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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. >

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. >

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. >