Valentina Moro

Mapping implied body actions in the human motor system.

The human visual system is highly tuned to perceive actual motion as well as to extrapolate dynamic information from static pictures of objects or creatures captured in the middle of motion. Processing of implied motion activates higher-order visual areas that are also involved in processing biological motion. Imagery and observation of actual movements performed by others engenders selective activation of motor and premotor areas that are part of a mirror-neuron system matching action observation and execution. By using single-pulse transcranial magnetic stimulation, we found that the mere observation of static snapshots of hands suggesting a pincer grip action induced an increase in corticospinal excitability as compared with observation of resting, relaxed hands, or hands suggesting a completed action. This facilitatory effect was specific for the muscle that would be activated during actual execution of the observed action. We found no changes in responsiveness of the tested muscles during observation of nonbiological entities with (e.g., waterfalls) or without (e.g., icefalls) implied motion. Thus, extrapolation of motion information concerning human actions induced a selective activation of the motor system. This indicates that overlapping motor regions are engaged in the visual analysis of physical and implied body actions. The absence of motor evoked potential modulation during observation of end posture stimuli may indicate that the observation–execution matching system is preferentially activated by implied, ongoing but not yet completed actions.

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.

Changes in spatial position of hands modify tactile extinction but not disownership of contralesional hand in two right brain-damaged patients

Abstract Somatic misperceptions and misrepresentations, like supernumerary phantom limb and denial of ownership of a given body part, have typically been reported following damage to the right side of the brain. These symptoms typically occur with personal or extrapersonal neglect and extinction of left-sided stimuli, suggesting that all these different symptoms may be linked to the same neural substrate. In the present research, we tested two right brain-damaged (RBD) patients to find out whether changing the position of the hands in space influences tactile extinction and denial of ownership to the same extent. Results showed that manipulation of the spatial position of the hands reduces tactile extinction but leaves denial of ownership of the left hand unaffected. Such a dissociation suggests that delusional misperceptions may be independent from somatic neglect and that representation of hands in space and attribution of ownership are dynamically mapped in at least partly separate neural substrates.

Phenomenology and neural correlates of implicit and emergent motor awareness in patients with anosognosia for hemiplegia

Anosognosia for hemiplegia (AH) is characterized by a lack of awareness of motor disorders and appears associated with fronto-temporal-parietal damage. Neuropsychological evidence indicates that behavioral indices of residual forms of motor awareness may co-exist with explicit denial of impairment. Here we explore whether the attempt by AH patients to perform an action may disclose residual forms of motor awareness and whether such forms are underpinned by different neural structures. Twelve hemiplegic patients affected by AH were tested in tasks assessing: (i) implicit awareness (IA), indexed by discrepancies between verbal reports and actual motor behavior; (ii) emergent awareness (EA), indexed by increased verbal awareness induced by the attempt to perform actions. IA and EA were found in five and three patients, respectively. Lesion analysis indicates that while the lack of IA is associated with damage to subcortical white matter anterior to the basal ganglia, lack of EA is linked to damage to cortical regions including insulo-frontal, temporal and parietal structures. Our results indicate that deficits in explicit and implicit awareness are associated with lesions involving different cortico-subcortical structures. Moreover, the results show that the attempt to perform an action may ameliorate body awareness deficits and have implications for rehabilitation.

Temporary and permanent signs of interhemispheric disconnection after traumatic brain injury

The corpus callosum is frequently damaged by closed head traumas, and the resulting deficits of interhemispheric communication may vary according to the specific position of the lesion within the corpus callosum. This paper describes a single case who suffered a severe traumatic brain injury resulting in a lesion of the posterior body of the corpus callosum. Among the classical symptoms of interhemispheric disconnection, left hand anomia, left upper limb ideomotor dyspraxia, left visual field dyslexia and dysnomia, and left ear suppression in a dichotic listening task were observed shortly after the injury but recovered completely or almost completely with the passage of time. The only symptom of interhemispheric disconnection which was found to persist more than 4 years after the injury was an abnormal prolongation of the crossed-uncrossed difference in a simple visuomotor reaction time task. This prolongation was comparable with that observed in subjects with complete callosal lesions or agenesis. The results suggest that the posterior body of the corpus callosum may be an obligatory interhemispheric communication channel for mediating fast visuo-motor responses. The transient nature of other symptoms of interhemispheric disconnection suggests a relatively wide dispersion of fibers with different functions through the callosal body, such that parts of them can survive a restricted lesion and allow functional recovery of hemispheric interactions. An assessment of the evolution in time of symptoms of interhemispheric disconnection following restricted callosal lesions may reveal fine and coarse features of the anatomo-functional topography of the corpus callosum.

Selective deficit of mental visual imagery with intact primary visual cortex and visual perception

There is a vigorous debate as to whether visual perception and imagery share the same neuronal networks, whether the primary visual cortex is necessarily involved in visual imagery, and whether visual imagery functions are lateralized in the brain. Two patients with brain damage from closed head injury were submitted to tests of mental imagery in the visual, tactile, auditory, gustatory, olfactory and motor domains, as well as to an extensive testing of cognitive functions. A computerized mapping procedure was used to localize the site and to assess the extent of the lesions. One patient showed pure visual mental imagery deficits in the absence of imagery deficits in other sensory domains as well as in the motor domain, while the other patient showed both visual and tactile imagery deficits. Perceptual, language, and memory deficits were conspicuously absent. Computerized analysis of the lesions showed a massive involvement of the left temporal lobe in both patients and a bilateral parietal lesion in one patient. In both patients the calcarine cortex with the primary visual area was bilaterally intact. Our study indicates that: (i) visual imagery deficits can occur independently from deficits of visual perception; (ii) visual imagery deficits can occur when the primary visual cortex is intact and (iii) the left temporal lobe plays an important role in visual mental imagery.

Rubber hand illusion induced by touching the face ipsilaterally to a deprived hand: evidence for plastic "somatotopic" remapping in tetraplegics

Background: Studies in animals and humans indicate that the interruption of body-brain connections following spinal cord injury (SCI) leads to plastic cerebral reorganization. Objective: To explore whether inducing the Rubber Hand Illusion (RHI) via synchronous multisensory visuo-tactile bodily stimulation may reveal any perceptual correlates of plastic remapping in SCI. Methods: In 16 paraplegic, 16 tetraplegic and 16 healthy participants we explored whether RHI may be induced by tactile stimuli involving not only the left hand but also the left hemi-face. Touching the participants actual hand or face was either synchronous or asynchronous with tactile stimuli seen on a rubber hand. We assessed two components of the illusion, namely perceived changes in the real hand in space (indexed by proprioceptive drift) and ownership of the rubber hand (indexed by subjective responses to an ad-hoc questionnaire). Results: Proprioceptive drift and ownership were found in the healthy group only in the condition where the left real and fake hand were touched simultaneously. In contrast, no drift was found in the SCI patients who, however, showed ownership after both synchronous and asynchronous hand stroking. Importantly, only tetraplegics showed the effect also after synchronous face stroking. Conclusions: RHI may reveal plastic phenomena in SCI. In hand representation-deprived tetraplegics, stimuli on the face (represented contiguously in the somatic and motor systems), drive the sense of hand ownership. This hand-face remapping phenomenon may be useful for restoring a sense of self in massively deprived individuals.

Visual body recognition in a prosopagnosic patient

Conspicuous deficits in face recognition characterize prosopagnosia. Information on whether agnosic deficits may extend to non-facial body parts is lacking. Here we report the neuropsychological description of FM, a patient affected by a complete deficit in face recognition in the presence of mild clinical signs of visual object agnosia. His deficit involves both overt and covert recognition of faces (i.e. recognition of familiar faces, but also categorization of faces for gender or age) as well as the visual mental imagery of faces. By means of a series of matching-to-sample tasks we investigated: (i) a possible association between prosopagnosia and disorders in visual body perception; (ii) the effect of the emotional content of stimuli on the visual discrimination of faces, bodies and objects; (iii) the existence of a dissociation between identity recognition and the emotional discrimination of faces and bodies. Our results document, for the first time, the co-occurrence of body agnosia, i.e. the visual inability to discriminate body forms and body actions, and prosopagnosia. Moreover, the results show better performance in the discrimination of emotional face and body expressions with respect to body identity and neutral actions. Since FMs lesions involve bilateral fusiform areas, it is unlikely that the amygdala-temporal projections explain the relative sparing of emotion discrimination performance. Indeed, the emotional content of the stimuli did not improve the discrimination of their identity. The results hint at the existence of two segregated brain networks involved in identity and emotional discrimination that are at least partially shared by face and body processing.

Massive somatic deafferentation and motor deefferentation of the lower part of the body impair its visual recognition: A psychophysical study of patients with spinal cord injury

Embodied cognition theories postulate that perceiving and understanding the body states of other individuals are underpinned by the neural structures activated during first‐hand experience of the same states. This suggests that one’s own sensorimotor system may be used to identify the actions and sensations of others. Virtual and real brain lesion studies show that visual processing of body action and body form relies upon neural activity in the ventral premotor and the extrastriate body areas, respectively. We explored whether visual body perception may also be altered in the absence of damage to the above cortical regions by testing healthy controls and spinal cord injury (SCI) patients whose brain was unable to receive somatic information from and send motor commands to the lower limbs. Participants performed tasks investigating the ability to visually discriminate changes in the form or action of body parts affected by somatosensory and motor disconnection. SCI patients showed a specific, cross‐modal deficit in the visual recognition of the disconnected lower body parts. This deficit affected both body action and body form perception, hinting at a pervasive influence of ongoing body signals on the brain network dedicated to visual body processing. Testing SCI patients who did or did not practise sports allowed us to test the influence of motor practice on visual body recognition. We found better upper body action recognition in sport‐practising SCI patients, indicating that motor practice is useful for maintaining visual representation of actions after deafferentation and deefferentation. This may be a potential resource to be exploited for rehabilitation.

Illusory movements of the contralesional hand in patients with body image disorders

Objectives: In the present study, we assess whether illusory sensations of movement can be elicited in patients with right brain damage (RBD). Methods: Ten RBD patients (three with disorders of bodily representations) were asked to report whether movements of their right hand induced any illusory somatic or motor sensations. Inquiries on anomalous sensation of movement of the left hand were carried out while subjects: 1) observed the moving hand in a mirror propped vertically along the parasagittal plane; 2) looked directly at the moving hand; 3) looked at the still hand; 4) kept their eyes closed. Twelve healthy subjects served as controls. Results: Movement of the right hand induced a very clear sensation of movement of the left, contralesional hand in two patients affected by body image disorders. Remarkably, this occurred mainly while subjects were looking in the mirror, that is, when conflicts between visual, somatic, and motor information were maximal. In no condition did control subjects report any consistent anomalous evoked movement or sensation. Conclusions: Illusory movements of the left, plegic hand contingent upon sensorimotor conflicts can be evoked in brain damaged patients with body image disorders.