Unit activity, evoked potentials and slow waves in the rat hippocampus and olfactory bulb recorded with a 24-channel microelectrode

Fabrication of a photo-crosslinkable fluoropolymer-passivated flexible neural probe and acute recording and stimulation performances in vivo

Herein, we fabricated fluorine-containing, polymer-based, flexible neural probes with fluorinated ethylene propylene (FEP) films as the substrates and photo-crosslinkable fluoropolymers as the passivation material. For fabrication, metal-free Au layer formation on the FEP film, the simultaneous photo-adhesion and photo-patterning technique, and the pulsed-laser scanning probe shaping technique were combined, followed by Au electrode surface modification. The resultant probes achieved a charge injection limit equal to 5.18 mC cm-2 by implementing iridium oxide-modified nanoporous Au (IrOx/NPG) structures. We performed simultaneous in vivo micro-stimulations of the Schaffer collateral fibres and recorded the evoked field excitatory postsynaptic potentials (fEPSPs) in the stratum radiatum layer of the hippocampal Cornu Ammonis 1 region using a single probe. Inducing the fEPSP at very low charge per pulse settings (3.2-3.6 nC/pulse) indicates the efficient charge injection capability of the IrOx/NPG electrode, thereby enabling safe, prolonged, and thrifty micro-stimulations. Furthermore, the single probe-induced and recorded long-term potentiation persisted for periods longer than 60 min following theta-burst stimulation. The materials used in this study are all biocompatible and chemically robust. The fabricated neural probes can be applied in chronic clinical trials in vivo.

Mechanical and Biological Interactions of Implants with the Brain and Their Impact on Implant Design

Neural prostheses have already a long history and yet the cochlear implant remains the only success story about a longterm sensory function restoration. On the other hand, neural implants for deep brain stimulation are gaining acceptance for variety of disorders including Parkinsons disease and obsessive-compulsive disorder. It is anticipated that the progress in the field has been hampered by a combination of technological and biological factors, such as the limited understanding of the longterm behavior of implants, unreliability of devices, biocompatibility of the implants among others. While the field's understanding of the cell biology of interactions at the biotic-abiotic interface has improved, relatively little attention has been paid on the mechanical factors (stress, strain), and hence on the geometry that can modulate it. This focused review summarizes the recent progress in the understanding of the mechanisms of mechanical interaction between the implants and the brain. The review gives an overview of the factors by which the implants interact acutely and chronically with the tissue: blood-brain barrier (BBB) breach, vascular damage, micromotions, diffusion etc. We propose some design constraints to be considered in future studies. Aspects of the chronic cell-implant interaction will be discussed in view of the chronic local inflammation and the ways of modulating it.

Testing for effects of different stimuli on neuronal firing relative to background activity

OBJECTIVE

Statistical testing for effects of stimuli on the responses of large populations of recorded neurons is a key technique for analyzing data generated using multi-channel recording systems. Combining statistical tests for differences of the responses to different stimuli and tests for changes from background firing, while appealing as apparently focusing analysis on neurons which react to the stimuli, can lead to significant overestimates of the magnitudes of the effects of stimulation or even erroneous identification of responses as being statistically significant.

APPROACH

As this type of combination is common in the neurophysiological literature, we derive a non-parametric hypothesis test for changes of responses to different types of stimuli relative to background firing.

MAIN RESULTS

This test can be used to avoid improperly combining two statistical tests. We also model the overestimation of effects of stimulus type that can be observed in typical single-unit recordings when applying two statistical tests sequentially.

SIGNIFICANCE

Over-estimation of the fraction of neurons with a response can range up to 1800% in plausible recording scenarios. The method of testing derived here avoids this type of error.

Role of the hippocampus in goal-oriented tasks requiring retrieval of spatial versus non-spatial information

The role of the hippocampus in non-spatial memory has been issue of some controversy. To investigate the nature of dorsal hippocampus engagement in spatial and non-spatial memory we performed discrete excitotoxic lesions of this region before mice (C57/BL6) were trained in one of two tasks that required the animals to retrieve a hidden food reward. In the visuospatial task animals had to remember a particular spatial location, independent of odor cues. In contrast, in a non-spatial olfactory task animals had to remember a particular odor, independent of spatial location. The mice were trained in one of these tasks over a period of three days. We found that lesions restricted to the dorsal hippocampus affected performance only in the spatial task. In contrast, lesions that also encompassed a larger portion of the ventral hippocampus caused a moderate deficit in the olfactory task. These results are consistent with the role of the dorsal hippocampus in long-term spatial episodic memory, and support the involvement of larger portions of the hippocampus on the encoding of non-spatial olfactory representations.

What is remembered? Role of attention on the encoding and retrieval of hippocampal representations

The hippocampus is critically involved in storing explicit memory such as memory for space. A defining feature of explicit memory storage is that it requires attention both for encoding and retrieval. Whereas, a great deal is now known about the mechanisms of storage, the mechanisms whereby attention modulates the encoding and retrieval of space and other hippocampus-dependent memory representations are not known. In this review we discuss recent studies, including our own, which show on the cellular level that attention is critical for the stabilization of spatial and reward-associated odour representations. Our findings support the view that in the hippocampus attention selects the reference frame for task-relevant information. This mechanism is in part mediated by dopamine acting through D1/D5 receptors and involves an increase in neuronal synchronization in the gamma band frequency. We propose that synchronous activity leads to enhancements in synaptic strength that mediate the stabilization of hippocampal representations.

A portable microelectrode array recording system incorporating cultured neuronal networks for neurotoxin detection

Cultured neuronal networks, which have the capacity to respond to a wide range of neuroactive compounds, have been suggested to be useful for both screening known analytes and unknown compounds for acute neuropharmacologic effects. Extracellular recording from cultured neuronal networks provides a means for extracting physiologically relevant activity, i.e. action potential firing, in a noninvasive manner conducive for long-term measurements. Previous work from our laboratory described prototype portable systems capable of high signal-to-noise extracellular recordings from cardiac myocytes. The present work describes a portable system tailored to monitoring neuronal extracellular potentials that readily incorporates standardized microelectrode arrays developed by and in use at the University of North Texas. This system utilizes low noise amplifier and filter boards, a two-stage thermal control system with integrated fluidics and a graphical user interface for data acquisition and control implemented on a personal computer. Wherever possible, off-the-shelf components have been utilized for system design and fabrication. During use with cultured neuronal networks, the system typically exhibits input referred noise levels of only 4-6 microVRMS, such that extracellular potentials exceeding 40 microV can be readily resolved. A flow rate of up to 1 ml/min was achieved while the cell recording chamber temperature was maintained within a range of 36-37 degrees C. To demonstrate the capability of this system to resolve small extracellular potentials, pharmacological experiments with cultured neuronal networks have been performed using ion channel blockers, tetrodotoxin and tityustoxin. The implications of the experiments for neurotoxin detection are discussed.

Massively parallel recording of unit and local field potentials with silicon-based electrodes

Parallel recording of neuronal activity in the behaving animal is a prerequisite for our understanding of neuronal representation and storage of information. Here we describe the development of micro-machined silicon microelectrode arrays for unit and local field recordings. The two-dimensional probes with 96 or 64 recording sites provided high-density recording of unit and field activity with minimal tissue displacement or damage. The on-chip active circuit eliminated movement and other artifacts and greatly reduced the weight of the headgear. The precise geometry of the recording tips allowed for the estimation of the spatial location of the recorded neurons and for high-resolution estimation of extracellular current source density. Action potentials could be simultaneously recorded from the soma and dendrites of the same neurons. Silicon technology is a promising approach for high-density, high-resolution sampling of neuronal activity in both basic research and prosthetic devices.

Recording and marking with silicon multichannel electrodes

This protocol describes an implementation of recording and analysis of evoked potentials in the hippocampal cortex, combined with lesioning using multichannel silicon probes. Multichannel recording offers the advantage of capturing a potential field at one instant in time. The potentials are then subjected to current source density (CSD) analysis, to reveal the layer-by-layer current sources and sinks. Signals from each channel of a silicon probe (maximum 16 channels in this study) were amplified and digitized at up to 40 kHz after sample-and-hold circuits. A modular lesion circuit board could be inserted between the input preamplifiers and the silicon probe, such that any one of the 16 electrodes could be connected to a DC lesion current. By making a lesion at the electrode showing a physiological event of interest, the anatomical location of the event can be precisely identified, as shown for the distal dendritic current sink in CA1 following medial perforant path stimulation. Making two discrete lesions through the silicon probe is useful to indicate the degree of tissue shrinkage during histological procedures. In addition, potential/CSD profiles were stable following small movements of the silicon probe, suggesting that the probe did not cause excessive damage to the brain.

Design and demonstration of an automated cell-based biosensor

Cell-based biosensors have the capacity to respond to a wide range of analytes in a physiologically relevant manner and appear well-suited for toxicity monitoring of both known and unknown analytes. One means of acquiring cellular functional information for biosensor applications involves extracellular recording from excitable cells, which can generate noninvasive and long-term measurements. Previous work from our laboratory described a prototype portable system capable of high signal-to-noise extracellular recordings, in spite of deficiencies in thermal control, fluidics handling, and absence of data acquisition (DAQ) capability. The present work describes a cell-based biosensor system that incorporates low noise amplifier and filter boards, a two-stage thermal control system with integrated fluidics and a flexible graphical user interface for DAQ and control implemented on a personal computer. Wherever possible, commercial off-the-shelf components have been utilized for system design and fabrication. The system exhibits input-referred noise levels of 5-10 microV(RMS), such that extracellular potentials exceeding 50-60 microV can be readily resolved. In addition, the biosensor system is capable of automated temperature and fluidics control. Flow rates can range from 0-2.5 ml/min, while the cell recording chamber temperature is maintained within a range of 36-37 degrees C. To demonstrate the capability of this system to resolve small extracellular potentials, recordings from embryonic chick cardiac myocytes have been performed.

Multiple microelectrode-recording system for human intracortical applications

The human brain is dominated by the neocortex, a large folded surface, whose cellular and synaptic elements are arranged in layers. Since cortical structure is relatively constant across its surface, local information processing can be inferred from multiple laminar recordings of its electrical activity along a line perpendicular to its surface. Such recordings need to be spaced at least as close together as the cortical layers, and need to be wideband in order to sample both low frequency synaptic currents as well as high-frequency action potentials. Finally, any device used in the human brain must comply with strict safety standards. The current paper presents details of a system meeting these criteria, together with sample results obtained from epileptic subjects undergoing acute or chronic intracranial monitoring for definition of the epileptogenic region.

Description and demonstration of a CMOS amplifier-based-system with measurement and stimulation capability for bioelectrical signal transduction

An extracellular recording system incorporating an electrode array and an amplifier/stimulator CMOS chip is described and characterized. Important features of this custom VLSI chip include 16 instrumentation amplifiers with a gain of 50 and the incorporation of a cross-point array allowing designation of an extracellular microelectrode as either a stimulator or sensor. The planar array consisted of 32 microelectrodes, 14 microns in diameter, and four larger reference electrodes. Microelectrodes, interconnecting traces, and bond pads were patterned with a 500-nm layer of gold. The interconnecting traces were passivated with a 1-micron thick layer of silicon nitride to provide chemical and electrical insulation and microelectrode impedance was lowered utilizing electrode position of platinum black. The amplifier exhibited a nearly flat frequency response with high pass and low pass corner frequencies of 0.7 Hz and 50 kHz, respectively. The input referred noise over the 50 kHz bandwidth was 12-16 microVRMS, well below the magnitude of previously reported extracellular potentials. Crosstalk between neighboring channels resulted in an output signal below the amplifier noise level, even for relatively large extracellular potentials. Using this system, extracellular recording were demonstrated yielding typical peak-to-peak biopotentials of magnitude 0.9-2.1 mV and 100-400 microV for chick cardiac myocytes and rat spinal cord neurons, respectively. The key components of this extracellular recording system can be manufactured using industry standard thin film photolithographic techniques.

Spatial distribution of field potential profiles in the cat cerebellar cortex evoked by peripheral and central inputs

The present study was designed to characterize the spread of excitation within the frontal plane of the cat cerebellar cortex following different types of stimuli. In particular, experiments were performed to determine whether the spread of excitation evoked by mossy fibre inputs proceeds primarily along the parallel fibres ("beam-like" spread) or whether these inputs activate non-propagated foci ("patches") in the cerebellar cortex. Field potentials were recorded within a frontal plane as a medial to lateral array at different depths in parallel tracks. The recordings were made following electrical stimulation of different forelimb nerves and functionally related areas of the sensorimotor cortex as well as during passive paw movements. The resulting spatial grid of responses provides discrete spatio-temporal information reflecting the activation of specific cerebellar afferents and the neuronal interactions they evoke. The method employed demonstrates the spatial distribution of the temporal sequence of excitability changes throughout all the cerebellar cortical layers. In general, the characteristics of the responses in the intermediate cerebellar cortex depended on the source of the signals. Activity patterns evoked by peripheral nerve stimulation showed more clustered foci compared with those following electrical stimulation of functionally related areas of the sensorimotor cortex. The centrally evoked profiles were generally more homogeneous. The largest number of foci were observed following passive movements around the wrist joint. The spread of excitation in the vertical direction was evaluated by the spatial shift of the line of reversal of the N3/P2-potential (zero-isopotential line). Lines of reversal for peripherally-evoked activity patterns were approximately 90 microns closer to the molecular layer than those evoked by central stimulation in animals in which recordings have been performed in lobule Vc. The opposite was found for recordings in lobule Vb, where potential reversals following peripheral stimulation were located 40 microns deeper than those evoked following central stimulation. Cortical inputs resulted in a more proximal activation of lobule Vc Purkinje cell dendrites than in lobule Vb. This type of input processing thus seems to be lobule dependent. A beam-like spread of excitation could not be demonstrated. For both climbing fibre and mossy fibre afferent systems multiple foci were found in the frontal plane. The foci due to mossy fibre activation arose from the granular layer and expanded vertically to the molecular layer. For the climbing fibre system the foci were restricted to the molecular layer, where they merged to form a superficial band of activation. Although the data presented in this paper favour a focal distribution of activity, they do not exclude beam-like propagation along the parallel fibres, because of the difficulty of detecting this pattern in response to the stimuli. The "beam"- and "patch"-like hypotheses need not be mutually exclusive. Each could contribute to a specific stage of the temporal-spatial processing in the cerebellar cortex in a functional and task-specific manner.

A neuron-silicon junction: a Retzius cell of the leech on an insulated-gate field-effect transistor

An identified neuron of the leech, a Retzius cell, has been attached to the open gate of a p-channel field-effect transistor. Action potentials, spontaneous or stimulated, modulate directly the source-drain current in silicon. The electronic signals match the shape of the action potential. The average voltage on the gate was up to 25 percent of the intracellular voltage change. Occasionally weak signals that resemble the first derivative of the action potential were observed. The junctions can be described by a model that includes capacitive coupling of the plasma membrane and the gate oxide and that accounts for variable resistance of the seal.

Inhibition in the nervous system: models of its roles in choice and context determination

Neural inhibition has often been regarded as playing an important role in stabilizing and tuning the responses of networks of excitatory neurons. Some partial quantitative bases for this qualitative notion are discussed in the context of current neural network models. Such neural network principles as associative learning, competition, opponent processing, and interlevel resonant feedback are explained and related to behavioral and neurochemical data. Tentative analogies of parts of these model networks with specific neurotransmitter systems are explored; these analogies are likely to become more precise as the networks are further refined.

Multisite recording of brain field potentials and unit activity in freely moving rats

A technique has been developed to record from 16 different brain sites of the freely moving rat using subminiature MOSFET preamplifiers. The high input impedance, small size, durability and light weight of the amplifiers and connecting cable allows high quality multisite recording of field potentials and unit activity. In addition, a movable headstage for positioning multiple microelectrodes is described. The compact recording system permits one to construct neocortical EEG maps, instant depth profiles of evoked and spontaneous field data, and to study neuronal synchrony of distant cell populations.

Simultaneous recording of local electrical activity, partial oxygen tension and temperature in the rat hippocampus with a chamber-type microelectrode. Effects of anaesthesia, ischemia and epilepsy

A miniature multiple thin-film recording sensor was used to measure simultaneously the electrical activity, oxygen content and temperature of brain tissue. The chamber-type potential sensor was an Ag/AgCl electrode covered by an Si3N4 (silicon nitride) chamber. The chamber-type oxygen sensor consisted of an Au-Ag/AgCl two-electrode electrochemical cell embedded in an electrolyte-filled Si3N4 chamber. The temperature sensor was a thin-film germanium resistor. The different sensors were spaced 300 microns apart. Anaesthetics (pentobarbital, chloral hydrate, chlornembutal, halothane) were shown to depress electrical activity and to increase local oxygen tension in the hippocampus. Halothane, but not the other anaesthetics, also increased the current output of the oxygen sensor when tested in saline bath, indicating that the apparent increase in measured oxygen levels during halothane anaesthesia was partly due to an electrochemical effect of halothane on the oxygen sensors. The decrease of tissue oxygen consumption produced by the other anaesthetics is likely to be the result of metabolic depression. Cerebral ischemia, evoked by cauterization of the vertebral arteries and occlusion of the carotid arteries for 30 min, resulted in the disappearance of both spontaneous and evoked electrical activity in the hippocampus and a decrease of both local temperature and oxygen tension. There was a marked overshoot of the oxygen tension to above preocclusion level following the release of the carotid arteries. As soon as electrical activity returned, the oxygen tension fell again, often below the lowest level seen during the ischemic period. This secondary decrease of oxygen level could be reversed by administration of supplementary small doses of anaesthetic. The anaesthetic-induced increase in oxygen tension was accompanied by a marked decrease in electroencephalogram amplitude and frequency. During electrically induced seizures a decrease in hippocampal oxygen content occurred and was accompanied by an increase of local temperature. Since the rectal temperature was kept constant, the changes in temperature are likely to reflect changes in blood perfusion of the recorded area. These findings are in agreement with previous observations made with conventional electrodes. In addition, the miniature size of the chamber-type microelectrode assembly allows a correlated monitoring of parallel physiological changes with high spatial and temporal resolution during anaesthesia, ischemia and epilepsy.