Birthe Rubehn

Attentional stimulus selection through selective synchronization between monkey visual areas.

A central motif in neuronal networks is convergence, linking several input neurons to one target neuron. In visual cortex, convergence renders target neurons responsive to complex stimuli. Yet, convergence typically sends multiple stimuli to a target, and the behaviorally relevant stimulus must be selected. We used two stimuli, activating separate electrocorticographic V1 sites, and both activating an electrocorticographic V4 site equally strongly. When one of those stimuli activated one V1 site, it gamma synchronized (60-80 Hz) to V4. When the two stimuli activated two V1 sites, primarily the relevant one gamma synchronized to V4. Frequency bands of gamma activities showed substantial overlap containing the band of interareal coherence. The relevant V1 site had its gamma peak frequency 2-3 Hz higher than the irrelevant V1 site and 4-6 Hz higher than V4. Gamma-mediated interareal influences were predominantly directed from V1 to V4. We propose that selective synchronization renders relevant input effective, thereby modulating effective connectivity.

A MEMS-based flexible multichannel ECoG-electrode array.

We present a micromachined 252-channel ECoG (electrocorticogram)-electrode array, which is made of a thin polyimide foil substrate enclosing sputtered platinum electrode sites and conductor paths. The array subtends an area of approximately 35 mm by 60 mm and is designed to cover large parts of a hemisphere of a macaque monkeys cortex. Eight omnetics connectors are directly soldered to the foil. This leads to a compact assembly size which enables a chronic implantation of the array and allows free movements of the animal between the recording sessions. The electrode sites are 1 mm in diameter and were characterized by electrochemical impedance spectroscopy. At 1 kHz, the electrode impedances vary between 1.5 kOmega and 5 kOmega. The yield of functioning electrodes in three assembled devices is 99.5%. After implantation of a device with 100% working electrodes, standard electrocorticographic signals can be obtained from every electrode. The response to visual stimuli can be measured with electrodes lying on the visual cortex. After an implantation time of 4.5 months, all electrodes are still working and no decline in signal quality could be observed.

Brain–computer interfaces: an overview of the hardware to record neural signals from the cortex

Brain-computer interfaces (BCIs) record neural signals from cortical origin with the objective to control a user interface for communication purposes, a robotic artifact or artificial limb as actuator. One of the key components of such a neuroprosthetic system is the neuro-technical interface itself, the electrode array. In this chapter, different designs and manufacturing techniques will be compared and assessed with respect to scaling and assembling limitations. The overview includes electroencephalogram (EEG) electrodes and epicortical brain-machine interfaces to record local field potentials (LFPs) from the surface of the cortex as well as intracortical needle electrodes that are intended to record single-unit activity. Two exemplary complementary technologies for micromachining of polyimide-based arrays and laser manufacturing of silicone rubber are presented and discussed with respect to spatial resolution, scaling limitations, and system properties. Advanced silicon micromachining technologies have led to highly sophisticated intracortical electrode arrays for fundamental neuroscientific applications. In this chapter, major approaches from the USA and Europe will be introduced and compared concerning complexity, modularity, and reliability. An assessment of the different technological solutions comparable to a strength weaknesses opportunities, and threats (SWOT) analysis might serve as guidance to select the adequate electrode array configuration for each control paradigm and strategy to realize robust, fast, and reliable BCIs.

Microprobe Array with Low Impedance Electrodes and Highly Flexible Polyimide Cables for Acute Neural Recording

This paper reports on a novel type of silicon- based microprobes with linear, two and three dimensional (3D) distribution of their recording sites. The microprobes comprise either single shafts, combs with multiple shafts or 3D arrays combining two combs with 9, 36 or 72 recording sites, respectively. The electrical interconnection of the probes is achieved through highly flexible polyimide ribbon cables attached using the MicroFlex Technology which allows a connection part of small lateral dimensions. For an improved handling, probes can be secured by a protecting canula. Low-impedance electrodes are achieved by the deposition of platinum black. First in vivo experiments proved the capability to record single action potentials in the motor cortex from electrodes close to the tip as well as body electrodes along the shaft.

Polymer-based shaft microelectrodes with optical and fluidic capabilities as a tool for optogenetics

In this work, we describe the fabrication of a polymer-based shaft electrode which can conduct light as well as fluids to a target brain region and record electrical neural signals from the same tissue volume simultaneously. This multifunctional neural probe is intended to facilitate optogenetic in vivo experiments.

Evaluation of polyimide as substrate material for electrodes to interface the peripheral nervous system

The introduction of micromachining technologies into design and development of interfaces to the peripheral nervous systems more than 20 years ago led to novel silicon and polymer based microimplants. Polyimide has been chosen as candidate for flexible implants. In this work, the investigations on different designs, the material stability and different aspects of biocompatibility as well as the functional outcome of different electrode designs will be presented. Sieve electrodes to interface regenerating nerves, multichannel cuff electrodes but also newer concepts like longitudinal and transverse intrafascicular multichannel electrodes (LIFE, TIME) have been developed with polyimide as substrate and insulation material. Evaluation in vitro as well as in vivo of the material itself but also of the different electrode designs and concepts showed good material stability, no material toxicity, only little foreign body reaction after implantation and good spatial selectivity of the electrodes depending on the chosen concept to interface the peripheral nerve.

MEMS-Technology for Large-Scale, Multichannel ECoG-Electrode Array Manufacturing

We present a micromachined ECoG (electrocorticogram)-electrode array which combines structures in the micrometer range with a total array area of approximately 60 mm by 35 mm. This enables the distribution of 252 electrodes over a complete hemisphere of a monkey’s cortex. The array and the integrated cables are processed on wafer level and are made of a 10 µm thick polyimide foil which encloses a 300 nm thick sputtered platinum layer serving as electrodes and conductor paths. The electrode sites are 1 mm in diameter, the conductor paths have a pitch of 30 µm. 8 Omnetics connectors (NPD series, Omnetics Connector Corp., Minneapolis, MN, USA) are directly soldered to the foil to maintain a small assembly size which allows free movements of the animal between experimental sessions. The electrodes are characterized by electrochemical impedance spectroscopy. At 1 kHz the electrode impedances vary between 1.5 kΩ and 5 kΩ. The yiel functionable electrodes is 99.5 %.

Measurement of Defects in Spin Coated Polyimide Films

Polyimide is a material often used in thin film neural microelectrodes. In this work, we processed thin films from three different polyimide precursors, Pyralin PI2611, Durimide 7510 and U-Varnish S, and tested them with respect to their defect density. We regard defects as areas of any size where the insulation of the thin film is damaged. All three materials were measured with a defect density less than 0.2 cm-2. To obtain a defect free polyimide layer it is crucial to process the precursor in a particle free environment.

Fatigue Testing of Polyimide-Based Micro Implants

Due to its excellent chemical and mechanical properties, polyimide is frequently used as substrate material for electrodes in biomedical applications. In addition, highly flexible polyimide ribbon cables are integrated in hybrid micro devices such as silicon-based neural probes to connect these recording and/or stimulation units with external instrumentation. Typically a metallization layer, e.g. gold or platinum, is sandwiched between two polyimide layers with an implant thickness in the range of 10 µm. This paper describes a bending setup constructed to perform fatigue tests with dedicated polyimide specimens in physiological environment. A pivot mounted rod bends the specimens around defined radii of 1, 1.5, 2 or 6 mm at a maximum frequency of 2 Hz. The system concept allows to freely choose angels between 0° and 90° in steps of 0.9°. The applied test method is adapted to t European standard DIN EN 45502-2-1 for pacemaker leads, stipulating the application of mechanical loads by repetitive bending of the specimens. The machine allows the simultaneous testing of up to five samples in a physiological environment. Additionally, online four-wire resistance measurements are performed to detect functional failures during the test. Control specimens are stored at equal conditions without mechanical load. The resistance of all tested samples remained stable longer then the 47,000 cycles demanded in the standard.