Forearm orientation guidance with a vibrotactile feedback bracelet: On the directionality of tactile motor communication

User-teacher interaction during the learning and the execution of motor tasks requires the employment of various sensory channels, of which the tactile is one of the most natural and effective. In this paper we present a wearable robotic teacher for predefined motor tasks, consisting of a localization system and a wearable stimulation unit. This unit embeds four vibrotactile stimulators which are activated in order to provide the user with a feedback about the movement direction of the forearm in the cartesian space. Stimulators were chosen in order to maximize tactile sensitivity and spatial resolution. Tactile interface performances in guiding 2 DOF forearm movements were comparatively evaluated with two different sensory modalities: visual and visuotactile, by using a Virtual Reality (VR) rendering of the motor task. The comparison among sensory modalities was based on two movement indexes ad hoc defined: positioning accuracy and directionality of motor communication. The experimental tests have shown that the system described hereafter is a valuable tool for human motor motion guidance, allowing a successful and useful weighting of concurrent sensory inputs without providing relevant sensory interferences. Compared to visually-guided trajectories, positioning accuracy was improved in visuotactile-guided trajectories. The comparative analysis of the directionality index in all sensory modalities suggests that increasing the number of stimulators could improve the directionality of tactile motor communication.