How touch rewires our sense of self—and what goes wrong in psychosis

The experience of my body and my actions as belonging to me is one of the most fundamental experiences of the self. While this may seem intuitive, it requires both the recognition of a self, and a self-other distinction, separating the self from the surrounding world. How this self-other distinction is influenced by touch and its properties is unclear. It is also unclear how the bodily self-other distinction influences higher-order self-aspects. This thesis first explored how self-touch perception and processing are influenced by touch characteristics. Then, it examined the relationship between the bodily and higher order self, using ketamine as a pharmacological model, and psychosis as a clinical model of multifactorial alterations in selfprocesses. Paper I explored social compared to self-touch in healthy volunteers, and found affective touch preferences to depend both on sensory, bottom-up processes, such as velocity and movements, and top-down processes, particularly agency. During unpredictable sensations, bottom-up processes were more influential. By contrast, a sense of agency attenuated role of bottom-up sensory signals. Paper II and III showed self-touch to be significantly different from simultaneous touching and being touched on behavioural, physiological, and neural levels. Paper III showed substantial overlap in neural processing between tactile self-other distinction and higher-order self-referential thinking. On the behavioural level, Paper III also found pleasantness preferences for social touch to be related to the self-concept. In Paper IV, a transient alteration in self-experience was induced using ketamine in a cross-over within subject design. A reduced self-other distinction was found during ketamine administration, as well as a hyperconnectivity between sensory-integration and processing areas. The altered self-other distinction was related to interoceptive changes following ketamine, while the hyperconnectivity was associated with the subjective dissociative experience. Paper V found patients with psychosis to have altered tactile self-other distinction compared to matched controls. Patients displayed elevated neural response to touch in general, but different modulations in spinal cord signalling for self- and other-touch. The current work showed that the self-other distinction is both dependent on bottom-up sensory experiences with associated lower-level sensorimotor predictions, and higher-order top-down modulation. Self-other distinction was related to the higher-order self. Altered self-experiences, both transient and chronic, were associated with alterations in self-other distinction. Understanding how these bodily and higher-order processes interact in shaping the experience of self is crucial for understanding the role of touch in social interaction and the pathophysiology of aspects in disorders where the boundaries of the self are less pronounced, such as psychosis.