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The degree of spatial selectivity which can be obtained with longitudinal dot tripoles in an insulating cuff was quantified in terms of the overlap between fiber populations activated by different tripoles. Previous studies have failed to take into account the relative influences of transverse current and longitudinal current on position-selective activation, and furthermore have not controlled for the differing sensitivities of large and small nerve fibers to electrical stimuli. In this study, these factors were taken into account. Transverse current from an anode positioned opposite the stimulating cathode was found to improve spatial selectivity, and selectivity was enhanced when the ratio of transverse current to longitudinal current was increased. Large fibers were excited before small fibers, irrespective of fiber position, indicating a combination of position and size selectivity.

We have developed a system for classifying action potentials in multi-unit recordings on the basis of three feature types: amplitude/duration features, 'reduced features', and FFT coefficients. We have compared the success of classification using each feature set and found that on average about 2 features of each type are sufficient for the best attainable classification. Somewhat better classification is obtained with the use of 'reduced features'.

Intrafascicular electrodes have been implanted in peripheral nerves of cats and have recorded neural activity from a small population of nerve fibers for as long as six months, suggesting that these electrodes can be used on a chronic basis to acquire information about limb position, muscle force and skin contact. Preliminary results suggest that the activity of a small population of nerve fibers can be reliably monitored on a long-term basis using intrafascicular electrodes. They appear to cause little or no permanent damage and so may be suitable for use in an functional neuromuscular stimulation feedback system. >

Threshold level, sinusoidal stimuli are used to elicit phase-locked activity from cutaneous mechanoreceptors selectively, for the purpose of evaluating single-unit activity in multiunit peripheral nerve recordings. Each receptor is characterized by its response to a range of mechanical stimuli and by the size and location of its receptive field. This information is used to determine the number and type of units present in multiunit recordings made with chronically implanted electrodes and to follow the presence of the units in the recordings over time. This approach can be used with any type of composite neural recording in which units can be selectively activated and classified. >

Computer simulations were performed to investigate the timing of action potential production and propagation in nerve fibers ranging in diameter from 5 to 15 /spl mu/m during stimulation with a tripolar cuff electrode. The influence of stimulus pulse amplitude and duration on size selective excitation and blocking was considered. Because the stimulus duration required to produce anodal blocking depends on the time at which the action potential arrives at the blocking anode, delays in fiber activation and action potential propagation were investigated. They were found to be dependent on fiber diameter as well as stimulus amplitude and duration. The total delay associated with events occurring at the cuff electrode could be expressed as the sum of the activation delay and the propagation delay. Simple exponential equations were proposed for calculating activation and propagation delay as functions of fiber diameter and stimulus amplitude. Estimates of delays in action potential production and propagation may be useful for the design of electrodes and selection of stimuli for producing selective blocking of nerve fibers, and also for the analysis of compound neural signals elicited by electrical stimulation. >

Computer simulations were perfonned to investigate the influence of stimulus amplitude on cathodal activation delay, propagation delay and blocking during stimulation with a bipolar cuff electrode. Activation and propagation delays were combined in a total delay term which was minimized between the excitation and blocking thresholds.

This paper deals with the first step in the modeling of newe stimulation: the calculation of the potential field in a 3D volume conductor model of the nerve. surroundings and electrodes. Because of its time efficiency, a multigrid method was used to calculate the field. Compared to a Gauss-Seidel relaxation (overrelaxation factor = 1.7), a calculation time reduction of a factor 20 was obtained

A system for extracting single-unit activity patterns from multi-unit neural recordings was tested using real and simulated neural data. The system provided reliable estimates of firing frequency for individual units in simulated multi-unit data and allowed reliable determinations of the responses of individual cutaneous mechanoreceptor units to ‘natural’ stimuli such as brushing or pressing on the skin. An implementation of the system, which operated online and in real time, was used to obtain estimates of multiple, single-unit responses from multi-unit intrafascicular electrode recordings. The pattern of activity across the population of units in a given recording gave a reliable indication of the type of stimulus that had evoked the activity. It was concluded that this system, used in combination with intrafascicular peripheral nerve recordings, could be used to provide online, real-time information about peripheral stimuli.

Classification of action potentials in multiunit recordings was based on the use of various types of features to uniquely characterize action potentials from different cells. The authors compared classification results obtained using three types of descriptive features: digitized data points, amplitude and duration (time domain) parameters, and fast Fourier transform (FFT) coefficients. Digitized data points used as descriptive features provided good classification success and required minimal computation. Time-domain features gave comparable results but required more computation. FFT coefficients were less effective than the other features. As the signal-to-noise ratio of the recordings increased, smaller differences in feature values could be discriminated. >

Multiunit recordings were made in anesthetized cats with chronically implanted intrafascicular electrodes over a period of six months. Neural signals recorded with these electrodes consisted of activity in sensory fibers innervating a variety of cutaneous mechanoreceptors. Mechanical stimuli were used to selectively activate individual nerve fibers, and the receptive field and receptor type were identified for each unit. Over a period of six months, there was a net shift in the recorded population, but the electrodes contained to provide a representative sample of the activity in the fascicle as a whole. The total number of units from which activity could be recorded remained roughly constant with time, and individual units persisted in the recordings for up to six months. These results indicate that intrafascicular electrodes could be used to sample information carried by individual somatosensory fibers on a long term basis. >

The authors are developing a computer-based method for identifying individual action potentials in multiunit peripheral nerve recordings. Threshold-level, sinusoidal stimuli are used to elicit phase-locked activity from single units. These action potentials are digitized, and a template is constructed using various waveform parameters. Once templates have been generated for all the units, the program is used in a sorting mode in which action potentials are detected and digitized, and parameter values are measured and compared to the template values. The output of the program is an indication of the instantaneous firing rate of each identified unit as a function of time. In principle, the sorting could be done online in real time. >