Sensory feedback in upper limb prosthetics

One of the challenges facing prosthetic designers and engineers is to restore the missing sensory function inherit to hand amputation. Several different techniques can be employed to provide amputees with sensory feedback: sensory substitution methods where the recorded stimulus is not only transferred to the amputee, but also translated to a different modality (modality-matched feedback), which transfers the stimulus without translation and direct neural stimulation, which interacts directly with peripheral afferent nerves. This paper presents an overview of the principal works and devices employed to provide upper limb amputees with sensory feedback. The focus is on sensory substitution and modality matched feedback; the principal features, advantages and disadvantages of the different methods are presented.

Optimization of single electrode tactile codes

The effects of a frequency modulated electrocutaneous signal's (code's) characteristics on the interpretability of the signal were investigated using an electrocutaneous tracking approach. The characteristics investigated include the functional relationship (exponential and hybrid) between an informational signal and the stimulation frequency, the range of stimulation (2-50 Hz and 2-100 Hz), and the impact of pulse width compensation on a code's efficacy. The interpretability of six different single bipolar electrode codes was examined by 30 subjects using a balanced incomplete block experimental design. Codes with exponentially shaped transfer functions resulted in generally lower electrocutaneous tracking errors than codes utilizing hybrid-shaped transfer functions. Hybrid codes had a transfer function that was linear in the lower frequency range and exponential in the higher frequency range. Codes with a 2-100 Hz frequency range were interpreted better than codes with a 2-50 Hz frequency range. The use of pulse width compensation to maintain a more even level of stimulation intensity had a slightly negative effect on the subjects' abilities to cutaneously track the information signal.

Cognitive feedback for use with FES upper extremity neuroprostheses

This paper describes the development of two sensory substitutions systems that provide cognitive feedback for FES hand grasp restoration neuroprostheses. One system uses an array of five electrodes to provide machine status information and a spatially encoded representation of the command signal that a quadriplegic individual generates to achieve proportional grasp control. Only one electrode site is active at any given instant, and a second informational channel is superimposed on the spatial position channel by modulating the frequency of the stimulus pulses. The frequency modulated feedback channel signals six levels of force developed at the finger tips during prehension activities. The second sensory system is an integral part of an implanted FES system and utilizes a single subdermally placed electrode to display machine status information and a five-level frequency code for feedback of the user generated grasp control signal. The multielectrode feedback system was implemented for laboratory studies using surface mounted electrodes, although its design will ultimately incorporate subdermal electrodes to provide a highly cosmetic and unencumbering system. An evaluation of the effectiveness of grasp force and command signal feedback provided by this multielectrode system in assisting an FES hand system user to regulate grasp force during a laboratory task, showed increased consistency of performance and an economy of grasp effort between 25 and 30%. Alternative strategies for feedback information and coding algorithms are discussed.

Rhythm perception equipment for skin vibratory stimulation

The characteristics of vibrotactile stimulation and an experimental device for the transmission of rhythm information contained in music and similar sounds by means of vibrotactile stimulation are described. The actuator and input/output behavior of the device are examined. Measurement of the intensity discrimination threshold and beat frequency perception threshold is discussed. Application to music education in schools for the deaf is described, and results of a clinical test are reported.

Effects of training on human tracking of electrocutaneous signals

The effects of training on a person's ability to perceive, interpret and utilize information presented via the tactile sense were examined by using a dual-channel electrocutaneous tracking approach. The electrocutaneous code studied was linear pulse rate encoding of information. The stimulus had a pulse width of 200 microseconds and a pulse rate between 2 and 50 pulses per second. Three tracking tasks--visual tracking, one-dimensional electrocutaneous tracking and two-dimensional electrocutaneous tracking--were performed by 20 subjects during eight to nine daily training sessions. The effects of this training regimen were found to be highly significant for both electrocutaneous tracking tasks (p less than 0.00005). The overall average improvement between successive training sessions was 21.6% for one-dimensional electrocutaneous tracking and 22.9% for two-dimensional electrocutaneous tracking. Furthermore, the rate of improvement was fastest during the initial training sessions with a slower rate of improvement seen in most subjects beginning with their fifth session. The cumulative effects of training were also reflected in the lesser amounts of practice and review required by the subjects with each succeeding test session. Results from this study can be used to estimate the amount and rate of improvement that one can expect using a typical training program for tactile sensory aids.

Information transmission across the skin: high-resolution tactile sensory aids for the deaf and the blind

New techniques are described for implementing high-resolution communication interfaces to the skin, by direct electrical stimulation of the sense of touch. The psychophysical properties of these displays are summarized, and a practical application is presented: an electrotactile sensory aid for profoundly deaf children, which displays acoustic information on the skin of the abdomen, via a belt of electrotactile stimulators. The resulting tactile patterns provide cues for recognizing environmental sounds, enhance the accuracy of lipreading, and enable the child to monitor his/her own voice. The tactile patterns corresponding to sounds must be learned, much like acquiring a second language; substantial training and experience is required. Other applications are discussed, both for the rehabilitation of sensory handicaps and for tactile communication in general.

Electrocutaneous code pairs for artificial sensory communication systems

Pairs of electrocutaneous codes suitable for dual-channel sensory communication systems were compared using a dual-channel electrocutaneous tracking task. The tracking task required the test subject to dynamically respond to changes in the tactile sensation being modulated by two independent pseudorandom signals, one for each channel. The rule (or method) by which the signals changed the tactile sensations was called an electrocutaneous code. Four frequency variation codes and two intensity variation codes were paired in different combinations and then checked as to their effectiveness for sensory communications. The experimental protocol used a balanced incomplete block design which involved 24 subjects testing 3 of 8 code pairs each. Although the variance in the tracking performances between subjects was larger than the differences between the code pairs, learning rates for the various pairs were significantly different. The easiest one to learn was the Low Pulse Rate Modulation Code paired with itself. other findings included the general superiority of monophasic stimulation code pairs over biphasic stimulation code pairs, the need for placement of the two electrodes on different dermatomes in order to achieve satisfactory dual-channel communications, and the greater sensitivity to electrocutaneous stimulation of the ventral side of the forearm versus its dorsal side.