Objective. To test a novel braided multi-electrode probe design with compliance exceeding that of a 50 µm microwire, thus reducing micromotion- and macromotion-induced tissue stress. Approach. We use up to 24 ultra-fine wires interwoven into a tubular braid to obtain a highly flexible multi-electrode probe. The tether-portion wires are simply non-braided extensions of the braid structure, allowing the microprobe to follow gross neural tissue movements. Mechanical calculation and direct measurements evaluated bending stiffness and axial compression forces in the probe and tether system. These were compared to 50 µm nichrome microwire standards. Recording tests were performed in decerebrate animals. Main results. Mechanical bending tests on braids comprising 9.6 or 12.7 µm nichrome wires showed that implants (braided portions) had 4 to 21 times better mechanical compliance than a single 50 µm wire and non-braided tethers were 6 to 96 times better. Braided microprobes yielded robust neural recordings from animals' spinal cords throughout cord motions. Significance. Microwire electrode arrays that can record and withstand tissue micro- and macromotion of spinal cord tissues are demonstrated. This technology may provide a stable chronic neural interface into spinal cords of freely moving animals, is extensible to various applications, and may reduce mechanical tissue stress.

17 SENSATION AND PERCEPTION | BRAIN FACTS SOCIETY FOR NEUROSCIENCE Although the process is not yet completely understood, recent findings suggest that visual signals are fed into at least three separate processing systems. One system appears to process information mainly about shape; a second, mainly about color; and a third, movement, location, and spatial organization. These findings of separate processing systems come from anatomical and physiological studies in monkeys. They are supported by human psychological studies showing that the perception of movement, depth, perspective, the relative size of objects, the relative movement of objects, shading, and gradations in texture all depend primarily on contrasts in light intensity rather than on color. Why movement and depth perception should be emphasized by one processing system may be explained by a school of thought called Gestalt psychology. Perception requires various elements to be organized so that related ones are grouped together. This stems from the brain’s ability to group the parts of an image together and also to separate images from one another and from their individual backgrounds. How do all these systems combine to produce the vivid images of solid objects that we perceive? This involves extracting biologically relevant information at each stage and associating firing patterns of neuron l populations with past experience. Vision studies also have led to better treatment for visual disorders. Information from research in cats and monkeys has improved the therapy for strabismus, or squint, a term for cross-eye or walleye. Children with strabismus initially have good vision in each eye. But because they cannot fuse the images in the two eyes, they tend to favor one eye and often lose useful vision in the other. Vision can be restored in such cases, but only during infancy or early childhood. Beyond the age of 6 or so, the blindness in one eye becomes permanent. Until a few decades ago, ophthalmologists waited until children reached the age of 4 before operating to align the eyes or prescribing exercises or an eye patch. Now strabismus is corrected very early in life — before age 4, when normal vision can still be restored.

The authors report the use of dense two-dimensional microelectrode array recordings to characterize fine resolution electrocortical activity ("microEEG") in epileptogenic human cortex. A 16-mm(2) 96 microelectrode array with 400-mum interelectrode spacing was implanted in five patients undergoing invasive EEG monitoring for medically refractory epilepsy. High spatial resolution data from the array were analyzed in conjunction with simultaneously acquired data from standard intracranial electrode grids and strips. microEEG recorded from within the epileptogenic zone demonstrates discharges resembling both interictal epileptiform activity ("microdischarges") and electrographic seizures ("microseizures") but confined to cortical regions as small as 200 microm(2). In two patients, this activity appeared to be involved in the initiation or propagation of electrographic seizures. The authors hypothesize that microdischarges and microseizures are generated by small cortical domains that form the substrate of epileptogenic cortex and play important roles in seizure initiation and propagation.

Real-time applications of spike sorting, e.g., neural decoding, generally require high numbers of channels, and manual spike sorting methods are extremely time consuming, subjective and, generally, do not perform well for low signal-to-noise ratio (SNR) signals. Hence, an automatic method is sought which is efficient and robust in both detecting neural spikes and constructing a classification model of spikes arriving with underlying statistics that are time-varying. We present such a system under study for application with a microelectrode array of 96 channels with typically three or four units (Le., neurons) per channel. There are several novel elements of the system including filtering the neural signal to a frequency band having better SNR for spike detection, a fixed feature space for simple implementation yet adequate resolving capabilities, a Gaussian statistics model also for simple implementation as a log-likelihood classifier, a systematic approach to determining the number of clusters in a pattern recognition problem, and a robust linear discriminant, histogram-based technique for determining boundaries between feature space clusters

Neuromotor prostheses ( NMPs) aim to replace or restore lost motor functions in paralysed humans by routeing movement-related signals from the brain, around damaged parts of the nervous system, to external effectors. To translate preclinical results from intact animals to a clinically useful NMP, movement signals must persist in cortex after spinal cord injury and be engaged by movement intent when sensory inputs and limb movement are long absent. Furthermore, NMPs would require that intention-driven neuronal activity be converted into a control signal that enables useful tasks. Here we show initial results for a tetraplegic human (MN) using a pilot NMP. Neuronal ensemble activity recorded through a 96-microelectrode array implanted in primary motor cortex demonstrated that intended hand motion modulates cortical spiking patterns three years after spinal cord injury. Decoders were created, providing a 'neural cursor' with which MN opened simulated e-mail and operated devices such as a television, even while conversing. Furthermore, MN used neural control to open and close a prosthetic hand, and perform rudimentary actions with a multi-jointed robotic arm. These early results suggest that NMPs based upon intracortical neuronal ensemble spiking activity could provide a valuable new neurotechnology to restore independence for humans with paralysis.

We are investigating the representation of simple visual objects by groups of retinal ganglion cells and are simultaneously recording the responses of ganglion cells in the isolated turtle retina with 15 out of an array of 100 penetrating microelectrodes. Stimulation is with circular spots of light of various intensities, diameters and colors. We have trained a three layer artificial neural network to estimate the stimulus parameters and have challenged it to classify the color, size and intensity of test stimuli. Individual ganglion cells are poor encoders of stimulus features, but the 15 cells in our sample allow one to classify intensity, color and spot diameter to within 0.6 log units, 61 nm, and 0.68 mm, respectively.

The restoration of graceful, yet powerful movements in paralyzed regions of the body has not been achieved in humans. The problems are manifold and each is quite complex. Normal motions around a single joint are typically produced by coordinated activation of many muscles. Further, graded force generation in a single muscle is achieved by successively recruiting more and more motoneurons that innervate the target muscle. Finally, normal physiological movements are often achieved without excessive fatigue, a condition difficult to replicate with conventional cuff-type, epimysial, or surface electrodes. We have conducted experiments using intrafascicular multielectrodc stimulation (IFMS) in the cat sciatic nerve that address many of these problems by D. McDonnall, et al. (2004). We have implanted Utah slanted electrode arrays (USEA's) in the sciatic nerve of cat, and have been able to selectively stimulate small, independent subpopulations of motoneurons that innervate the muscles of the lower leg. The USEA consists of a 10 /spl times/ 10 grid of graded-length microelectrodes separated by 400 microns. We have shown that motoneuron stimulation via the USEA is highly selective and that stimulation via specific electrodes can evoke a maximal force in each of these muscles without producing substantive forces in the others. Further, because different electrodes excite different motoneurons innervating a particular muscle, we have been able to interleave low-frequency stimulation through these electrodes to produce ripple-free, fatigue-resistant forces over a large range of forces. These experiments demonstrate that IFMS can be achieved with high-count penetrating electrode arrays with a slanted architecture. Such arrays, when inserted into the nerves of the peripheral nervous system, could be used to recreate graceful, yet powerful, fatigue-resistant motions in the musculo-skeletal system of paralyzed subjects. The large number of muscles that can be activated via the 100 electrodes in a single USCA implantation, together with the relative ease of surgical implantation of these devices, offers a promising approach to reanimation of paralyzed limbs.

We studied the consequences of long-term implantation of a penetrating microelectrode array in peripheral nerve over the time course of 4-6 mo. Electrode arrays without lead wires were implanted to test the ability of different containment systems to protect the array and nerve during contractions of surrounding muscles. Treadmill walking was monitored and the animals showed no functional deficits as a result of implantation. In a different set of experiments, electrodes with lead wires were implanted for up to 7 mo and the animals were tested at 2-4 week intervals at which time stimulation thresholds and recorded sensory activity were monitored for every electrode. It was shown that surgical technique highly affected the long-term stimulation results. Results between measurement sessions were compared, and in the best case, the stimulation properties stabilized in 80% of the electrodes over the course of the experiment (162 days). The recorded sensory signals, however, were not stable over time. A histological analysis performed on all implanted tissues indicated that the morphology and fiber density of the nerve around the electrodes were normal.

The recording capability of a microelectrode array in the cat dorsal root ganglion (DRG) was studied in 11 acute experiments, 373 single, discriminable sensory units were recorded on 587 electrodes (0.64 units/electrode). Sensory action potentials as large as 1750 μV were obtained (mean=132 μV). These were comparable to literature reports of the best DRG extracellular recordings made with conventional electrodes. We were able simultaneously to activate and record over 50 discriminable, time-varying units from L6 and L7 DRGs during a cyclic ankle displacement. We also successfully recorded stable, phase dependent multiple sensory units with very little artifact or electromyographic (EMG) contamination during decerebrate walking. Thus, the array is capable of recording more effectively from more DRGs neurons than has been achieved by conventional recording techniques. The recording selectivity and stability of the array, coupled with the large number of neurons that can be recorded simultaneously, provide attractive features for better understanding sensorimotor control principles.

Restoration of motor function to individuals who have had spinal cord injuries or stroke has been hampered by the lack of an interface to the peripheral nervous system. A suitable interface should provide selective stimulation of a large number of individual muscle groups with graded recruitment of force. We have developed a new neural interface, the Utah Slanted Electrode Array (USEA), that was designed to be implanted into peripheral nerves. Its goal is to provide such an interface that could be useful in rehabilitation as well as neuroscience applications. In this study, the stimulation capabilities of the USEA were evaluated in acute experiments in cat sciatic nerve. The recruitment properties and the selectivity of stimulation were examined by determining the target muscles excited by stimulation via each of the 100 electrodes in the array and using force transducers to record the force produced in these muscles. It is shown in the results that groups of up to 15 electrodes were inserted into individual fascicles. Stimulation slightly above threshold was selective to one muscle group for most individual electrodes. At higher currents, co-activation of agonist but not antagonist muscles was observed in some instances. Recruitment curves for the electrode array were broader with twitch thresholds starting at much lower currents than for cuff electrodes. In these experiments, it is also shown that certain combinations of electrode pairs, inserted into an individual fascicle, excite fiber populations with substantial overlap, whereas other pairs appear to address independent populations. We conclude that the USEA permits more selective stimulation at much lower current intensities with more graded recruitment of individual muscles than is achieved by conventional cuff electrodes.

The feasibility of implanting an array of penetrating electrodes into peripheral nerves is studied in acute experiments in the cat sciatic nerve. A novel, silicon-based array of microelectrodes, the Utah Electrode Array, was used, which contains 25 or 100 1-mm long electrodes that project out from a silicon substrate. Electrode arrays of this complexity, when inserted in the peripheral nerve, could cause significant compression of the nerve and block the conduction of action potentials. Using a high velocity insertion technique, the electrode array was implanted into the sciatic nerve. Compound action potentials were evoked by and recorded with cuff electrodes. Compound action potentials recorded 1 h after insertion were only slightly altered from those recorded before insertion. Single units were readily extracted from evoked multiunit neural recordings in response to cutaneous stimulation and limb rotation around joints. Current injections into the nerve through the electrodes evoked muscle twitches with currents in the 10 μA range. Recording and stimulation stability were demonstrated for periods of up to 60 h. We have shown that high density arrays of electrodes can be inserted into the peripheral nerve and can provide a stable recording and stimulating interface to individual peripheral nerve axons. Such an array may be useful in future neuroscience research and potential neuroprosthetic applications.

A practical interface to the peripheral nervous system (or to the spinal cord) that could be used in a neuroprosthetic application must have two features: it should be able to selectively stimulate a large number of individual muscle groups in a highly specific fashion; and it should also be able to stimulate these muscles in a broadly graded fashion in a manner similar to the way they are stimulated naturally. We have developed a new type of silicon based microelectrode array which consists of 100 penetrating needles that project out from a thin silicon substrate. We have developed such an electrode array and have evaluated it in a series of acute implantations in cat sciatic nerve. Using a high velocity insertion technique, the array can be inserted through the epineurium and perineurium with little damage to the propagation of neural activity through the implantation site. The array records single- and multi-unit sensory information. It is capable of generating muscle twitches with single, biphasic 200 usec current pulses in the 1-10 microampere range

The authors have developed a neural interface, the Utah Slant Array (the USA), that provides unprecedented access to a large number of sensory and motor neurons. The array has been designed to be implanted into fascicles of peripheral nerves, and it provides up to 100 channels of neural communication. The authors are currently evaluating the recording and stimulation capabilities of the USA in acute experiments in cat sciatic nerve. Very high selectivity even for high forces can be reached by near simultaneous stimulation of two or more electrodes innervating the same muscles. The authors suspect that, using the same methods, fatigue resistant stimulation is also possible

Visual stimuli are represented in neuronal firing of a 2D highly interconnected network of ganglion cells driven by the stimuli. As a result, the network is able to encode more features of the visual stimuli than can the single neurons that make up the network. We have recorded the simultaneous responses of 15 ganglion cells in the isolated, perfused turtle retina to simple visual stimuli circles of various diameters, intensities and colors. We trained a three-layer neural network to classify the responses in terms of stimulus color, size and intensity. The network performed poorly in the classification of the test stimuli. Further, concurrently recorded responses did not do significantly better at specifying the stimulus than did sequentially recorded responses. The approach provides a formalism that allows one to assess the effects of response variability on the ability of the network to classify responses