I am pleased to have a chance to present part of my recent work, which I have conducted at the Laboratory of Cognitive Neuroscience at EPFL. The main focus of my research is how the pre-reflective sense of self emerges from multimodal bodily signals.
The sense of self is an important human feature, giving us a sense of identity, unity, and continuity. It enables us to distinguish ourselves from others and to relate to the world that surrounds us. Understanding what the sense of self is, and how it is constructed, is not only relevant to satisfy scientific and philosophical curiosity, but even more so to comprehend several psychiatric and neurological conditions where the sense of self is severely altered.
The importance of multisensory and sensorimotor body information for the scientific study of consciousness has been widely recognized by cognitive neuroscience in the last decade. It has been suggested that the fundamental, pre-reflective form of self-consciousness, i.e. the immediate experience of being a subject of experience, is grounded in sensory and motor processing [1], [2]. This approach investigates the self by studying how we perceive our body and targets the brain mechanisms that process bodily information, i.e. bodily self-consciousness [1]–[6].
Bodily self-consciousness is defined as the pre-reflective, immediate awareness to be the subject, the “I” of experience, to reside in a body, and to have a control over that body [1], [3]. Experimental research has focused on its main components: the embodiment and the sense of agency [1], [2]. The fundamental aspects of embodiment are the sense of owning a body and self-identification with that body (body ownership), the sense of being located within the physical boundaries of that body (self-location) and perception of the world from the visuo-spatial perspective of that body (first person perspective) [1], [3]. The sense of agency is the sense of being the author of generated actions or thoughts, and includes the feeling of being in control of one’s own body movements and thoughts [1], [2], [7]–[10]. While the sense of embodiment has been recognized to crucially depend on the accurate integration of body-related multimodal sensory information (vision, touch, sound, proprioception, vestibular signals, visceral signals) [3], [11], [12], the sense of agency involves a strong efferent component as it depends on integration of centrally generated motor commands for voluntary actions and sensory consequences of those actions[7], [13]–[15].
As the body is normally perceived to be always there and is usually inseparable from the mind [16], it has been difficult to manipulate experimentally the components of bodily self-consciousness described above until recent years. However, with the technological advances of video, virtual reality (VR) and robotics, cognitive neuroscience has developed powerful approaches to investigate separate components of bodily self-consciousness by inducing “bodily illusions”, through on-line presentation of conflicting sensory information regarding one’s own body.
For example, in the full-body illusion (Lenggenhager et al., 2007) the participant wears a VR headset, onto which a real-time recording of a video camera, positioned behind the participant, is displayed. At the same time, the experimenter touches the participant’s back with a stick. Thus, the participant sees through the VR headset that her/his own body is being touched in front of oneself, while simultaneously feeling touched on her/his physical back. Thus, there is a spatial discrepancy between what it is seen and what is felt, i.e. a visuo-tactile conflict. When the temporal congruence between the multisensory stimuli is maintained, this spatial visuo-tactile conflict is usually resolved in favour of visual information, which biases the process of multisensory integration [17]. The resolution of the visuo-tactile conflict results in the subjective experience of ownership for the virtual body, as well as in the biased perception of self-location, i.e. participants perceive being located outside the borders of their physical body and closer to the virtual body. The dissociation between one’s self and the physical body is abolished when the visuo-tactile cues are not temporally correlated (i.e. when there is a sufficient delay between what is seen and what is felt). In this body illusion paradigm (as well as in several other body illusion paradigms, such as the rubber hand illusion [18], out-of-body illusion [11], and enfacement illusion [19]), the participant is a passive receiver of tactile stimulation. However, the integration of motor (efferent) signals with the sensory (afferent) feedback, as in the case of active self-touch, plays an important role in the bodily self-consciousness. Self-touch represents a particular form of self-produced action, where the body part administrating tactile stimulation can also be, at the same time, the body part receiving the tactile input. Thus, during self-touch the body is the agent of the action, as well as the object of the performed action. During active self-touch motor signals are integrated with multisensory feedback (simultaneous tactile cues from two different body surfaces and proprioceptive signals) and, as such, self-touch represents a unique interaction between the sense of agency and the sense of body ownership. It has been argued that self-touch importantly contributes to the generation and restoration of body representation [20].
To investigate the role of self-touch in the body illusions, we designed an experimental setup to manipulate sensorimotor integration, where the participant could administer tactile stimulation to and by himself, while the temporal congruency between his own movements and received tactile feedback is controlled by the experimenter (for the details of the study, see [21]). This was possible through a custom-built robotic master-slave system [22]. Blindfolded participants moved the master device in front of them, while trajectories and force of their movements were sent to the slave robot, which applied tactile stimuli in real time to the participant’s backs. Participants were instructed to move the robot for 3 minutes in the synchronous mode, where their arm movements were temporally matched with the touch applied by the robot, or in the asynchronous mode, where the touch was delayed by 500 milliseconds. During synchronous stimulation, participants experienced the sensation of touching themselves (i.e. self-touch), despite the spatial conflict (they were extending their arms in front, while they felt the touch on their backs). Synchronous stimulation was also associated with a drift in self-location towards the front position, as measured by an implicit self-location task. These results extend previous findings using the body illusion or rubber hand illusion paradigms, induced by multisensory conflicts.
However, a very important effect was observed in the asynchronous condition. Participants reported weaker sensation of self-touch and stronger experience of being touched by someone else. Thus, their sense of agency for the tactile stimulation decreased. Participants also reported strong feelings that another person was standing behind them, although no one was there (feeling of a presence – FoP). The feeling of a presence was also associated with a backward drift in perceived self-location as measured by the implicit task. In order to obtain an objective indicator of FoP we have designed a behavioural task, namely a person numerosity task. For this experiment, the participant received the following instructions: they would operate the robot, while any of the four experimenters in the room could be present at any time in the part of the room close to him. Blindfolded and sound-masked, the participant had to judge how many people were present in their part of the room, while no person was actually present at any time. As the study showed, participants made elevated numerosity judgments when they operated the robotic system in the asynchronous mode, i.e. when a delay was inserted between the performed movements and received tactile feedback. Thus, although in reality nobody was ever present next to them, they nevertheless judged there are more people near them when they were operating the robot in the asynchronous mode. We explain our findings in the framework of predictive models [23]–[25], and argue that alterations in the congruency between sensorimotor predictions and feedback signals - due to brain lesions or appropriate experimental manipulations - cause the misperception of the source and identity of one’s own sensorimotor signals, resulting in the feeling of a presence of another agent. These findings are relevant for understanding schizophrenia, where abnormal integration of sensorimotor signals have been associated with hallucinatory and delusional symptoms, such as the delusions of control and auditory hallucinations (Mellor, 1970; Schneider, 1959). Moreover, this work extends existing knowledge on the sense of body ownership and the sense of agency by demonstrating that afferent and efferent signals (and their interaction) play an essential role in the basic sense of self. We demonstrate that sensorimotor body representations play an important role in the construction of boundaries between self and other and as such also affect social perception.
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