The role of action knowledge in the comprehension of artefacts--a PET study

Action representations in robotics: A taxonomy and systematic classification

Understanding and defining the meaning of “action” is substantial for robotics research. This becomes utterly evident when aiming at equipping autonomous robots with robust manipulation skills for action execution. Unfortunately, to this day we still lack both a clear understanding of the concept of an action and a set of established criteria that ultimately characterize an action. In this survey, we thus first review existing ideas and theories on the notion and meaning of action. Subsequently, we discuss the role of action in robotics and attempt to give a seminal definition of action in accordance with its use in robotics research. Given this definition we then introduce a taxonomy for categorizing action representations in robotics along various dimensions. Finally, we provide a meticulous literature survey on action representations in robotics where we categorize relevant literature along our taxonomy. After discussing the current state of the art we conclude with an outlook towards promising research directions.

Imagined motor action and eye movements in schizophrenia

Visual exploration and planning of actions are reported to be abnormal in schizophrenia. Most of the studies monitoring eye movements in patients with schizophrenia have been performed under free-viewing condition. The present study was designed to assess whether mentally performing an action modulates the visuomotor behavior in patients with schizophrenia and in healthy controls. Visual scan paths were monitored in eighteen patients with schizophrenia and in eighteen healthy controls. Participants performed two tasks in which they were asked either to (1) look at a scene on a computer screen (free viewing), or (2) picture themselves making a sandwich in front of a computer screen (active viewing). The scenes contained both task-relevant and task-irrelevant objects. Temporal and spatial characteristics of scan paths were compared for each group and each task. The results indicate that patients with schizophrenia exhibited longer fixation durations, and fewer fixations, than healthy controls in the free viewing condition. The patients' visual exploration improved in the active viewing condition. However, patients looked less at task-relevant objects and looked more at distractors than controls in the active viewing condition in which they were asked to picture themselves making a sandwich in moving their eyes to task-relevant objects on an image. These results are consistent with the literature on deficits in motor imagery in patients with schizophrenia and it extends the impairment to visual exploration in an action imagery task.

The relationship between visual object exploration and action processing in schizophrenia

INTRODUCTION

Abnormalities in visual exploration and action processing are widely reported in schizophrenia. The aim of this study was to investigate whether object exploration (in order to recognise an action or the object) modulates visuomotor behaviour differently in schizophrenic patients and controls.

METHODS

Visual scan paths were monitored in 36 patients and 36 controls. Participants performed three tasks, in which they were asked to either (1) name the object (the object-naming task), (2) picture themselves interacting with the object and then name the action (the action-naming task), or (3) explore the object (the free-viewing task).

RESULTS

Patients explored objects less than controls did. Controls explored the part needed to identify an object in the object-naming task and the whole object in the action-naming and free-viewing tasks. In contrast, the patients maintained their gaze on the "identity" part of the object in all three tasks.

CONCLUSION

Our results were consistent with the literature findings on impaired action processing in schizophrenia but also extend the known impairment to implicit action processing when the subject is visually exploring an object. We discuss our results in terms of motivation, the effect of dopamine on eye movement, attentional capture, and frontal lobe dysfunction.

Graspable objects shape number processing

The field of numerical cognition represents an interesting case for action-based theories of cognition, since number is a special kind of abstract concept. Several studies have shown that within the parietal lobes adjacent neural regions code numerical magnitude and grasping-related information. This anatomical proximity between brain areas involved in number and sensorimotor processes may account for interactions between numerical magnitude and action. In particular, recent studies have demonstrated a causal role of action perception on numerical magnitude processing. If objects are represented in terms of actions (affordances), the causal role of action on number processing should extend to the case of objects affordances. This study investigates the relationship between numbers and objects affordances in two experiments, without (Experiment 1) or with (Experiment 2) the requirement of an action (i.e., participants were asked to hold an object in their hands during the task). The task consisted in repeating aloud the odd or even digit within a pair depending on the type of the preceding or following object. Order of presentation (object-number vs. number-object), Object type (graspable vs. ungraspable), Object size (small vs. large), and Numerical magnitude (small vs. large) were manipulated for each experiment. Experiment 1 showed a facilitation - in terms of quicker responses - for graspable over ungraspable objects preceded by numbers, and an effect of numerical magnitude after the presentation of graspable objects. Experiment 2 demonstrated that the action execution enhanced overall the sensitivity to numerical magnitude, and that at the same time it interfered with the effects of objects affordances on number processing. Overall, these findings demonstrate that numbers and graspable objects are strongly interrelated, supporting the view that abstract concepts may be grounded in the motor experience.

Nouns referring to tools and natural objects differentially modulate the motor system

While increasing evidence points to a critical role for the motor system in language processing, the focus of previous work has been on the linguistic category of verbs. Here we tested whether nouns are effective in modulating the motor system and further whether different kinds of nouns - those referring to artifacts or natural items, and items that are graspable or ungraspable - would differentially modulate the system. A Transcranial Magnetic Stimulation (TMS) study was carried out to compare modulation of the motor system when subjects read nouns referring to objects which are Artificial or Natural and which are Graspable or Ungraspable. TMS was applied to the primary motor cortex representation of the first dorsal interosseous (FDI) muscle of the right hand at 150 ms after noun presentation. Analyses of Motor Evoked Potentials (MEPs) revealed that across the duration of the task, nouns referring to graspable artifacts (tools) were associated with significantly greater MEP areas. Analyses of the initial presentation of items revealed a main effect of graspability. The findings are in line with an embodied view of nouns, with MEP measures modulated according to whether nouns referred to natural objects or artifacts (tools), confirming tools as a special class of items in motor terms. Additionally our data support a difference for graspable versus non graspable objects, an effect which for natural objects is restricted to initial presentation of items.

Language sensorimotor specificity modulates the motor system

Embodied approaches to language understanding hold that comprehension of linguistic material entails a situated simulation of the situation described. Some recent studies have shown that implicit, explicit, and relational properties of objects implied in a sentence are part of this simulation. However, the issue concerning the extent to which language sensorimotor specificity expressed by linguistic constituents of a sentence, contributes to situating the simulation process has not yet been adequately addressed. To fill this gap, we combined a concrete action verb with a noun denoting a graspable or non-graspable object, to form a sensible or non-sensible sentence. Verbs could express a specific action with low degrees of freedom (DoF) or an action with high DoF. Participants were asked to respond indicating whether the sentences were sensible or not. We found that simulation was active in understanding both sensible and non-sensible sentences. Moreover, the simulation was more situated with sentences containing a verb referring to an action with low DoF. Language sensorimotor specificity expressed by the noun, played a role in situating the simulation, only when the noun was preceded by a verb denoting an action with high DoF in sensible sentences. The simulation process in understanding non-sensible sentences evoked both the representations related to the verb and to the noun, these remaining separated rather than being integrated as in sensible sentences. Overall our findings are in keeping with embodied approaches to language understanding and suggest that the language sensorimotor specificity of sentence constituents affects the extent to which the simulation is situated.

Action observation can prime visual object recognition

Observing an action activates action representations in the motor system. Moreover, the representations of manipulable objects are closely linked to the motor systems at a functional and neuroanatomical level. Here, we investigated whether action observation can facilitate object recognition using an action priming paradigm. As prime stimuli we presented short video movies showing hands performing an action in interaction with an object (where the object itself was always removed from the video). The prime movie was followed by a (briefly presented) target object affording motor interactions that are either similar (congruent condition) or dissimilar (incongruent condition) to the prime action. Participants had to decide whether an object name shown after the target picture corresponds with the picture or not (picture–word matching task). We found superior accuracy for prime–target pairs with congruent as compared to incongruent actions across two experiments. Thus, action observation can facilitate recognition of a manipulable object typically involving a similar action. This action priming effect supports the notion that action representations play a functional role in object recognition.

Neural substrates of object identification: Functional magnetic resonance imaging evidence that category and visual attribute contribute to semantic knowledge

Recent findings suggest that neural representations of semantic knowledge contain information about category, modality, and attributes. Although an object's category is defined according to shared attributes that uniquely distinguish it from other category members, a clear dissociation between visual attribute and category representation has not yet been reported. We investigated the contribution of category (living and nonliving) and visual attribute (global form and local details) to semantic representation in the fusiform gyrus. During functional magnetic resonance imaging (fMRI), 40 adults named pictures of animals, tools, and vehicles. In a preliminary study, identification of objects in these categories was differentially dependent on global versus local visual feature processing. fMRI findings indicate that activation in the lateral and medial regions of the fusiform gyrus distinguished stimuli according to category, that is, living versus nonliving, respectively. In contrast, visual attributes of global form (animals) were associated with higher activity in the right fusiform gyrus, while local details (tools) were associated with higher activity in the left fusiform gyrus. When both global and local attributes were relevant to processing (vehicles), cortex in both left and right medial fusiform gyri was more active than for other categories. Taken together, results support distinctions in the role of visual attributes and category in semantic representation.

The neural systems of tool and action semantics: a perspective from functional imaging

This review discusses the contributions of functional imaging (fMRI/PET) to our understanding of how action and tool concepts are represented and processed in the human brain. Category-selective deficits in neuropsychological patients have suggested a fine-grained functional specialization within the neural systems of semantics. However, the underlying principles of semantic organization remain controversial. The feature-based account of semantic memory (or 'sensory-motor theory') predicates category-selective effects (e.g. tool vs. animals) on anatomical segregation for different semantic features (e.g. action vs. visual). Within this framework, we will review functional imaging evidence that semantic processing of tools and actions may rely on activations within the visuo-motor system.

Functional Representation of Living and Nonliving Domains across the Cerebral Hemispheres: A Combined Event-related Potential/Transcranial Magnetic Stimulation Study

Transcranial magnetic stimulation (TMS) over the left hemisphere has been shown to disrupt semantic processing but, to date, there has been no direct demonstration of the electrophysiological correlates of this interference. To gain insight into the neural basis of semantic systems, and in particular, study the temporal and functional organization of object categorization processing, we combined repetitive TMS (rTMS) and ERPs. Healthy volunteers performed a picture–word matching task in which Snodgrass drawings of natural (e.g., animal) and artifactual (e.g., tool) categories were associated with a word. When short trains of high-frequency rTMS were applied over Wernicke's area (in the region of the CP5 electrode) immediately before the stimulus onset, we observed delayed response times to artifactual items, and thus, an increased dissociation between natural and artifactual domains. This behavioral effect had a direct ERP correlate. In the response period, the stimuli from the natural domain elicited a significant larger late positivity complex than those from the artifactual domain. These differences were significant over the centro-parietal region of the right hemisphere. These findings demonstrate that rTMS interferes with postperceptual categorization processing of natural and artifactual stimuli that involve separate subsystems in distinct cortical areas.

The influence of action perception on object recognition: a developmental study

Two experiments explored the existence and the development of relations between action representations and object representations. A priming paradigm was used in which participants viewed an action pantomime followed by the picture of a tool, the tool being either associated or unassociated with the preceding action. Overall, we observed that the perception of an action pantomime can facilitate the recognition of a corresponding tool. Experiment 1 was based on a naming task and was conducted with 9- to 12-year-old children and a group of young adults. While substantial priming effects were obtained for all age groups, they were especially important for the youngest participants. Smaller priming effects were obtained in Experiment 2, using a categorization task and conducted on 5- to 11-year-old children and young adults, but again the results suggest that these action priming effects diminish with increasing age. Implications of these results for the organization and development of conceptual knowledge are discussed.

Functional Interactions during the Retrieval of Conceptual Action Knowledge: An fMRI Study

Impaired retrieval of conceptual knowledge for actions has been associated with lesions of left premotor, left parietal, and left middle temporal areas [Tranel, D., Kemmerer, D., Adolphs, R., Damasio, H., & Damasio, A. R. Neural correlates of conceptual knowledge for actions. Cognitive Neuropsychology, 409–432, 2003]. Here we aimed at characterizing the differential contribution of these areas to the retrieval of conceptual knowledge about actions. During functional magnetic resonance imaging (fMRI), different categories of pictograms (whole-body actions, manipulable and nonmanipulable objects) were presented to healthy subjects. fMRI data were analyzed using SPM2. A conjunction analysis of the neural activations elicited by all pictograms revealed ( p < .05, corrected) a bilateral inferior occipito-temporal neural network with strong activations in the right and left fusiform gyri. Action pictograms contrasted to object pictograms showed differential activation of area MT+, the inferior and superior parietal cortex, and the premotor cortex bilaterally. An analysis of psychophysiological interactions identified contribution-dependent changes in the neural responses when pictograms triggered the retrieval of conceptual action knowledge: Processing of action pictograms specifically enhanced the neural interaction between the right and left fusiform gyri, the right and left middle temporal cortices (MT+), and the left superior and inferior parietal cortex. These results complement and extend previous neuropsychological and neuroimaging studies by showing that knowledge about action concepts results from an increased coupling between areas concerned with semantic processing (fusiform gyrus), movement perception (MT+), and temporospatial movement control (left parietal cortex).

Motion, emotion and empathy in esthetic experience

The implications of the discovery of mirroring mechanisms and embodied simulation for empathetic responses to images in general, and to works of visual art in particular, have not yet been assessed. Here, we address this issue and we challenge the primacy of cognition in responses to art. We propose that a crucial element of esthetic response consists of the activation of embodied mechanisms encompassing the simulation of actions, emotions and corporeal sensation, and that these mechanisms are universal. This basic level of reaction to images is essential to understanding the effectiveness both of everyday images and of works of art. Historical, cultural and other contextual factors do not preclude the importance of considering the neural processes that arise in the empathetic understanding of visual artworks.

A review of functional imaging studies on category specificity

A review of 20 functional imaging studies that compared visual processing of natural objects and artifacts in normal subjects is presented. The studies included fulfilled three criteria: (i) they used pictures as stimuli, (ii) they were based on direct contrasts between categories, and (iii) they reported findings in Talairach space. Not a single area is consistently activated for a given category across all studies. In contrast, 11 out of 29 regions are reported activated by both artifacts and natural objects. It is argued that the inconsistency is unlikely to reflect differences between studies in task requirements alone. Rather, the most likely causes of the inconsistency are: (i) adoption of liberal statistical thresholds that may yield false-positive activations, (ii) limited sensitivity due to few observations, and (iii) failure to match categories on confounding variables such as familiarity and visual complexity. Of the most consistent activations found, none appear to be selective for natural objects or artifacts. The findings reviewed are compatible with theories of category specificity that assume a widely distributed conceptual system not organized by category.

Lefties get it "right" when hearing tool sounds

Our ability to manipulate and understand the use of a wide range of tools is a feature that sets humans apart from other animals. In right-handers, we previously reported that hearing hand-manipulated tool sounds preferentially activates a left hemisphere network of motor-related brain regions hypothesized to be related to handedness. Using functional magnetic resonance imaging, we compared cortical activation in strongly right-handed versus left-handed listeners categorizing tool sounds relative to animal vocalizations. Here we show that tool sounds preferentially evoke activity predominantly in the hemisphere "opposite" the dominant hand, in specific high-level motor-related and multisensory cortical regions, as determined by a separate task involving pantomiming tool-use gestures. This organization presumably reflects the idea that we typically learn the "meaning" of tool sounds in the context of using them with our dominant hand, such that the networks underlying motor imagery or action schemas may be recruited to facilitate recognition.

Hand actions and speech representation in Broca's area

This paper presents data and theoretical framework supporting a new interpretation of the role played by Broca's area. Recent brain imaging studies report that, in addition to speech-related activation, Broca's area is also significantly involved during tasks devoid of verbal content. In consideration of the large variety of experimental paradigms inducing Broca's activation, here we present some neurophysiological data from the monkey homologue of Brodmann's areas (BA) 44 and 45 aiming to integrate on a common ground these apparently different functions. Finally, we will report electrophysiological data on humans which connect speech perception to the more general framework of other's action understanding.

Cortical networks related to human use of tools

Greater manual dexterity and greater conceptual knowledge of tool use represent two main features that distinguish humans from other primates. Studies of human brain lesions suggest that the left hemisphere (at least in right-handed people) includes a system for processing manual skills that is specialized for tool use that interacts with another system involved more with conceptualizing, planning, and accessing knowledge associated with tool use. Growing evidence from recent neuroimaging studies supports this organization, and studies have begun to highlight specific brain regions and pathways that may be necessary for tool use. This review compares and summarizes results from 64 paradigms published over the past decade that have examined cortical regions associated with tool use skills and tool knowledge. A meta-analysis revealed cortical networks in both hemispheres, though with a clear left hemisphere bias, which may be organized to optimally represent action knowledge. Portions of this network appear to represent part of a system that is tightly linked with language systems, which is discussed together with the effects that handedness may have on the cortical organization for tool use.

Category-specific effects on the identification of non-manipulable objects

Theories of category-specific effects on visual object identification predict easier identification of non-living than living objects. The Sensory-Functional theory credits greater representational weighting of the visual properties of living objects independent of greater weighting of the functional properties of non-living objects. It predicts a lost or reversed non-living advantage for non-manipulable objects. Normal participants matched pictures of non-manipulable objects with words describing three levels of identity while visual object similarity, and concept familiarity were controlled. Consistent with the Sensory-Functional theory, living objects were matched faster than non-living objects. Concept familiarity facilitated subordinate matches. Visual similarity hampered subordinate matches and facilitated basic matches.

Premotor activations in response to visually presented single letters depend on the hand used to write: a study on left-handers

In a previous fMRI study on right-handers (Rhrs), we reported that part of the left ventral premotor cortex (BA6) was activated when alphabetical characters were passively observed and that the same region was also involved in handwriting [Longcamp, M., Anton, J. L., Roth, M., & Velay, J. L. (2003). Visual presentation of single letters activates a premotor area involved in writing. NeuroImage, 19, 1492-1500]. We therefore suggested that letter-viewing may induce automatic involvement of handwriting movements. In the present study, in order to confirm this hypothesis, we carried out a similar fMRI experiment on a group of left-handed subjects (Lhrs). We reasoned that if the above assumption was correct, visual perception of letters by Lhrs might automatically activate cortical motor areas coding for left-handed writing movements, i.e., areas located in the right hemisphere. The visual stimuli used here were either single letters, single pseudoletters, or a control stimulus. The subjects were asked to watch these stimuli attentively, and no response was required. The results showed that a ventral premotor cortical area (BA6) in the right hemisphere was specifically activated when Lhrs looked at letters and not at pseudoletters. This right area was symmetrically located with respect to the left one activated under the same circumstances in Rhrs. This finding supports the hypothesis that visual perception of written language evokes covert motor processes. In addition, a bilateral area, also located in the premotor cortex (BA6), but more ventrally and medially, was found to be activated in response to both letters and pseudoletters. This premotor region, which was not activated correspondingly in Rhrs, might be involved in the processing of graphic stimuli, whatever their degree of familiarity.

Structural similarity and category-specificity: a refined account

It has been suggested that category-specific recognition disorders for natural objects may reflect that natural objects are more structurally (visually) similar than artefacts and therefore more difficult to recognize following brain damage. On this account one might expect a positive relationship between blood flow and structural similarity in areas involved in visual object recognition. Contrary to this expectation we report a negative relationship in that identification of articles of clothing cause more extensive activation than identification of vegetables/fruit and animals even though items from the categories of animals and vegetables/fruit are rated as more structurally similar than items from the category of articles of clothing. Given that this pattern cannot be explained in terms of a tradeoff between activation and accuracy, we interpret these findings within a model where the matching of visual forms to memory incorporates two operations: (i) the integration of stored object features into whole object representations (integral units), and (ii) the competition between activated integral units for selection (i.e. identification). In addition, we suggest that these operations are differentially affected by structural similarity in that high structural similarity may be beneficial for the integration of stored features into integral units, thus explaining the greater activation found with articles of clothing, whereas it may be harmful for the selection process proper because a greater range of candidate integral units will be activated and compete for selection, thus explaining the higher error rate associated with animals. We evaluate the model based on previous evidence from both normal subjects and patients with category-specific disorders and argue that this model can help reconcile otherwise conflicting data.

Category-specific medial temporal lobe activation and the consolidation of semantic memory: evidence from fMRI

Semantic memory consolidation was studied by comparing medial temporal lobe (MTL) fMRI activation to ANIMAL, IMPLEMENT and ABSTRACT nouns in healthy seniors to that of young adults. Relative to healthy seniors, young adults were predicted to show greater MTL activation for IMPLEMENTS, but not ANIMALS, because the ANIMALS category consists of highly intercorrelated and overlapping features that should require less MTL-mediated binding than IMPLEMENTS over a shorter period of time during concept consolidation. ABSTRACT meanings are context-dependent and do not consist of fixed feature sets. Thus it was predicted that ABSTRACT words would not involve age-related feature binding mediated by the MTL. These predictions were confirmed by the results. Our observations are consistent with the hypothesis that the structure of a category influences the consolidation of knowledge in semantic memory.

Shared and dissociated cortical regions for object and letter processing

The present study determined the extent to which object and letter recognition recruit similar or dissociated neural resources. Participants passively viewed and silently named line drawings of objects, single letters, and visual noise patterns and centrally fixated an asterisk. We used whole-brain functional MRI and a very conservative approach to hypothesis testing that distinguished among brain regions that were selectively activated by different experimental conditions and those that were conjointly activated. The left fusiform gyrus (BA 19 & 37) and left inferior frontal cortex BA(44/6) showed a greater degree of conjoined activation for objects and letters than selective activation for either category, whereas left inferior parietal cortex (BA 40) and the left insula showed a strong letter-selective response. Equal recruitment of left fusiform and inferior frontal regions by objects and letters reflects similar demands on cognitive processing by these two categories and argues against category-specific modules in these regions. However, cortical systems for object and letter processing are not completely shared given the exclusive activation of left inferior parietal cortex by letters.

Activations related to "mirror" and "canonical" neurones in the human brain: an fMRI study

In the macaque monkey ventral premotor cortex (F5), "canonical neurones" are active when the monkey observes an object and when the monkey grasps that object. In the same area, "mirror neurones" fire both when the monkey observes another monkey grasping an object and when the monkey grasps that object. We used event-related fMRI to investigate where in the human brain activation can be found that reflects both canonical and mirror neuronal activity. There was activation in the intraparietal and ventral limbs of the precentral sulcus when subjects observed objects and when they executed movements in response to the objects (canonical neurones). There was activation in the dorsal premotor cortex, the intraparietal cortex, the parietal operculum (SII), and the superior temporal sulcus when subjects observed gestures (mirror neurones). Finally, activations in the ventral premotor cortex and inferior frontal gyrus (area 44) were found when subjects imitated gestures and executed movements in response to objects. We suggest that in the human brain, the ventral limb of the precentral sulcus may form part of the area designated F5 in the macaque monkey. It is possible that area 44 forms an anterior part of F5, though anatomical studies suggest that it may be a transitional area between the premotor and prefrontal cortices.

When action turns into words. Activation of motor-based knowledge during categorization of manipulable objects

Functional imaging studies have demonstrated that processing of man-made objects activate the left ventral premotor cortex, which is known to be concerned with motor function. This has led to the suggestion that the comprehension of man-made objects may rely on motor-based knowledge of object utilization (action knowledge). Here we show that the left ventral premotor cortex is activated during categorization of "both" fruit/vegetables and articles of clothing, relative to animals and nonmanipulable man-made objects. This observation suggests that action knowledge may not be important for the processing of man-made objects per se, but rather for the processing of manipulable objects in general, whether natural or man-made. These findings both support psycholinguistic theories suggesting that certain lexical categories may evolve from, and the act of categorization rely upon, motor-based knowledge of action equivalency, and have important implications for theories of category specificity. Thus, the finding that the processing of vegetables/fruit and articles of clothing give rise to similar activation is difficult to account for should knowledge representations in the brain be truly categorically organized. Instead, the data are compatible with the suggestion that categories differ in the weight they put on different types of knowledge.

Motor distal component and pragmatic representation of objects

In this study, we tested whether in certain experimental conditions the distal and proximal motor components may dissociate in action imitation, and how the distal component and the object representation may interact. Experiments 1-3 were carried out using a dual task procedure in which an action span and a motor suppression were coupled. In Experiment 1, it was shown that action spans were affected more by a distal movement (e.g. distal motor suppression) than by a proximal movement (proximal motor suppression). In Experiment 2, it was demonstrated that the reduction of the action spans was not due to the distal motor suppression being more difficult than the proximal motor suppression. In Experiment 3, pantomimes were replaced with the corresponding objects and the participants were asked to mime the objects, reproducing the appropriate actions. During the presentation of the objects, the participants were engaged in a motor suppression that was performed either by the hand or by the arm. The distal motor suppression reduced the pantomime span more significantly than the proximal motor suppression. Experiment 4 showed that the spans of pantomimes of objects shown in Experiment 3 are longer when participants do not perform motor suppressions. Our results suggest that in normal subjects a difference between the distal and the proximal motor component can be observed under dual task conditions and that distal motor suppression seems to interfere both with the encoding of action and with pragmatic representation of objects.