When does action comprehension need motor involvement? Evidence from upper limb aplasia

When does imagery require motor resources? A commentary on Bach et al., 2022

Bach, Frank, and Kunde introduce a hypothesis that encompasses two main claims: (1) motor imagery relies primarily on representations of the perceptual effects of actions, and (2) the engagement of motor resources provides access to the specific timing, kinematic or internal bodily state that characterize an action. In this commentary, I argue that the first claim is compelling and suggest some alternatives to the second one.

Effector-specific motor simulation supplements core action recognition processes in adverse conditions

Observing other people acting activates imitative motor plans in the observer. Whether, and if so when and how, such 'effector-specific motor simulation' contributes to action recognition remains unclear. We report that individuals born without upper limbs (IDs)-who cannot covertly imitate upper-limb movements-are significantly less accurate at recognizing degraded (but not intact) upper-limb than lower-limb actions (i.e. point-light animations). This finding emphasizes the need to reframe the current controversy regarding the role of effector-specific motor simulation in action recognition: instead of focusing on the dichotomy between motor and non-motor theories, the field would benefit from new hypotheses specifying when and how effector-specific motor simulation may supplement core action recognition processes to accommodate the full variety of action stimuli that humans can recognize.

Typically Efficient Lipreading without Motor Simulation

All it takes is a face-to-face conversation in a noisy environment to realize that viewing a speaker's lip movements contributes to speech comprehension. What are the processes underlying the perception and interpretation of visual speech? Brain areas that control speech production are also recruited during lipreading. This finding raises the possibility that lipreading may be supported, at least to some extent, by a covert unconscious imitation of the observed speech movements in the observer's own speech motor system-a motor simulation. However, whether, and if so to what extent, motor simulation contributes to visual speech interpretation remains unclear. In two experiments, we found that several participants with congenital facial paralysis were as good at lipreading as the control population and performed these tasks in a way that is qualitatively similar to the controls despite severely reduced or even completely absent lip motor representations. Although it remains an open question whether this conclusion generalizes to other experimental conditions and to typically developed participants, these findings considerably narrow the space of hypothesis for a role of motor simulation in lipreading. Beyond its theoretical significance in the field of speech perception, this finding also calls for a re-examination of the more general hypothesis that motor simulation underlies action perception and interpretation developed in the frameworks of motor simulation and mirror neuron hypotheses.

Putting Yourself in the Skin of In- or Out-Group Members: No Effect of Implicit Biases on Egocentric Mental Transformation

Previous studies suggest that visual encoding of ethnicity of in-group/out-group members might influence empathy and sensorimotor sharing. Here, we investigated whether mental perspective taking, presumably a precursor of empathy, is also influenced by in-group/out-group perception and the implicit attitudes toward it. We used an embodied egocentric visual-perspective taking task, the full body rotation task (FBR), in which participants were asked to mentally rotate themselves into the position of dark- or light-skinned bodies. FBR was contrasted to a pure sensorimotor task, the hand laterality task (HLT), in which participants were asked to mentally rotate their hand to the posture of seen light- or dark-skinned hands, which does not require mental simulation of another person's perspective. We expected the FBR but not the HLT to be influenced by the skin color of the stimuli and by the individual implicit biases toward out-group members. Contrary to this hypothesis, we found that neither skin color nor implicit biases modulated reaction times (RTs) in either task. The data thus suggest that unlike other empathy tasks, skin color does not influence visuospatial perspective taking.

Large-Scale Organization of the Hand Action Observation Network in Individuals Born Without Hands

The human high-level visual cortex comprises regions specialized for the processing of distinct types of stimuli, such as objects, animals, and human actions. How does this specialization emerge? Here, we investigated the role of effector-specific visuomotor coupling experience in shaping the organization of the action observation network (AON) as a window on this question. Observed body movements are frequently coupled with corresponding motor codes, e.g., during monitoring one’s own movements and imitation, resulting in bidirectionally connected circuits between areas involved in body movements observation (e.g., of the hand) and the motor codes involved in their execution. If the organization of the AON is shaped by this effector-specific visuomotor coupling, then, it should not form for body movements that do not belong to individuals’ motor repertoire. To test this prediction, we used fMRI to investigate the spatial arrangement and functional properties of the hand and foot action observation circuits in individuals born without upper limbs. Multivoxel pattern decoding, pattern similarity, and univariate analyses revealed an intact hand AON in the individuals born without upper limbs. This suggests that the organization of the AON does not require effector-specific visuomotor coupling.

Typical predictive eye movements during action observation without effector-specific motor simulation

When watching someone reaching to grasp an object, we typically gaze at the object before the agent's hand reaches it-that is, we make a "predictive eye movement" to the object. The received explanation is that predictive eye movements rely on a direct matching process, by which the observed action is mapped onto the motor representation of the same body movements in the observer's brain. In this article, we report evidence that calls for a reexamination of this account. We recorded the eye movements of an individual born without arms (D.C.) while he watched an actor reaching for one of two different-sized objects with a power grasp, a precision grasp, or a closed fist. D.C. showed typical predictive eye movements modulated by the actor's hand shape. This finding constitutes proof of concept that predictive eye movements during action observation can rely on visual and inferential processes, unaided by effector-specific motor simulation.

Persistent sparing of action conceptual processing in spite of increasing disorders of action production: A case against motor embodiment of action concepts

In this study, we addressed the issue of whether the brain sensorimotor circuitry that controls action production is causally involved in representing and processing action-related concepts. We examined the three-year pattern of evolution of brain atrophy, action production disorders, and action-related concept processing in a patient (J.R.) diagnosed with corticobasal degeneration. During the period of investigation, J.R. presented with increasing action production disorders resulting from increasing bilateral atrophy in cortical and subcortical regions involved in the sensorimotor control of actions (notably, the superior parietal cortex, the primary motor and premotor cortex, the inferior frontal gyrus, and the basal ganglia). In contrast, the patient's performance in processing action-related concepts remained intact during the same period. This finding indicated that action concept processing hinges on cognitive and neural resources that are mostly distinct from those underlying the sensorimotor control of actions.

Typical action perception and interpretation without motor simulation

Every day, we interact with people synchronously, immediately understand what they are doing, and easily infer their mental state and the likely outcome of their actions from their kinematics. According to various motor simulation theories of perception, such efficient perceptual processing of others' actions cannot be achieved by visual analysis of the movements alone but requires a process of motor simulation--an unconscious, covert imitation of the observed movements. According to this hypothesis, individuals incapable of simulating observed movements in their motor system should have difficulty perceiving and interpreting observed actions. Contrary to this prediction, we found across eight sensitive experiments that individuals born with absent or severely shortened upper limbs (upper limb dysplasia), despite some variability, could perceive, anticipate, predict, comprehend, and memorize upper limb actions, which they cannot simulate, as efficiently as typically developed participants. We also found that, like the typically developed participants, the dysplasic participants systematically perceived the position of moving upper limbs slightly ahead of their real position but only when the anticipated position was not biomechanically awkward. Such anticipatory bias and its modulation by implicit knowledge of the body biomechanical constraints were previously considered as indexes of the crucial role of motor simulation in action perception. Our findings undermine this assumption and the theories that place the locus of action perception and comprehension in the motor system and invite a shift in the focus of future research to the question of how the visuo-perceptual system represents and processes observed body movements and actions.

Decoding actions at different levels of abstraction

Brain regions that mediate action understanding must contain representations that are action specific and at the same time tolerate a wide range of perceptual variance. Whereas progress has been made in understanding such generalization mechanisms in the object domain, the neural mechanisms to conceptualize actions remain unknown. In particular, there is ongoing dissent between motor-centric and cognitive accounts whether premotor cortex or brain regions in closer relation to perceptual systems, i.e., lateral occipitotemporal cortex, contain neural populations with such mapping properties. To date, it is unclear to which degree action-specific representations in these brain regions generalize from concrete action instantiations to abstract action concepts. However, such information would be crucial to differentiate between motor and cognitive theories. Using ROI-based and searchlight-based fMRI multivoxel pattern decoding, we sought brain regions in human cortex that manage the balancing act between specificity and generality. We investigated a concrete level that distinguishes actions based on perceptual features (e.g., opening vs closing a specific bottle), an intermediate level that generalizes across movement kinematics and specific objects involved in the action (e.g., opening different bottles with cork or screw cap), and an abstract level that additionally generalizes across object category (e.g., opening bottles or boxes). We demonstrate that the inferior parietal and occipitotemporal cortex code actions at abstract levels whereas the premotor cortex codes actions at the concrete level only. Hence, occipitotemporal, but not premotor, regions fulfill the necessary criteria for action understanding. This result is compatible with cognitive theories but strongly undermines motor theories of action understanding.

The bodily self and its disorders: neurological, psychological and social aspects

PURPOSE OF REVIEW

The experience of ourselves as an embodied agent with a first-person perspective is referred to as 'bodily self'. We present a selective overview of relevant clinical and experimental studies.

RECENT FINDINGS

Sharing multisensory body space with others can be observed in patients with structurally altered bodies (amputations, congenital absence of limbs), with altered functionality after hemiplegia, such as denial of limb ownership (somatoparaphrenia) and with alterations in bodily self-consciousness on the level of the entire body (e.g. in autoscopic phenomena). In healthy participants, the mechanisms underpinning body ownership and observer perspective are empirically investigated by multisensory stimulation paradigms to alter the bodily self. The resulting illusions have promoted the understanding of complex disturbances of the bodily self, such as out-of-body experiences. We discuss the role of interoception in differentiating between self and others and review current advances in the study of body integrity identity disorder, a condition shaped as much by neurological as by social-psychological factors.

SUMMARY

We advocate a social neuroscience approach to the bodily self that takes into account the interactions between body, mind and society and might help close the divide between neurology and psychiatry.

Decoding actions at different levels of abstraction

Brain regions that mediate action understanding must contain representations that are action specific and at the same time tolerate a wide range of perceptual variance. Whereas progress has been made in understanding such generalization mechanisms in the object domain, the neural mechanisms to conceptualize actions remain unknown. In particular, there is ongoing dissent between motor-centric and cognitive accounts whether premotor cortex or brain regions in closer relation to perceptual systems, i.e., lateral occipitotemporal cortex, contain neural populations with such mapping properties. To date, it is unclear to which degree action-specific representations in these brain regions generalize from concrete action instantiations to abstract action concepts. However, such information would be crucial to differentiate between motor and cognitive theories. Using ROI-based and searchlight-based fMRI multivoxel pattern decoding, we sought brain regions in human cortex that manage the balancing act between specificity and generality. We investigated a concrete level that distinguishes actions based on perceptual features (e.g., opening vs closing a specific bottle), an intermediate level that generalizes across movement kinematics and specific objects involved in the action (e.g., opening different bottles with cork or screw cap), and an abstract level that additionally generalizes across object category (e.g., opening bottles or boxes). We demonstrate that the inferior parietal and occipitotemporal cortex code actions at abstract levels whereas the premotor cortex codes actions at the concrete level only. Hence, occipitotemporal, but not premotor, regions fulfill the necessary criteria for action understanding. This result is compatible with cognitive theories but strongly undermines motor theories of action understanding.