns.

Neural correlates of differential finger gesture imitation deficits in left hemisphere stroke

Behavioural studies in apraxic patients revealed dissociations between the processing of meaningful (MF) and meaningless (ML) gestures. Consequently, the existence of two differential neural mechanisms for the imitation of either gesture type has been postulated. While the indirect (semantic) route exclusively enables the imitation of MF gestures, the direct route can be used for the imitation of any gesture type, irrespective of meaning, and thus especially for ML gestures. Concerning neural correlates, it is debated which of the visuo-motor streams (i.e., the ventral steam, the ventro-dorsal stream, or the dorso-dorsal stream) supports the postulated indirect and direct imitation routes. To probe the hypotheses that regions of the dorso-dorsal stream are involved differentially in the imitation of ML gestures and that regions of the ventro-dorsal stream are involved differentially in the imitation of MF gestures, we analysed behavioural (imitation of MF and ML finger gestures) and lesion data of 293 patients with a left hemisphere (LH) stroke. Confirming previous work, the current sample of LH stroke patients imitated MF finger gestures better than ML finger gestures. The analysis using voxel-based lesion symptom mapping (VLSM) revealed that LH damage to dorso-dorsal stream areas was associated with an impaired imitation of ML finger gestures, whereas damage to ventro-dorsal regions was associated with a deficient imitation of MF finger gestures. Accordingly, the analyses of the imitation of visually uniform and thus highly comparable MF and ML finger gestures support the dual-route model for gesture imitation at the behavioural and lesion level in a substantial patient sample. Furthermore, the data show that the direct route for ML finger gesture imitation depends on the dorso-dorsal visuo-motor stream while the indirect route for MF finger gesture imitation is related to regions of the ventro-dorsal visuo-motor stream.

Action and semantic tool knowledge - Effective connectivity in the underlying neural networks

Evidence from neuropsychological and imaging studies indicate that action and semantic knowledge about tools draw upon distinct neural substrates, but little is known about the underlying interregional effective connectivity. With fMRI and dynamic causal modeling (DCM) we investigated effective connectivity in the left-hemisphere (LH) while subjects performed (i) a function knowledge and (ii) a value knowledge task, both addressing semantic tool knowledge, and (iii) a manipulation (action) knowledge task. Overall, the results indicate crosstalk between action nodes and semantic nodes. Interestingly, effective connectivity was weakened between semantic nodes and action nodes during the manipulation task. Furthermore, pronounced modulations of effective connectivity within the fronto-parietal action system of the LH (comprising lateral occipito-temporal cortex, intraparietal sulcus, supramarginal gyrus, inferior frontal gyrus) were observed in a bidirectional manner during the processing of action knowledge. In contrast, the function and value knowledge tasks resulted in a significant strengthening of the effective connectivity between visual cortex and fusiform gyrus. Importantly, this modulation was present in both semantic tasks, indicating that processing different aspects of semantic knowledge about tools evokes similar effective connectivity patterns. Data revealed that interregional effective connectivity during the processing of tool knowledge occurred in a bidirectional manner with a weakening of connectivity between areas engaged in action and semantic knowledge about tools during the processing of action knowledge. Moreover, different semantic tool knowledge tasks elicited similar effective connectivity patterns.

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.

Task- and domain-specific modulation of functional connectivity in the ventral and dorsal object-processing pathways

A whole-brain network of regions collectively supports the ability to recognize and use objects-the Tool Processing Network. Little is known about how functional interactions within the Tool Processing Network are modulated in a task-dependent manner. We designed an fMRI experiment in which participants were required to either generate object pantomimes or to carry out a picture matching task over the same images of tools, while holding all aspects of stimulus presentation constant across the tasks. The Tool Processing Network was defined with an independent functional localizer, and functional connectivity within the network was measured during the pantomime and picture matching tasks. Relative to tool picture matching, tool pantomiming led to an increase in functional connectivity between ventral stream regions and left parietal and frontal-motor areas; in contrast, the matching task was associated with an increase in functional connectivity among regions in ventral temporo-occipital cortex, and between ventral temporal regions and the left inferior parietal lobule. Graph-theory analyses over the functional connectivity data indicated that the left premotor cortex and left lateral occipital complex were hub-like (exhibited high betweenness centrality) during tool pantomiming, while ventral stream regions (left medial fusiform gyrus and left posterior middle temporal gyrus) were hub-like during the picture matching task. These results demonstrate task-specific modulation of functional interactions among a common set of regions, and indicate dynamic coupling of anatomically remote regions in task-dependent manner.

[Networks involved in motor cognition : Physiology and pathophysiology of apraxia]

Apraxia is an umbrella term for different disorders of higher motor abilities that are not explained by elementary sensorimotor deficits (e. g. paresis or ataxia). Characteristic features of apraxia that are easy to recognize in clinical practice are difficulties in pantomimed or actual use of tools as well as in imitation of meaningless gestures. Apraxia is bilateral, explaining the cognitive motor disorders and occurs frequently (but not exclusively) after left hemispheric lesions, as well as in neurodegenerative diseases, such as corticobasal syndrome and Alzheimer's disease. Apraxic deficits can seriously impair activities of daily living, which is why the appropriate diagnosis is of great relevance. At the functional anatomical level, different cognitive motor skills rely on at least partly different brain networks, namely, a ventral processing pathway for semantic components, such as tool-action associations, a ventro-dorsal pathway for sensorimotor representations of learnt motor acts, as well as a dorso-dorsal pathway for on-line motor control and, probably, imitation of meaningless gestures. While these networks partially overlap with language-relevant regions, more clear cut dissociations are found between apraxia deficits and disorders of spatial attention. In addition to behavioral interventions, noninvasive neuromodulation approaches, as well as human-computer interface assistance systems are a growing focus of interest for the treatment of apraxia.