Min-Chi Hsiao

Custom-designed high-density conformal planar multielectrode arrays for brain slice electrophysiology

Multielectrode arrays have enabled electrophysiological experiments exploring spatio-temporal dynamics previously unattainable with single electrode recordings. The finite number of electrodes in planar MEAs (pMEAs), however, imposes a trade-off between the spatial resolution and the recording area. This limitation was circumvented in this paper through the custom design of experiment-specific tissue-conformal high-density pMEAs (cMEAs). Four configurations were presented as examples of cMEAs designed for specific stimulation and recording experiments in acute hippocampal slices. These cMEAs conformed in designs to the slice cytoarchitecture whereas their high-density provided high spatial resolution for selective stimulation of afferent pathways and current source density (CSD) analysis. The cMEAs have 50 or 60 microm center-to-center inter-electrode distances and were manufactured on glass substrates by photolithographically defining ITO leads, insulating them with silicon nitride and SU-8 2000 epoxy-based photoresist and coating the etched electrode tips with gold or platinum. The ability of these cMEAs to stimulate and record electrophysiological activity was demonstrated by recording monosynaptic, disynaptic, and trisynaptic field potentials. The conformal designs also facilitated the selection of the optimal electrode locations for stimulation of specific afferent pathways (Schaffer collaterals; medial versus lateral perforant path) and recording the corresponding responses. In addition, the high-density of the arrays enabled CSD analysis of laminar profiles obtained through sequential stimulation along the CA1 pyramidal tree.

An in vitro seizure model from human hippocampal slices using multi-electrode arrays.

Temporal lobe epilepsy is a neurological condition marked by seizures, typically accompanied by large amplitude synchronous electrophysiological discharges, affecting a variety of mental and physical functions. The neurobiological mechanisms responsible for the onset and termination of seizures are still unclear. While pharmacological therapies can suppress the symptoms of seizures, typically 30% of patients do not respond well to drug control. Unilateral temporal lobectomy, a procedure in which a substantial part of the hippocampal formation and surrounding tissue is removed, is a common surgical treatment for medically refractory epilepsy. In this study, we have developed an in vitro model of epilepsy using human hippocampal slices resected from patients suffering from intractable mesial temporal lobe epilepsy. We show that using a planar multi-electrode array system, spatio-temporal inter-ictal like activity can be consistently recorded in high-potassium (8 mM), low-magnesium (0.25 mM) artificial cerebral spinal fluid with 4-aminopyridine (100 μM) added. The induced epileptiform discharges can be recorded in different subregions of the hippocampus, including dentate, CA1 and subiculum. This new paradigm will allow the study of seizure generation in different subregions of hippocampus simultaneously, as well as propagation of seizure activity throughout the intrinsic circuitry of hippocampus. This experimental model also should provide insights into seizure control and prevention, while providing a platform to develop novel, anti-seizure therapeutics.

A method for unit recording in the lumbar spinal cord during locomotion of the conscious adult rat.

Extracellular recordings from single units in the brain, for example the neocortex, have proven feasible in moving, awake rats, but have not yet been possible in the spinal cord. Single-unit activity during locomotor-like activity in reduced preparations from adult cats and rats have provided valuable insights for the development of hypotheses about the organization of functional networks in the spinal cord. However, since reduced preparations could result in spurious conclusions, it is crucial to test these hypotheses in animals that are awake and behaving. Furthermore, unresolved issues such as how muscle force precision is achieved by motoneurons as well as how spinal neurons are spatio-temporally correlated are better to investigate in the conscious and behaving animal. We have therefore developed procedures to implant arrays of extracellular recording electrodes in the lumbar spinal cord of the adult rat for long-term studies. In addition, we implanted pairs of electromyographic electrodes in the hindlimbs for the purpose of monitoring locomotion. With our technique, we obtained stable long-term recordings of spinal units, even during locomotion. We suggest this as a novel method for investigating motor pattern-generating circuitry in the spinal cord.

Real time hardware neural spike amplitude extraction

A novel algorithm for population spike (PS) amplitude extraction suitable for real time hardware processing was developed. The extraction method was implemented digitally and experimentally tested on a field programmable gate array (FPGA) device using 16-bit quantization. The accuracy of the implementation was tested using PS signals recorded from hippocampal slices. The PS response of the dentate gyrus granule cells were generated in a multi-electrode array (MEA) setup. Spike amplitudes extracted in real time by the hardware were compared with values resulting from floating-point computation in software. Results showed successful implementation of hardware algorithm with average normalized mean square error (NMSE) less than 2%.

Nonlinear dynamical model based control of in vitro hippocampal output

This paper describes a modeling-control paradigm to control the hippocampal output (CA1 response) for the development of hippocampal prostheses. In order to bypass a damaged hippocampal region (e.g., CA3), downstream hippocampal signal (e.g., CA1 responses) needs to be reinstated based on the upstream hippocampal signal (e.g., dentate gyrus responses) via appropriate stimulations to the downstream (CA1) region. In this approach, we optimize the stimulation signal to CA1 by using a predictive DG-CA1 nonlinear model (i.e., DG-CA1 trajectory model) and an inversion of the CA1 input–output model (i.e., inverse CA1 plant model). The desired CA1 responses are first predicted by the DG-CA1 trajectory model and then used to derive the optimal stimulation intensity through the inverse CA1 plant model. Laguerre-Volterra kernel models for random-interval, graded-input, contemporaneous-graded-output system are formulated and applied to build the DG-CA1 trajectory model and the CA1 plant model. The inverse CA1 plant model to transform desired output to input stimulation is derived from the CA1 plant model. We validate this paradigm with rat hippocampal slice preparations. Results show that the CA1 responses evoked by the optimal stimulations accurately replicate the CA1 responses recorded in the hippocampal slice with intact trisynaptic pathway.

Control theory-based regulation of hippocampal CA1 nonlinear dynamics

We are developing a biomimetic electronic neural prosthesis to replace regions of the hippocampal brain area that have been damaged by disease or insult. Our previous study has shown that the VLSI implementation of a CA3 nonlinear dynamic model can functionally replace the CA3 subregion of the hippocampal slice [1]. As a result, the propagation of temporal patterns of activity from DG→VLSI→CA1 reproduces the activity observed experimentally in the biological DG→CA3→CA1 circuit. In this project, we incorporate an open-loop controller to optimize the output (CA1) response. Specifically, we seek to optimize the stimulation signal to CA1 using a predictive dentate gyrus (DG)-CA1 nonlinear model (i.e., DG-CA1 trajectory model) and a CA1 input-output model (i.e., CA1 plant model), such that the ultimate CA1 response (i.e., desired output) can be first predicted by the DG-CA1 trajectory model and then transformed to the desired stimulation through the inversed CA1 plant model. Lastly, the desired CA1 output is evoked by the estimated optimal stimulation. This study will be the first stage of formulating an integrated modeling-control strategy for the hippocampal neural prosthetic system.

Design of a flexible parylene-based multi-electrode array for multi-region recording from the rat hippocampus.

The hippocampus is a critical deep brain structure in several aspects. It is directly related to the formation of new long-term declarative memory. The malfunction of the hippocampus closely relates to various disease and pathological conditions. It is also a model structure for the study of cortical function and synaptic plasticity in general because of its special neuro-anatomical structure and intrinsic connections within the hippocampus formation. Both the understanding of roles that the hippocampus plays in recognition memory and the study of neural plasticity require simultaneously recording of neural activities from multiple sub-regions of the hippocampus from behavioral animals. However the distribution of cells in the hippocampus make the recording from multiple sub-regions a big challenge with the traditional uni-length micro-wire arrays. Well-designed electrode arrays are required to reach multiple regions simultaneously because of the distinctive double C shape of the hippocampus cell body layers. In this work, we designed a multi-shanks electrode which uses Parylene C, a highly biocompatible and flexible polymer, as a base and has multiple recording sites specially positioned along the longitudinal axis to fit the curvy shape of the rat hippocampus.

A flexible parylene probe for in vivo recordings from multiple subregions of the rat hippocampus

The hippocampus is crucial to the formation of long-term memory and declarative memory. It is divided into three sub-fields the CA1, the CA3 and the DG. To understand the neuronal circuitry within the hippocampus and to study the role of the hippocampus in memory function requires the collection of neural activities from multiple subregions of the hippocampus simultaneously. Micro-wire electrode arrays are commonly used as an interface with neural systems. However, recording from multiple deep brain regions with curved anatomical structures such as the thin cell body layers of the hippocampus requires the micro-wires to be arranged into a highly accurate, complex layout that is difficult to fabricated manually. In this work, we designed and developed a flexible parylene-C based neural probe which can be easily micro-machined to the desired dimensions. Sixty-four electrical recording sites are micromachined on to 8 parylene shanks and spaced according to the distribution of hippocampal principal neurons in different hippocampus subregions. Together with our collaborators, we developed and optimized the implantation procedure of the flexible parylene probe and tested the insertion method both in brain tissue phantom and in vivo with a sham device. Immunohistochemistry (IHC) staining post-implantation of the sham probe was used to verify the location of the probe and to evaluate immune responses to the probe. Fully functional devices were fabricated and, in future studies, functional probes will be chronically implanted into the rat hippocampus, and neural activities will be recorded and compared with signals obtained with micro-wire arrays.

Hippocampal Microcircuits, Functional Connectivity, and Prostheses

Hippocampus is a brain region critical for the formation of new long-term declarative memories. It transmits and processes memory information with its distinct feedforward trisynaptic pathway. Identifying functional properties of the hippocampal circuits is important for understanding the mechanisms of memory formation and building hippocampal prostheses for restoring memory functions lost in diseases or injuries. In hippocampal slices, trisynaptic responses can be elicited and recorded using conformal multi-electrode arrays. A proof-of-principle hippocampal prosthetic system has been successfully developed based on a computational model that accurately describes the input-output properties of the hippocampal circuit. In behaving animals, hippocampal functional connectivities are analyzed with a nonlinear dynamical multi-input, multi-output (MIMO) model using behaviorally-driven spiking data. Results show that the hippocampal CA3-CA1 functional connection is diffusive along the septo-temporal axis, as opposed to strictly laminar. There are strong causal relations between the CA3 and CA1 spiking activities. The MIMO model can accurately predict the spatio-temporal patterns of the CA1 output spikes based on the ongoing spatio-temporal patterns of the CA3 input spikes. MIMO model-based electrical stimulation to the CA1 region effectively restores the hippocampal memory function by reinstating the CA1 activities. The recording component, the nonlinear dynamical MIMO model, and the stimulation component essentially constitute a closed-loop prosthetic system that bypasses the impaired hippocampal region.

Recording place cells from multiple sub-regions of the rat hippocampus with a customized micro-electrode array.

The hippocampus is a subcortical structure which is involved in memory function. There is a considerable amount of evidence available which indicates that the hippocampal system is necessary for effective spatial learning in rodents and short-term topographical memory in human. Recordings of neural activities from the hippocampus of behaving animals can help us to understand how spatial information is encoded and processed by the hippocampus. In this work, we designed a triple-region microelectrode array (MEA) which took into concern the anatomical structures of the rat hippocampus. The array was composed of 16 stainless steel wires which were arranged into three groups that differed in length. Each group targeted one subregion of the hippocampus. The array was chronically implanted into the rat hippocampus through craniotomy. Neural activities were monitored both during the implantation and after recovery. The triple-region MEA was capable of recording unitary activities from multiple subregions of the rat hippocampus and the spatial distribution of firing rates were analyzed while the animal freely explored in the environment.