Primate reaching cued by multichannel spatiotemporal cortical microstimulation

Neural Plasticity in Sensorimotor Brain-Machine Interfaces

Brain-machine interfaces (BMIs) aim to treat sensorimotor neurological disorders by creating artificial motor and/or sensory pathways. Introducing artificial pathways creates new relationships between sensory input and motor output, which the brain must learn to gain dexterous control. This review highlights the role of learning in BMIs to restore movement and sensation, and discusses how BMI design may influence neural plasticity and performance. The close integration of plasticity in sensory and motor function influences the design of both artificial pathways and will be an essential consideration for bidirectional devices that restore both sensory and motor function.

Neuroprosthetics: from sensorimotor to cognitive disorders

Neuroprosthetics is a multidisciplinary field at the interface between neurosciences and biomedical engineering, which aims at replacing or modulating parts of the nervous system that get disrupted in neurological disorders or after injury. Although neuroprostheses have steadily evolved over the past 60 years in the field of sensory and motor disorders, their application to higher-order cognitive functions is still at a relatively preliminary stage. Nevertheless, a recent series of proof-of-concept studies suggest that electrical neuromodulation strategies might also be useful in alleviating some cognitive and memory deficits, in particular in the context of dementia. Here, we review the evolution of neuroprosthetics from sensorimotor to cognitive disorders, highlighting important common principles such as the need for neuroprosthetic systems that enable multisite bidirectional interactions with the nervous system.

A Bioactive and Photoresponsive Platform for Wireless Electrical Stimulation to Promote Neurogenesis

Delivering electrical signals to neural cells and tissue has attracted increasing attention in the treatment of nerve injuries. Unlike traditional wired electrical stimulation, wireless and remote light stimulation provides less invasive and longer-lasting interfaces, holding great promise in the treatment of nerve injuries and neurodegenerative diseases, as well as human-computer interaction. Additionally, a bioactive matrix that bridges the injured gap and induces nerve regeneration is essential for injured nerve repair. However, it is still challenging to construct a 3D biomimetic cell niche with optoelectrical responsiveness. Herein, a bioactive platform for remote and wireless optoelectrical stimulation is established by incorporating hydrophilic poly(3-hexylthiophene) nanoparticles (P3HT NPs) into a biomimetic hydrogel matrix. Moreover, the hydrogel matrix is modified by varying the composition and/or the crosslinking degree to meet the needs of different application scenarios. When exposed to pulsed green light, P3HT NPs in hydrogels convert light signals into electrical signals, resulting in the generation of tens of picoampere photocurrent, which is proved to promote the growth of cortical neurons that covered by hydrogels and the neuronal differentiation of bone marrow mesenchymal stem cells (BMSCs) encapsulated in hydrogels. This work is of great significance for the design of next-generation neural electrodes and scaffolds.

Similarities Between Somatosensory Cortical Responses Induced via Natural Touch and Microstimulation in the Ventral Posterior Lateral Thalamus in Macaques

Lost sensations, such as touch, could be restored by microstimulation (MiSt) along the sensory neural substrate. Such neuroprosthetic sensory information can be used as feedback from an invasive brain-machine interface (BMI) to control a robotic arm/hand, such that tactile and proprioceptive feedback from the sensorized robotic arm/hand is directly given to the BMI user. Microstimulation in the human somatosensory thalamus (Vc) has been shown to produce somatosensory perceptions. However, until recently, systematic methods for using thalamic stimulation to evoke naturalistic touch perceptions were lacking. We have recently presented rigorous methods for determining a mapping between ventral posterior lateral thalamus (VPL) MiSt, and neural responses in the somatosensory cortex (S1), in a rodent model (Choi et al., 2016; Choi and Francis, 2018). Our technique minimizes the difference between S1 neural responses induced by natural sensory stimuli and those generated via VPL MiSt. Our goal is to develop systems that know what neural response a given MiSt will produce and possibly allow the development of natural “sensation.” To date, our optimization has been conducted in the rodent model and simulations. Here, we present data from simple non-optimized thalamic MiSt during peri-operative experiments, where we used MiSt in the VPL of macaques, which have a somatosensory system more like humans, as compared to our previous rat work (Li et al., 2014; Choi et al., 2016). We implanted arrays of microelectrodes across the hand area of the macaque S1 cortex as well as in the VPL. Multi and single-unit recordings were used to compare cortical responses to natural touch and thalamic MiSt in the anesthetized state. Post-stimulus time histograms were highly correlated between the VPL MiSt and natural touch modalities, adding support to the use of VPL MiSt toward producing a somatosensory neuroprosthesis in humans.

Development of a magnetically aligned regenerative tissue-engineered electronic nerve interface for peripheral nerve applications

Peripheral nerve injuries can be debilitating to motor and sensory function, with severe cases often resulting in complete limb amputation. Over the past two decades, prosthetic limb technology has rapidly advanced to provide users with crude motor control of up to 20° of freedom; however, the nerve-interfacing technology required to provide high movement selectivity has not progressed at the same rate. The work presented here focuses on the development of a magnetically aligned regenerative tissue-engineered electronic nerve interface (MARTEENI) that combines polyimide "threads" encapsulated within a magnetically aligned hydrogel scaffold. The technology exploits tissue-engineered strategies to address concerns over traditional peripheral nerve interfaces including poor axonal sampling through the nerve and rigid substrates. A magnetically templated hydrogel is used to physically support the polyimide threads while also promoting regeneration in close proximity to the electrode sites on the polyimide. This work demonstrates the utility of magnetic templating for use in tuning the mechanical properties of hydrogel scaffolds to match the stiffness of native nerve tissue while providing an aligned substrate for Schwann cell migration in vitro. MARTEENI devices were fabricated and implanted within a 5-mm-long rat sciatic-nerve transection model to assess regeneration at 6 and 12 weeks. MARTEENI devices do not disrupt tissue remodeling and show axon densities equivalent to fresh tissue controls around the polyimide substrates. Devices are observed to have attenuated foreign-body responses around the polyimide threads. It is expected that future studies with functional MARTEENI devices will be able to record and stimulate single axons with high selectivity and low stimulation regimes.

Magnetoelectric Nanoparticles Incorporated Biomimetic Matrix for Wireless Electrical Stimulation and Nerve Regeneration

Electrical stimulation is regarded pivotal to promote repair of nerve injuries, however, failed to get extensive application in vivo due to the challenges in noninvasive electrical loading accompanying with construction of biomimetic cell niche. Herein, a new concept of magneto responsive electric 3D matrix for remote and wireless electrical stimulation is demonstrated. By the preparation of magnetoelectric core/shell structured Fe₃ O₄ @BaTiO₃ NPs-loaded hyaluronan/collagen hydrogels, which recapitulate considerable magneto-electricity and vital features of native neural extracellular matrix, the enhancement of neurogenesis both in cellular level and spinal cord injury in vivo with external pulsed magnetic field applied is proved. The findings pave the way for a novel class of remote controlling and delivering electricity through extracellular niches-mimicked hydrogel network, arising prospects not only in neurogenesis but also in human-computer interaction with higher resolution.

Injecting Information into the Mammalian Cortex: Progress, Challenges, and Promise

For 150 years artificial stimulation has been used to study the function of the nervous system. Such stimulation-whether electrical or optogenetic-eventually may be used in neuroprosthetic devices to replace lost sensory inputs and to otherwise introduce information into the nervous system. Efforts toward this goal can be classified broadly as either biomimetic or arbitrary. Biomimetic stimulation aims to mimic patterns of natural neural activity, so that the subject immediately experiences the artificial stimulation as if it were natural sensation. Arbitrary stimulation, in contrast, makes no attempt to mimic natural patterns of neural activity. Instead, different stimuli-at different locations and/or in different patterns-are assigned different meanings randomly. The subject's time and effort then are required to learn to interpret different stimuli, a process that engages the brain's inherent plasticity. Here we will examine progress in using artificial stimulation to inject information into the cerebral cortex and discuss the challenges for and the promise of future development.

A Brain to Spine Interface for Transferring Artificial Sensory Information

Lack of sensory feedback is a major obstacle in the rapid absorption of prosthetic devices by the brain. While electrical stimulation of cortical and subcortical structures provides unique means to deliver sensory information to higher brain structures, these approaches require highly invasive surgery and are dependent on accurate targeting of brain structures. Here, we propose a semi-invasive method, Dorsal Column Stimulation (DCS) as a tool for transferring sensory information to the brain. Using this new approach, we show that rats can learn to discriminate artificial sensations generated by DCS and that DCS-induced learning results in corticostriatal plasticity. We also demonstrate a proof of concept brain-to-spine interface (BTSI), whereby tactile and artificial sensory information are decoded from the brain of an “encoder” rat, transformed into DCS pulses, and delivered to the spinal cord of a second “decoder” rat while the latter performs an analog-to-digital conversion during a sensory discrimination task. These results suggest that DCS can be used as an effective sensory channel to transmit prosthetic information to the brain or between brains, and could be developed as a novel platform for delivering tactile and proprioceptive feedback in clinical applications of brain-machine interfaces.

Creating a neuroprosthesis for active tactile exploration of textures

Intracortical microstimulation (ICMS) of the primary somatosensory cortex (S1) can produce percepts that mimic somatic sensation and, thus, has potential as an approach to sensorize prosthetic limbs. However, it is not known whether ICMS could recreate active texture exploration-the ability to infer information about object texture by using one's fingertips to scan a surface. Here, we show that ICMS of S1 can convey information about the spatial frequencies of invisible virtual gratings through a process of active tactile exploration. Two rhesus monkeys scanned pairs of visually identical screen objects with the fingertip of a hand avatar-controlled first via a joystick and later via a brain-machine interface-to find the object with denser virtual gratings. The gratings consisted of evenly spaced ridges that were signaled through individual ICMS pulses generated whenever the avatar's fingertip crossed a ridge. The monkeys learned to interpret these ICMS patterns, evoked by the interplay of their voluntary movements and the virtual textures of each object, to perform a sensory discrimination task. Discrimination accuracy followed Weber's law of just-noticeable differences (JND) across a range of grating densities; a finding that matches normal cutaneous sensation. Moreover, 1 monkey developed an active scanning strategy where avatar velocity was integrated with the ICMS pulses to interpret the texture information. We propose that this approach could equip upper-limb neuroprostheses with direct access to texture features acquired during active exploration of natural objects.

A Robust Encoding Scheme for Delivering Artificial Sensory Information via Direct Brain Stimulation

Innovations for creating somatosensation via direct electrical stimulation of the brain will be required for the next generation of bi-directional cortical neuroprostheses. The current lack of tactile perception and proprioceptive input likely imposes a fundamental limit on speed and accuracy of brain-controlled prostheses or re-animated limbs. This study addresses the unique challenge of identifying a robust, high bandwidth sensory encoding scheme in a high-dimensional parameter space. Previous studies demonstrated single dimensional encoding schemes delivering low bandwidth sensory information, but no comparison has been performed across parameters, nor with update rates suitable for real-time operation of a neuroprosthesis. Here, we report the first comprehensive measurement of the resolution of key stimulation parameters such as pulse amplitude, pulse width, frequency, train interval and number of pulses. Surprisingly, modulation of stimulation frequency was largely undetectable. While we initially expected high frequency content to be an ideal candidate for passing high throughput sensory signals to the brain, we found only modulation of very low frequencies were detectable. Instead, the charge-per-phase of each pulse yields the highest resolution sensory signal, and is the key parameter modulating perceived intensity. The stimulation encoding patterns were designed for high-bandwidth information transfer that will be required for bi-directional brain interfaces. Our discovery of the stimulation features which best encode perceived intensity have significant implications for design of any neural interface seeking to convey information directly to the brain via electrical stimulation.

A wireless and artefact-free 128-channel neuromodulation device for closed-loop stimulation and recording in non-human primates

Closed-loop neuromodulation systems aim to treat a variety of neurological conditions by delivering and adjusting therapeutic electrical stimulation in response to a patient's neural state, recorded in real time. Existing systems are limited by low channel counts, lack of algorithmic flexibility, and the distortion of recorded signals by large and persistent stimulation artefacts. Here, we describe an artefact-free wireless neuromodulation device that enables research applications requiring high-throughput data streaming, low-latency biosignal processing, and simultaneous sensing and stimulation. The device is a miniaturized neural interface capable of closed-loop recording and stimulation on 128 channels, with on-board processing to fully cancel stimulation artefacts. In addition, it can detect neural biomarkers and automatically adjust stimulation parameters in closed-loop mode. In a behaving non-human primate, the device enabled long-term recordings of local field potentials and the real-time cancellation of stimulation artefacts, as well as closed-loop stimulation to disrupt movement preparatory activity during a delayed-reach task. The neuromodulation device may help advance neuroscientific discovery and preclinical investigations of stimulation-based therapeutic interventions.

Transparent arrays of bilayer-nanomesh microelectrodes for simultaneous electrophysiology and two-photon imaging in the brain

Transparent microelectrode arrays have emerged as increasingly important tools for neuroscience by allowing simultaneous coupling of big and time-resolved electrophysiology data with optically measured, spatially and type resolved single neuron activity. Scaling down transparent electrodes to the length scale of a single neuron is challenging since conventional transparent conductors are limited by their capacitive electrode/electrolyte interface. In this study, we establish transparent microelectrode arrays with high performance, great biocompatibility, and comprehensive in vivo validations from a recently developed, bilayer-nanomesh material composite, where a metal layer and a low-impedance faradaic interfacial layer are stacked reliably together in a same transparent nanomesh pattern. Specifically, flexible arrays from 32 bilayer-nanomesh microelectrodes demonstrated near-unity yield with high uniformity, excellent biocompatibility, and great compatibility with state-of-the-art wireless recording and real-time artifact rejection system. The electrodes are highly scalable, with 130 kilohms at 1 kHz at 20 μm in diameter, comparable to the performance of microelectrodes in nontransparent Michigan arrays. The highly transparent, bilayer-nanomesh microelectrode arrays allowed in vivo two-photon imaging of single neurons in layer 2/3 of the visual cortex of awake mice, along with high-fidelity, simultaneous electrical recordings of visual-evoked activity, both in the multi-unit activity band and at lower frequencies by measuring the visual-evoked potential in the time domain. Together, these advances reveal the great potential of transparent arrays from bilayer-nanomesh microelectrodes for a broad range of utility in neuroscience and medical practices.

In vitro characterization of intrinsic properties and local synaptic inputs to pyramidal neurons in macaque primary motor cortex

Primates (including humans) have a highly developed corticospinal tract, and specialized motor cortical areas which differ in key ways from rodents. Much work on motor cortex has therefore used macaque monkeys as a good animal model for human motor control. However, there is a paucity of data describing the fundamental functional architecture of primate primary motor cortex, which is best addressed with in vitro approaches. In this study we examined the cellular properties and the micro-circuitry of the adult macaque primary motor cortex by carrying out in-vitro intracellular recordings. We aimed to characterize the basic properties of the cortical circuitry by studying the intrinsic properties of its pyramidal neurons and their physiological interconnectivity. We studied the passive and active electrophysiological properties of pyramidal neurons in both superficial and deep cortical layers. Both superficial and deep pyramidal neurons exhibited bursting behaviour that could act as powerful excitation for downstream targets. Synaptic connections were lamina specific. Neurons in the deep layers had convergent excitatory inputs from all cortical layers whereas superficial neurons had only significant inputs from superficial layers. This sheds light on the functional architecture of the primate primary motor cortex and how its output is shaped. We also took the unique opportunity in our recording technique to characterize the relationship between intracellular and extracellular spike waveforms, with implications for cell-type identification in studies in awake behaving monkey. Our results will aid the interpretation of primate studies into motor control involving extracellular spike recordings and electrical stimulation in primary motor cortex.

Development of an Activity-Dependent Epidural Stimulation System in Freely Moving Spinal Cord Injured Rats: A Proof of Concept Study

Purpose: Extensive pre-clinical and clinical experimentation has yielded data on the robustness and versatility of epidural stimulation (ES) strategies to activate spinal neural circuitry to produce functional benefits. Increasing studies are now reporting that closed-loop electrical stimulation delivery methods significantly enhance the neuromodulation effects of stimulation, to in turn, improve physiological outcomes of the intervention. No studies have yet explored the feasibility and usage of closed-loop systems to neuromodulate the cervical spinal cord using ES. Methods: We developed an activity-dependent system that utilizes electromyography (EMG) activity to trigger epidural stimulation (tES) of the cervical spinal cord in awake, freely moving rats. Experiments were performed on rats that were implanted with chronic forelimb EMG and cervical epidural implants, with (n = 7) and without (n = 2) a complete C4 spinal hemisection. Results: Our results show that the EMG triggered activity-dependent system can be reliably applied and reproduced for: (i) stimulating multiple rats simultaneously throughout the night during free home-cage activity and (ii) use as a mobile system for testing and training during various short-term behavioral testing conditions. The system was able to consistently generate stimulation pulse trains in response to attempted EMG activity that crossed a user-defined threshold in all rats for all experiments, including the overnight experiments that lasts for 7 h/session for 6 days/week through the 3-month period. Conclusion: The developed closed-loop system can be considered to represent a class of bidirectional neural prostheses via a circuit that enables two-way interactions between neural activity (real-time processing of EMG activity) and external devices (such as a stimulator). It can operate autonomously for extended periods of time in unrestrained rats, allowing its use as a long-term therapeutic tool. It can also enable us to study the long-term physiological effects of incorporating electrical stimulation techniques into the nervous system. The system can also be experimented for connecting several neural systems into a Brainet by combining neural signals from multiple rats dynamically and in real-time so as to enhance motor performance. Studies are ongoing in our laboratory to test the usefulness of this system in the recovery of hand function after cervical spinal cord injuries.

Optimization of Stimulation Parameters for Targeted Activation of Multiple Neurons Using Closed-Loop Search Methods

Differential activation of neuronal populations can improve the efficacy of clinical devices such as sensory or cortical prostheses. Improving stimulus specificity will facilitate targeted neuronal activation to convey biologically realistic percepts. In order to deliver more complex stimuli to a neuronal population, stimulus optimization techniques must be developed that will enable a single electrode to activate subpopulations of neurons. However, determining the stimulus needed to evoke targeted neuronal activity is challenging. To find the most selective waveform for a particular population, we apply an optimization-based search routine, Powell's conjugate direction method, to systematically search the stimulus waveform space. This routine utilizes a 1-D sigmoid activation model and a 2-D strength-duration curve to measure neuronal activation throughout the stimulus waveform space. We implement our search routine in both an experimental study and a simulation study to characterize potential stimulus-evoked populations and the associated selective stimulus waveform spaces. We found that for a population of five neurons, seven distinct sub-populations could be activated. The stimulus waveform space and evoked neuronal activation curves vary with each new combination of neuronal culture and electrode array, resulting in a unique selectivity space. The method presented here can be used to efficiently uncover the selectivity space, focusing experiments in regions with the desired activation pattern.

Injecting Instructions into Premotor Cortex

The premotor cortex (PM) receives inputs from parietal cortical areas representing processed visuospatial information, translates that information into programs for particular movements, and communicates those programs to the primary motor cortex (M1) for execution. Consistent with this general function, intracortical microstimulation (ICMS) in the PM of sufficient frequency, amplitude, and duration has been shown to evoke complex movements of the arm and hand that vary systematically depending on the locus of stimulation. Using frequencies and amplitudes too low to evoke muscle activity, however, we found that ICMS in the PM can provide instructions to perform specific reach, grasp, and manipulate movements. These instructed actions were not fixed but rather were learned through associations between the arbitrary stimulation locations and particular movements. Low-amplitude ICMS at different PM locations thus evokes distinguishable experiences that can become associated with specific movements arbitrarily, providing a novel means of injecting information into the nervous system.

Selectivity and Longevity of Peripheral-Nerve and Machine Interfaces: A Review

For those individuals with upper-extremity amputation, a daily normal living activity is no longer possible or it requires additional effort and time. With the aim of restoring their sensory and motor functions, theoretical and technological investigations have been carried out in the field of neuroprosthetic systems. For transmission of sensory feedback, several interfacing modalities including indirect (non-invasive), direct-to-peripheral-nerve (invasive), and cortical stimulation have been applied. Peripheral nerve interfaces demonstrate an edge over the cortical interfaces due to the sensitivity in attaining cortical brain signals. The peripheral nerve interfaces are highly dependent on interface designs and are required to be biocompatible with the nerves to achieve prolonged stability and longevity. Another criterion is the selection of nerves that allows minimal invasiveness and damages as well as high selectivity for a large number of nerve fascicles. In this paper, we review the nerve-machine interface modalities noted above with more focus on peripheral nerve interfaces, which are responsible for provision of sensory feedback. The invasive interfaces for recording and stimulation of electro-neurographic signals include intra-fascicular, regenerative-type interfaces that provide multiple contact channels to a group of axons inside the nerve and the extra-neural-cuff-type interfaces that enable interaction with many axons around the periphery of the nerve. Section Current Prosthetic Technology summarizes the advancements made to date in the field of neuroprosthetics toward the achievement of a bidirectional nerve-machine interface with more focus on sensory feedback. In the Discussion section, the authors propose a hybrid interface technique for achieving better selectivity and long-term stability using the available nerve interfacing techniques.

Rapid Integration of Artificial Sensory Feedback during Operant Conditioning of Motor Cortex Neurons

Neuronal motor commands, whether generating real or neuroprosthetic movements, are shaped by ongoing sensory feedback from the displacement being produced. Here we asked if cortical stimulation could provide artificial feedback during operant conditioning of cortical neurons. Simultaneous two-photon imaging and real-time optogenetic stimulation were used to train mice to activate a single neuron in motor cortex (M1), while continuous feedback of its activity level was provided by proportionally stimulating somatosensory cortex. This artificial signal was necessary to rapidly learn to increase the conditioned activity, detect correct performance, and maintain the learned behavior. Population imaging in M1 revealed that learning-related activity changes are observed in the conditioned cell only, which highlights the functional potential of individual neurons in the neocortex. Our findings demonstrate the capacity of animals to use an artificially induced cortical channel in a behaviorally relevant way and reveal the remarkable speed and specificity at which this can occur.

•

All-optical brain-machine-brain interface for neuroprosthetic control

•

Mice rapidly learn to activate single neurons under optogenetically evoked feedback

•

Population imaging reveals that learning is restricted to the conditioned neuron

•

Novel “in cerebro” learning paradigm for neural circuit dissection

Somatosensory encoding with cuneate nucleus microstimulation: Detection of artificial stimuli

The sense of touch and proprioception are critical to movement control. After spinal cord injury, these senses may be restored with direct, electrical microstimulation of the brain as part of a complete sensorimotor neuroprosthesis. The present study was designed to test, in part, the hypothesis that the cuneate nucleus (CN) of the brainstem is a suitable site to encode somatosensory information. Two rhesus macaques were implanted with microelectrode arrays providing chronic access to the CN. The monkeys were trained on an active touch oddity task to detect vibrotactile stimuli. When the vibrotactile stimuli were replaced with electrical stimuli delivered to the CN, initial detection probabilities were near chance. Detection performance improved over time, reaching a plateau after about 10 daily sessions. At plateau performance, the monkeys exhibited detection probabilities that were 68-80% higher than the chance probability. Finally, detection probability was quantified as a function of stimulus amplitude. The resulting psychometric curve showed a detection threshold of 45 μA for 100-Hz stimulus trains. These behavioral data are the first to show that artificial CN activation is sufficient for perception. The results are consistent with our hypothesis and motivate future tests of the CN as a somatosensory encoding site.

Brain-Machine Interfaces: From Basic Science to Neuroprostheses and Neurorehabilitation

Brain-machine interfaces (BMIs) combine methods, approaches, and concepts derived from neurophysiology, computer science, and engineering in an effort to establish real-time bidirectional links between living brains and artificial actuators. Although theoretical propositions and some proof of concept experiments on directly linking the brains with machines date back to the early 1960s, BMI research only took off in earnest at the end of the 1990s, when this approach became intimately linked to new neurophysiological methods for sampling large-scale brain activity. The classic goals of BMIs are 1) to unveil and utilize principles of operation and plastic properties of the distributed and dynamic circuits of the brain and 2) to create new therapies to restore mobility and sensations to severely disabled patients. Over the past decade, a wide range of BMI applications have emerged, which considerably expanded these original goals. BMI studies have shown neural control over the movements of robotic and virtual actuators that enact both upper and lower limb functions. Furthermore, BMIs have also incorporated ways to deliver sensory feedback, generated from external actuators, back to the brain. BMI research has been at the forefront of many neurophysiological discoveries, including the demonstration that, through continuous use, artificial tools can be assimilated by the primate brain's body schema. Work on BMIs has also led to the introduction of novel neurorehabilitation strategies. As a result of these efforts, long-term continuous BMI use has been recently implicated with the induction of partial neurological recovery in spinal cord injury patients.

Interfacing to the brain's motor decisions

It has been long known that neural activity, recorded with electrophysiological methods, contains rich information about a subject's motor intentions, sensory experiences, allocation of attention, action planning, and even abstract thoughts. All these functions have been the subject of neurophysiological investigations, with the goal of understanding how neuronal activity represents behavioral parameters, sensory inputs, and cognitive functions. The field of brain-machine interfaces (BMIs) strives for a somewhat different goal: it endeavors to extract information from neural modulations to create a communication link between the brain and external devices. Although many remarkable successes have been already achieved in the BMI field, questions remain regarding the possibility of decoding high-order neural representations, such as decision making. Could BMIs be employed to decode the neural representations of decisions underlying goal-directed actions? In this review we lay out a framework that describes the computations underlying goal-directed actions as a multistep process performed by multiple cortical and subcortical areas. We then discuss how BMIs could connect to different decision-making steps and decode the neural processing ongoing before movements are initiated. Such decision-making BMIs could operate as a system with prediction that offers many advantages, such as shorter reaction time, better error processing, and improved unsupervised learning. To present the current state of the art, we review several recent BMIs incorporating decision-making components.

Key considerations in designing a somatosensory neuroprosthesis

In recent years, a consensus has emerged that somatosensory feedback needs to be provided for upper limb neuroprostheses to be useful. An increasingly promising approach to sensory restoration is to electrically stimulate neurons along the somatosensory neuraxis to convey information about the state of the prosthetic limb and about contact with objects. To date, efforts toward artificial sensory feedback have consisted mainly of demonstrating that some sensory information could be conveyed using a small number of stimulation patterns, generally delivered through single electrodes. However impressive these achievements are, results from different studies are hard to compare, as each research team implements different stimulation patterns and tests the elicited sensations differently. A critical question is whether different stimulation strategies will generalize from contrived laboratory settings to activities of daily living. Here, we lay out some key specifications that an artificial somatosensory channel should meet, discuss how different approaches should be evaluated, and caution about looming challenges that the field of sensory restoration will face.

Encoding and Decoding of Multi-Channel ICMS in Macaque Somatosensory Cortex

Naturalistic control of brain-machine interfaces will require artificial proprioception, potentially delivered via intracortical microstimulation (ICMS). We have previously shown that multi-channel ICMS can guide a monkey reaching to unseen targets in a planar workspace. Here, we expand on that work, asking how ICMS is decoded into target angle and distance by analyzing the performance of a monkey when ICMS feedback was degraded. From the resulting pattern of errors, we found that the animal's estimate of target direction was consistent with a weighted circular-mean strategy-close to the optimal decoding strategy given the ICMS encoding. These results support our previous finding that animals can learn to use this artificial sensory feedback in an efficient and naturalistic manner.

Eliciting naturalistic cortical responses with a sensory prosthesis via optimized microstimulation

OBJECTIVE

Lost sensations, such as touch, could one day be restored by electrical stimulation along the sensory neural pathways. Such stimulation, when informed by electronic sensors, could provide naturalistic cutaneous and proprioceptive feedback to the user. Perceptually, microstimulation of somatosensory brain regions produces localized, modality-specific sensations, and several spatiotemporal parameters have been studied for their discernibility. However, systematic methods for encoding a wide array of naturally occurring stimuli into biomimetic percepts via multi-channel microstimulation are lacking. More specifically, generating spatiotemporal patterns for explicitly evoking naturalistic neural activation has not yet been explored.

APPROACH

We address this problem by first modeling the dynamical input-output relationship between multichannel microstimulation and downstream neural responses, and then optimizing the input pattern to reproduce naturally occurring touch responses as closely as possible.

MAIN RESULTS

Here we show that such optimization produces responses in the S1 cortex of the anesthetized rat that are highly similar to natural, tactile-stimulus-evoked counterparts. Furthermore, information on both pressure and location of the touch stimulus was found to be highly preserved.

SIGNIFICANCE

Our results suggest that the currently presented stimulus optimization approach holds great promise for restoring naturalistic levels of sensation.

Biomimetic rehabilitation engineering: the importance of somatosensory feedback for brain-machine interfaces

OBJECTIVE

Brain-machine interfaces (BMIs) re-establish communication channels between the nervous system and an external device. The use of BMI technology has generated significant developments in rehabilitative medicine, promising new ways to restore lost sensory-motor functions. However and despite high-caliber basic research, only a few prototypes have successfully left the laboratory and are currently home-deployed.

APPROACH

The failure of this laboratory-to-user transfer likely relates to the absence of BMI solutions for providing naturalistic feedback about the consequences of the BMI's actions. To overcome this limitation, nowadays cutting-edge BMI advances are guided by the principle of biomimicry; i.e. the artificial reproduction of normal neural mechanisms.

MAIN RESULTS

Here, we focus on the importance of somatosensory feedback in BMIs devoted to reproducing movements with the goal of serving as a reference framework for future research on innovative rehabilitation procedures. First, we address the correspondence between users' needs and BMI solutions. Then, we describe the main features of invasive and non-invasive BMIs, including their degree of biomimicry and respective advantages and drawbacks. Furthermore, we explore the prevalent approaches for providing quasi-natural sensory feedback in BMI settings. Finally, we cover special situations that can promote biomimicry and we present the future directions in basic research and clinical applications.

SIGNIFICANCE

The continued incorporation of biomimetic features into the design of BMIs will surely serve to further ameliorate the realism of BMIs, as well as tremendously improve their actuation, acceptance, and use.

An automatic experimental apparatus to study arm reaching in New World monkeys

BACKGROUND

Several species of the New World monkeys have been used as experimental models in biomedical and neurophysiological research. However, a method for controlled arm reaching tasks has not been developed for these species.

NEW METHOD

We have developed a fully automated, pneumatically driven, portable, and reconfigurable experimental apparatus for arm-reaching tasks suitable for these small primates.

RESULTS

We have utilized the apparatus to train two owl monkeys in a visually-cued arm-reaching task. Analysis of neural recordings demonstrates directional tuning of the M1 neurons.

COMPARISON WITH EXISTING METHOD(S)

Our apparatus allows automated control, freeing the experimenter from manual experiments.

CONCLUSION

The presented apparatus provides a valuable tool for conducting neurophysiological research on New World monkeys.

Implications of the Dependence of Neuronal Activity on Neural Network States for the Design of Brain-Machine Interfaces

Brain-machine interfaces (BMIs) can improve the quality of life of patients with sensory and motor disabilities by both decoding motor intentions expressed by neural activity, and by encoding artificially sensed information into patterns of neural activity elicited by causal interventions on the neural tissue. Yet, current BMIs can exchange relatively small amounts of information with the brain. This problem has proved difficult to overcome by simply increasing the number of recording or stimulating electrodes, because trial-to-trial variability of neural activity partly arises from intrinsic factors (collectively known as the network state) that include ongoing spontaneous activity and neuromodulation, and so is shared among neurons. Here we review recent progress in characterizing the state dependence of neural responses, and in particular of how neural responses depend on endogenous slow fluctuations of network excitability. We then elaborate on how this knowledge may be used to increase the amount of information that BMIs exchange with brain. Knowledge of network state can be used to fine-tune the stimulation pattern that should reliably elicit a target neural response used to encode information in the brain, and to discount part of the trial-by-trial variability of neural responses, so that they can be decoded more accurately.

PET Mapping for Brain-Computer Interface Stimulation of the Ventroposterior Medial Nucleus of the Thalamus in Rats with Implanted Electrodes

Brain-computer interface (BCI) technology has great potential for improving the quality of life for neurologic patients. This study aimed to use PET mapping for BCI-based stimulation in a rat model with electrodes implanted in the ventroposterior medial (VPM) nucleus of the thalamus.

METHODS

PET imaging studies were conducted before and after stimulation of the right VPM.

RESULTS

Stimulation induced significant orienting performance. (18)F-FDG uptake increased significantly in the paraventricular thalamic nucleus, septohippocampal nucleus, olfactory bulb, left crus II of the ansiform lobule of the cerebellum, and bilaterally in the lateral septum, amygdala, piriform cortex, endopiriform nucleus, and insular cortex, but it decreased in the right secondary visual cortex, right simple lobule of the cerebellum, and bilaterally in the somatosensory cortex.

CONCLUSION

This study demonstrated that PET mapping after VPM stimulation can identify specific brain regions associated with orienting performance. PET molecular imaging may be an important approach for BCI-based research and its clinical applications.

Implantable neurotechnologies: electrical stimulation and applications

Neural stimulation using injected electrical charge is widely used both in functional therapies and as an experimental tool for neuroscience applications. Electrical pulses can induce excitation of targeted neural pathways that aid in the treatment of neural disorders or dysfunction of the central and peripheral nervous system. In this review, we summarize the recent trends in the field of electrical stimulation for therapeutic interventions of nervous system disorders, such as for the restoration of brain, eye, ear, spinal cord, nerve and muscle function. Neural prosthetic applications are discussed, and functional electrical stimulation parameters for treating such disorders are reviewed. Important considerations for implantable packaging and enhancing device reliability are also discussed. Neural stimulators are expected to play a profound role in implantable neural devices that treat disorders and help restore functions in injured or disabled nervous system.

Toward a Proprioceptive Neural Interface that Mimics Natural Cortical Activity

The dramatic advances in efferent neural interfaces over the past decade are remarkable, with cortical signals used to allow paralyzed patients to control the movement of a prosthetic limb or even their own hand. However, this success has thrown into relief, the relative lack of progress in our ability to restore somatosensation to these same patients. Somatosensation, including proprioception, the sense of limb position and movement, plays a crucial role in even basic motor tasks like reaching and walking. Its loss results in crippling deficits. Historical work dating back decades and even centuries has demonstrated that modality-specific sensations can be elicited by activating the central nervous system electrically. Recent work has focused on the challenge of refining these sensations by stimulating the somatosensory cortex (S1) directly. Animals are able to detect particular patterns of stimulation and even associate those patterns with particular sensory cues. Most of this work has involved areas of the somatosensory cortex that mediate the sense of touch. Very little corresponding work has been done for proprioception. Here we describe the effort to develop afferent neural interfaces through spatiotemporally precise intracortical microstimulation (ICMS). We review what is known of the cortical representation of proprioception, and describe recent work in our lab that demonstrates for the first time, that sensations like those of natural proprioception may be evoked by ICMS in S1. These preliminary findings are an important first step to the development of an afferent cortical interface to restore proprioception.

Implantable neurotechnologies: bidirectional neural interfaces--applications and VLSI circuit implementations

A bidirectional neural interface is a device that transfers information into and out of the nervous system. This class of devices has potential to improve treatment and therapy in several patient populations. Progress in very large-scale integration has advanced the design of complex integrated circuits. System-on-chip devices are capable of recording neural electrical activity and altering natural activity with electrical stimulation. Often, these devices include wireless powering and telemetry functions. This review presents the state of the art of bidirectional circuits as applied to neuroprosthetic, neurorepair, and neurotherapeutic systems.

Multi-walled carbon nanotubes change morpho-functional and GABA characteristics of mouse cortical astrocytes

BACKGROUND

Multi-walled carbon nanotubes (MW-CNTs) have been extensively explored for their possible beneficial use in the nervous system. CNTs have shown to modulate neuronal growth and electrical properties, but its effect that varying length of MW-CNTs on primary astrocyte roles have not been clearly demonstrated yet.

RESULTS

We investigate here the effect of MW-CNTs on astrocytic morphology, cell-cell interaction and the distribution of intracellular GABA (gamma-amino butyric acid). Primary cultured cortical astrocytes on MW-CNT-coated glass coverslips grow rounder and make more cell-cell interactions, with many cell processes, compared to astrocytes on poly-D-lysine (PDL) coverslips. In addition, intracellular GABA spreads into the cell processes of astrocytes on MW-CNT coverslips. When this GABA spreads into cell processes from the cell body GABA can be released more easily and in larger quantities compared to astrocytes on PDL coverslips.

CONCLUSIONS

Our result confirm that MW-CNTs modulate astrocytic morphology, the distribution of astrocytic GABA, cell-cell interactions and the extension of cell processes. CNTs look to be a promising material for use neuroprosthetics such as brain-machine interface technologies.

Strategies for optical control and simultaneous electrical readout of extended cortical circuits

BACKGROUND

To dissect the intricate workings of neural circuits, it is essential to gain precise control over subsets of neurons while retaining the ability to monitor larger-scale circuit dynamics. This requires the ability to both evoke and record neural activity simultaneously with high spatial and temporal resolution.

NEW METHOD

In this paper we present approaches that address this need by combining micro-electrocorticography (μECoG) with optogenetics in ways that avoid photovoltaic artifacts.

RESULTS

We demonstrate that variations of this approach are broadly applicable across three commonly studied mammalian species - mouse, rat, and macaque monkey - and that the recorded μECoG signal shows complex spectral and spatio-temporal patterns in response to optical stimulation.

COMPARISON WITH EXISTING METHODS

While optogenetics provides the ability to excite or inhibit neural subpopulations in a targeted fashion, large-scale recording of resulting neural activity remains challenging. Recent advances in optical physiology, such as genetically encoded Ca(2+) indicators, are promising but currently do not allow simultaneous recordings from extended cortical areas due to limitations in optical imaging hardware.

CONCLUSIONS

We demonstrate techniques for the large-scale simultaneous interrogation of cortical circuits in three commonly used mammalian species.

Neural stimulation and recording with bidirectional, soft carbon nanotube fiber microelectrodes

The development of microelectrodes capable of safely stimulating and recording neural activity is a critical step in the design of many prosthetic devices, brain-machine interfaces, and therapies for neurologic or nervous-system-mediated disorders. Metal electrodes are inadequate prospects for the miniaturization needed to attain neuronal-scale stimulation and recording because of their poor electrochemical properties, high stiffness, and propensity to fail due to bending fatigue. Here we demonstrate neural recording and stimulation using carbon nanotube (CNT) fiber electrodes. In vitro characterization shows that the tissue contact impedance of CNT fibers is remarkably lower than that of state-of-the-art metal electrodes, making them suitable for recording single-neuron activity without additional surface treatments. In vivo chronic studies in parkinsonian rodents show that CNT fiber microelectrodes stimulate neurons as effectively as metal electrodes with 10 times larger surface area, while eliciting a significantly reduced inflammatory response. The same CNT fiber microelectrodes can record neural activity for weeks, paving the way for the development of novel multifunctional and dynamic neural interfaces with long-term stability.

Intracortical Microstimulation (ICMS) Activates Motor Cortex Layer 5 Pyramidal Neurons Mainly Transsynaptically

BACKGROUND

Intracortical microstimulation (ICMS) is a technique used for a number of purposes including the derivation of cortical movement representations (motor maps). Its application can activate the output layer 5 of motor cortex and can result in the elicitation of body movements depending upon the stimulus parameters used.

OBJECTIVE

The extent to which pyramidal tract projection neurons of the motor cortex are activated transsynaptically or directly by ICMS remains an open question. Given this uncertainty in the mode of activation, we used a preparation that combined patch clamp whole-cell recordings from single layer 5 pyramidal neurons and extracellular ICMS in slices of motor cortex as well as a standard in vivo mapping technique to ask how ICMS activated motor cortex pyramidal neurons.

METHODS

We measured changes in synaptic spike threshold and spiking rate to ICMS in vitro and movement threshold in vivo in the presence or absence of specific pharmacological blockers of glutamatergic (AMPA, NMDA and Kainate) receptors and GABAA receptors.

RESULTS

With major excitatory and inhibitory synaptic transmission blocked (with DNQX, APV and bicuculline methiodide), we observed a significant increase in the ICMS current intensity required to elicit a movement in vivo as well as to the first spike and an 85% reduction in spiking responses in vitro. Subsets of neurons were still responsive after the synaptic block, especially at higher current intensities, suggesting a modest direct activation.

CONCLUSION

Taken together our data indicate a mainly synaptic mode of activation to ICMS in layer 5 of rat motor cortex.

Investigating the interfacial properties of electrochemically roughened platinum electrodes for neural stimulation

Platinum electrodes have been electrochemically roughened (roughness factors up to 430) and evaluated for use as neural stimulation electrodes. The roughened electrodes show superior interfacial properties with increasing surface roughness. The roughened electrode (fR = 250) has a charge injection limit of 1.0 mC cm(-2) (400 μs pulse width), which is superior to that of titanium nitride (0.87 mC cm(-2)) but comparable to that of carbon nanotubes (1.0-1.6 mC cm(-2)). The surface roughness can also be optimized for different neural stimulation applications based on the available charge density at a particular pulse width of stimulation. The roughened platinum electrodes demonstrated good mechanical stability under harsh ultrasonication and electrochemical stability under continuous biphasic stimulation, indicating the potential of this biological interface to be safe and stable.

Assessing vibrotactile feedback strategies by controlling a cursor with unstable dynamics

Brain computer interface (BCI) control predominately uses visual feedback. Real arm movements, however, are controlled under a diversity of feedback mechanisms. The lack of additional BCI feedback modalities forces users to maintain visual contact while performing tasks. Such stringent requirements result in poor BCI control during tasks that inherently lack visual feedback, such as grasping, or when visual attention is diverted. Using a modified version of the Critical Tracking Task which we call the Critical Stability Task (CST), we tested the ability of 9 human subjects to control an unstable system using either free arm movements or pinch force. The subjects were provided either visual feedback, 'proportional' vibrotactile feedback, or 'on-off' vibrotactile feedback about the state of the unstable system. We increased the difficulty of the control task by making the virtual system more unstable. We judged the effectiveness of a particular form of feedback as the maximal instability the system could reach before the subject lost control of it. We found three main results. First, subjects can use solely vibrotactile feedback to control an unstable system, although control was better using visual feedback. Second, 'proportional' vibrotactile feedback provided slightly better control than 'on-off' vibrotactile feedback. Third, there was large intra-subject variability in terms of the most effective input and feedback methods. This highlights the need to tailor the input and feedback methods to the subject when a high degree of control is desired. Our new task can provide a complement to traditional center-out paradigms to help boost the real-world relevance of BCI research in the lab.

Short-term dynamics of causal information transfer in thalamocortical networks during natural inputs and microstimulation for somatosensory neuroprosthesis

Recording the activity of large populations of neurons requires new methods to analyze and use the large volumes of time series data thus created. Fast and clear methods for finding functional connectivity are an important step toward the goal of understanding neural processing. This problem presents itself readily in somatosensory neuroprosthesis (SSNP) research, which uses microstimulation (MiSt) to activate neural tissue to mimic natural stimuli, and has the capacity to potentiate, depotentiate, or even destroy functional connections. As the aim of SSNP engineering is artificially creating neural responses that resemble those observed during natural inputs, a central goal is describing the influence of MiSt on activity structure among groups of neurons, and how this structure may be altered to affect perception or behavior. In this paper, we demonstrate the concept of Granger causality, combined with maximum likelihood methods, applied to neural signals recorded before, during, and after natural and electrical stimulation. We show how these analyses can be used to evaluate the changing interactions in the thalamocortical somatosensory system in response to repeated perturbation. Using LFPs recorded from the ventral posterolateral thalamus (VPL) and somatosensory cortex (S1) in anesthetized rats, we estimated pair-wise functional interactions between functional microdomains. The preliminary results demonstrate input-dependent modulations in the direction and strength of information flow during and after application of MiSt. Cortico-cortical interactions during cortical MiSt and baseline conditions showed the largest causal influence differences, while there was no statistically significant difference between pre- and post-stimulation baseline causal activities. These functional connectivity changes agree with physiologically accepted communication patterns through the network, and their particular parameters have implications for both rehabilitation and brain—machine interface SSNP applications.

A cognitive neuroprosthetic that uses cortical stimulation for somatosensory feedback

OBJECTIVE

Present day cortical brain-machine interfaces (BMIs) have made impressive advances using decoded brain signals to control extracorporeal devices. Although BMIs are used in a closed-loop fashion, sensory feedback typically is visual only. However medical case studies have shown that the loss of somesthesis in a limb greatly reduces the agility of the limb even when visual feedback is available.

APPROACH

To overcome this limitation, this study tested a closed-loop BMI that utilizes intracortical microstimulation to provide 'tactile' sensation to a non-human primate.

MAIN RESULT

Using stimulation electrodes in Brodmann area 1 of somatosensory cortex (BA1) and recording electrodes in the anterior intraparietal area, the parietal reach region and dorsal area 5 (area 5d), it was found that this form of feedback can be used in BMI tasks.

SIGNIFICANCE

Providing somatosensory feedback has the poyential to greatly improve the performance of cognitive neuroprostheses especially for fine control and object manipulation. Adding stimulation to a BMI system could therefore improve the quality of life for severely paralyzed patients.

Modifying cognition and behavior with electrical microstimulation: implications for cognitive prostheses

A fundamental goal of cognitive neuroscience is to understand how brain activity generates complex mental states and behaviors. While neuronal activity may predict or correlate with behavioral responses in a cognitive task, the use of electrical microstimulation presents the possibility to augment such correlational findings with direct evidence for causal relationships. Although microstimulation has been used for many years as a tool for mapping sensory and motor function, its role in learning, memory and decision-making has emerged only recently. Focal microstimulation of higher cortical areas can produce complex mental states and sequences of action. However, the relationship between the locus of stimulation and the percepts or actions evoked is often stereotyped and inflexible. The challenge is to develop stimulation systems that do not have fixed output but can flexibly contribute to complex cognitive and behavioral tasks. We discuss how microstimulation has been instrumental in manipulating a wide spectrum of cognitive functions including working memory, perceptual decisions and executive control by enhancing attention, re-ordering temporal sequence of saccades, improving associative learning or cognitive performance. For example, stimulation in prefrontal, parietal and sensory cortices may establish causal effects on decision-making, while microstimulation of inferotemporal cortex or caudate nucleus enhances associative learning. Building cognitive prosthetics based on the insights gleaned from such studies may depend on the development of multiple-input, multiple-output (MIMO) devices that allow subjects to control stimulation with their own thoughts in a closed-loop system.

Comparing temporal aspects of visual, tactile, and microstimulation feedback for motor control

OBJECTIVES

Current brain-computer interfaces (BCIs) rely on visual feedback, requiring sustained visual attention to use the device. Improvements to BCIs may stem from the development of an effective way to provide quick feedback independent of vision. Tactile stimuli, either delivered on the skin surface, or directly to the brain via microstimulation in somatosensory cortex, could serve that purpose. We examined the effectiveness of vibrotactile stimuli and microstimulation as a means of non-visual feedback by using a fundamental element of feedback: the ability to react to a stimulus while already in motion.

APPROACH

Human and monkey subjects performed a center-out reach task which was, on occasion, interrupted with a stimulus cue that instructed a change in reach target.

MAIN RESULTS

Subjects generally responded faster to tactile cues than to visual cues. However, when we delivered cues via microstimuation in a monkey, its response was slower on average than for both tactile and visual cues.

SIGNIFICANCE

Tactile and microstimulation feedback can be used to rapidly adjust movements mid-flight. The relatively slow speed of microstimulation is surprising and warrants further investigation. Overall, these results highlight the importance of considering temporal aspects of feedback when designing alternative forms of feedback for BCIs.

Restoring sensorimotor function through intracortical interfaces: progress and looming challenges

The loss of a limb or paralysis resulting from spinal cord injury has devastating consequences on quality of life. One approach to restoring lost sensory and motor abilities in amputees and patients with tetraplegia is to supply them with implants that provide a direct interface with the CNS. Such brain-machine interfaces might enable a patient to exert voluntary control over a prosthetic or robotic limb or over the electrically induced contractions of paralysed muscles. A parallel interface could convey sensory information about the consequences of these movements back to the patient. Recent developments in the algorithms that decode motor intention from neuronal activity and in approaches to convey sensory feedback by electrically stimulating neurons, using biomimetic and adaptation-based approaches, have shown the promise of invasive interfaces with sensorimotor cortices, although substantial challenges remain.

Cerebral Baylisascaris larva migrans in a cynomolgus macaque (Macaca fascicularis)

An incidental, asymptomatic, focal inflammatory lesion was detected in brain cerebrum of an approximately 6-year-old, female cynomolgus macaque from a chronic toxicology study. No gross lesions were noted at necropsy. Microscopically, the lesion contained a cross-section of larvae approximately 70-80 μm in diameter, a centrally located intestine flanked on either side by large triangular excretory columns, and prominent single lateral cuticular alae. Mixed inflammatory cells of eosinophils, macrophages, and lymphocytes admixed with abundant connective tissue stroma and necrosis surrounded the larvae. Histochemical stains for trichrome revealed significant amount of fibrous connective tissue. The morphology of the larvae was compatible with Baylisascaris spp. Based on the microscopic and histochemical examination, a diagnosis of neural Baylisascaris spp. larva migrans was made.