Recording spikes from a large fraction of the ganglion cells in a retinal patch

To understand a neural circuit completely requires simultaneous recording from most of the neurons in that circuit. Here we report recording and spike sorting techniques that enable us to record from all or nearly all of the ganglion cells in a patch of the retina. With a dense multi-electrode array, each ganglion cell produces a unique pattern of activity on many electrodes when it fires an action potential. Signals from all of the electrodes are combined with an iterative spike sorting algorithm to resolve ambiguities arising from overlapping spike waveforms. We verify that we are recording from a large fraction of ganglion cells over the array by labeling the ganglion cells with a retrogradely transported dye and by comparing the number of labeled and recorded cells. Using these methods, we show that about 60 receptive fields of ganglion cells cover each point in visual space in the salamander, consistent with anatomical findings.

A multi-channel, implantable microdrive system for use with sharp, ultra-fine "Reitboeck" microelectrodes

Arrays of closely spaced quartz-insulated, platinum-tungsten microelectrodes are widely used to obtain acute recordings from chronically prepared subjects. These electrodes have excellent recording characteristics and can be fabricated to a wide variety of tip specifications. Typically, in such experiments, electrodes are introduced into, and removed from, the brain on a daily basis and, over many months of study, hundreds of penetrations may be made through an intact dura. This procedure has benefits as well as problems and risks. For some experimental aims, it might be desirable to leave the microelectrodes within the brain so that the penetrations could be continued on subsequent days. This would allow a more thorough and systematic exploration of the neurons that lie along the trajectory of each of the closely aligned electrodes and would minimize risks and preparation time associated with daily electrode insertions. Here we present a means for achieving this aim using arrays of sharp, flexible Reitboeck electrodes of extremely fine diameter (40-microm shaft diameter, pulled and ground to a fine tip). We show that these electrodes retain their excellent recording characteristics and can remain under microdrive control within the brain for periods of many months and, in one remarkable case, for >4 years.

Méthodes et intérêt clinique de la mesure de la période réfractaire nerveuse périphérique chez l'homme

Immediately after action potential occurrence, owing to transient sodium channel inactivation, axon excitability is reduced for a short period of time, including the absolute refractory period, a first period of total inexcitability, followed by the relative refractory period. There are basically two different stimulation protocols to estimate axonal refractoriness, i.e. "paired-pulse" and "collision" techniques. Refractory period has been assessed in various conditions and appeared to depend on several physiological or methodological factors, featuring the type of nerve or the characteristics of the subject, but also the technique of stimulation or the method of data analysis. In addition, refractory periods can be altered by pathological conditions. Several studies showed prolonged refractory periods in patients suffering from alcoholic, diabetic or toxic neuropathies. Refractory period abnormality is a sensitive marker of axonal dysfunction as observed in Guillain-Barré syndrome, carpal tunnel syndrome or multiple sclerosis. Thus, the measurement of the refractory periods is a valuable tool to study the pathophysiology of peripheral nerves, complementary to standard nerve conduction studies. However, the application of these techniques in the routine practice of clinical neurophysiology remains limited.

The repeatability of corticomotor threshold measurements

Threshold (Th) is a neurophysiological parameter frequently used in TMS studies. The present study was designed to investigate the repeatability of the Th measurements by reexamining healthy subjects over various time points. Overall, 82 subjects (median age: 19 years, range: 12-65) entered the study. Following a baseline examination, there were six retest sessions: S0 (n = 8 hemispheres reexamined, mean interval x = 19 min), S1 (n = 34 hemispheres reexamined, mean interval x = 4 days), S2 (n = 32 hemispheres, x = 29 days), S3 (n = 30 hemispheres, x = 106 days), S4 (n = 30 hemispheres, x = 183 days) and S5 (n = 30 hemispheres, x = 1867 days). Stimulation was performed with a figure of eight coil and Th was defined at 1% steps. At baseline, controls had an MT of 41.1 +/- 8. Mean difference of MT from baseline was 0.62 on S0 (95% confidence interval (CI) of the difference: -1.04 to +2.29), 0.13 on S1 (95% CI: -1.2 to +1.5), -0.03 on S2 (95% CI: -1.1 to +1.06), -2.07 on S3 (95% CI: -4.33 to +0.19), 0.15 on S4 (95% CI: -0.98 to +1.28) and 0.87 on S5 (95% CI: -0.49 to +2.23). None of these differences were statistically significant (repeated measures ANOVA, P > 0.05). The upper limit of MT difference that an individual subject might have with a probability of 95% (measurement error) was 8. The repeatability of the method was found to be independent from the age of the subjects, the magnitude of threshold or the test-retest interval. The topography of corticomotor threshold was also investigated. Minimal threshold values were obtained from a restricted area of scalp sites that always included the fixed stimulation point of the current protocol. Therefore, using a fixed stimulation point is an adequate technique for measuring threshold. In conclusion, threshold is a stable parameter on an individual and group basis. These data quantify the repeatability of the method and may prove useful in the interpretation of findings during longitudinal studies.

Effect of intensive neurodevelopmental treatment in gross motor function of children with cerebral palsy

This study examined the effect of neurodevelopmental treatment (NDT) and differences in its intensity on gross motor function of children with cerebral palsy (CP). Participants were 34 children (12 females, 22 males; mean age 7y 3mo [SD 3y 6mo], age range 3 to 14y) with mild to moderate spasticity and hemiplegia (n=10), diplegia (n=12), and tetraplegia (n=12). Gross Motor Function Classification System levels were: I (n=10), II (n=10), and III (n=14). The paired sample, which was obtained by ratio stratification and matching by sex, age, and distribution of impairment from a total of 114 children with CP, was assigned randomly to two groups: group A underwent NDT twice a week and group B five times a week for 16 weeks. The outcome measure used was the Gross Motor Function Measure, which assessed the performance of the children before and after intervention. The paired-sample t-test revealed that gross motor function of children from both groups improved significantly after intervention (p<0.05). Children in group B performed better and showed significantly greater improvement than those in group A (p<0.05). Results support the effectiveness of NDT and underline the need for intensive application of the treatment.

Strategies for cellular identification in nucleus tractus solitarius slices

The indistinct regional anatomy and intermixing of second order neurons with projection and interneurons make cellular studies more difficult within the nucleus tractus solitarius (NTS). Here, we outline experimental strategies to join in vitro electrophysiological with neuroanatomical protocols to discriminate specific subpopulations of NTS neurons. Horizontally cutting the brain stem produces slices in which electrical activation of the solitary tract (ST) is free of local interneuron contamination. Such ST excitatory synaptic currents (EPSCs) functionally identify second order NTS neurons by their minimal variation of latency (jitter). Sapphire blades, cold cutting temperatures and a mechanically stable microtome were critical to consistently obtain viable slices that were optimized for infrared and fluorescence microscopy. Anterogradely transported carbocyanine dye implanted on the aortic depressor nerve anatomically identified second order NTS neurons and their ST synaptic performance conformed to the minimal jitter signature of second order neurons. Retrograde tracers and green fluorescent protein labeled neurons afford two additional promising approaches for discriminating NTS neuron phenotypes in broader system contexts. Detailed methods and troubleshooting are described. Coupling tracing techniques with electrophysiology adds important new dimensions to NTS studies and such strategies provide bridging information between cellular mechanisms, neuroanatomy and systems integration.

An in vivo mouse spinal cord preparation for patch-clamp analysis of nociceptive processing

The laboratory mouse is now considered the preferred mammalian species for molecular and genetic analysis in neurobiology. In part, this is due to the existence, in the mouse, of several well characterised naturally occurring mutations in ligand gated ion channels and recent knockout, knockin, and transgenic techniques, which facilitate the manipulation of key molecules. These techniques have recently been applied to pain research with in vitro electrophysiological and behavioural techniques traditionally developed for the rat, now being adapted for the mouse particularly at the level of the spinal cord. Here, we describe an in vivo preparation of the mouse spinal cord for patch-clamp recording of nociceptive processing in the superficial dorsal horn (SDH) that permits analysis in the intact nervous system. We have recorded from SDH neurons and characterised their background synaptic activity, discharge properties, and evoked synaptic responses following controlled application of innocuous and noxious stimuli to the hind paw. Application of these techniques along with genetic, biomolecular, in vitro and behavioural approaches will allow future studies to comprehensively analyse the contributions of specific molecules involved in nociceptive processing in the spinal cord of a single species.

An optical telemetry system for underwater recording of electromyogram and neuronal activity from non-tethered crayfish

We have developed an optical telemetry system for recording electrical signals associated with muscle and neuronal activities from freely walking crayfish under water. The device was made from conventional electronic parts which are commercially available, utilizing infrared light (880 nm) for signal transmission. Two or four channels of biological signals were multiplexed, the voltage of each data point modulated to the duration of subcarrier pulses and further to the interval of narrower carrier pulses that directly drove the infrared light emission diode (IRLED) under water. The light-pulse modulated signals were received by photodiodes and demodulated to restore the original two or four channel signals. Electrical recordings using wired electrodes and conventional amplifiers revealed that the optically transmitted signals were consistent with the wire-transmitted ones. In order to test the performance of this system, we recorded electromyograms (EMGs) from the second and third walking legs on each side of crayfish together with the neuronal activity in the ventral nerve cord. The results confirmed our previous observation in tethered crayfish that the background tonus of leg muscles showed an increase preceding their rhythmic activation.

A method for direct thalamic stimulation in fMRI studies using a glass-coated carbon fiber electrode

Recent fMRI studies are of interest in exploring long-range interactions between different brain structures and the functional activation of specific brain regions by known neuroanatomical pathways. One of the experimental approaches requires the invasive implantation of an intracranial electrode to excite specific brain structures. In the present report, we describe a procedure for the production of a glass-coated carbon fiber electrode and the use of this electrode for direct activation of the brain in fMRI studies. The glass-coated carbon fiber microelectrode was implanted in the medial thalamus of anaesthetized rats and T2*-weighted gradient echo images in the sagittal plane obtained on a 4.7 T system (Biospec BMT 47/40) during electrical stimulation of the medial thalamus. The image quality obtained using this electrode was acceptable without reduction of the signal-to-noise ratio and image distortion. Cross-correlation analysis showed that the signal intensities of activated areas in the ipsilateral anterior cingulate cortex were significantly increased by about 4-5% during medial thalamus stimulation. The present study shows that glass-coated carbon fiber electrodes are suitable for fMRI studies and can be used to investigate functional thalamocingulate activation.

Method for in situ detection of the mitochondrial function in neurons

Conventional studies of neuronal mitochondria have been limited to the use of purified preparations of isolated mitochondria, neural cell homogenates, living neurons, or brain slices. However, each technique has several drawbacks. Here, we demonstrate that the neuronal cell's membrane can be effectively permeabilized by saponin-treatment and that these permeabilized neurons can be used for qualitative and quantitative assessments of oxygen consumption in combination with registration of mitochondrial membrane potential and free [Ca2+] in the matrix. Under these conditions, the mitochondrial function can be studied without removing the mitochondria from their natural milieu thus avoiding the damage of the associated cytoskeleton and outer membrane. At the same time, the method allows the estimation of the mitochondrial function independently of other processes in the cell, and the easy manipulation of the milieu surrounding the mitochondria. Thus, the presented method offers the opportunity to study the neuronal mitochondrial function in situ and can also be applied to examine the mitochondrial function by other commonly used methods.

Adaptive neural models of queuing and timing in fluent action

In biological cognition, specialized representations and associated control processes solve the temporal problems inherent in skilled action. Recent data and neural circuit models highlight three distinct levels of temporal structure: sequence preparation, velocity scaling, and state-sensitive timing. Short sequences of actions are prepared collectively in prefrontal cortex, then queued for performance by a cyclic competitive process that operates on a parallel analog representation. Successful acts like ball-catching depend on coordinated scaling of effector velocities, and velocity scaling, mediated by the basal ganglia, may be coupled to perceived time-to-contact. Making acts accurate at high speeds requires state-sensitive and precisely timed activations of muscle forces in patterns that accelerate and decelerate the effectors. The cerebellum may provide a maximally efficient representational basis for learning to generate such timed activation patterns.

An in vivo method for recording single unit activity in lumbar spinal cord in mice anesthetized with a volatile anesthetic

We describe a method to record single unit neuronal activity from mouse spinal cord using volatile anesthesia. The small size of the mouse can complicate usual methods that are used for single-unit recording in rats, but simple modifications can significantly increase the number of successful recordings. Stabilization of the vertebral column is particularly important, as are adequate ventilation of the animal, control of body temperature and accurate determination of anesthetic concentrations in respiratory gas samples.

A novel mouse brain slice preparation of the hippocampo–accumbens pathway

The nucleus accumbens (NAc) is an important component of circuitry that underlies reward related behaviors and the rewarding properties of drugs of abuse. Glutamatergic afferents to the nucleus are critical for its normal function and for behaviors related to drug addiction. An angled, sagittal mouse brain slice preparation has been designed to facilitate concurrent stimulation of two major glutamatergic afferent pathways to the nucleus accumbens. Medium spiny neurons at the medial core/shell boundary of the accumbens were depolarized by stimulation of either hippocampal or limbic cortical afferents through activation of AMPA-type glutamate receptors. High frequency but not low frequency stimulation of hippocampal afferents depolarized medium spiny neurons to a membrane potential that resembled the up state observed upon high frequency stimulation in vivo. The magnitude of the membrane depolarization was positively correlated with the amplitude of the stimulus-evoked EPSP. Concurrent stimulation of hippocampal and limbic cortical afferents at theta frequency selectively induced a long-term depression (LTD) in the magnitude of stimulus-evoked EPSPs on the hippocampal afferent only. These data suggest that this brain slice preparation can be used to study mechanisms underlying synaptic plasticity at two of the critical glutamatergic afferent synapses in the nucleus accumbens as well as characterizing potential interactions between afferents. Additionally, LTD at hippocampo-accumbens synapses can be induced at a stimulus frequency known to support reinstatement of drug seeking behavior.

An algorithm for real-time extraction of population EPSP and population spike amplitudes from hippocampal field potential recordings

A new method is presented for extracting the amplitude of excitatory post synaptic potentials (EPSPs) and spikes in real time. It includes a low pass filter (LPF), a differentiator, a threshold function, and an intelligent integrator. It was applied to EPSP and population spike data recorded in the Dentate Gyrus and the CA1 hippocampus in vitro. The accuracy of the extraction algorithm was evaluated via the extraction normalized mean square error (eNMSE) and was found to be very high (eNMSE < 5%). The preservation of neuronal information was confirmed using the Volterra-Poisson modeling approach. Volterra-Poisson kernels were computed using amplitudes extracted with both proposed and traditional methods. The accuracy of the computed kernels and the resulting model was evaluated via the prediction normalized mean square error (pNMSE) and was found to be very high (pNMSE < 5%). The similarity between the kernels computed when the proposed method was used to extract the field potential amplitude and their counterparts when the traditional method was used to extract the field potential amplitude confirms the preservation of the neuronal dynamics. The proposed method represents a new class of real time field potential amplitude extraction algorithms with complexity that can be included in hardware implementations.

Optimising coherence estimation to assess the functional correlation of tremor-related activity between the subthalamic nucleus and the forearm muscles

Application of coherence estimation needs not only to correctly estimate coherence values but also to efficiently test the statistical significance of the estimates. In the present report, we have explained the approach of optimising a coherence estimator by restricting its normalised bias error and random error. In addition to the commonly used independence threshold, two more tests based on the probability of detection and the exact confidence interval have been proposed for detecting the significance of the coherence estimates. All three methods have been used to evaluate the significant functional correlation between oscillatory field potentials (FPs) in the subthalamic nucleus (STN) and the surface electromyogram (EMG) of the forearm muscles during tremor in Parkinson's disease.

[Drug-resistant partial epilepsy. Invasive electrophysiological explorations]

During the past 20 years, advances in neuroimaging techniques have greatly improved the surgery of epilepsy. Nevertheless, for fifty percent of epileptic patients undergoing such surgery procedures, recordings with chronically implanted intracerebral electrodes are necessary to localize the epileptic focus. Since the fifties, these electrodes have been used to record the cerebral cortex during the surgical procedure. Due to technological progress, these electrodes can now be left implanted for several days to record the inter- and per-ictal electroencephalogram (EEG). In comparison with scalp recordings, intracortical assessment of epileptic patients allows exploring the cortical epileptic network with a higher spatial resolution and less artefacts. Moreover, this technique enables performing direct cortical electrical stimulation to map functionally eloquent cortices and epileptogenic areas. Each type of depth electrodes has been developed for a specific use. In this work, we review the different solutions used at the present time, their specific indications and finally their advantages and disadvantages. Moreover, we mention the new emerging therapeutic indications.

A dual RF resonator system for high-field functional magnetic resonance imaging of small animals

A new apparatus has been developed that integrates an animal restrainer arrangement for small animals with an actively tunable/detunable dual radio-frequency (RF) coil system for in vivo anatomical and functional magnetic resonance imaging of small animals at 4.7 T. The radio-frequency coil features an eight-element microstrip line configuration that, in conjunction with a segmented outer copper shield, forms a transversal electromagnetic (TEM) resonator structure. Matching and active tuning/detuning is achieved through fixed/variable capacitors and a PIN diode for each resonator element. These components along with radio-frequency chokes (RFCs) and blocking capacitors are placed on two printed circuit boards (PCBs) whose copper coated ground planes form the front and back of the volume coil and are therefore an integral part of the resonator structure. The magnetic resonance signal response is received with a dome-shaped single-loop surface coil that can be height-adjustable with respect to the animal's head. The conscious animal is immobilized through a mechanical arrangement that consists of a Plexiglas body tube and a head restrainer. This restrainer has a cylindrical holder with a mouthpiece and position screws to receive and restrain the head of the animal. The apparatus is intended to perform anatomical and functional magnetic resonance imaging in conscious animals such as mice, rats, hamsters, and marmosets. Cranial images acquired from fully conscious rats in a 4.7 T Bruker 40 cm bore animal scanner underscore the feasibility of this approach and bode well to extend this system to the imaging of other animals.

Design of a twin tetrode microdrive and headstage for hippocampal single unit recordings in behaving mice

A new, easy to construct electrode, microdrive and headstage for electrophysiological recording system which is specifically adapted for freely behaving mice is described. The system uses printed circuit boards and light, flexible cables to enable the animal's free movement for behavioral testing. A clip attachment system permits rapid and secure connection of the headstage and cables to the microdrive assembly on the animal's head. The current system provides eight recording channels, but the design can be modified to accommodate additional channels.

MRCI: a flexible real-time dynamic clamp system for electrophysiology experiments

We present a real-time simulation system that enables modeled dynamical systems to interact with physical experimental systems, and is specifically aimed towards execution of the dynamic clamp protocol. Model reference current injection (MRCI) operates under Real-Time Linux (RT-Linux or RTL) and provides a simple equation-oriented language for describing dynamical system models. Features include scripting of commands to implement repeatable protocols, the ability to output pre-computed waveforms through any variable or parameter of the model, the means to conduct time measurements and assess the computational performance of the real-time system, and an installation program that installs the software and accompanying device drivers with minimal input from the user. Tested models operate as fast as 30 kHz, with actual maximum rates dependent on model complexity. We present sample models that exhibit the main features of the modeling language. Experiments demonstrate the abilities of the system by creating a hybrid network of real and simulated neurons, and playing a pre-defined synaptic waveform into a synaptic conductance variable. We conclude by introducing a waveform reconstruction technique that is useful for establishing the presence of significant experimental error in implementations of the dynamic clamp protocol.

A state-dependent trigger for electrophysiological recording at predetermined membrane potentials

This paper describes the circuitry and construction of a novel electronic threshold discriminator, and details its specific application to in vivo intracellular recording. The discriminator reliably triggers electrophysiological recording at pre-selectable membrane potentials in neuronal systems that exhibit membrane potential oscillations. It has been used successfully whilst recording from spiny projection neurons of the striatum to measure membrane properties and trigger electrical stimulation within either of two discrete membrane potential "states". The device works by comparing the analogue membrane potential waveform with a user-defined threshold membrane potential, and outputs a logic signal to flag the occurrence of a threshold-crossing event. This signal is used to trigger the commencement of episodic recording and the application of current injection or electrical stimulation at a consistent membrane potential. Thus, the discriminator acts as a functional clamp to isolate evoked responses from endogenous fluctuations in membrane potential. The unit uses cheap and easily available components and can be constructed with the minimum of electronics experience. It could be adapted to isolate discrete events within any oscillatory system.

A simple method for fabricating horizontal and vertical microwire arrays

Microwire array electrodes are important in multi-site, multiple single-unit recording experiments. A simple method is described herein for the construction of microwire arrays consisting of evenly spaced insulated microwires in either horizontal or vertical orientations. Several key steps in the fabrication of a good microwire array electrode are made easier with this method. These steps include (1) arranging microwires into a desirable configuration, (2) keeping track of microwire sequences, and (3) soldering microwires to closely packed slots. This method needs only general mechanical tools and is relatively simple even for inexperienced workers.

Pattern Filtering for Detection of Neural Activity, with Examples from HVc Activity During Sleep in Zebra Finches

The detection of patterned spiking activity is important in the study of neural coding. A pattern filtering approach is developed for pattern detection under the framework of point processes, which offers flexibility in combining temporal details and firing rates. The detection combines multiple steps of filtering in a coarse-to-fine manner. Under some conditional Poisson assumptions on the spiking activity, each filtering step is equivalent to classifying by likelihood ratios all the data segments as targets or as background sequences. Unlike previous studies, where global surrogate data were used to evaluate the statistical significance of the detected patterns, a localizedp-test procedure is developed, which better accounts for firing modulation and nonstationarity in spiking activity. Common temporal structures of patterned activity are learned using an entropy-based alignment procedure, without relying on metrics or pair-wise alignment. Applications of pattern filtering to single, presumptive interneurons recorded in the nucleus HVc of zebra finch are illustrated. These demonstrate a match between the auditory-evoked response to playback of the individual bird's own song and spontaneous activity during sleep. Small temporal compression or expansion, or both, is required for optimal matching of spontaneous patterns to stimulus-evoked activity.

Wavelet analysis can sensitively describe dynamics of ethanol evoked local field potentials of the slug (Limax marginatus) brain

Odorants evoke characteristic, but complex, local field potentials (LFPs) in the molluscan brain. Wavelet tools in combination with Fourier analysis can detect and characterize hitherto unknown discrete, slow potentials underlying the conspicuous oscillations. Ethanol was one of the odorants that we have extensively studied (J. Neurosci. Methods, 119 (2002) 89). To detect new features and to elucidate their functions, we tested the wavelet tools on the ethanol-evoked LFP responses of the slug (Limax) procerebrum. Recordings were made in vitro from the neuropile and the cell layer. The present study led to the following findings: (i) Mutual exclusion. Energy concentrated mainly in two ranges, (a) 0.1-0.4 Hz and (b) 1.56-12.5 Hz, and the sum of energy remained constant throughout experiments regardless of the condition. A redistribution of relative energy within this sum seemed to occur in the course of main, possible interactions between the two components excluding each other ('mutual exclusion'). (ii) Transient signal ordering and disordering. Ethanol stimulation alternatingly evoked periods of strongly time evolving oscillation dominated by the energy of 1.56-12.5 Hz (increase of entropy=disordered or complexly ordered state) and those of near-silence were predominated by the energy of 0.1-0.4 Hz (decrease of entropy=ordered state). (iii) About 0.1 Hz slow wave oscillation. It was robust. The dominant energy oscillation and the resulting large entropy fluctuation were negatively correlated to each other, and revealed strong frequency-tuning or synchronization at this frequency. Our findings suggest that discrete slow waves play functionally important roles in the invertebrate brain, as widely known in vertebrate EEG. Wavelet tools allow an easy interpretation of several minutes of frequency variations in a single display and give precise information on stimulus-evoked complex change of the neural system describing the new state 'more ordered' or 'non-ordered or more complexly ordered'.

Techniques for recording renal sympathetic nerve activity in awake, freely moving animals

The methods described here allow recording of sympathetic nerve discharge in awake, freely moving animals, and follow a historical perspective of the different techniques developed over the years to record sympathetic discharge. The length of time each system is viable for recording varies from days to weeks. Also included are special hints for the surgical implantation of recording electrodes, types of recording electrodes and cables, as well as the minimum equipment necessary for recording sympathetic discharge. Lastly, a section on troubleshooting includes how to remove movement artifacts and extraneous noise, and minimize the destruction of leads common in recording in awake, freely moving animals. This article is written for the beginner or novice with an emphasis on what is needed when embarking on recording sympathetic nerve discharge in awake, freely moving animals.

Brain-machine interfaces: computational demands and clinical needs meet basic neuroscience

As long as 150 years ago, when Fritz and Hitzig demonstrated the electrical excitability of the motor cortex, scientists and fiction writers were considering the possibility of interfacing a machine with the human brain. Modern attempts have been driven by concrete technological and clinical goals. The most advanced of these has brought the perception of sound to thousands of deaf individuals by means of electrodes implanted in the cochlea. Similar attempts are underway to provide images to the visual cortex and to allow the brains of paralyzed patients to re-establish control of the external environment via recording electrodes. This review focuses on two challenges: (1) establishing a 'closed loop' between sensory input and motor output and (2) controlling neural plasticity to achieve the desired behavior of the brain-machine system. Meeting these challenges is the key to extending the impact of the brain-machine interface.