Effects of co-vibrotactile stimulations around the torso on non-volitional postural responses.

The purpose of this study was to characterize the effects of co-vibrotactile stimulations around the torso on non-volitional postural responses in the absence of instructions. Four healthy young adults maintained an upright, erect posture with their eyes closed in two different stance conditions: normal and Romberg stance. Six vibrotactile transducers (tactors) were placed on the skin over the right and left external oblique, internal oblique, and erector spinae muscles. Either a combination of vibration at two locations or all locations around the torso was applied for 5 s during each experimental trial. Regardless of stance condition, vibration applied concurrently over the right and left internal oblique muscle locations and the right and left erector spinae muscle locations induced a postural shift in the anterior and posterior directions, respectively. For these two stimulation conditions, the root-mean-square of sway in the anterior-posterior direction was significantly greater during vibration than before or after stimulation. However, simultaneous activation of all tactors, a combination of right internal oblique and right erector spinae locations, and a combination of left internal oblique and left erector spinae locations did not produce significant directional postural shifts or increases in sway, regardless of the stance condition. These findings suggest that stimuli combinations contribute to a vector summation of individual postural responses described in our previous work and that they could be leveraged in balance-related applications of sensory augmentation vibrotactile displays.