Salvatore Maria Aglioti

The body in the brain revisited

Corporeal awareness is a difficult concept which refers to perception, knowledge and evaluation of one’s own body as well as of other bodies. We discuss here some controversies regarding the significance of the concepts of body schema and body image, as variously entertained by different authors, for the understanding of corporeal awareness, and consider some newly proposed alternatives. We describe some recent discoveries of cortical areas specialized for the processing of bodily forms and bodily actions, as revealed by neuroimaging, neurophysiological, and lesion studies. We further describe new empirical and theoretical evidence for the importance of interoception, in addition to exteroception and proprioception, for corporeal awareness, and discuss how itch, a typical interoceptive input, has been wrongly excluded from the classic concept of the proprioceptive–tactile body schema. Finally, we consider the role of the insular cortex as the terminal cortical station of interoception and other bodily signals, along with Craig’s proposal that the human insular cortex sets our species apart from other species by supporting consciousness of the body and the self. We conclude that corporeal awareness depends on the spatiotemporally distributed activity of many bodies in the brain, none of which is isomorphic with the actual body.

Somatic and Motor Components of Action Simulation

Seminal studies in monkeys report that the viewing of actions performed by other individuals activates frontal and parietal cortical areas typically involved in action planning and execution. That mirroring actions might rely on both motor and somatosensory components is suggested by reports that action observation and execution increase neural activity in motor and in somatosensory areas. This occurs not only during observation of naturalistic movements but also during the viewing of biomechanically impossible movements that tap the afferent component of action, possibly by eliciting strong somatic feelings in the onlooker. Although somatosensory feedback is inherently linked to action execution, information on the possible causative role of frontal and parietal cortices in simulating motor and sensory action components is lacking. By combining low-frequency repetitive and single-pulse transcranial magnetic stimulation, we found that virtual lesions of ventral premotor cortex (vPMc) and primary somatosensory cortex (S1) suppressed mirror motor facilitation contingent upon observation of possible and impossible movements, respectively. In contrast, virtual lesions of primary motor cortex did not influence mirror motor facilitation. The reported double dissociation suggests that vPMc and S1 play an active, differential role in simulating efferent and afferent components of observed actions.

Putting yourself in the skin of a black avatar reduces implicit racial bias

Although it has been shown that immersive virtual reality (IVR) can be used to induce illusions of ownership over a virtual body (VB), information on whether this changes implicit interpersonal attitudes is meager. Here we demonstrate that embodiment of light-skinned participants in a dark-skinned VB significantly reduced implicit racial bias against dark-skinned people, in contrast to embodiment in light-skinned, purple-skinned or with no VB. 60 females participated in this between-groups experiment, with a VB substituting their own, with full-body visuomotor synchrony, reflected also in a virtual mirror. A racial Implicit Association Test (IAT) was administered at least three days prior to the experiment, and immediately after the IVR exposure. The change from pre- to post-experience IAT scores suggests that the dark-skinned embodied condition decreased implicit racial bias more than the other conditions. Thus, embodiment may change negative interpersonal attitudes and thus represent a powerful tool for exploring such fundamental psychological and societal phenomena.

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.

Magnetic Stimulation of Extrastriate Body Area Impairs Visual Processing of Nonfacial Body Parts

Functional magnetic resonance imaging indicates that observation of the human body induces a selective activation of a lateral occipitotemporal cortical area called extrastriate body area (EBA). This area is responsive to static and moving images of the human body and parts of it, but it is insensitive to faces and stimulus categories unrelated to the human body. With event-related repetitive transcranial magnetic stimulation, we tested the possible causal relation between neural activity in EBA and visual processing of body-related, nonfacial stimuli. Facial and noncorporeal stimuli were used as a control. Interference with neural activity in EBA induced a clear impairment, consisting of a significant increase in discriminative reaction time, in the visual processing of body parts. The effect was selective for stimulus type, because it affected responses to nonfacial body stimuli but not to noncorporeal and facial stimuli, and for locus of stimulation, because the effect from the interfering stimulation of EBA was absent during a corresponding stimulation of primary visual cortex. The results provide strong evidence that neural activity in EBA is not only correlated with but also causally involved in the visual processing of the human body and its parts, except the face.

My eyes want to look where your eyes are looking: exploring the tendency to imitate another individual's gaze.

In this study we investigated the tendency of humans to imitate the gaze direction of other individuals. Distracting gaze stimuli or non biological directional cues (arrows) were presented to observers performing an instructed saccadic eye movement task. Eye movement recordings showed that observers performed less accurately when the distracting gaze and the instructed saccade had opposite directions, with a substantial number of saccades matching the direction of the distracting gaze. Static (Experiment 1) and dynamic (Experiment 2) gaze distracters, but not pointing arrows (Experiment 3), produced the effect. Results show a strong predisposition of humans to imitate somebody elses oculomotor behaviour, even when detrimental to task performance. This is likely linked to a strong tendency to share attentional states of other individuals, known as joint attention.

Visually Induced Analgesia: Seeing the Body Reduces Pain

Given previous reports of strong interactions between vision and somatic senses, we investigated whether vision of the body modulates pain perception. Participants looked into a mirror aligned with their body midline at either the reflection of their own left hand (creating the illusion that they were looking directly at their own right hand) or the reflection of a neutral object. We induced pain using an infrared laser and recorded nociceptive laser-evoked potentials (LEPs). We also collected subjective ratings of pain intensity and unpleasantness. Vision of the body produced clear analgesic effects on both subjective ratings of pain and the N2/P2 complex of LEPs. Similar results were found during direct vision of the hand, without the mirror. Furthermore, these effects were specific to vision of ones own hand and were absent when viewing another persons hand. These results demonstrate a novel analgesic effect of non-informative vision of the body.

Transcranial magnetic stimulation reveals two cortical pathways for visual body processing

Visual recognition of human bodies is more difficult for upside down than upright presentations. This body inversion effect implies that body perception relies on configural rather than local processing. Although neuroimaging studies indicate that the visual processing of human bodies engages a large fronto-temporo-parietal network, information about the neural underpinnings of configural body processing is meager. Here, we used repetitive transcranial magnetic stimulation (rTMS) to study the causal role of premotor, visual, and parietal areas in configural processing of human bodies. Eighteen participants performed a delayed matching-to-sample task with upright or inverted static body postures. Event-related, dual-pulse rTMS was applied 150 ms after the sample stimulus onset, over left ventral premotor cortex (vPMc), right extrastriate body area (EBA), and right superior parietal lobe (SPL) and, as a control site, over the right primary visual cortex (V1). Interfering stimulation of vPMc significantly reduced accuracy of matching judgments for upright bodies. In contrast, EBA rTMS significantly reduced accuracy for inverted but not for upright bodies. Furthermore, a significant body inversion effect was observed after interfering stimulation of EBA and V1 but not of vPMc and SPL. These results demonstrate an active contribution of the fronto-parietal mirror network to configural processing of bodies and suggest a novel, embodied aspect of visual perception. In contrast, the local processing of the body, possibly based on the form of individual body parts instead of on the whole body unit, appears to depend on EBA. Therefore, we propose two distinct cortical routes for the visual processing of human bodies.

The influence of hands posture on mental rotation of hands and feet

Behavioural and functional neuroanatomy studies demonstrate that mental rotation of body parts is carried out through a sort of inner motor simulation. Here we examined whether changes of hands posture influence the mental rotation of hands and feet. Twenty healthy subjects were asked to verbally judge the laterality of hands and feet pictures in two different postural conditions. In one condition, subjects kept hands on their knees in anatomical position; in the other, their hands were kept in an unusual posture with intertwined fingers, behind the back. Results show that mental rotation of hands but not of feet was influenced by changes in hands posture. Indeed, while mental rotation of hands was faster in the front than in the back hands position, no similar effect was found when mentally rotating feet. Thus, sensory-motor and postural information coming from the body may influence mental rotation of body parts according to specific, somatotopic rules.

Redundant target effect and intersensory facilitation from visual-tactile interactions in simple reaction time

In a simple reaction time (RT) task, normal observers responded faster to simultaneous visual and tactile stimuli than to single visual or tactile stimuli. RT to simultaneous visual and tactile stimuli was also faster than RT to simultaneous dual visual or tactile stimuli. The advantage for RT to combined visual-tactile stimuli over RT to the other types of stimulation could be accounted for by intersensory neural facilitation rather than by probability summation. The direction of gaze (and presumably of visual attention) to space regions near to or far from the site of tactile stimulation had no effect on tactile RT. However, RT to single or dual tactile stimuli was fastest when observers could see the sites of tactile stimulation on their hands both directly and through a mirror at the same time. All these effects can be ascribed to the convergence of tactile and visual inputs onto neural centers which contain flexible multimodal representations of body parts.