Inspired by studies of interpersonal coordinations, we assumed that unintentional synchrony is a fundamental parameter to initiate and maintain Human Machine interactions. We developed a neural model allowing a robot to synchronize its behavior depending on human movement frequency, and thus to choose this partner on the basis of synchrony detection between its own learned dynamic and the visual stimuli induced by the human. To confirm or deny our assumptions we presented here a psychological study to measure unintentional synchronization during Unidirectional and Bidirectional Human Robot Interaction using our previously proposed model for initiating the interaction and focusing the robot attention on a selected partner. The experimental results demonstrated that bidirectional intuitive interaction leading to possible unintentional synchronization is primordial to obtain natural human robot interactions using minimal cognitive load (unintentional behavior).