Contact cues are an effective substitute for the loss of vestibular function

The Lackner et al. study in 1998 clearly indicates how light touch fingertip contact improves sensory information in patients with vestibular loss. These patients suffering from a severe bilateral loss of the vestibular system had a defective labyrinthine. The preliminary studies consisted of testing the patients in a semi-tandem Romberg stance with one foot in front of the other and horizontally fifteen centimetres apart.  Fingertip contact helped reduce the sway in these patients and the normal subjects when they were standing in a heel-to-toe, tandem Romberg stance.2 

Figure 1: Tandem Romberg Stance.2 

        The results from the measure of head (MSA of Headx) and center of foot pressure displacement (CFPx) demonstrated that vestibular loss subjects were not able to stand for more than a few seconds in the dark without falling. In the vision-no touch trials, the patients fell before the trial was over. However, a light touch of the index finger stabilized the postural sway of these patients. All the subjects without vestibular function (i.e. normal subjects) exhibited increased postural sway in the dark, but it was reduced with touch. In addition, their postural sway was less than that of the patients with the dysfunction.  Furthermore, the vestibular loss subjects also showed more stability in a dark-touch condition than the normal subjects in a dark – no touch condition.2 

        Both subject groups were able to maintain the applied forces in touch condition.  However, the vestibular loss subjects applied significantly less vertical force. Thus, this article provides sufficient evidence that there is a correlation between CFPx displacement and lateral or vertical fingertip contact force.  The study demonstrates that precision contact of the index finger at mechanically non-supportive force levels is an effective substitute for labyrinthine function in subjects with vestibular loss.2 

        A recently developed technique is the artificial use of the vibrotactile feedback (VTTF) device that improves postural stability in vestibular patients. It indicates head sway to the patient via a vibrational frequency that humans are sensitive to5. This device provides information about head motion for patients with a loss of vestibular function5. The device consists of a motion sensing system placed on the lower back of the subject6. The device senses body tilt, which causes vibrations to the subject’s anterior and posterior torso via three rows of tactors7.

Figure 2: Vibrotactile Feedback Device8

According to an experiment performed by Dr. Wall et al., the use of dynamic computerized posturagraphy illustrated the vibrotactile effect on postural sway in patients with vestibular loss.7 The VTTF of body tilt significantly decreased sway.7 Wall tested subjects on an uneven surface. The tilt caused the activation of tactile vibrators on the left and right sides of the subjects. Perturbation during locomotion was also tested with or without VTTF of body tilt. These tests occurred after the subjects were trained to use the device. In both events, there was a reduction in body sway due to the use of the vibrotactile feedback relative to body tilt. It helped control the mediolateral body sway while subjects walked on uneven surfaces and during perturbations while walking.1, 8

        The latter experiment performed by Wall et al hypothesized that vibrotactile balance prosthesis helps postural stability. The experiment showed how the vibrotactile balance prosthesis increased postural stability in patients with a vestibular loss. The subjects with severe vestibular loss were able to reduce their tilt using the VTTF. This further controlled posture and substituted for the vestibular system.1, 8, 9

These devices have also been used in aviation to provide awareness to pilots. When a pilot finds himself in an unstable situation (i.e. acrobatic maneuvers), these devices help him return to a stable, level flight. Hence, the device provides navigational cues in the cockpit.10 

In 2003, Wall III and Kentala evaluated the effect of vibrotactile display upon postural stability in uni and bilateral vestibular loss patients. The subjects were divided up into two groups. Group one consisted of patients with vestibular schwannoma, who were compensating for unilateral loss. Group two patients were suffering from severe balance problems (i.e. bilateral vestibular loss). The sensory organization test (SOT) performance results demonstrated that the use of a VTTF device in Group 1 slightly increased the percentage of falls, while Group 2 illustrated a greatly reduced percentage of falls. No falls were recorded in any groups for the motor control test (MCT) runs. However, the response trajectories varied from subject to subject. The onset of perturbation and returning close to the pre-disturbance baseline rapidly increased the anterior-posterior (A/P) tilt response trajectory of the MCT. Some subjects displayed non-oscillating responses while others displayed oscillating responses. The oscillating responses were so small that they resulted in a rapid return to baseline with tactors on. To obtain an indication of the potential usefulness of the VTTF, the subjects scored the VTTF on a scale of 0 to 10. The results from Group 1 ranged from 0 to 7, while the results for Group 2 ranged from 7 to 10. The experiment concluded that VTTF information was sensed by skin sensors, which were processed by the sensorimotor control of the CNS.  This control was used in less than one second. Under both conditions, that is, distorting sensory inputs and perturbation, the VTTF reduces tilt along one axis. At the time of the experiment, the device was bulky and large. Hence, a more practical form would need to be developed for use in everyday life.7 

Figure 3: SOT Results with and without VTTF 7

A study performed in 2004 by Wall III et al illustrated how applications of the VTTF reduced sway in vestibulopathic patients. The subjects were taught to use the device in fifteen minutes, to control their body tilt. Two further experiments were done to test the patients, using the device. A comparison of with and without the balance aid in response to toes up pitch during quiet standing demonstrated increased stability of the trunk in space and center of mass, with the help of VTTF. Block trials were performed, where the same tilt velocity was repeated then the peak tilt reduced. Peak tilt responses were reduced via tilt reduction and sway feedback. Even after the device was removed, the patients were still able to maintain the progress they had made. Hence, a long-term benefit from training may be plausible.10

The next step was to administer perturbations on vestibulopathic patients to examine the effects of VTTF on tilt. No feedback was present in this case. Blocks of VTTF training trials were done next. A greater reduction in sway was observed in subjects after training on the VTTF, as opposed to subjects that underwent perturbation trials with no vibrotactile tilt feedback. Therefore, Wall proposed the application of VTTF technology as a method of rehabilitation. The vestibulopathic patients used the vibrotactile display of A/P tilt to reduce postural sway. Hence, they displayed an improvement in postural control.10

The observations reported by many of these scientists provide sufficient evidence that tactile input and artificial means of vibrotactile feedback can be used as a way to partially compensate for vestibular loss. The various experiments performed by Jeka, Lackner and other scientists provide substantial proof for partial vestibular compensation via tactile inputs. Wall III, Kentala and other scientists have evaluated the effects of vibrotactile feedback of body tilt in patients with vestibular loss. Further research may be helpful in shedding light on the present situation. These recent discoveries may lead to further advancements in the area of sensory compensation for vestibular loss. This understanding should lead to better and more practical devices in the future.

References:

1. Wall, Conrad et al. Vestibular prostheses: The Engineering and Biomedical Issues. Journal of Vestibular Research. 12, 95-113 (2002/2003).
2. Lackner, James R. et al. Precision Contact of the fingertip reduces postural sway of individuals with bilateral vestibular loss. Experimental Brain Research. 126, 459-466 (1999).
3. Jeka, John J. Light Touch Contact as a Balance Aid. Physical Therapy. 77, 476-487 (1997).
4. Lackner, James R. and Dizio, Paul. Vestibular, Proprioceptive, and Haptic Contributions to Spatial Orientation. Annual Review of Psychology. 56, 117-147 (2005).
5. Wall, Conrad et al. Balance Prosthesis Based on Micromechanical Sensors Using Vibrotactile Feedback of Tilt. IEEE Transactions on Biomedical Engineering. 48, 1153-1161 (2001).
6. Sienko, K.H. et al. A Human Performance Crossover Model for Balance Control with Vibrotactile Feedback. Journal of Vestibualr Research. 14, 126 (2004).
7. Wall, Conrad. Control of Sway Using Vibrotactile Feedback of Body Tilt in Patients with Moderate and Severe Postural Control Deficits. Journal of Vestibular Research. 15, 313-325 (2005).
8. Wall, Conrad and Weinberg, Marc S. Balance Prostheses for Postural Control. IEEE Engineering in Medicine and Biology Magazine. 84-90 (2003).
9. Wall, Conrad. Vibrotactile Sensory Substitution Approach to Restoration of Balance: Transition for Standing to Walking. Journal of Vestibular Research. 14, 77-97 (2004).
10. Wall, Conrad et al. Applications of Vibrotactile Display of Body Tilt for Rehabilitation. Proceedings of the 26th Annual International Conference of the IEEE EMBS. 4763-4765 (2004).