Mariella Pazzaglia

Neural Underpinnings of Gesture Discrimination in Patients with Limb Apraxia

Limb apraxia (LA), is a neuropsychological syndrome characterized by difficulty in performing gestures and may therefore be an ideal model for investigating whether action execution deficits are causatively linked to deficits in action understanding. We tested 33 left brain-damaged patients and 8 right brain-damaged patients for the presence of the LA. Importantly, we also tested all the patients in an ad hoc developed gesture recognition task wherein an actor performs, either correctly or incorrectly, transitive (using objects) or intransitive (without objects) meaningful conventional limb gestures. Patients were instructed to judge whether the observed gesture was correct or incorrect. Lesion analysis enabled us to evaluate the relationship between specific brain regions and behavioral performance in gesture execution and gesture comprehension. We found that LA was present in 21 left brain-damaged patients and it was linked to frontal and parietal lesions. Moreover, we found that recognition of correct execution of familiar gestures performed by others was more impaired in patients with LA than in nonapraxic patients. Crucially, the gesture comprehension deficit correlated with damage to the opercular and triangularis portions of the inferior frontal gyrus, two regions that are involved in complex aspects of action-related processing. In contrast, no such relationship was observed with lesions centered on the inferior parietal cortex. The present findings suggest that lesions to left frontal regions that are involved in planning and performing actions are causatively associated with deficits in the recognition of the correct execution of meaningful gestures.

The Neural Basis of Body Form and Body Action Agnosia

Visual analysis of faces and nonfacial body stimuli brings about neural activity in different cortical areas. Moreover, processing body form and body action relies on distinct neural substrates. Although brain lesion studies show specific face processing deficits, neuropsychological evidence for defective recognition of nonfacial body parts is lacking. By combining psychophysics studies with lesion-mapping techniques, we found that lesions of ventromedial, occipitotemporal areas induce face and body recognition deficits while lesions involving extrastriate body area seem causatively associated with impaired recognition of body but not of face and object stimuli. We also found that body form and body action recognition deficits can be double dissociated and are causatively associated with lesions to extrastriate body area and ventral premotor cortex, respectively. Our study reports two category-specific visual deficits, called body form and body action agnosia, and highlights their neural underpinnings.

The Sound of Actions in Apraxia

Studies in nonhuman and human primates have demonstrated that sound-producing actions are mapped on the same mirror circuits that are activated during the visual recognition and execution of actions [1-12]. However, no causative link between the auditory recognition and execution of actions has been provided thus far. Here, we sought to determine whether patients with apraxia, who are by definition impaired in performing specific gestures, are also impaired in recognizing sounds specifically linked to human actions. Twenty-eight left-hemisphere-damaged patients with or without limb and/or buccofacial apraxia and seven right-hemisphere-damaged patients with no apraxia were asked to match sounds evoking human-related actions or nonhuman action sounds with specific visual pictures. Hand and mouth action-related sound recognition were specifically impaired in limb and buccofacial apraxia patients, respectively. Lesional mapping revealed that the left frontoparietal cortex is crucial for recognizing the sound of limb movements. By contrast, the left inferior frontal gyrus and adjacent insular cortex are causatively associated with recognition of buccofacial-related action sounds. These behavioral and neural double dissociations indicate that a left-lateralized multimodal mirror network is actively involved in the body-part-specific motor mapping of limb and mouth action-related sounds, as well as in the execution of the very same actions.

Representing actions through their sound

Since the discovery of ‘mirror neurons’ in the monkey premotor and parietal cortex, an increasing body of evidence in animals and humans alike has supported the notion of the inextricable link between action execution and action perception. Although research originally focused on the relationship between performed and viewed actions, more recent studies highlight the importance of representing the actions of others through audition. In the first part of this article, we discuss animal studies, which provide direct evidence that action is inherently linked to multi-sensory cues, as well as the studies carried out on healthy subjects by using state-of-the-art cognitive neuroscience techniques such as functional magnetic resonance imaging (fMRI), event-related potentials (ERP), magnetoencephalography (MEG), and transcranial magnetic stimulation (TMS). In the second section, we review the lesion analysis studies in brain-damaged patients demonstrating the link between ‘resonant’ fronto-parieto-temporal networks and the ability to represent an action by hearing its sound. Moreover, we examine the evidence in favour of somatotopy as a possible representational rule underlying the auditory mapping of actions and consider the links between language and audio-motor action mapping. We conclude with a discussion of some outstanding questions for future research on the link between actions and the sounds they produce.

The Sense of the Body in Individuals with Spinal Cord Injury

Increasing evidence suggests that the basic foundations of the self lie in the brain systems that represent the body. Specific sensorimotor stimulation has been shown to alter the bodily self. However, little is known about how disconnection of the brain from the body affects the phenomenological sense of the body and the self. Spinal cord injury (SCI) patients who exhibit massively reduced somatomotor processes below the lesion in the absence of brain damage are suitable for testing the influence of body signals on two important components of the self–the sense of disembodiment and body ownership. We recruited 30 SCI patients and 16 healthy participants, and evaluated the following parameters: (i) depersonalization symptoms, using the Cambridge Depersonalization Scale (CDS), and (ii) measures of body ownership, as quantified by the rubber hand illusion (RHI) paradigm. We found higher CDS scores in SCI patients, which show increased detachment from their body and internal bodily sensations and decreasing global body ownership with higher lesion level. The RHI paradigm reveals no alterations in the illusory ownership of the hand between SCI patients and controls. Yet, there was no typical proprioceptive drift in SCI patients with intact tactile sensation on the hand, which might be related to cortical reorganization in these patients. These results suggest that disconnection of somatomotor inputs to the brain due to spinal cord lesions resulted in a disturbed sense of an embodied self. Furthermore, plasticity-related cortical changes might influence the dynamics of the bodily self.

Sounds and scents in (social) action.

Although vision seems to predominate in triggering the simulation of the behaviour and mental states of others, the social perception of actions might rely on auditory and olfactory information not only when vision is lacking (e.g. in congenitally blind individuals), but also in daily life (e.g. hearing footsteps along a dark street prompts an appropriate fight-or-fly reaction and smelling the scent of coffee prompts the act of grasping a mug). Here, we review recent evidence showing that non-visual, telereceptor-mediated motor mapping might occur as an autonomous process, as well as within the context of the multimodal perceptions and representations that characterize real-world experiences. Moreover, we discuss the role of auditory and olfactory resonance in anticipating the actions of others and, therefore, in shaping social interactions.

Body image distortions following spinal cord injury

Background Following spinal cord injury (SCI) or anaesthesia, people may continue to experience feelings of the size, shape and posture of their body, suggesting that the conscious body image is not fully determined by immediate sensory signals. How this body image is affected by changes in sensory inputs from, and motor outputs to, the body remains unclear. Methods We tested paraplegic and tetraplegic SCI patients on a task that yields quantitative measures of body image. Participants were presented with an anchoring stimulus on a computer screen and told to imagine that the displayed body part was part of a standing mirror image of themselves. They then identified the position on the screen, relative to the anchor, where each of several parts of their body would be located. Veridical body dimensions were identified based on measurements and photographs of participants. Results Compared with age matched controls, paraplegic and tetraplegic patients alike perceived their torso and limbs as elongated relative to their body width. No effects of lesion level were found. Conclusions The common distortions in body image across patient groups, despite differing SCI levels, imply that a body image may be maintained despite chronic sensory and motor loss. Systematic alterations in body image follow SCI although our results suggest these may reflect changes in body posture, rather than loss of specific sensorimotor pathways. These findings provide new insight into how the body image is maintained, and may prove useful in treatments that intervene to manipulate the body image.

The embodiment of assistive devices-from wheelchair to exoskeleton

Spinal cord injuries (SCIs) place a heavy burden on the healthcare system and have a high personal impact and marked socio-economic consequences. Clinically, no absolute cure for these conditions exists. However, in recent years, there has been an increased focus on new robotic technologies that can change the frame we think about the prognosis for recovery and for treating some functions of the body affected after SCIs. This review has two goals. The first is to assess the possibility of the embodiment of functional assistive tools after traumatic disruption of the neural pathways between the brain and the body. To this end, we will examine how altered sensorimotor information modulates the sense of the body in SCI. The second goal is to map the phenomenological experience of using external tools that typically extend the potential of the body physically impaired by SCI. More specifically, we will focus on the difference between the perception of ones physically augmented and non-augmented affected body based on observable and measurable behaviors. We discuss potential clinical benefits of enhanced embodiment of the external objects by way of multisensory interventions. This review argues that the future evolution of human robotic technologies will require adopting an embodied approach, taking advantage of brain plasticity to allow bionic limbs to be mapped within the neural circuits of physically impaired individuals.

Gesture Discrimination in Primary Progressive Aphasia: The Intersection between Gesture and Language Processing Pathways

The issue of the relationship between language and gesture processing and the partial overlap of their neural representations is of fundamental importance to neurology, psychology, and social sciences. Patients suffering from primary progressive aphasia, a clinical syndrome characterized by comparatively isolated language deficits, may provide direct evidence for anatomical and functional association between specific language deficits and gesture discrimination deficits. A consecutive series of 16 patients with primary progressive aphasia and 16 matched control subjects participated. Our nonverbal gesture discrimination task consisted of 19 trials. In each trial, participants observed three video clips showing the same gesture performed correctly in one clip and incorrectly in the other two. Subjects had to indicate which of the three versions was correct. Language and gesture production were evaluated by means of conventional tasks. All participants underwent high-resolution structural and diffusion tensor magnetic resonance imaging. Ten of the primary progressive aphasia patients showed a significant deficit on the nonverbal gesture discrimination task. A factor analysis revealed that this deficit clustered with gesture imitation, word and pseudoword repetition, and writing-to-dictation. Individual scores on this cluster correlated with volume in the left anterior inferior parietal cortex extending into the posterior superior temporal gyrus. Probabilistic tractography indicated this region comprised the cortical relay station of the indirect pathway connecting the inferior frontal gyrus and the superior temporal cortex. Thus, the left perisylvian temporoparietal area may underpin verbal imitative behavior, gesture imitation, and gesture discrimination indicative of a partly shared neural substrate for language and gesture resonance.

Restoring tactile awareness through the rubber hand illusion in cervical spinal cord injury.

Background. Bodily sensations are an important component of corporeal awareness. Spinal cord injury can leave affected body parts insentient and unmoving, leading to specific disturbances in the mental representation of one’s own body and the sense of self. Objective. Here, we explored how illusions induced by multisensory stimulation influence immediate sensory signals and tactile awareness in patients with spinal cord injuries. Methods. The rubber hand illusion paradigm was applied to 2 patients with chronic and complete spinal cord injury of the sixth cervical spine, with severe somatosensory impairments in 2 of 5 fingers. Results. Both patients experienced a strong illusion of ownership of the rubber hand during synchronous, but not asynchronous, stroking. They also, spontaneously reported basic tactile sensations in their previously numb fingers. Tactile awareness from seeing the rubber hand was enhanced by progressively increasing the stimulation duration. Conclusions. Multisensory illusions directly and specifically modulate the reemergence of sensory memories and enhance tactile sensation, despite (or as a result of) prior deafferentation. When sensory inputs are lost, and are later illusorily regained, the brain updates a coherent body image even several years after the body has become permanently unable to feel. This particular example of neural plasticity represents a significant opportunity to strengthen the sense of the self and the feelings of embodiment in patients with spinal cord injury.