Spencer L. BeMent

Performance of planar multisite microprobes in recording extracellular single-unit intracortical activity

The microprobes consist of a thin-film electrode array supported by a silicon micromachined substrate and insulated using deposited dielectrics. Microprobes with multiple recording sites spaced from 30 mu m to 200 mu m apart are used to record spontaneous single-unit activity from rat cerebral cortex. Additionally, a theoretical model is used to establish a basis for interpreting the multisite single-unit data. The results suggest that the microprobes (1) couple tightly to the neural tissue with relatively little disturbance to the neural system, (2) facilitate the identification of single units in multiunit records through the use of spatially-separate recording sites, and (3) can be used to detect the cell position in tissue and observe events such as the propagation of electrical activity from the soma to the dendritic tree.<<ETX>>

A direct brain interface based on event-related potentials

Cross-correlation between a trigger-averaged event-related potential (ERP) template and continuous electrocorticogram was used to detect movement-related ERPs. The accuracy of ERP detection for the five best subjects (of 17 studied), had hit percentages >90% and false positive percentages <10%. These cases were considered appropriate for operation of a direct brain interface.

THE SPECIFIC IMPEDANCE OF THE DORSAL COLUMNS OF CAT: AN ANISOTROPIC MEDIUM,

Abstract Low frequency, nonstimulating current was passed from a small electrode on the surface of the dorsal columns in the cervical cord of cats. A glass microelectrode was used to record the voltage at distances of 0.5 to 3 mm from the current electrode. The voltage fell off more rapidly in depth and across the dorsal columns than it did longitudinally—the resistance was lower in the longitudinal direction. Accordingly, the dorsal columns are anisotropic. An approximate equation is presented which describes the data fairly well and which is consistent with the anatomy. From this equation, the resistivity in the longitudinal direction was 138 to 212 ohm-cm and in the transverse direction, 1,211 ohm-cm. These values are shown to be consistent with the view that the anisotropy is primarily due to current flowing longitudinally in axons. The frequency dependence of the specific impedance was also measured. Some features of this frequency dependence have no clear explanation, but some of them are consistent with a nodal membrane having a time constant of roughly 50 μsec.

A Quantitative Study of Electrical Stimulation of Central Myelinated Fibers

Received Jammy 29,1969 Quantitative extracellular electrical stimulation data were obtained from single fibers in the dorsal columns of anesthetized and paralyzed cats. Single-fiber responses were recorded with glass microelectrodes. The threshold current was found for fibers stimulated with a 100-p diameter monopolar electrode at the surface of the dorsal columns 0.5-2.0 cm from the recording electrode. Shock artifact was greatly reduced because of the geometry of the stimulating and recording scheme. The distance of the stimulating electrode from a recorded fiber could be determined since fibers in the medial dorsal columns maintain a relatively constant relation to the midline and the surface for several segments. The stimulus was usually a single cathodal current pulse of SO-psec duration. The conduction velocity was also determined for each fiber. Current-distance relations for threshold stimulation were determined for these fibers. In general, the further away the stimulating electrode, the greater the current required for stimulation. Moreover, fibers with larger conduction velocities required less current for stimulation. The fibers were also shown to be periodic in their longitudinal stimulation properties which probably was related to the length of their internodes. The time constant of the nodal membrane was determined from strength-duration data to be about 100120 @sec. The mean of the ratio of anodal to cathodal stimulating current was 4.57 + 1.00 (SD) and ranged from 3.19 to 7.70 for 50-,usec pulse durations.

Solid-State Electrodes for Multichannel Multiplexed Intracortical Neuronal Recording

Thin-film arrays of extracellular recording electrodes have been developed for use in studies of information processing in neural structures and eventual use in closed-loop control of neural prostheses. These probes consist of a silicon substrate which supports an array of thin-film conductors. The conductors are insulated above and below with deposited dielectrics. The electrode sites are defined by openings in the upper dielectric layer and are inlaid with gold to form low-impedance recording surfaces. The probes are typically 15 pim in thickness with shank widths as narrow as 20 ¿m. The probe fabrication process is compatible with the inclusion of signal processing circuitry directly on the probe substrate. A 12 channel on-chip signal processor design with per-channel gain of 100, bandwidth of 100 Hz-6 kHz, multiplexed output, and recording-site impedance check capability is described. The probes have adequate strength to penetrate the gerbil pia-arachnoid layer and have recorded single neuron activity of over 500 ¿V peak-to-peak from tip, side, and mid-carrier sites. Signal-to-noise ratios as high as 10:1 have been achieved. An equivalent circuit model for the conducting leads, the recording site, and the electrode-electrolyte interface is described. Development of biocompatible insulation and encapsulation materials for long-term implantation of active probes is underway.

Kinematics, dynamics and control of wheeled mobile robots

The controllability of nonholonomic robot systems is proved for six common wheel and axle configurations that possess two or three degrees of freedom. After the kinematics and dynamics were modeled using the synchro-drive vehicle as an example, it was proved that continuous feedback stabilization of the vehicle to an equilibrium point is impossible. The dynamic model so developed is also used to prove that simple controllers are sufficient to guarantee stability for the drive- and steering-angle components of synchro-drive vehicles. Although the underlying control systems are stable, the experimental results demonstrated that potential-field navigation can lead robot trajectories to an unexpected invariant set. The results reported can easily be extended to the modeling and control of other mobile robot systems.<<ETX>>

Identification of electrocorticogram patterns as the basis for a direct brain interface.

This study reports on the first step in the development of a direct brain interface based on the identification of event-related potentials (ERPs) from an electrocorticogram obtained from the surface of the cortex. Ten epilepsy surgery patients, undergoing monitoring with subdural electrode strips and grid arrays, participated in this study. Electrocorticograms were continuously recorded while subjects performed multiple repetitions for each of several motor actions. ERP templates were identified from action-triggered electrocorticogram averages using an amplitude criterion. At least one ERP template was identified for all 10 subjects and in 56% of all electrode-recording sets resulting from a subject performing an action. These results were obtained with electrodes placed solely for clinical purposes and not for research needs. Eighty-two percent of the identified ERPs began before the trigger, indicating the presence of premovement ERP components. The regions yielding the highest probability of valid ERP identification were the sensorimotor cortex (precentral and postcentral gyri) and anterior frontal lobe, although a number were recorded from other areas as well. The recording locations for multiple ERPs arising from the performance of a specific action were usually found on close-by electrodes. ERPs associated with different actions were occasionally identified from the same recording site but often had noticeably different characteristics. The results of this study support the use of ERPs recorded from the cortical surface as a basis for a direct brain interface.

Automatic glottal inverse filtering from speech and electroglottographic signals

The flow of air through the glottis, the glottal volume-velocity, reflects the action of the vocal folds and is thus an important indicator of laryngeal function. However, it cannot be measured directly because of vocal tract filtering. We have developed an automated on-line method to determine the glottal volume-velocity waveform from normal and pathological speech based on digital inverse filtering. The method developed addresses the problems of accurate identification of vocal tract parameters and reduction of low-frequency noise. The vocal tract filter is estimated by analysis of the undriven vocal tract response during closed glottis, as identified from an electroglottographic signal. Low-frequency noise is attenuated by a high-pass filtering operation followed by a low-pass compensation. The complete inverse filtering method provides reliable glottal volume-velocity waveforms for both normal and pathological speech.

A model for electrcal stimulation of central myelinated fibers with monopolar electrodes

Abstract A mathematical model for monopolar cathodal stimulation of myelinated fibers in an anisotropic medium is developed. This model includes all of the factors known to affect significantly the stimulation properties of myelinated fibers. The quantitative relationship between stimulation current and electrode distance predicted by the model compares favorably with experimental data for stimulating electrode distances greater than about 500 μ. The model is used to predict electrical stimulation properties for myelinated fibers in other regions of the central nervous system.

Multichannel semiconductor-based electrodes for in vivo electrochemical and electrophysiological studies in rat CNS

Five-channel silicon-based microprobes were sputter-coated with carbon, coated with Nafion, and used for both in vivo electrochemical and single-unit electrophysiological recordings. High-speed electrochemical studies were performed in vitro and in vivo, which demonstrated that these multisite probes were capable of monitoring the evoked overflow of monoamines in selected brain regions of the rat. In addition, action potentials from Purkinje cells in the rat cerebellum, identified electrophysiologically, were recorded from different sites on the same probe. Spontaneous firing rates could be monitored for up to 2 hours in order to investigate the effects of systemic administration of phencyclidine. These results provide preliminary evidence that solid-state multi-site probes can be utilized for both in vivo electrochemical and electrophysiological studies in the rat brain.