Electroencephalogram (EEG) recording is a widely used method to measure electrical activity in the brain. Rodent EEG brain recording not only is noninvasive but also has the advantages to accomplish full brain monitoring, compared with that of the invasive techniques like micro-electrode-arrays. In comparison to other noninvasive recording techniques, EEG is the only technique that can achieve sub-ms scale time resolution, which is essential to obtain causal relationship. In this work, we demonstrated a simple microfabrication process for developing a high-density polyimide-based rodent EEG recording cap. A 34-channel rodent electrode array with a total size of 11mmx8mm, individual electrode diameter 240μm and interconnect wire linewidth 35μm was designed and fabricated. For the fabrication process, we first deposit 350nm SiO2 on a silicon substrate. We then fabricate 6-7μm thick first layer polyimide caps with fingers and contact holes. Gold deposition and then lithography etching of 34 channel contact-electrodes and their interconnects were fabricated in the second step. The third step was to cover metal interconnects with a 10μm thick second layer polyimide, which was fabricated with photolithography before the final film released by HF undercutting etching of SiO2 layer. Then the fabricated EEG cap is interfaced with a commercial 34-channel female connector, which is soldered with 34-line wires. These wires are then connected to an ADC to record the EEG data in computer for post-processing. With polyimide, the EEG cap is biocompatible, and flexible which makes it suitable for good contact with rodent skulls.
There is a rise in the study of functional connectivity among various cortical regions and investigations to uncover causal links between a stimulus and the corresponding neural dynamics through electrophysiological imaging of the human brain. Animal model that exhibit simplistic representations of such networks open a doorway for such investigations and are gaining rapid popularity. In this study, we investigate and compare resting state network and auditory stimulus related activity with minimal invasive technology along computational spectral analysis on a C57/BL6 based mouse model. Somatosensory, motor and visual cortex are observed to be highly active and significantly correlated (p-value<0.05). Moreover, given the spatial limitation due to small size of the mouse head, we also describe a low-cost and effective fabrication process for the mouse EEG Polyimide Based Microelectrodes (PBM) array. The easy-to-implement fabrication process involves transfer of the pattern on a copper layer of the Kapton film followed by gold electroplating and application of insulation paint. Acoustic stimulation is done by using tube extensions for avoiding electrical coupling to EEG signals. Unlike multi-electrode array type of invasive methods that are local to a cortical region, the methods established in this study can be used for examining functional connectivity analysis, neural dynamics and cortical response at a global level.
Brain simulation techniques have demonstrated undisputable therapeutic effects on neural diseases. Invasive stimulation techniques like deep brain stimulation (DBS) and noninvasive techniques like transcranial magnetic stimulation (TMS) have been approved by FDA as treatments for many drug resist neural disorders and diseases. Developing noninvasive, deep, and targeted brain stimulation techniques is currently one of the important tasks in brain researches. Transcranial direct current stimulation (tDCS) and transcranial alternative current stimulation (tACS) techniques have the advantages of low cost and portability. However, neither of them can produce targeted stimulation due to lacking of electrical field focusing mechanism. Recently, Grossman et al. reported using the down beating signals of two tACS signals to accomplish focused stimulation. By sending two sine waves running at slightly different high frequencies (~2kHz), they demonstrated that they can modulate a “localized” neuron group at the difference frequency of the two sine waves and at the same time avoid excitation of neurons at other locations. As a result, equivalent focusing effect was accomplished by such beating mechanism. In this work, we show neither theoretically nor experimentally the beating mechanism can produce “focusing effect” and the beating signal spread globally across the full brain. The localized modulation effect likely happened right at the electrode contact sites when the electrode contact area is small and the current is concentrated. We conclude that to accomplish noninvasive and focused stimulation at current stage the only available tool is the focused TMS system we recently demonstrated.
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