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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.

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. >