Microinvasive Probes for Monitoring Electrical and Chemical Neural Activity in Nonhuman Primates

Abstract

We leveraged carbon fiber materials for creating sensors that provide dual neurochemical and electrical neural activity recording at microinvasive (10 μm) spatial footprints proximal to recording sites, and enabling these measurements from deep brain targets of primates with conventional cranial chambers. These shaft-assisted microinvasive probes (s-μIPs) are approximately 10 μm in diameter along the distal length (1-15 mm) immediately surrounding the targeted recording site. This microinvasive portion ensures that the recording site is isolated from tissue damage induced by the wider shaft portion of the device. The shaft (150-165 μm in diameter) within the device stiffens the remaining length of the probe (>100 mm), and provides compatibility with standard intracranial insertion protocols (e.g., guide tubes and chamber setups) that require a sufficiently rigid and long shaft for deep brain insertion in monkeys. The s-μIP was further expanded to provide dual-channel chemical and electrical neural activity recording with micrometer spatial resolution. Measurements of reward- and movement- related dopamine, spikes, and local field potentials were made from single and dual-channel s-μIPs implanted in task-performing monkeys. Recordings from chronically implanted s-μIPs display the capability of functional multimodal (chemical and electrical) neural activity measurements over 1-year postimplantation from microinvasive devices.

Keywords: dopamine recording; fast-scan cyclic voltammetry; microinvasive neural implant.

1

Overview of the s-μIP. The s-μIP is capable of recording both neurochemical and electrical neural activity via electrochemical (FSCV) and electrophysiological (EPhys) recording, respectively. Left inset shows a magnified photo of the tip of a fabricated device, to visualize the recording tips on two flexible CF electrode threads (CFETs) (arrowheads) emerging from the tapered end of the silica tube shaft. The cellular scale (∼10 μm) diameter of the CFETs (1–15 mm long) ensures minimal trauma to the tissue near the targeted recording site while the stiff shaft (165 μm diameter, 90–100 mm long) helps reinforce the device for insertion toward deep brain targets, such as the monkey striatum (illustrated on the bottom).

2

s-μIP fabrication process. Individual steps are further detailed in “Methods: s-μIP Fabrication Process”. (A) 150 mm long tungsten (W) rods (50 μm diameter) are electrochemically etched to reduce their thicknesses and create a tapered tip for subsequent connection of the carbon fiber (CF). (B) The tungsten wire is attached to a 20–30 mm long CF using silver epoxy. Multiple tungsten-CF assemblies (W-CF) are mounted on a custom 3D-printed filament separating fixture and are processed for conformal parylene-C (Ppy) deposition. (C) The py insulation is stripped from the tips of the coated CFs (py-CFs) to expose the recording tips by flame etching with a torch while maintaining thermal insulation of the remainder of the probes using a freeze-anchoring platform. (D) The assembled and patterned CF electrode thread (CFET) is tested in vitro in 0.9% saline to validate dopamine detection functionality. (E) Single or pairs of CFETs are threaded into silica tubes (165 μm outer diameter) with isopropanol (IPA) to reduce friction and facilitate the threading process. (F) The CFETs are permanently attached to the silica tube using structural epoxy. The tail end of the CFETs are stripped using flame-etching and used to connect to external instrumentation (not shown). These exposed wires are further protected with heat shrink tubing.

3

In vivo FSCV and EPhys measurements from s-μIP from task-performing monkey T. (A) FSCV data collected from two neighboring sites using a dual-channel s-μIP, showing single-trial measurements as the monkey performed a forced choice large reward trial in the shape task (sites c8dg and c8ds: 483 days postimplant). Color plot shows clear dopamine redox current (i.e., color changes ∼0.6 and −0.2 V). PCA extracted dopamine concentration change ([ΔDA]) is plotted below the color plot and highlights changes around task events (e.g., increases in both channels after the value cue, V). (B) Single-trial LFP signals measured as monkey performed a large reward trial (top panel) in the direction task, with time plotted relative to value cue display at 0 s (site c3bs-c3a: 496 days postimplant). Top inset displays a close-up of the LFP signal where β bursts are visible (orange traces). The bottom panel is the β-band power to highlight task relevant changes in β signaling. For all figure panels, C indicates central cue, V is value cue display, and RW is reward outcome.

4

Dopamine and LFP activity related to specific behavioral conditions in monkey T. (A) Trial-averaged dopamine for large (red) and small reward (blue) conditions for successful forced choice trials in the shape task. Higher dopamine levels are observed following the display of large reward associated value cues as compared to small reward cues in monkey T (site c8dg: 483 days postimplant). (B) Trial-averaged β-band LFP power for large and small reward conditions in the direction task, showing increased suppression (i.e., ERD, highlighted in yellow) for large as compared to small reward cues during the brief time window following the value cue at 0 s (site c3bs-c3a: 496 days postimplant). (C) Trial-averaged β-band LFP power for a fixed large reward condition for left (contralateral) and right (ipsilateral) cues for the same session and site as (B). Increased suppression for contralateral is observed as compared to ipsilateral movement conditions. For all figure panels, C indicates central cue, V is value cue display, and RW is reward outcome. Shading represents ± standard error.

5

Spike activity recordings from s-μIP implanted in monkey P in the direction task (site c33:390 days postimplant). (A) Raster plot showing detected spikes as a function of time relative to the value cue display (V at 0 s) for each trial (y-axis) with peristimulus time histogram (PSTH) plotted in the bottom panel showing average spike firing rate (y-axis) for the large (red) and small (blue) reward conditions relative to the same V event. C is the central cue and RW is reward outcome. (B) Average spike waveform ± standard deviation. (C) Interspike interval (ISI) histogram showing the distribution of the timing between spikes detected. Bin widths are 1 ms.