Non-invasive brain stimulation of motor cortex induces embodiment when integrated with virtual reality feedback.

Previous evidence highlighted the multisensory-motor origin of embodiment - that is, the experience of having a body and of being in control of it - and the possibility of experimentally manipulating it. For instance, an illusory feeling of embodiment towards a fake hand can be triggered by providing synchronous visuo-tactile stimulation to the hand of participants and to a fake hand or by asking participants to move their hand and observe a fake hand moving accordingly (rubber hand illusion). Here, we tested whether it is possible to manipulate embodiment not through stimulation of the participant's hand, but by directly tapping into the brain's hand representation via non-invasive brain stimulation. To this aim, we combined transcranial magnetic stimulation (TMS), to activate the hand corticospinal representation, with virtual reality (VR), to provide matching (as contrasted to non-matching) visual feedback, mimicking involuntary hand movements evoked by TMS. We show that the illusory embodiment occurred when TMS pulses were temporally matched with VR feedback, but not when TMS was administered outside primary motor cortex, (over the vertex) or when stimulating motor cortex at a lower intensity (that did not activate peripheral muscles). Behavioural (questionnaires) and neurophysiological (motor-evoked-potentials, TMS-evoked-movements) measures further indicated that embodiment was not explained by stimulation per se, but depended on the temporal coherence between TMS-induced activation of hand corticospinal representation and the virtual bodily feedback. This reveals that non-invasive brain stimulation may replace the application of external tactile hand cues and motor components related to volition, planning and anticipation.

Self in motion: sensorimotor and cognitive mechanisms in gait agency.

Acting in our environment and experiencing ourselves as conscious agents are fundamental aspects of human selfhood. While large advances have been made with respect to understanding human sensorimotor control from an engineering approach, knowledge about its interaction with cognition and the conscious experience of movement (agency) is still sparse, especially for locomotion. We investigated these relationships by using life-size visual feedback of participants' ongoing locomotion, thereby extending agency research previously limited to goal-directed upper limb movements to continuous movements of the entire body. By introducing temporal delays and cognitive loading we were able to demonstrate distinct effects of bottom-up visuomotor conflicts as well as top-down cognitive loading on the conscious experience of locomotion (gait agency) and gait movements. While gait agency depended on the spatial and temporal congruency of the avatar feedback, gait movements were solely driven by its temporal characteristics as participants nonconsciously attempted to synchronize their gait with their avatar's gait. Furthermore, gait synchronization was suppressed by cognitive loading across all tested delays, whereas gait agency was only affected for selective temporal delays that depended on the participant's step cycle. Extending data from upper limb agency and auditory gait agency, our results are compatible with effector-independent and supramodal control of agency; they show that both mechanisms are dissociated from automated sensorimotor control and that cognitive loading further enhances this dissociation.

Visuo-tactile integration and body ownership during self-generated action.

Although there is increasing knowledge about how visual and tactile cues from the hands are integrated, little is known about how self-generated hand movements affect such multisensory integration. Visuo-tactile integration often occurs under highly dynamic conditions requiring sensorimotor updating. Here, we quantified visuo-tactile integration by measuring cross-modal congruency effects (CCEs) in different bimanual hand movement conditions with the use of a robotic platform. We found that classical CCEs also occurred during bimanual self-generated hand movements, and that such movements lowered the magnitude of visuo-tactile CCEs as compared to static conditions. Visuo-tactile integration, body ownership and the sense of agency were decreased by adding a temporal visuo-motor delay between hand movements and visual feedback. These data show that visual stimuli interfere less with the perception of tactile stimuli during movement than during static conditions, especially when decoupled from predictive motor information. The results suggest that current models of visuo-tactile integration need to be extended to account for multisensory integration in dynamic conditions.

"Self pop-out": agency enhances self-recognition in visual search.

In real-life situations, we are often required to recognize our own movements among movements originating from other people. In social situations, these movements are often correlated (for example, when dancing or walking with others) adding considerable difficulty to self-recognition. Studies from visual search have shown that visual attention can selectively highlight specific features to make them more salient. Here, we used a novel visual search task employing virtual reality and motion tracking to test whether visual attention can use efferent information to enhance self-recognition of one's movements among four or six moving avatars. Active movements compared to passive movements allowed faster recognition of the avatar moving like the subject. Critically, search slopes were flat for the active condition but increased for passive movements, suggesting efficient search for active movements. In a second experiment, we tested the effects of using the participants' own movements temporally delayed as distractors in a self-recognition discrimination task. We replicated the results of the first experiment with more rapid self-recognition during active trials. Importantly, temporally delayed distractors increased reaction times despite being more perceptually different than the spatial distractors. The findings demonstrate the importance of agency in self-recognition and self-other discrimination from movement in social settings.

The neuropsychology and neuroanatomy of reduplicative paramnesia.

Reduplicative paramnesia refers to the delusional belief that there are identical places in different locations. In this case-control study we investigated the clinical, phenomenological, neuropsychological and neuroanatomical data of eleven patients with reduplicative paramnesia and compared them against a control group of eleven patients with severe spatial disorientation without signs of reduplicative paramnesia. We show that most patients with reduplicative paramnesia report that a current place is reduplicated and/or relocated to an other familiar place. Patients with reduplicative paramnesia show a higher prevalence of deficits in the executive functions compared to the control patients, while mnestic and visuo-spatial deficits were both frequent in patients with reduplicative paramnesia and the control group. Patients with reduplicative paramnesia mostly suffer from right hemispheric lesions with a maximal overlap in the dorsolateral prefrontal cortex. Using lesion network mapping we show that lesions causing reduplicative paramnesia are connected to bilateral anterior insula and the right cingulate cortex. We argue that patients with reduplicative paramnesia fail to integrate the actual context with visuo-spatial memories and personal relevant emotional information due to a disruption of the neural network within the anterior temporal lobe, the cingulate cortex and the anterior insula. Also patients with reduplicative paramnesia are not able to resolve this conflict due to the lesion of the dorsolateral prefrontal cortex and executive dysfunction.

I feel who I see: visual body identity affects visual-tactile integration in peripersonal space.

Recent studies have shown the importance of integrating multisensory information in the body representation for constituting self-consciousness. However, one idea that has received only scant attention is that our body representation is also constituted by knowledge of bodily visual characteristics (i.e. 'what I look like'). Here in two experiments we used a full body crossmodal congruency task in which visual distractors were presented on a photograph of the participant, another person, who was either familiar or unfamiliar, or an object. Results revealed that during the 'self-condition' CCEs were enhanced compared to the 'other condition'. The CCE was similar for unfamiliar and familiar others. CCEs for the object condition were significantly smaller. The results show that presentation of an irrelevant image of a body affects multimodal processing and that the effect is enhanced when that image is of the self. The results hold intriguing implications for body representation in social situations.

Psychogenic amnesia and self-identity: a multimodal functional investigation.

BACKGROUND: Patients with psychogenic amnesia generally suffer from episodic memory deficits associated with an impairment of self-identity. While the first is generally attributed to limbic dysfunction, the latter might be related to posterior parietal cortex. METHODS AND RESULTS: In a patient with acute repetitive psychogenic amnesia, three different functional investigations (fMRI, electrical-neuroimaging, PET) during both resting-state and a behavioural paradigm testing 'time-travel' showed left posterior parietal activation, unlike in 12 control subjects. CONCLUSION: Impairment of self-identity and episodic memory in psychogenic amnesia may be associated with functional alterations of left posterior parietal cortex.

Carbon outclasses wood in racket paddles: Ratings of expert and intermediate tennis players.

Wooden racket paddles were modified with rubber and carbon fibre laminates and their differences tested in terms of flexural, damping, and coefficient of restitution properties. Four rackets types were designed: a wood reference, wood with rubber, carbon fibre 0°, and carbon fibre 90°. Seven expert and eight intermediate tennis players tested the rackets. To determine which of the four rackets suited the players best, we asked the players to compare the rackets two by two. After each pair tested, participants had to fill out a 4-item questionnaire in which different aspects of the rackets' performance were judged. The most preferred racket was the 0° carbon fibre racket, followed by the 90° carbon fibre racket, the wood racket and, finally, the 1-mm rubber racket. Thus, rackets with the highest stiffness, least damping, and highest coefficient of restitution were the most preferred. Interestingly, although experts and intermediate players overall judged the rackets in very similar ways according to force, vibration, and control, they were sensitive to quite different physical characteristics of the rackets.

Gender bending: auditory cues affect visual judgements of gender in biological motion displays.

The movement of an organism typically provides an observer with information in more than one sensory modality. The integration of information modalities reduces the likelihood that the observer will be confronted with a scene that is perceptually ambiguous. With that in mind, observers were presented with a series of point-light walkers each of which varied in the strength of the gender information they carried. Presenting those stimuli with auditory walking sequences containing ambiguous gender information had no effect on observers' ratings of visually perceived gender. When the visual stimuli were paired with auditory cues that were unambiguously female, observers' judgments of walker gender shifted such that ambiguous walkers were judged to look more female. To show that this is a perceptual rather than a cognitive effect, we induced visual gender after-effects with and without accompanying female auditory cues. The pairing of gender-neutral visual stimuli with unambiguous female auditory cues during adaptation elicited male after-effects. These data suggest that biological motion processing mechanisms can integrate auditory and visual cues to facilitate the extraction of higher-order features like gender. Possible neural substrates are discussed.

Augmented manipulation ability in humans with six-fingered hands.

Neurotechnology attempts to develop supernumerary limbs, but can the human brain deal with the complexity to control an extra limb and yield advantages from it? Here, we analyzed the neuromechanics and manipulation abilities of two polydactyly subjects who each possess six fingers on their hands. Anatomical MRI of the supernumerary finger (SF) revealed that it is actuated by extra muscles and nerves, and fMRI identified a distinct cortical representation of the SF. In both subjects, the SF was able to move independently from the other fingers. Polydactyly subjects were able to coordinate the SF with their other fingers for more complex movements than five fingered subjects, and so carry out with only one hand tasks normally requiring two hands. These results demonstrate that a body with significantly more degrees-of-freedom can be controlled by the human nervous system without causing motor deficits or impairments and can instead provide superior manipulation abilities.

Body ownership and embodiment: vestibular and multisensory mechanisms.

Body ownership and embodiment are two fundamental mechanisms of self-consciousness. The present article reviews neurological data about paroxysmal illusions during which body ownership and embodiment are affected differentially: autoscopic phenomena (out-of-body experience, heautoscopy, autoscopic hallucination, feeling-of-a-presence) and the room tilt illusion. We suggest that autoscopic phenomena and room tilt illusion are related to different types of failures to integrate body-related information (vestibular, proprioceptive and tactile cues) in addition to a mismatch between vestibular and visual references. In these patients, altered body ownership and embodiment has been shown to occur due to pathological activity at the temporoparietal junction and other vestibular-related areas arguing for a key importance of vestibular processing. We also review the possibilities of manipulating body ownership and embodiment in healthy subjects through exposition to weightlessness as well as caloric and galvanic stimulation of the peripheral vestibular apparatus. In healthy subjects, disturbed self-processing might be related to interference of vestibular stimulation with vestibular cortex leading to disintegration of bodily information and altered body ownership and embodiment. We finally propose a differential contribution of the vestibular cortical areas to the different forms of altered body ownership and embodiment.

Distinct mechanisms of form-from-motion perception in human extrastriate cortex.

The exquisite sensitivity of the human visual system to form-from-motion (FfM) cues is well documented. However, identifying the neural correlates of this sensitivity has proven difficult, particularly determining the respective contributions of different motion areas in extrastriate visual cortex. Here we measured visual FfM perception and more elementary visual motion (VM) perception in a group of 32 patients suffering from acute posterior brain damage, and performed MRI-based lesion analysis. Our results suggest that severe FfM perception deficits without an associated deficit of VM perception are due to damage to ventral occipito-temporal cortex (VOT), whereas associated deficits of FfM and VM perception are due to damage either in proximity to area MT+/V5 or an area including lateral occipital complex (LOC) and VOT. These data suggest the existence of at least three functionally and anatomically distinct regions in human visual cortex that process FfM signals.

Auditory motion affects visual biological motion processing.

The processing of biological motion is a critical, everyday task performed with remarkable efficiency by human sensory systems. Interest in this ability has focused to a large extent on biological motion processing in the visual modality (see, for example, Cutting, J. E., Moore, C., & Morrison, R. (1988). Masking the motions of human gait. Perception and Psychophysics, 44(4), 339-347). In naturalistic settings, however, it is often the case that biological motion is defined by input to more than one sensory modality. For this reason, here in a series of experiments we investigate behavioural correlates of multisensory, in particular audiovisual, integration in the processing of biological motion cues. More specifically, using a new psychophysical paradigm we investigate the effect of suprathreshold auditory motion on perceptions of visually defined biological motion. Unlike data from previous studies investigating audiovisual integration in linear motion processing [Meyer, G. F. & Wuerger, S. M. (2001). Cross-modal integration of auditory and visual motion signals. Neuroreport, 12(11), 2557-2560; Wuerger, S. M., Hofbauer, M., & Meyer, G. F. (2003). The integration of auditory and motion signals at threshold. Perception and Psychophysics, 65(8), 1188-1196; Alais, D. & Burr, D. (2004). No direction-specific bimodal facilitation for audiovisual motion detection. Cognitive Brain Research, 19, 185-194], we report the existence of direction-selective effects: relative to control (stationary) auditory conditions, auditory motion in the same direction as the visually defined biological motion target increased its detectability, whereas auditory motion in the opposite direction had the inverse effect. Our data suggest these effects do not arise through general shifts in visuo-spatial attention, but instead are a consequence of motion-sensitive, direction-tuned integration mechanisms that are, if not unique to biological visual motion, at least not common to all types of visual motion. Based on these data and evidence from neurophysiological and neuroimaging studies we discuss the neural mechanisms likely to underlie this effect.

Functional and neural mechanisms of embodiment: importance of the vestibular system and the temporal parietal junction.

Embodiment, the sense of being localized within one's physical body, is a fundamental aspect of the self. Recent research shows that self and body processing as well as embodiment require distinct brain mechanisms. Here, we review recent clinical and neuroimaging research on multisensory perception and integration as well as mental imagery, pointing out their importance for the coding of embodiment at the temporo-parietal junction (TPJ). Special reference is given to vestibular mechanisms that are relevant for self and embodiment and to methods that interfere experimentally with normal embodiment. We conclude that multisensory and vestibular coding at the TPJ mediates humans' experience as being embodied and spatially situated, and argue that pathologies concerning the disembodied self, such as out-of-body experience or other autoscopic phenomena, are due to deficient multisensory integration at the TPJ.

Changing motor perception by sensorimotor conflicts and body ownership.

Experimentally induced sensorimotor conflicts can result in a loss of the feeling of control over a movement (sense of agency). These findings are typically interpreted in terms of a forward model in which the predicted sensory consequences of the movement are compared with the observed sensory consequences. In the present study we investigated whether a mismatch between movements and their observed sensory consequences does not only result in a reduced feeling of agency, but may affect motor perception as well. Visual feedback of participants' finger movements was manipulated using virtual reality to be anatomically congruent or incongruent to the performed movement. Participants made a motor perception judgment (i.e. which finger did you move?) or a visual perceptual judgment (i.e. which finger did you see moving?). Subjective measures of agency and body ownership were also collected. Seeing movements that were visually incongruent to the performed movement resulted in a lower accuracy for motor perception judgments, but not visual perceptual judgments. This effect was modified by rotating the virtual hand (Exp.2), but not by passively induced movements (Exp.3). Hence, sensorimotor conflicts can modulate the perception of one's motor actions, causing viewed "alien actions" to be felt as one's own.

MRI, (1)H-MRS, and functional MRI during and after prolonged nonconvulsive seizure activity.

BACKGROUND: Various structural and functional changes, such as focal edema, blood flow, and metabolism, occur in the cerebral cortex after focal status epilepticus. These changes can be assessed noninvasively by means of MRI techniques, such as fluid-attenuated inversion recovery (FLAIR), EEG-triggered functional MRI (EEG-fMRI), and proton MR spectroscopy (MRS). METHODS: The authors report on a 40-year-old patient with nonlesional partial epilepsy in the left posterior quadrant in whom these MRI techniques were applied in an active seizure focus and repeated during a follow-up of 1 year. RESULTS: FLAIR imaging taken at the time of status epilepticus showed a signal hyperintensity in the occipital region. (1)H-MRS of this cortical region showed elevated lactate, decreased N:-acetylaspartate (NAA), and elevated choline (Cho). In the same region, EEG-fMRI revealed an area of signal enhancement. After seizure control, recovery of lactate and Cho was observed, whereas the NAA level remained reduced. The structural abnormality demonstrated on FLAIR disappeared within 3 months. CONCLUSIONS: Repetitive MRI with sensitive sequences during clinically critical periods may disclose the structural correlate in a previously nonlesional epilepsy case. Corresponding to the clinical evolution, reversible and irreversible focally abnormal metabolism can be determined with (1)H-MRS, reflecting both increased neuronal activity and neuronal damage.

Internally driven vs. externally cued movement selection: a study on the timing of brain activity.

Brain imaging studies in man and single cell recordings in monkey have suggested that medial supplementary motor areas (SMA) and lateral pre-motor areas (PMA) are functionally dissociated concerning their involvement in internally driven and externally cued movements. This dichotomy, however, seems to be relative rather than absolute. Here, we searched for further evidence of relative differences and aimed to determine by what aspect of brain activity (duration, strength, or both) these might be accounted for. Event-related potentials (ERPs) were recorded while healthy, right-handed subjects selected one of three possible right hand digit movements based either on 'internal' choice or 'external' cues. The results obtained from ERP mapping suggest that movement selection evokes the same electrical brain activity patterns in terms of surface potential configurations in the same order and at the same strength independent of the selection mode. These identical configurations, however, differed in their duration. Combined with the results of a distributed source localization procedure, our data are suggestive of longer lasting activity in SMA during the 'internal' and longer lasting activity in PMA during the 'external' condition. Our results confirm previous findings in showing that SMA and PMA are distinctively involved in the two tasks and that this functional dichotomy is relative rather than absolute but indicate that such a dissociation can result from differences in duration rather than pure strength of activation.

Location of the human frontal eye field as defined by electrical cortical stimulation: anatomical, functional and electrophysiological characteristics.

Electrical cortical stimulation of the human frontal gyri and the precentral gyrus has been shown to induce eye movements and it has classically been assumed that these stimulation-induced eye movements result from electrical interference with the human homologue of the monkey frontal eye field (FEF). However, amplitude of electrical current and induced type of eye movement, which are essential for the determination of eye fields in the monkey, have not been investigated systematically in man. We applied electrical cortical stimulation in the lateral frontal cortex in six epileptic patients. Sites whose stimulation resulted in eye movements were determined with respect to gyral and sulcal patterns, Talairach coordinates and neighboring functions as found by electrical cortical stimulation. Based on this approach, a restricted location of the electrically defined FEF is proposed within a larger oculomotor region on the posterior part of the middle frontal gyrus.

Visual activity in the human frontal eye field.

Although visual information processing in the monkey frontal eye field (FEF) has been well demonstrated, the contribution of its human homologue to vision is still unknown. Here we report a study of intracranial visual evoked potentials (VEPs) recorded from the human FEF which was identified by electrical cortical stimulation. Electrical stimulations and EEG recordings were carried out via subdural grid electrodes placed over the frontal cortex in three epileptic patients. Evoked eye movements were mainly horizontal and always directed to the hemispace contralateral to the stimulation site. Intracranial VEPs showed responses predominately to stimuli in the contralateral visual field. Our findings demonstrate a close relationship between the direction of the electrically elicited eye movements and the visual stimulus location which predominantly leads to neural responses in the FEF. These findings provide evidence for the functional role of the human FEF in the analysis of visual stimuli from the contralateral visual field as well as in the generation of eye movements towards these conspicuous targets.