What is the role of the haptic generator in motor and cognitive performance modification using visual-haptic interfaces?

Updated: Aug 20, 2019
  • Author: Morris Steffin, MD; Chief Editor: Jonathan P Miller, MD  more...
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The counterforce presented to a body part (in this case, the wrist) at any instant can be represented by a vector whose characteristics must be determined by the constraints of the spatial domain and the conditions for movement stability. The computational system provides a value for each ROI in the force corridor region proportional to the level of encroachment by a body part (eg, wrist, fingers) into the corridor limit zone delineated by that ROI.

The generated values for each ROI can be incorporated into a transfer matrix to determine the counterforce vector components. The encroachment matrix values must be processed to generate the specific force components. To continue the example of the reaching arm, application of force by a transducer at a single point on the upper extremity, such as the wrist, is assumed for simplicity.

Consider a haptic device with 3° of freedom output; that is, the force takes the form of a vector, F = F[x(D,t),y(D,t),z(D,t)], in which x, y, and z are functions of the spatial domain matrix, D, and time, t. By formulating the force transfer characteristic in this way, the haptic generator can produce a stabilizing, rather than destabilizing, corrective output to the patient. Bioengineering concepts and principles involved in the construction of such a force vector from spatial data have been described. Implementation of the computational subroutines is proceeding in the author's laboratory. [9, 10]

The application of appropriate counterforce can appreciably decrease tremor and inaccuracy of movement in a patient with cerebellar deficit, as indicated in the images below, the latter showing the complete epoch.

Patient with cerebellar tremor with suitable count Patient with cerebellar tremor with suitable counterforce. Force corridor is represented by 3 regions of interest (ROIs) as corridor limits.
Patient with cerebellar tremor with suitable count Patient with cerebellar tremor with suitable counterforce. Force corridor is represented by 3 regions of interest (ROIs) as corridor limits. Target (ie, glass) is grasped successfully.

In this case, a stabilizing force was applied as a preliminary test of the idea. Note the markedly decreased perturbation in trajectory demonstrated by the much flatter curves in the encroachment graphs of the images above than in those of the images farther above.

Application of such a counterforce can be achieved by tethering a haptic device of 3° of freedom directly to the wrist. [9, 10] This general approach also appears to be effective in improving movement accuracy in certain cases of spasticity.

This visual-to-haptic transfer approach has several advantages. Because the functional spatial domain is constructed from the patient's videospace, the acquisition technology for the spatial domain data is primarily a function of software engineering. This reduces the overall complexity of the hardware for integrating electromagnetic or multiple infrared detectors into the patient's environment to achieve this result. Likewise, the transduction to force output, at least for the paradigmatic case outlined here, involves relatively simple interaction between the computer and the force generator. The goal of such an approach is construction of a practical instrument that would be available in a typical patient environment. By extension, finer movements (eg, of the fingers) ultimately may be incorporated into the approach using this and other stimulation modalities.

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