Helen N. Schwerdt

Slowly varying internal states govern our ability to sustain goal-directed behavior over minutes to hours, imposing fundamental constraints on cognitive performance, yet the neural signals that track these states remain poorly defined. Dopamine is a powerful modulator of motivated behavior, but its best-characterized signals are in the form of reward prediction errors (RPEs) that operate at seconds timescales. Whether these fast signals also carry information about slower, ongoing states of behavioral performance has not been directly tested. Here, we recorded subsecond dopamine concentration changes across multiple sites in the caudate nucleus (CN) and putamen of rhesus monkeys performing a reward-guided saccade task. Task performance oscillated over tens to hundreds of trials, demonstrating fluctuating internal states of sustained engagement. We found that single-trial dopamine signals were strongly modulated by these slowly evolving performance states, in a manner dissociable from both trial-level RPE and cumulative reward rate. This relationship was biased toward future rather than past performance windows, indicating that dopamine tracks motivational state prospectively rather than simply reflecting the reward history on which prediction errors are computed. Furthermore, performance-state modulation was concentrated in the CN rather than the putamen, consistent with the distinct roles of these regions in oculomotor and skeletomotor control and suggesting that state-dependent dopamine signals are organized according to the behavioral demands of the task. These findings demonstrate that phasic dopamine signals in the dorsal striatum simultaneously reflect fast learning signals and slowly evolving internal states, linking dopamine’s canonical RPE function to its broader role in sustaining motivated performance.

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.

We developed an implantable neural interface for monitoring both chemical and electrical forms of brain activity in monkeys that maintains aseptic properties for year-long periods while leveraging the modular functions (e.g., sensor moveability) provided by a chamber system. Invasive electrophysiological recordings, especially in subcortical structures from nonhuman primates usually involves implanting electrodes into the brain through a skull-mounted chamber. These electrodes may be attached temporarily for several hours of recording, or permanently. Permanent attachments are favorable to allow for sealing the chamber completely from externally originating pathogenic species that can infiltrate and compromise the health of the animal. A sealed chamber also reduces the need for frequent chamber cleaning required to minimize the accumulation of pathogenic organisms. However, neurochemical measurements require specialized electrodes with extremely fragile carbon fiber tips and are not compatible with recently developed sealed chamber systems. Here, we leveraged osseointegrating materials and hermetic sealing strategies to enable both neurochemical and electrical neural activity measurements from a sealed chamber with an aspirating port for culturing chamber fluid to ensure an aseptic environment. The system was shown to provide successful recordings of neural activity in two monkeys while maintaining negative bacteria culture results for over a year post-implant.

We leveraged carbon fiber materials for creating sensors that provide dual neurochemical and electrical neural activity recording at micro-invasive (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 micro-invasive probes (s-µIPs) are approximately 10 µm in diameter along the distal length (1 – 15 mm) immediately surrounding the targeted recording site. This micro-invasive 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 multi-modal (chemical and electrical) neural activity measurements over 1-year post-implantation from micro-invasive devices.

Measuring the dynamic relationship between neuromodulators, such as dopamine, and neuronal action potentials is imperative to understand how these fundamental modes of neural signaling interact to mediate behavior. We developed methods to measure concurrently dopamine and extracellular action potentials (i.e., spikes) in monkeys. Standard fast-scan cyclic voltammetric (FSCV) electrochemical (EChem) and electrophysiological (EPhys) recording systems are combined and used to collect spike and dopamine signals, respectively, from an array of carbon fiber (CF) sensors implanted in the monkey striatum. FSCV requires the application of small voltages at the implanted sensors to measure redox currents generated from target molecules, such as dopamine. These applied voltages create artifacts at neighboring EPhys measurement sensors which may lead to misclassification of these signals as physiological spikes. Therefore, simple automated temporal interpolation algorithms were designed to remove these artifacts and enable accurate spike extraction. We validated these methods using simulated artifacts and demonstrated an average spike recovery rate of 84.5%. We identified and discriminated cell type-specific units in the monkey striatum that were shown to correlate to specific behavioral task parameters related to reward size and eye movement direction. Synchronously recorded spike and dopamine signals displayed contrasting relations to the task variables, suggesting a complex relationship between these two modes of neural signaling. Future application of our methods will help advance our understanding of the interactions between neuromodulator signaling and neuronal activity, to elucidate more detailed mechanisms of neural circuitry and plasticity mediating behaviors in health and in disease.

Highlights: We present a compact MRI-compatible stereotaxic frame for large nonhuman primates. The design is 3D printable, inexpensive, and matches size of an adult human head. Enabled real-time, accurate, MRI-guided deep-brain viral vector injection. Facilitated multimodal alignment for deep-brain electrophysiology planning. All computer-aided-design files are modularized and publicly available and editable. Background: High-precision neurosurgical targeting in nonhuman primates (NHPs) often requires presurgical anatomy mapping with noninvasive neuroimaging techniques (MRI, CT, PET), allowing for translation of individual anatomical coordinates to surgical stereotaxic apparatus. Given the varied tissue contrasts that these imaging techniques produce, precise alignment of imaging-based coordinates to surgical apparatus can be cumbersome. MRI-compatible stereotaxis with radiopaque fiducial markers offer a straight-forward and reliable solution, but existing commercial options do not fit in conformal head coils that maximize imaging quality. New method: We developed a compact MRI-compatible stereotaxis suitable for a variety of NHP species ( Macaca mulatta , Macaca fascicularis , and Cebus apella ) that allows multimodal alignment through technique-specific fiducial markers. Comparison with existing methods: With the express purpose of compatibility with clinically available MRI, CT, and PET systems, the frame is no larger than a human head, while allowing for imaging NHPs in the supinated position. This design requires no marker implantation, special software, or additional knowledge other than the operation of a common large animal stereotaxis. Results: We demonstrated the applicability of this 3D-printable apparatus across a diverse set of experiments requiring presurgical planning: 1) We demonstrate the accuracy of the fiducial system through a within-MRI cannula insertion and subcortical injection of viral vectors. 2) We also demonstrated accuracy of multimodal (MRI and CT) alignment and coordinate transfer to guide a surgical robot electrode implantation for deep-brain electrophysiology. Conclusions: The computer-aided design files and engineering drawings are publicly available, with the modular design allowing for low cost and manageable manufacturing.

Measuring the dynamic relationship between neuromodulators, such as dopamine, and neuronal action potentials is imperative to understand how these fundamental modes of neural signaling interact to mediate behavior. Here, we developed methods to measure concurrently dopamine and extracellular action potentials (i.e., spikes) and applied these in a monkey performing a behavioral task. Standard fast-scan cyclic voltammetric (FSCV) electrochemical (EChem) and electrophysiological (EPhys) recording systems are combined and used to collect spike and dopamine signals, respectively, from an array of carbon fiber (CF) sensors implanted in the monkey striatum. FSCV requires the application of small voltages at the implanted sensors to measure redox currents generated from target molecules, such as dopamine. These applied voltages create artifacts at neighboring EPhys-measurement sensors, producing signals that may falsely be classified as physiological spikes. Therefore, simple automated temporal interpolation algorithms were designed to remove these artifacts and enable accurate spike extraction. We validated these methods using simulated artifacts and demonstrated an average spike recovery rate of 84.5%. This spike extraction was performed on data collected from concurrent EChem and EPhys recordings made in a task-performing monkey to discriminate cell-type specific striatal units. These identified units were shown to correlate to specific behavioral task parameters related to reward size and eye-movement direction. Synchronous measures of spike and dopamine signals displayed contrasting relations to the behavioral task parameters, as taken from our small set of representative data, suggesting a complex relationship between these two modes of neural signaling. Future application of our methods will help advance our understanding of the interactions between neuromodulator signaling and neuronal activity, to elucidate more detailed mechanisms of neural circuitry and plasticity mediating behaviors in health and in disease. Significance statement: We present a simple method for recording synchronous molecular and neuronal spike signals. Conventional electrophysiological and electrochemical instruments are combined without the need for additional hardware. A custom-designed algorithm was made and validated for extracting neuronal action potential signals with high fidelity. We were able to compute cell-type specific spike activity along with molecular dopamine signals related to reward and movement behaviors from measurements made in the monkey striatum. Such combined measurements of neurochemical and extracellular action potentials may help pave the way to elucidating mechanisms of plasticity, and how neuromodulators and neurons are orchestrated to mediate behavior.

We developed a flexible “electrode-thread” array for recording dopamine neurochemical activity from a lateral distribution of subcortical targets (up to 16) transverse to the axis of insertion. Ultrathin (∼ 10 µm diameter) carbon fiber (CF) electrode-threads (CFETs) are clustered into a tight bundle to introduce them into the brain from a single entry point. The individual CFETs splay laterally in deep brain tissue during insertion due to their innate flexibility. This spatial redistribution allows navigation of the CFETs towards deep brain targets spreading horizontally from the axis of insertion. Commercial “linear” arrays provide single entry insertion but only allow measurements along the axis of insertion. Horizontally configured neurochemical recording arrays inflict separate penetrations for each individual channel (i.e., electrode). We tested functional performance of our CFET arrays in vivo for recording dopamine neurochemical dynamics and for providing lateral spread to multiple distributed sites in the striatum of rats. Spatial spread was further characterized using agar brain phantoms to measure electrode deflection as a function of insertion depth. We also developed protocols to slice the embedded CFETs within fixed brain tissue using standard histology techniques. This method allowed extraction of the precise spatial coordinates of the implanted CFETs and their recording sites as integrated with immunohistochemical staining for surrounding anatomical, cytological, and protein expression labels. Neurochemical recording operations tested here can be integrated with already widely established capabilities of CF-based electrodes to record single neuron activity and local field potentials, to enable multi-modal recording functions. Our CFET array has the potential to unlock a wide range of applications, from uncovering the role of neuromodulators in synaptic plasticity, to addressing critical safety barriers in clinical translation towards diagnostic and adaptive treatment in Parkinson’s disease and major mood disorders.