Tobias Holzhammer

A wireless multi-channel recording system for freely behaving mice and rats.

To understand the neural basis of behavior, it is necessary to record brain activity in freely moving animals. Advances in implantable multi-electrode array technology have enabled researchers to record the activity of neuronal ensembles from multiple brain regions. The full potential of this approach is currently limited by reliance on cable tethers, with bundles of wires connecting the implanted electrodes to the data acquisition system while impeding the natural behavior of the animal. To overcome these limitations, here we introduce a multi-channel wireless headstage system designed for small animals such as rats and mice. A variety of single unit and local field potential signals were recorded from the dorsal striatum and substantia nigra in mice and the ventral striatum and prefrontal cortex simultaneously in rats. This wireless system could be interfaced with commercially available data acquisition systems, and the signals obtained were comparable in quality to those acquired using cable tethers. On account of its small size, light weight, and rechargeable battery, this wireless headstage system is suitable for studying the neural basis of natural behavior, eliminating the need for wires, commutators, and other limitations associated with traditional tethered recording systems.

Oscillatory Activity in the Medial Prefrontal Cortex and Nucleus Accumbens Correlates with Impulsivity and Reward Outcome

Actions expressed prematurely without regard for their consequences are considered impulsive. Such behaviour is governed by a network of brain regions including the prefrontal cortex (PFC) and nucleus accumbens (NAcb) and is prevalent in disorders including attention deficit hyperactivity disorder (ADHD) and drug addiction. However, little is known of the relationship between neural activity in these regions and specific forms of impulsive behaviour. In the present study we investigated local field potential (LFP) oscillations in distinct sub-regions of the PFC and NAcb on a 5-choice serial reaction time task (5-CSRTT), which measures sustained, spatially-divided visual attention and action restraint. The main findings show that power in gamma frequency (50–60 Hz) LFP oscillations transiently increases in the PFC and NAcb during both the anticipation of a cue signalling the spatial location of a nose-poke response and again following correct responses. Gamma oscillations were coupled to low-frequency delta oscillations in both regions; this coupling strengthened specifically when an error response was made. Theta (7–9 Hz) LFP power in the PFC and NAcb increased during the waiting period and was also related to response outcome. Additionally, both gamma and theta power were significantly affected by upcoming premature responses as rats waited for the visual cue to respond. In a subgroup of rats showing persistently high levels of impulsivity we found that impulsivity was associated with increased error signals following a nose-poke response, as well as reduced signals of previous trial outcome during the waiting period. Collectively, these in-vivo neurophysiological findings further implicate the PFC and NAcb in anticipatory impulsive responses and provide evidence that abnormalities in the encoding of rewarding outcomes may underlie trait-like impulsive behaviour.

Application of floating silicon-based linear multielectrode arrays for acute recording of single neuron activity in awake behaving monkeys.

Abstract One of the fundamental challenges in behavioral neurophysiology in awake animals is the steady recording of action potentials of many single neurons for as long as possible. Here, we present single neuron data obtained during acute recordings mainly from premotor cortices of three macaque monkeys using a silicon-based linear multielectrode array. The most important aspect of these probes, compared with similar models commercially available, is that, once inserted into the brain using a dedicated insertion device providing an intermediate probe fixation by means of vacuum, they can be released and left floating in the brain. On the basis of our data, these features appear to provide (i) optimal physiological conditions for extracellular recordings, (ii) good or even excellent signal-to-noise ratio depending on the recorded brain area and cortical layer, and (iii) extreme stability of the signal over relatively long periods. The quality of the recorded signal did not change significantly after several penetrations into the same restricted cortical sector, suggesting limited tissue damage due to probe insertion. These results indicate that these probes offer several advantages for acute neurophysiological experiments in awake monkeys, and suggest the possibility to employ them for semichronic or even chronic studies.

Integration of silicon-based neural probes and micro-drive arrays for chronic recording of large populations of neurons in behaving animals.

OBJECTIVE Understanding how neuronal assemblies underlie cognitive function is a fundamental question in system neuroscience. It poses the technical challenge to monitor the activity of populations of neurons, potentially widely separated, in relation to behaviour. In this paper, we present a new system which aims at simultaneously recording from a large population of neurons from multiple separated brain regions in freely behaving animals. APPROACH The concept of the new device is to combine the benefits of two existing electrophysiological techniques, i.e. the flexibility and modularity of micro-drive arrays and the high sampling ability of electrode-dense silicon probes. MAIN RESULTS Newly engineered long bendable silicon probes were integrated into a micro-drive array. The resulting device can carry up to 16 independently movable silicon probes, each carrying 16 recording sites. Populations of neurons were recorded simultaneously in multiple cortical and/or hippocampal sites in two freely behaving implanted rats. SIGNIFICANCE Current approaches to monitor neuronal activity either allow to flexibly record from multiple widely separated brain regions (micro-drive arrays) but with a limited sampling density or to provide denser sampling at the expense of a flexible placement in multiple brain regions (neural probes). By combining these two approaches and their benefits, we present an alternative solution for flexible and simultaneous recordings from widely distributed populations of neurons in freely behaving rats.

Novel method for the assembly and electrical contacting of out-of-plane microstructures

This paper reports on the assembly of out-of-plane microstructures with multiple electrical contacts established at the same time. The reported structures are intended for neuroscientific applications. Dedicated bays in a platform ensure the orthogonal orientation of the microstructure. In addition, a 10-µm-thick polymer cable is clamped between bays and microstructures and establishes the fixation of the probes. This cable, enhanced by gold bumps, ensures the transfer of conducting lines from the horizontal into the vertical direction. It simultaneously provides the electrical contacts to the microstructures. The contact resistance of individual contacts was found to be smaller than 2 Ω.

CMOS-based neural probe with enhanced electronic depth control

This paper reports on the design, fabrication, and testing of complementary-metal-oxide-semiconductor (CMOS)-based high-density neural probes. An enhanced electronic depth control (EDC) scheme is implemented using a switch matrix integrated in the slender probe shaft, allowing simultaneous reconfiguration and recording on unaffected channels. The number of simultaneously available analog output channels is increased to 16 compared to the initial EDC probes of our group. Probe shaft length up to 10 mm with a shaft width and thickness of 100 μm and 40 μm, respectively, have been realized. A maximum number of 334 electrodes is achieved on the longest probe variant with an inter-electrode spacing of 30 μm along the probe shafts. The addressing and switching scheme is realized using the commercial 0.18 μm six-metal, double-poly CMOS process from XFAB combined with an in-house post processing of the electrode metallization and probe patterning. The probe functionality is verified using bench tests and the electrode impedance characterization revealing 1.4±0.2 MΩ at 1 kHz for platinum electrodes with a diameter of 30 μm.

Compact wireless neural recording system for small animals using silicon-based probe arrays

This paper reports on a compact, small-scale neural recording system combining state-of-art silicon-based probe arrays with a light-weight 32-channel wireless head stage. The system is equipped with two- and four-shaft, comb-shaped probe arrays connected to highly flexible ribbon cables enabling a reliable and controlled insertion of probe arrays through the intact dura mater into the medial prefrontal cortex and nucleus accumbens of rats. The in vivo experiments applied the 5-choice serial reaction time task (5-CSRTT) using freely behaving rats in order to understand the neural basis of sustained visual attention and impulsivity. The long-term stability of the system allowed local field potential (LFP) activity to be recorded without a significant decrement in signal quality for up to 28 weeks, and similarly, we were able to follow single unit activity for up to 4 weeks.

Out-of-plane assembly of 3D neural probe arrays using a platform with SU-8-based thermal actuators

This paper describes an optimized out-of-plane assembly method based on a platform with integrated thermal actuators. The platform comprises cavities with overhanging gold clips embedded into a flexible polyimide (PI) cable. Planar micro components are inserted perpendicularly into the platform cavities and electrically connected to the PI cable by bending the gold clips into the cavities. In contrast to previous approaches using platforms of fixed cavity sizes [1–3], the new approach applies SU-8-based thermal actuators capable of widening the cavities during the assembly process by up to 9 µm for a temperature increase of 155 K. The directionally effective thermal expansion of nine actuator designs is evaluated experimentally and compared with finite element (FE) simulations. The optimized system design is successfully demonstrated with a three-dimensional (3D) neural probe array comprising 40 individually addressable electrodes.

Ultrathin, dual-sided silicon neural microprobes realized using BCB bonding and aluminum sacrificial etching

This paper presents an innovative fabrication process for dual-sided silicon-based microprobe arrays using (i) temporary wafer bonding applying B-staged bisbenzo-cyclobutene (BCB), (ii) wafer grinding, (iii) deep reactive ion etching (DRIE), and (iv) the electrochemical removal of a sacrificial aluminum layer. The dual-sided microprobes comprise aligned electrodes on the front and rear of 120-μm-wide and only 50-μm-thick probe shafts. The temporary BCB bonding to a glass substrate is compatible with process temperatures up to 300°C and with DRIE. Furthermore, dual-side mask alignment is enabled by the high optical transparency of both the glass substrate and the BCB bonding layer. Even at this exploratory stage, probes realized using this process sequence have exhibited a yield of functional electrodes of better than 96% after probe assembly. Initial in vivo electrophysiology recordings in a rat brain have demonstrated an satisfactory probe performance.

Why not record from every channel with a CMOS scanning probe

Neural recording devices normally require one output connection for each electrode. This constrains the number of electrodes that can be accommodated by the thin shafts of implantable probes. Sharing a single output connection between multiple electrodes relaxes this constraint and permits designs of ultra-high density neural probes. Here we report the design and in vivo validation of such a device, a complementary metal-oxide-semiconductor (CMOS) scanning probe with 1344 electrodes and 12 reference electrodes along an 8.1 mm × 100 μm × 50 μm shaft; the outcome of the European research project NeuroSeeker. This technology presented new challenges for data management and visualization, and we also report new methods addressing these challenges developed within NeuroSeeker. Scanning CMOS technology allows the fabrication of much smaller, denser electrode arrays. To help design electrode configurations for future probes, several recordings from many different brain regions were made with an ultra-dense passive probe fabricated using CMOS process. All datasets are available online.