First demonstration of velocity selective recording from the pig vagus using a nerve cuff shows respiration afferents.

Neural interfaces have great potential to treat disease and disability by modulating the electrical signals within the nervous system. However, whilst neural stimulation is a well-established technique, current neural interfaces are limited by poor recording ability. Low signal amplitudes necessitate the use of highly invasive techniques that divide or penetrate the nerve, and as such are unsuitable for chronic implantation. In this paper, we present the first application of the velocity selective recording technique to the detection of respiration activity in the vagus nerve, which is involved with treatments for epilepsy, depression, and rheumatoid arthritis. Further, we show this using a chronically implantable interface that does not divide the nerve. We also validate our recording setup using electrical stimulation and we present an analysis of the recorded signal amplitudes. The recording interface was formed from a cuff containing ten electrodes implanted around the intact right vagus nerve of a Danish Landrace pig. Nine differential amplifiers were connected to adjacent electrodes, and the resulting signals were processed to discriminate neural activity based on conduction velocity. Despite the average single channel signal-to-noise ratio of - 5.8 dB, it was possible to observe distinct action potentials travelling in both directions along the nerve. Further, contrary to expectation given the low signal-to-noise ratio, we have shown that it was possible to identify afferent neural activity that encoded respiration. The significance of this is the demonstration of a chronically implantable method for neural recording, a result that will transform the capabilities of future neuroprostheses.

A new method for spike extraction using velocity selective recording demonstrated with physiological ENG in Rat.

BACKGROUND: This paper describes a series of experiments designed to verify a new method of electroneurogram (ENG) recording that enables the rate of neural firing within prescribed bands of propagation velocity to be determined in real time. Velocity selective recording (VSR) has been proposed as a solution to the problem of increasing the information available from an implantable neural interface (typically with electrodes in circumferential nerve cuffs) and has been successful in transforming compound action potentials into the velocity domain. NEW METHOD: The new method extends VSR to naturally-evoked (physiological) ENG in which the rate of neural firing at particular velocities is required in addition to a knowledge of the velocities present in the recording. RESULTS: The experiments, carried out in rats required individual spikes to be distinct and non-overlapping, which could be achieved by a microchannel or small-bore cuff. In these experiments, strands of rat nerve were laid on ten hook electrodes in oil to demonstrate the principle. COMPARISON WITH EXISTING METHOD: The new method generates a detailed overview of the firing rates of neurons based on their conduction velocity and direction of propagation. In addition it allows real time working in contrast to existing spike sorting methods using statistical pattern processing techniques. CONCLUSIONS: Results show that by isolating neural activity based purely on conduction velocity it was possible to determine the onset of direct cutaneous stimulation of the L5 dermatome.

FES cycling.

Spinal cord injury (SCI) leads to a partial or complete disruption of motor, sensory, and autonomic nerve pathways below the level of the lesion. In paraplegic patients, functional electrical stimulation (FES) was originally widely considered as a means to restore walking function but this was proved technically very difficult because of the numerous degrees of freedom involved in walking. FES cycling was developed for people with SCI and has the advantages that cycling can be maintained for reasonably long periods in trained muscles and the risk of falls is low. In the article, we review research findings relevant to the successful application of FES cycling including the effects on muscle size, strength and function, and the cardiovascular and bone changes. We also describe important practical considerations in FES cycling regarding the application of surface electrodes, training and setting up the stimulator limitations, implanted stimulators and FES cycling including FES cycling in groups and other FES exercises such as FES rowing.

Implantable telemeter for long-term electroneurographic recordings in animals and humans.

A system is described that amplifies an electroneurographic signal (ENG) from a tripolar electrode nerve cuff and transmits it from the implanted amplifier to an external drive box. The output was raw ENG, bandpass filtered from 800 to 8000 Hz. The implant was powered by radio-frequency induction and operated for coil-to-coil separations up to 30 mm. The testing and performance of the system is described. The input-referred noise was never more than 1 microV RMS, and, at some positions of the radio-frequency field, was 0.7 microV, close to the expected value for the amplifier used. The common-mode rejection ratio (CMRR) depended on the impedance imbalance from the cuff and the length of input cable. Devices with a short cable and low source impedance had CMRR of 84 dB, but, with 31 cm of cable and a real cuff, the CMRR fell to 66 dB. Recovery from a stimulus artifact took 5 ms. The responses of the cuff to external potential gradients and to common-mode signals are described theoretically or by simulation. The devices are available for use in neuroprosthetic or neurophysiological research.

Recruitment by motor nerve root stimulators: significance for implant design.

Three paraplegics have been implanted with stimulators of the lumbar anterior roots. Twelve roots were trapped in slots, each with three electrodes, a central cathode and two anodes, but the anodes in all the slots were connected together to reduce the number of wires. Cross-talk between roots was observed at lower levels than expected. Cross-talk was assessed from the ratio of the root's threshold to the threshold of the contralateral response (expected ratio: 72). Two hypothetical reasons for this low ratio were: that the cathode current was not equally shared by the anodes; or that the contralateral responses were reflex. Experiments showed that neither explanation was valid. The ratio of the contralateral to ipsilateral threshold for individual slots (K(1)) was sometimes low because the ipsilateral threshold was high. By taking the ratio of the lowest contralateral response to lowest ipsilateral response, for all roots in each subject (K(2)), the ratio should approach the theoretical value. However, for the two subjects with small slots, it was 7.9 and 15.3, much less than 72, suggesting that the original theory was incorrect. Approximate calculations of the activation function suggest that the reason may be that roots which run close to a slot, but not through it, may pass through a virtual anode region outside the ends of the slots, and that anodal break stimulation in those regions causes the cross-talk. Our estimate is that this cross-talk would be expected to occur at intensities above 5.3 times the cathodal threshold. If the roots are stimulated in pairs, below the levels of cross-talk, experimental results show that the moments obtained in response are additive to within 5%.

A reconnectable multiway implantable connector.

A well-tried plug-and-socket connector system designed for connecting multichannel implanted cables was adapted so as to allow disconnection and reconnection during surgery. Five different sealing techniques were tested in vitro, and it was found that only one of them had the required qualities of high leakage path impedance (taken as more than one megaohm for the worst sample) after three months of saline soak, together with demountability under surgical conditions. The system has subsequently been successfully implemented in a patient in whom reconnection was required two years after implantation.

Implantable FES Stimulation Systems: What is Needed?

Since their initial development, the performance gains in functional electrical stimulation (FES) systems have been modest. Conceptually, the replacement of normal neural function by artificial electronic systems is attractive, considering the continued technologic advancements in electronics, communication, and control. It is likely that efficacious FES systems will require complete implantation and activation of large numbers of motor units. One approach is to develop a neural interface that has a one-to-one relationship between stimulating electrodes and lower motor neurons. While technology may offer solutions to the design of miniaturized implantable stimulators, the high-density neural interface remains more elusive. During the past 20 years, research in the stimulation of peripheral motor systems has been primarily constrained by progress in two areas of research: strategies for the control of paralyzed muscle and sophistication of implantable stimulation systems. Often, a debate concerning which of these two areas is a "critical path" element yields no strategic ideas. It has been stated that a need must be demonstrated for a specific number of electrode channels before it is warranted to invest effort into the engineering of implantable systems that are capable of driving large numbers of electrodes. Indeed it is a logical approach to problem solving that the need should drive the development of function. Even study sections, in review of FES grant applications, often resort to this logic. In our opinion, when applied to FES, this argument is often fallacious and ignores the reality that research frequently requires that a threshold of experimental methodology be reached before any meaningful work can be accomplished. Practical trials of stimulation control strategies, long-term patient acceptance, and achievable function for FES systems cannot begin without the capability of stimulation. And, in order to determine whether or not stimulating large numbers of muscle groups can lead to more natural control of movement, suitable stimulation hardware must first exist, and be reliable. In the specific case of lower extremity FES research, it is likely that without a quantum advance in technologic capabilities, the practical utility of FES systems will continue to only be marginally close to normal function. To reach the level of being considered a routine treatment for spinal cord injury, FES systems should be able to offer improved functionality, ease of use, and near-equal reliability, compared to wheelchairs. At present, no FES systems attain this combination. The functional reliability of FES systems must approach 100%. As a trivial example, consider that for a standing, or walking, system, the perspectives of bioengineers and users may be quite different. While an engineer might be pleased to design a system that functions, as intended, 99% of the time, if a user falls down 1 time out of every 100, this is likely to be unacceptable. The minimal threshold of functional utility for FES systems is unclear, and will not be addressed here. Rather, we consider the issues of what features and capabilities are desirable for next generation implantable systems, and to what degree these desires approach engineering feasibility.

Technical note. Estimated electrode operating conditions of the first London Mk V implanted stimulator.

The electrodes of an implanted stimulator must be operated in such a way that the task of converting an electron current in metal to an ion current in tissue is achieved without release of noxious chemical species into the tissue, and without significant corrosion of the electrode metal. Both these effects depend on the charge density at which the electrodes are operated. In bringing a newly designed multichannel stimulator into service, the charge densities can only be known when the operating stimulus strengths have been determined. Even then, unless the stimulator is arranged to telemeter out what it is doing, any charge-density figures obtained have the character of estimates, rather than measurements. The first London Mk. V stimulator is used to enable a paraplegic woman to stand up. This note provides estimates of the average charge density and corrosion rate for the Pt--Ir electrodes used. These fall within limits which we believe to be generally accepted as safe.

Selecting candidates for a lower limb stimulator implant programme: a patient-centred method.

OBJECTIVE: To develop an effective selection procedure for lower limb functional neurostimulation (LLFNS) for standing in paraplegia. DESIGN: The selection procedure and exclusion criteria were based on the previous experience for two clinical centres with experience of LLFNS. SETTING: Two Regional Spinal Injuries units in southern England. SUBJECTS: 254 fully rehabilitated paraplegics living in the community. INTERVENTION: Patients were invited to participate in the programme, and if suitable to subject themselves to a rigorous staged selection procedure from which they could withdraw at any time. OUTCOME MEASURE: Functionally successful home standing using closed-loop surface electrical stimulation. RESULTS: 57/254 patients were suitable on paper and were accessible. 19 of these (CI = 10-28) were interested in the project and attended one of the spinal centres for details. Twelve (CI = 5-19) of these fulfilled the selection criteria and started on the training programme; and 10 of them completed the muscle training programme successfully. Seven patients (CI = 2-12) achieved closed-loop standing in the laboratory and four patients (CI = 1-8) did so at home.

Apparatus and methods for studying artificial feedback-control of the plantarflexors in paraplegics without interference from the brain.

Apparatus has been built to explore the practical feasibility of using automatic control with electrical stimulation of paralysed legs to restore function. The experiments are performed with paraplegics with the aim of achieving a realistic postural task: to see whether the body may be maintained upright by stimulation of the plantarflexors when the other joints are braced. Significantly, the intact upper body, under natural control of the brain, cannot interfere with the automatic control. The "Wobbler" apparatus allows measurement of the ankle muscle properties in isometric conditions or in sinusoidal motion. Using the biomechanical properties of the body, which are also measured, controllers for stabilising the body can be designed. Controllers can be dynamically tested, imitating anterior-posterior sway, while the body is held upright, before "actual standing" is attempted.

FES standing: control by handle reactions of leg muscle stimulation (CHRELMS).

Many paraplegics can stand using functional electrical stimulation (FES) of their extensor muscles without any modulation of the stimulus intensity by feedback. This is possible because, using handles and the intact upper-body muscles, the position of the pelvis can be controlled and this determines the positions of the two legs. Nevertheless, it is highly desirable that some practical method of modulating the stimulus intensities is used to reduce muscle fatigue and improve posture. We propose that the goal of the controller should be the minimization of the handle reaction forces by referring these forces to the equivalent leg joint moments and, as far as possible, making corresponding changes to the stimulation of the leg muscles. If force sensors are used only at the handles, the joint moments in the two legs are indeterminate. We hypothesize that in some patients acceptable results may still be obtained if we treat the two legs as one. This method will control standing up, standing, and sitting down; the user will be able to "feel" that his leg muscles are tiring and will be able to "posture switch" without any explicit instruction to the controller to change mode.