Record Neural Activity Research Articles

SPARC: Soft silicone electrode net for modulating bladder function

Bladder wall stimulation has been attempted for many decades to restore bladder function in people with spinal cord injury and other voiding dysfunctions. However, these efforts were limited by current spread to urethral structures, activation of the legs and co‐recruitment of other pelvic organs. Neural interfaces for the detrusor muscle itself face several challenges due to its unique structure and function. To directly modulate the bladder and urethra, we designed a stretchable electrode array that interfaces directly with the surface of these organs. With a stretchable electrode array that conforms to the bladder wall, we aim to create direct organ interface electrodes to stimulate and record from the bladder wall.Acute experiments in isoflurane‐anesthetized cats (n=9, 8 males, 1 female) were performed to characterize the functional and electromechanical properties of this polymer electrode. Nerve cuffs were placed bilaterally on the hypogastric and pelvic nerves to record neural activity during bladder wall stimulation. Fine wire electromyography (EMG) electrodes were placed in gluteal muscle as well as the external anal and urethral sphincters to measure current spread. A pressure catheter was placed transurethrally to record pressure and infuse saline into the bladder. Elastic electrodes were placed on the dorsal, ventral, and both lateral aspects of the bladder. In an isovolumetric bladder state, each electrode was stimulated (at amplitude: 2–8 mA, frequency: 3 – 33 Hz, pulse width: 1ms) while recording bladder pressure and compound action potentials in the instrumented nerves. To determine the mechanism of action, pelvic and hypogastric nerves were transected sequentially while measuring bladder pressure.We found that electrodes positioned near the bladder neck around the ureterovesical junction elicited the largest bladder contractions up to ~45 cmH2O while other locations such as the bladder dome generated significantly smaller contractions (~10 cmH2O). Bipolar stimulation on the bladder limited activation of the urethral sphincter and hind limbs through current spread compared to monopolar stimulation as shown by significant reduction in EMG activity with similar increases in bladder pressure. EMG recorded from the bladder wall had a directly proportional relation between frequency and volume thereby depicting detrusor muscle activity. Finally, we found that the passive mechanical effects of the silicone net around the bladder had a minimal impact (< 5 cmH2O) on bladder pressure. Following nerve transection, single electrode stimulation on the ureterovesical junction resulted in smaller pressure increases compared to before transection, whereas there was no change in the pressures evoked by multi‐electrode stimulation.These soft silicone electrode nets can be used as a neural interface to generate bladder contractions in conditions where the bladder is underactive or atonic and enables continuous monitoring of detrusor EMG to monitor bladder state. Bladder wall stimulation acts through both direct‐efferent and reflex‐driven afferent mechanisms depending on stimulation location and charge.Support or Funding InformationNIH SPARC OT2OD025297

MP54-01 SOFT SILICONE ELECTRODE NET FOR MODULATING BLADDER FUNCTION

INTRODUCTION AND OBJECTIVE: Bladder wall stimulation has been attempted for many decades to restore bladder function in people with spinal cord injury and other voiding dysfunctions. However, these efforts were limited by current spread to urethral structures, activation of the legs and co-recruitment of other pelvic organs. Neural interfaces for the detrusor muscle itself face several challenges due to its unique structure and function. To directly modulate the bladder and urethra, we designed a stretchable electrode array that interfaces directly with the surface of these organs. With a stretchable electrode array that conforms to the bladder wall, we aim to create direct organ interface electrodes to stimulate and record from the bladder wall. METHODS: Acute experiments in isoflurane-anesthetized cats (n=9, 8 males, 1 female) were performed to characterize the functional and electromechanical properties of this polymer electrode. Nerve cuffs were placed bilaterally on the hypogastric and pelvic nerves to record neural activity during bladder wall stimulation. Fine wire electromyography (EMG) electrodes were placed in gluteal muscle as well as the external anal and urethral sphincters to measure current spread. A pressure catheter was placed transurethrally to record pressure and infuse saline into the bladder. Elastic electrodes were placed on the dorsal, ventral, and both lateral aspects of the bladder. In an isovolumetric bladder state, each electrode was stimulated (at amplitude: 2-8 mA, frequency: 3 -33 Hz, pulse width: 1ms) while recording bladder pressure and compound action potentials in the instrumented nerves. RESULTS: We found that electrodes positioned near the bladder neck around the ureterovesical junction elicited the largest bladder contractions up to ∼45 cmH2O while other locations such as the bladder dome generated significantly smaller contractions (∼10 cmH2O). Bipolar stimulation on the bladder limited activation of the urethral sphincter and hind limbs through current spread compared to monopolar stimulation as shown by significant reduction in EMG activity with similar increases in bladder pressure. EMG recorded from the bladder wall had a directly proportional relation between frequency and volume thereby depicting detrusor muscle activity. Finally, we found that the passive mechanical effects of the silicone net around the bladder had a minimal impact (< 5 cmH2O) on bladder pressure. CONCLUSIONS: These soft silicone electrode nets can be used as a neural interface to generate bladder contractions in conditions where the bladder is underactive or atonic and enables continuous monitoring of detrusor EMG to monitor bladder state. Source of Funding: This project is supported through NIH SPARC OT2OD025297

Interpreting Dynamic Interfacial Changes at Carbon Fiber Microelectrodes Using Electrochemical Impedance Spectroscopy.

Carbon-fiber microelectrodes are instrumental tools in neuroscience used for the electroanalysis of neurochemical dynamics and recordings of neural activity. However, performance is variable and dependent on fabrication strategies, the biological response to implantation, and the physical and chemical composition of the recording environment. This presents an analytical challenge, as electrode performance is difficult to quantitatively assess in situ, especially when electrodes are permanently implanted or cemented in place. We previously reported that electrode impedance directly impacts electrochemical performance for molecular sensing. In this work, we investigate the impacts of individual components of the electrochemical system on impedance. Equivalent circuit models for glass- and silica-insulated carbon-fiber microelectrodes were determined using electrochemical impedance spectroscopy (EIS). The models were validated based on the ability to assign individual circuit elements to physical properties of the electrochemical system. Investigations were performed to evaluate the utility of the models in providing feedback on how changes in ionic strength and carbon fiber material alter impedance properties. Finally, EIS measurements were used to investigate the electrode/solution interface prior to, during, and following implantation in live brain tissue. A significant increase in impedance and decrease in capacitance occur during tissue exposure and persist following implantation. Electrochemical conditioning, which occurs continually during fast-scan cyclic voltammetry recordings, etches and renews the carbon surface, mitigating these effects. Overall, the results establish EIS as a powerful method for characterization of carbon-fiber microelectrodes, providing unprecedented insight into how real-world factors affect the electrode/solution interface.

Massively parallel microwire arrays integrated with CMOS chips for neural recording.

Multi-channel electrical recordings of neural activity in the brain is an increasingly powerful method revealing new aspects of neural communication, computation, and prosthetics. However, while planar silicon-based CMOS devices in conventional electronics scale rapidly, neural interface devices have not kept pace. Here, we present a new strategy to interface silicon-based chips with three-dimensional microwire arrays, providing the link between rapidly-developing electronics and high density neural interfaces. The system consists of a bundle of microwires mated to large-scale microelectrode arrays, such as camera chips. This system has excellent recording performance, demonstrated via single unit and local-field potential recordings in isolated retina and in the motor cortex or striatum of awake moving mice. The modular design enables a variety of microwire types and sizes to be integrated with different types of pixel arrays, connecting the rapid progress of commercial multiplexing, digitisation and data acquisition hardware together with a three-dimensional neural interface.

Light Modulation of Brain and Development of Relevant Equipment.

Light modulation plays an important role in understanding the pathology of brain disorders and improving brain function. Optogenetic techniques can activate or silence targeted neurons with high temporal and spatial accuracy and provide precise control, and have recently become a method for quick manipulation of genetically identified types of neurons. Photobiomodulation (PBM) is light therapy that utilizes non-ionizing light sources, including lasers, light emitting diodes, or broadband light. It provides a safe means of modulating brain activity without any irreversible damage and has established optimal treatment parameters in clinical practice. This manuscript reviews 1) how optogenetic approaches have been used to dissect neural circuits in animal models of Alzheimer's disease, Parkinson's disease, and depression, and 2) how low level transcranial lasers and LED stimulation in humans improves brain activity patterns in these diseases. State-of-the-art brain machine interfaces that can record neural activity and stimulate neurons with light have good prospects in the future.

Systematic Integration of Structural and Functional Data into Multi-scale Models of Mouse Primary Visual Cortex

Structural rules underlying functional properties ofcortical circuits are poorly understood. To explore these rules systematically, we integrated information from extensive literature curation and large-scale experimental surveys into a data-driven, biologically realistic simulation of the awake mouse primary visual cortex. The model was constructed at two levels of granularity, using either biophysically detailed or point neurons. Both variants have identical network connectivity and were compared to each other and to experimental recordings of visual-driven neural activity. While tuning these networks to recapitulate experimental data, we identified rules governing cell-class-specific connectivity and synaptic strengths. These structural constraints constitute hypotheses that can be tested experimentally. Despite their distinct single-cell abstraction, both spatially extended and point models perform similarly at the level of firing rate distributions for the questions we investigated. All data and models are freely available as a resource for the community.

Probing Cortical Activity During Head-Fixed Behavior

The cortex is crucial for many behaviors, ranging from sensory-based behaviors to working memory and social behaviors. To gain an in-depth understanding of the contribution to these behaviors, cellular and sub-cellular recordings from both individual and populations of cortical neurons are vital. However, techniques allowing such recordings, such as two-photon imaging and whole-cell electrophysiology, require absolute stability of the head, a requirement not often fulfilled in freely moving animals. Here, we review and compare behavioral paradigms that have been developed and adapted for the head-fixed preparation, which together offer the needed stability for live recordings of neural activity in behaving animals. We also review how the head-fixed preparation has been used to explore the function of primary sensory cortices, posterior parietal cortex (PPC) and anterior lateral motor (ALM) cortex in sensory-based behavioral tasks, while also discussing the considerations of performing such recordings. Overall, this review highlights the head-fixed preparation as allowing in-depth investigation into the neural activity underlying behaviors by providing highly controllable settings for precise stimuli presentation which can be combined with behavioral paradigms ranging from simple sensory detection tasks to complex, cross-modal, memory-guided decision-making tasks.

Inference of synaptic connectivity and external variability in neural microcircuits.

A major goal in neuroscience is to estimate neural connectivity from large scale extracellular recordings of neural activity in vivo. This is challenging in part because any such activity is modulated by the unmeasured external synaptic input to the network, known as the common input problem. Many different measures of functional connectivity have been proposed in the literature, but their direct relationship to synaptic connectivity is often assumed or ignored. For in vivo data, measurements of this relationship would require a knowledge of ground truth connectivity, which is nearly always unavailable. Instead, many studies use in silico simulations as benchmarks for investigation, but such approaches necessarily rely upon a variety of simplifying assumptions about the simulated network and can depend on numerous simulation parameters. We combine neuronal network simulations, mathematical analysis, and calcium imaging data to address the question of when and how functional connectivity, synaptic connectivity, and latent external input variability can be untangled. We show numerically and analytically that, even though the precision matrix of recorded spiking activity does not uniquely determine synaptic connectivity, it is in practice often closely related to synaptic connectivity. This relation becomes more pronounced when the spatial structure of neuronal variability is jointly considered.

Biomechanical Stresses Due to Tissue Micromotion at the Neural Interface Modulate Intracellular Membrane Potentials

Micromotion in brain tissue at a neural interface can arise from respiration and vascular pulsation besides behavioral perturbations. We have reported 2-4 µm of brain tissue displacements in rodents due to vascular pulsation and 10-30 µm due to respiration. It has been hypothesized that the periodic membrane potential (MP) changes observed in neuronal intracellular recordings correlated to tissue micromotion were an artifact. In this study, using penetrating microelectrodes for intracellular recordings in an Aplysia californica abdominal ganglion, we show that the membrane potentials and firing rates change due to periodic cell compression. We show that these membrane potential changes are modulated by calcium, potassium, and mechanosensitive channel blockers. There were no membrane potential changes in response to periodic stresses <1.5 kPa in n=5 cells. Increases in amplitude of periodic stresses resulted in a linear increase in MP fluctuations; action potentials were generated in response to stresses >3 kPa for n=2 silent neurons suggesting the role of mechanosensitive receptors. Neurons that were already firing changed their firing rate in n=3 cells indicating that MP fluctuations are physiological and not artifacts. Application of 5-HT to the ganglion cell, resulted in decreased firing rate indicating that MP fluctuations are potentially modulated by mechanosensitive SK-channels in Aplysia. Preliminary in vivo rodent experiments show that periodic, intracellular membrane potentials fluctuate in the order of 10-30 mV. Using a custom double barrel pipette to (1) record neural activity and (2) to deliver drugs locally, these periodic membrane potential changes due to breathing and heart rate are inhibited by the mechanosensitive ion channel blocker gadolinium chloride. Results of this study suggest that cells at the electrode-brain tissue interface could be potentially undergoing chronic sub-threshold neuromodulation that could impact its long-term functionality and viability.

System for recording from multiple flexible polyimide neural probes in freely behaving animals

Objective. Long-term electrophysiological recordings of neural activity in freely behaving animals are indispensable to advance the understanding of complex brain function. It is a technical challenge to chronically monitor the detailed activity across multiple distributed brain regions in freely behaving animals over a period of months. Here we present a new implant for inserting multiple flexible polyimide probes into freely behaving rats for monitoring the brain activity over a long time period. Approach. This brain implant integrates multiple flexible probes in small micromanipulator devices that ensure free behaviour of the animal. The probes are micromachined and the positioning mechanism is 3D-printed using stereolithography. Each probe is lowered by a screw-driven shuttle and guided through an exit tip before penetrating the rat’s brain. Main results. The brain implant consists of 16 individually lowerable flexible polyimide probes that contain 16 embedded electrodes adding up to a total of 256 recording channels. The total travel distance is 8 mm. The assembly time of the device was only one day. The electrode impedance values had a mean of 335 kΩ and sample standard deviation of 107 kΩ after gold plating, excluding outliers. Significance. For the first time, hyperdrive-assisted insertion of flexible multichannel probes was demonstrated. Local field potentials and neuronal spiking activity from freely behaving rats were recorded over months.

Electrophysiological and Motor Responses to Chemosensory Stimuli in Isolated Cephalopod Arms.

While there is behavioral and anatomical evidence that coleoid cephalopods use their arms to "taste" substances in the environment, the neurophysiology of chemosensation has been largely unexamined. The range and sensitivity of detectable chemosensory stimuli, and the processing of chemosensory information, are unknown. To begin to address these issues, we developed a technique for recording neurophysiological responses from isolated arms, allowing us to test responses to biologically relevant stimuli. We tested arms from both a pelagic species (Doryteuthis pealeii) and a benthic species (Octopus bimaculoides) by attaching a suction electrode to the axial nerve cord to record neural activity in response to chemical stimuli. Doryteuthis pealeii arms showed anecdotal responses to some stimuli but generally did not tolerate the preparation; tissue was nonviable within minutes ex vivo. Octopus bimaculoides arms were used successfully, with tissue remaining healthy and responsive for several hours. Arms responded strongly to fish skin extract, glycine, methionine, and conspecific skin extract but not to cephalopod ink or seawater controls. Motor responses were also observed in response to detected stimuli. These results suggest that chemosensory receptor cells on O. bimaculoides arms were able to detect environmentally relevant chemicals and drive local motor responses within the arm. Further exploration of potential chemical stimuli for O. bimaculoides arms, as well as investigations into the neural processing within the arm, could enhance our understanding of how this species uses its arms to explore its environment. While not successful in D. pealeii, this technique could be attempted with other cephalopod species, as comparative questions remain of interest.

Open source silicon microprobes for high throughput neural recording

Objective. Microfabricated multielectrode arrays are widely used for high throughput recording of extracellular neural activity, which is transforming our understanding of brain function in health and disease. Currently there is a plethora of electrode-based tools being developed at higher education and research institutions. However, taking such tools from the initial research and development phase to widespread adoption by the neuroscience community is often hindered by several obstacles. The objective of this work is to describe the development, application, and open dissemination of silicon microprobes for recording neural activity in vivo. Approach. We propose an open source dissemination platform as an alternative to commercialization. This framework promotes recording tools that are openly and inexpensively available to the community. The silicon microprobes are designed in house, but the fabrication and assembly processes are carried out by third party companies. This enables mass production, a key requirement for large-scale dissemination. Main results. We demonstrate the operation of silicon microprobes containing up to 256 electrodes in conjunction with optical fibers for optogenetic manipulations or fiber photometry. These data provide new insights about the relationship between calcium activity and neural spiking activity. We also describe the current state of dissemination of these tools. A file repository of resources related to designing, using, and sharing these tools is maintained online. Significance. This paper is likely to be a valuable resource for both current and prospective users, as well as developers of silicon microprobes. Based on their extensive usage by a number of labs including ours, these tools present a promising alternative to other types of electrode-based technologies aimed at high throughput recording in head-fixed animals. This work also demonstrates the importance of validating fiber photometry measurements with simultaneous electrophysiological recordings.

Differential Roles of Mediodorsal Nucleus of the Thalamus and Prefrontal Cortex in Decision-Making and State Representation in a Cognitive Control Task Measuring Deficits in Schizophrenia.

The mediodorsal nucleus of the thalamus (MD) is reciprocally connected with the prefrontal cortex (PFC), and although the MD has been implicated in a range of PFC-dependent cognitive functions (Watanabe and Funahashi, 2012; Mitchell and Chakraborty, 2013; Parnaudeau et al., 2018), little is known about how MD neurons in the primate participate specifically in cognitive control, a capability that reflects the ability to use contextual information (such as a rule) to modify responses to environmental stimuli. To learn how the MD-PFC thalamocortical network is engaged to mediate forms of cognitive control that are selectively disrupted in schizophrenia, we trained male monkeys to perform a variant of the AX continuous performance task, which reliably measures cognitive control deficits in patients (Henderson et al., 2012) and used linear multielectrode arrays to record neural activity in the MD and PFC simultaneously. We found that the two structures made clearly different contributions to distributed processing for cognitive control: MD neurons were specialized for decision-making and response selection, whereas prefrontal neurons were specialized to preferentially encode the environmental state on which the decision was based. In addition, we observed that functional coupling between MD and PFC was strongest when the decision as to which of the two responses in the task to execute was being made. These findings delineate unique contributions of MD and PFC to distributed processing for cognitive control and characterized neural dynamics in this network associated with normative cognitive control performance.SIGNIFICANCE STATEMENT Cognitive control is fundamental to healthy human executive functioning (Miller and Cohen, 2001) and deficits in patients with schizophrenia relate to decreased functional activation of the MD thalamus and the prefrontal cortex (Minzenberg et al., 2009), which are reciprocally linked (Goldman-Rakic and Porrino, 1985; Xiao et al., 2009). We carry out simultaneous neural recordings in the MD and PFC while monkeys perform a cognitive control task translated from patients with schizophrenia to relate thalamocortical dynamics to cognitive control performance. Our data suggest that state representation and decision-making computations for cognitive control are preferentially performed by PFC and MD, respectively. This suggests experiments to parse decision-making and state representation deficits in patients while providing novel computational targets for future therapies.

Supramolecular Peptide Hydrogel-Based Soft Neural Interface Augments Brain Signals through a Three-Dimensional Electrical Network

Recording neural activity from the living brain is of great interest in neuroscience for interpreting cognitive processing or neurological disorders. Despite recent advances in neural technologies, development of a soft neural interface that integrates with neural tissues, increases recording sensitivity, and prevents signal dissipation still remains a major challenge. Here, we introduce a biocompatible, conductive, and biostable neural interface, a supramolecular β-peptide-based hydrogel that allows signal amplification via tight neural/hydrogel contact without neuroinflammation. The non-biodegradable β-peptide forms a multihierarchical structure with conductive nanomaterial, creating a three-dimensional electrical network, which can augment brain signal efficiently. By achieving seamless integration in brain tissue with increased contact area and tight neural tissue coupling, the epidural and intracortical neural signals recorded with the hydrogel were augmented, especially in the high frequency range. Overall, our tissuelike chronic neural interface will facilitate a deeper understanding of brain oscillation in broad brain states and further lead to more efficient brain-computer interfaces.

Viral vectors for manipulation and recording of neural activity

Viral vectors for manipulation and recording of neural activity

Implanting and Recycling Neuropixels Probes for Recordings in Freely Moving Mice.

Recording neural activity in unrestricted animals is necessary to unravel the neural basis of ethological behaviors. Recently, Neuropixels probes have made important strides in improving yield and lowering noise, but have limited use cases in freely moving animals. Although there are a number of studies demonstrating the use of these probes in headfixed mice, there are not established protocols for the use and reuse of them in a freely moving mouse. We therefore designed a novel device (the AMIE) that maximizes the potential value of these powerful probes. Here, we provide the technical drawings for the AMIE and detail its preparation, implantation, and explantation. With our approach, researchers can record hundreds of neurons during freely moving behavior across weeks of experiments, and then recycle valuable probes for future use.

Cyberattacks on Miniature Brain Implants to Disrupt Spontaneous Neural Signaling

Brain-Computer Interfaces (BCI) arose as systems that merge computing systems with the human brain to facilitate recording, stimulation, and inhibition of neural activity. Over the years, the development of BCI technologies has shifted towards miniaturization of devices that can be seamlessly embedded into the brain and can target single neuron or small population sensing and control. We present a motivating example highlighting vulnerabilities of two promising micron-scale BCI technologies, demonstrating the lack of security and privacy principles in existing solutions. This situation opens the door to a novel family of cyberattacks, called neuronal cyberattacks, affecting neuronal signaling. This article defines the first two neural cyberattacks, Neuronal Flooding (FLO) and Neuronal Scanning (SCA), where each threat can affect the natural activity of neurons. This work implements these attacks in a neuronal simulator to determine their impact over the spontaneous neuronal behavior, defining three metrics: number of spikes, percentage of shifts, and dispersion of spikes. Several experiments demonstrate that both cyberattacks produce a reduction of spikes compared to spontaneous behavior, generating a rise in temporal shifts and a dispersion increase. Mainly, SCA presents a higher impact than FLO in the metrics focused on the number of spikes and dispersion, where FLO is slightly more damaging, considering the percentage of shifts. Nevertheless, the intrinsic behavior of each attack generates a differentiation on how they alter neuronal signaling. FLO is adequate to generate an immediate impact on the neuronal activity, whereas SCA presents higher effectiveness for damages to the neural signaling in the long-term.

Diffusion Maps Kalman Filter for a Class of Systems With Gradient Flows

In this paper, we propose a non-parametric method for state estimation of high-dimensional nonlinear stochastic dynamical systems, which evolve according to gradient flows with isotropic diffusion. We combine diffusion maps, a manifold learning technique, with a linear Kalman filter and with concepts from Koopman operator theory. More concretely, using diffusion maps, we construct data-driven virtual state coordinates, which linearize the system model. Based on these coordinates, we devise a data-driven framework for state estimation using the Kalman filter. We demonstrate the strengths of our method with respect to both parametric and non-parametric algorithms in three tracking problems. In particular, applying the approach to actual recordings of hippocampal neural activity in rodents directly yields a representation of the position of the animals. We show that the proposed method outperforms competing non-parametric algorithms in the examined stochastic problem formulations. Additionally, we obtain results comparable to classical parametric algorithms, which, in contrast to our method, are equipped with model knowledge.

From Neurons to Cognition: Technologies for Precise Recording of Neural Activity Underlying Behavior.

Understanding how brain activity encodes information and controls behavior is a long-standing question in neuroscience. This complex problem requires converging efforts from neuroscience and engineering, including technological solutions to perform high-precision and large-scale recordings of neuronal activity in vivo as well as unbiased methods to reliably measure and quantify behavior. Thanks to advances in genetics, molecular biology, engineering, and neuroscience, in recent decades, a variety of optical imaging and electrophysiological approaches for recording neuronal activity in awake animals have been developed and widely applied in the field. Moreover, sophisticated computer vision and machine learning algorithms have been developed to analyze animal behavior. In this review, we provide an overview of the current state of technology for neuronal recordings with a focus on optical and electrophysiological methods in rodents. In addition, we discuss areas that future technological development will need to cover in order to further our understanding of the neural activity underlying behavior.

Silent Speech Interfaces for Speech Restoration: A Review

This review summarises the status of silent speech interface (SSI) research. SSIs rely on non-acoustic biosignals generated by the human body during speech production to enable communication whenever normal verbal communication is not possible or not desirable. In this review, we focus on the first case and present latest SSI research aimed at providing new alternative and augmentative communication methods for persons with severe speech disorders. SSIs can employ a variety of biosignals to enable silent communication, such as electrophysiological recordings of neural activity, electromyographic (EMG) recordings of vocal tract movements or the direct tracking of articulator movements using imaging techniques. Depending on the disorder, some sensing techniques may be better suited than others to capture speech-related information. For instance, EMG and imaging techniques are well suited for laryngectomised patients, whose vocal tract remains almost intact but are unable to speak after the removal of the vocal folds, but fail for severely paralysed individuals. From the biosignals, SSIs decode the intended message, using automatic speech recognition or speech synthesis algorithms. Despite considerable advances in recent years, most present-day SSIs have only been validated in laboratory settings for healthy users. Thus, as discussed in this paper, a number of challenges remain to be addressed in future research before SSIs can be promoted to real-world applications. If these issues can be addressed successfully, future SSIs will improve the lives of persons with severe speech impairments by restoring their communication capabilities.