Tactile Feedback for Motion Guidance
In the past, researchers have used two dimensional tactor arrays as a means of displaying the spatial distribution of information in a stationary location. The visual analog to such a system would be a two dimensional rasterized image with a fixed pixel density. A great deal of information can be conveyed through the traditional approach by presenting different tactile patterns or saltatory cues (often on the torso and more recently on the tongue), particularly in sensory substitution for visually impaired individuals . Unfortunately, the learning curve is steep and processing is slow because the skin is not naturally adapted to receive this type of information.
More recently, some researchers have used skin stretch as a means of communicating information to subjects. In contrast to the spatiotemporal stimulation discussed previously, these skin strech devices that apply shear forces on the skin can be understood as tactile vectors, composed of a magnitude and a direction. In  lateral skin strech on the fingertip was used to provide two d.o.f. directional cues to subjects. In  rotational skin strech was used to provide feedback regarding the movement of a virtual object. Though promising, these devices presently are bulky, requires extra effort to attach to a user, and can cause discomfort if the skin is streched beyond a threshold.
In other wearable applications, vibrotactile feedback has been used for proximity or contact detection. Bloomfield and Badler  created a tactile sleeve embedded with actuators and motion-tracking markers. Subjects reached within 3D virtual puzzles while trying to avoid collisions with the walls. They received spatial tactile collision feedback, visual feedback, both, or no feedback at all. The tactors (eccentric-mass DC motors) were driven at three different levels: off for no contact, on at fixed frequency and amplitude for medium-depth collisions; and pulsing for deep collisions. Despite this simple tactor feedback algorithm and the fact that users could not adequately distinguish the second and third types of tactile feedback, the associated human-subject experiment showed a significant collision avoidance benefit to the tactile feedback over the three other tested modalities.
Finally, several recent VR systems have capitalized on the concept of learning to imitate a virtual teacher who performs movements many times over within the context of virtual functional tasks (e.g., wiping a virtual tabletop or lifting a cup to the mouth)[ 6,9]. To facilitate movement matching, a pre-recorded trajectory of the correct movement is displayed alongside the real-time trajectory of the patient's own movement. The degree of match may also be quantified to provide augmented feedback in the form of a score or other verbal feedback. Lieberman and Breazeal developed a similar system for training subjects to mimic poses or trajectories demonstrated by an expert, using tactile feedback. The Tactile Interaction for Kinesthetic Learning (T.I.K.L)  provides vibrotactile feedback on 5 d.o.f. of the user's arm for training students to mimic motions demonstrated by an expert. Subjects wear a suit embedded with Tactaid voice coil vibrotactile actuators. Subjects try to mimic motions performed by the teacher while being tracked by a Vicon optical motion capture system. When the subject deviates from the target trajectory, vibrotactile feedback proportional to the amount of error is provided. Feedback on forearm pronation/supination and shoulder internal/external rotation is accomplished using the sensory saltation effect . A sequence of successive tactor activations is generated around the wrist at a fixed rate of 9 Hz. The sequence is either clockwise or anti-clockwise depending upon the sign of error for the twist joint. The authors report 15% gain in subject performance and 7% increase in learning with the addition of tactile feedback over subjects with only visual feedback. However this study uses an expensive Vicon motion capture system, which suffers from problems of marker occlusion and large workspace requirements, making it unsuitable for clinical environments. In  we presented a hands-on demonstration of a low cost prototype tactile feedback system for motion guidance on elbow flexion/extension and forearm pronation/supination degrees of freedom. Users held the handle of a two d.o.f. passive exoskeleton (fitted with potentiometers) and were provided tactile cues through a wrist band (embedded with shaftless eccentric mass vibration actuators). Graphical feedback was provided about the user's current forearm configuration and the desired pose on a screen.
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