To develop and maintain the long-lasting social relationships crucial for our wellbeing, people must be able not only to identify and connect with others, but also to differentiate their own thoughts and feelings from those of other people. This process of distinguishing self from other (self-other distinction) is important, for example, when sharing the affective state of another person- where emotions of the other influence those of the self (e.g. your sadness makes me feel sad) - but also when responding to the person in distress, where mental representations of self and other must be kept separate (e.g. it is you who is in pain, not me; therefore I will try to comfort you). If the ability to distinguish self from other is impaired, this can result in personal distress, which in turn can hinder the ability to respond empathically to the person in need. The experiments in this proposal seek to gain a better understanding of the processes involved in self-other distinction within social interaction and will employ a range of behavioural and social neuroscience methods in three convergent streams of research. The first one will assess the empathic abilities of individuals with autism spectrum disorders (ASD) and mirror-touch synaesthesia (MTS; individuals who experience touch on their own bodies when observing another being touched), in the context of both sharing the emotional state of others and providing an appropriate empathic response to the person in need. The results from these studies will provide a better understanding of the level of social impairment in both ASD and MTS and could potentially provide empirical support to the view that individuals with autism do not lack empathy per se, but that instead, impaired self-other distinction might prevent them from responding empathically to others. Furthermore, these findings could also shed light on how self-other processes typically operate. The second stream will use non-invasive brain stimulation to enhance self-other processing in both atypical groups (MTS and ASD) and in healthy individuals. Finally, the third stream will combine the same brain stimulation technique with neuroimaging in order to gain a better understanding of how brain stimulation affects the connectivity patterns between different brain regions involved in self-other processing. The findings from Streams 2 and 3 will provide a better insight into the underlying cognitive and neural mechanisms supporting a range of social abilities including empathising with others, imitating another's action, controlling the tendency to imitate others and distinguishing between our own face and that of another person. This proposal affords a unique opportunity to understanding key processes underlying our ability to interact with others. Although most of us take this ability for granted, little is known about how precisely this happens. Furthermore, for individuals with known social impairments such as those with ASD, difficulties in controlling self-other representations in different social contexts can have severe impacts in their personal, family and professional lives. Therefore, the questions addressed by these experiments are important for both advancing scientific theories of social interaction and for devising future therapeutic interventions in clinical populations where self-other processes have broken down.