Single Unit publications

The first-spike latency (FSL) is an important temporal characteristic of a neurophysiological response. We study the FSLs of sound-driven responses in individual neurons in the rat’s auditory midbrain. Responses were elicited by a train of stimuli created using multiple presentations of two tone bursts with different frequencies. Presentations of the two sounds were interleaved temporally in a random order. We found that the mean and the temporal variation of FSL of the response elicited by a sound was increased when the sound was presented more frequently or when it was moved from the ear that drove an excitatory response to the ear that drove an inhibitory effect. Furthermore, the FSL of response to one sound was dependent on the spatial location of the other sound. Results suggested that the timing of the first spike could be used by midbrain neurons to encode information related to the probability of occurrence and spatial location of a sound. It could also be used to gauge how the sound was related to the other sound in spatial location. These results enhance our understanding of neural bases of binaural hearing, especially in an environment with temporally separated competing sounds.

Auditory processing in the cerebral cortex is considered to begin with thalamocortical inputs to layer 4 (L4) of the primary auditory cortex (A1). In this canonical model, A1 L4 inputs initiate a hierarchical cascade that propagates to higher-order cortices for slower integration of complex sounds. Here, we identify parallel ascending pathways in mice that bypass A1 and directly reach the secondary auditory cortex (A2), alongside the canonical hierarchical route. We found that layer 6 (L6) of both A1 and A2 receive short-latency (<10 ms) sound inputs via higher-order thalamic nuclei. Additionally, A2 L4 is innervated by a caudal subdivision of the traditionally defined primary thalamus, which we now re-classify as non-primary. Notably, both identified thalamic regions receive projections from distinct subdivisions of the higher-order inferior colliculus, which in turn receive direct projections from cochlear nucleus neurons. Thus, higher-order auditory cortex integrates both slower, pre-processed information and rapid, direct sensory inputs, enabling parallel processing of fast sensory information across cortical areas.

Atypical sensory processing, particularly in the auditory domain, is one of the most common and quality-of-life affecting symptoms seen in autism spectrum disorders (ASD). Fragile X Syndrome (FXS) is a leading inherited cause of ASD and a majority of FXS individuals present with auditory processing alterations. While auditory hypersensitivity is a common phenotype observed in FXS and Fmr1 knockout (KO) rodent models, it is important to consider other auditory coding impairments that could contribute to sound processing difficulties and disrupted language comprehension in FXS. We have shown previously that a Fmr1 KO rat model of FXS exhibits heightened sound sensitivity that coincided with abnormal perceptual integration of stimulus bandwidth, indicative of altered spectral processing. Frequency discrimination is a fundamental aspect of sound encoding that is important for a range of auditory processes, such as source segregation and speech comprehension, and disrupted frequency coding could thus contribute to a range of auditory issues in FXS and ASD. Here we explicitly characterized spectral processing deficits in male Fmr1 KO rats using an operant conditioning tone discrimination assay and in vivo electrophysiological recordings from the auditory cortex and inferior colliculus. We found that Fmr1 KO rats exhibited poorer frequency resolution, which corresponded with neuronal hyperactivity and broader frequency tuning in auditory cortical but not collicular neurons. Using an experimentally informed population model, we show that these cortical physiological differences can recapitulate the observed behavior discrimination deficits, with decoder performance being tightly linked to differences in cortical tuning width and signal-to-noise ratios. Together, these findings indicate that cortical hyperexcitability in Fmr1 KO rats may act to preserve signal-to-noise ratios and signal detection threshold at the expense of sound sensitivity and fine feature discrimination, highlighting a potential mechanistic locus for a range of auditory behavioral phenotypes in FXS.

The prefrontal cortex plays a crucial role in procedural rule learning; however, the specific neuronal mechanism through which it represents rules is unknown. We hypothesized that sequential neuronal activities in the prefrontal cortex encode these rules. To investigate this, we recorded neuronal activities in the medial prefrontal cortex of mice during rule learning using Ca 2+ imaging. We utilized a method based on convolutional negative matrix factorization, iSeq, to automatically detect temporal neuronal sequences in the recorded data. As rule learning advanced, these neuronal sequences began to encode critical information for rule execution. In mice that had mastered the rule, the dynamics of neuronal sequences could predict success and failure of reward acquisition. Furthermore, the composition of cell populations within the neuronal sequences was rearranged throughout the learning process. These findings suggest that as animals learn a rule, the medial prefrontal cortex continually updates its neuronal sequences to assign significance to behavioural actions crucial for reward acquisition.

Introduction Prenatal exposure to valproic acid (VPA) is a common environmental cause of autism spectrum disorder (ASD) and often leads to expressive and receptive language impairments. Similar communication difficulties among individuals with ASD are often linked to abnormal subcortical and cortical sound processing. Rodents prenatally exposed to VPA exhibit degraded cortical responses to speech and an impaired ability to behaviorally discriminate speech sounds. Methods We sought to determine whether sound processing could be restored with paired vagus nerve stimulation (VNS). In a first experiment, we evaluated whether sound-paired VNS would alter in vivo extracellular multi-unit responses to tones, noise burst trains, and speech sounds from the anterior auditory field. We next sought to evaluate whether improvements to neural sound processing led to improvements in sound discrimination ability. In a second experiment, rats underwent go/no-go sound discrimination testing where VNS was paired with successful trials. Results We found that VPA-exposed rats had degraded spectral, temporal, and speech sound processing compared to saline-exposed control rats. VPA-exposed rats which received sound-paired VNS exhibited a partial or full restoration of processing across sound types. However, across several sound discrimination tasks, we did not observe changes in behavioral performance in response to prenatal exposure to VPA or VNS. Discussion Our study is the first to show that speech-paired VNS leads to a generalized improvement in cortical sound processing across sound types, rescuing neural processing among VPA-exposed rats. These results provide a framework for future studies to develop VNS-based interventions for communication disorders.

The mediodorsal thalamic nucleus (MD) forms neural circuits with various brain regions, including the prefrontal cortex (PFC), the reticular thalamic nucleus (TRN), and the midbrain reticular nucleus (MRN). However, the specific roles and underlying mechanisms in auditory information processing remain unclear. Here, we perform multi-channel electrophysiological recordings in awake mice to investigate the response patterns of the MD to auditory stimuli, as well as the regulatory effects of PFC, MRN, and TRN inputs. We identify two distinct types of sound-evoked responses. The Phasic-response features a transient burst firing to the stimulus with short latency, rapidly adapting to baseline and corresponding to the onset fluctuation of the local field potential. The Sustained-response is marked by prolonged firing with longer latency and is accompanied by persistent enhancement of oscillatory power following stimulus offset. The response patterns of MD neurons remain consistent across different types of auditory stimuli. Optogenetic inactivation of the MRN suppresses both response types in the MD. The Sustained-response is attenuated by PFC inactivation but enhanced by TRN inactivation, while the Phasic-response remains unaffected by inactivation of either the PFC or TRN. Our findings expand the understanding of the MD's role in sound information integration and auditory cognitive regulation.

Electroanalytical methods are used to understand, modify, and control bionic devices. Bionic devices can record or stimulate cells to understand and/or control normal or abnormal biological processes. These devices contain electrodes that transduce electrical current within the electrical circuit into ionic current within a tissue. Despite the similarity between electroanalysis and electrophysiology, there remains a poor understanding of the relationship between the two techniques, including their methodology and theory. This paper investigates the electrochemical and acute electrophysiological recording performance of neural electrodes. A range of behaviors is achieved by modifying electrodes with the conducting polymer poly(3,4‐ethylenedioxythiophene) (PEDOT) doped with chondroitin sulfate, dextran sulfate, or para ‐toluene sulfonate. The results support previous studies showing that increased electrode area reduced total impedance below the Maxwell–Wagner relaxation frequency and thermal noise while increasing the signal‐to‐noise ratio and neural spike count. The results allowed novel investigation of relative contributions of biological and electrode properties to electrophysiological performance, with increased electrode area having a larger impact on neural population within recording range rather than reducing thermal noise. The utility of measuring electrode impedance for predicting electrophysiological performance is mainly for an indirect measure of electrode area. The results provide insight into noise sources from electrophysiological recordings and limitations in cable theory in neuroscience.

Animals make decisions based on the value of potential outcomes. This perceived value is not fixed; it changes depending on internal needs, such as hunger or thirst, and past experiences. The basolateral amygdala (BLA) is known to be crucial for updating predicted reward values. However, it has been unclear how the BLA represents the specific value of different rewards. Two-photon calcium imaging in male mice showed that population response magnitude scaled with subjective value, and different rewards recruited distinct neuronal subpopulations. Value representations quickly re-scaled when a novel, higher-value reward appeared, and internal state shaped them: thirst selectively boosted responses to water, whereas aversive experience dampened sucrose responses. Thus, BLA circuits carry flexible, stimulus-specific value signals that integrate relative value and current affective or homeostatic conditions, providing a neural basis for adaptive decision making and learning. Our findings reveal that the BLA maintains adaptable, reward-specific value signals, essential for guiding choices according to current needs and changing circumstances.

Extracellular recordings in freely moving mice, especially those with movable electrodes (microdrives), are crucial for understanding brain function. However, existing microdrives are often heavy, expensive, fragile, and unsuited for long-term studies with multichannel recordings. The OptoDrive is a new, lightweight (3.2 g), low-cost system for chronic neural recordings and optogenetic manipulation in mice. It features a detachable, 16-channel tungsten-wire electrode assembly with a 3 mm stroke (15 μm step displacement) and an integrated optical fiber. This system enables repeated implantation and explantation without surgery, requiring only gas anesthesia. The OptoDrive has demonstrated stable recordings from the lateral hypothalamus of freely behaving mice for nearly 1 month and successful optogenetic silencing of neuronal activity. In conclusion, OptoDrive offers a cost-effective, compact solution for long-term electrophysiology and optogenetics in freely moving mice.

The neural processes that change when falling asleep are only partially understood. At the cortical level, features of both spontaneous neural activity and sensory responses change between wakefulness and sleep. For example, in the early auditory cortex, sleep increases the occurrence of postonset silent (OFF) periods and elevates population synchrony. However, it remains unknown whether such changes occur abruptly or gradually around sleep onset and awakening. Here, we recorded spontaneous and sound-evoked neuronal spiking activity in the early auditory cortex along with polysomnography during thousands of episodes when male rats fell asleep or woke up. We found that when falling asleep, stimulus-induced neuronal silent periods (OFF periods), characteristic of nonrapid eye movement sleep, increased within a few seconds around sleep onset. In contrast, a gradual increase in neuronal population synchrony built up over tens of seconds until reaching maximal levels. EEG auditory-evoked potentials likely representing stimulus-triggered “K-complexes” changed along with postonset neuronal firing, whereas ongoing EEG slow-wave activity was associated with neuronal population synchrony. Similar effects, but with opposite direction, were observed around awakenings. The results highlight late stimulus-induced neuronal silence as a key feature changing abruptly around transitions between vigilance states, likely reflecting neuronal bistability and manifesting also in EEG-evoked potentials. More generally, these findings emphasize the added value of going beyond monitoring ongoing activity and perturbing the nervous system to reveal its state—an insight that could also help guide the development of more sensitive noninvasive monitors of falling asleep in humans.

Advancements in the field of implantable neurotechnologies have enabled the integration of hundreds of microelectrodes on ultra-thin and flexible substrates. Besides implantable components, also connectors, headstages and cables have to comply with the high-count demand, resulting in a complex and compact chain with reduced line spacing and smaller safety margins. Here, we show that epicortical recordings acquired from anesthetized rat brains with a state-of-art neural acquisition system are undoubtedly compromised by crosstalk, with signal coherence maps exhibiting a strong dependency to the routing layout. A crosstalk back-correction algorithm is developed, allowing to infer on how signals would look like under a zero-crosstalk scenario. We found that signal coherence between closely routed channels effectively drops after correction, corroborating crosstalk contamination. Our work stresses the importance of validating recorded data against the routing layout as a crucial step of data quality control, helping to come closer to ground truth data.

Purpose Present-day cochlear-implant (CI) users can achieve high levels of speech reception in quiet surroundings. Nevertheless, sensitivity to the temporal pitch of sounds is limited, which contributes to deficits in speech reception amid multiple talkers and in appreciation of musical melodies. Short-term, invasive neurophysiological studies in animals have demonstrated limitations in neural phase locking in the tonotopic range of the auditory pathway that is activated by CIs. It remains an open question, however, whether those neural limitations can account for perceptual deficits in those animal species, let alone in human CI users. For that reason, we have evaluated non-invasive recordings of phase locking from cats chronically implanted with a CI. Methods Ten deafened cats (eight female) were implanted with an animal version of a clinical CI array. The electrically evoked frequency following response (eFFRs) was recorded from the scalps of sedated animals at ≥ 10 weeks post-implantation. Stimuli consisted of constant-amplitude electrical pulse trains at rates from ~ 40 to 640 pulses per second. Results Recordings of the eFFR demonstrated robust responses synchronized to electrical pulse trains across all stimulus rates. Analyses of eFFR amplitude and phase transfer functions confirmed that the eFFR, as with its normal-hearing counterpart, originates from multiple subcortical and cortical generators. The slopes of segments of eFFR phase transfer functions revealed stimulus-to-response latencies of generators that dominated the eFFR across restricted ranges of pulse rates. Those rate ranges must coincide with the limits of phase locking by putative generators at successive levels of the auditory neuroaxis and could inform our understanding of the limits to perceptual temporal acuity. Conclusion The eFFR demonstrated here in an animal model provides a valuable non-invasive measure of temporal processing in electric cochlear stimulation that can be related to ongoing perceptual measures in the same animals and is well-suited to evaluate novel modes of auditory prosthesis for enhancing temporal acuity.

Transcranial focused ultrasound stimulation (tFUS) is a promising neuromodulation technique capable of noninvasively modulating focal neuronal activities and neural circuits in both animals and humans. The cell-type selectivity of tFUS within targeted areas such as the somatosensory cortex (S1) during sonication has been shown to be parameter related. However, it remains unclear how tFUS affects neural circuits by changing the correlation between neurons and how to optimize the tFUS parameters to modulate neural pathways. In this study, multisite intracranial recordings are used in anesthetized male rats to quantify the neuronal responses to tFUS stimulation recorded from S1 and posterior medial thalamic nucleus (POm) of cortico-thalamo-cortical (CTC) pathway. Different tFUS parameters including ranges of pulse repetition frequencies (PRFs) and duty cycles (DCs) are tested. We find that when targeting at S1, only regular-spiking units (RSUs) respond to specific tFUS parameters during sonication (DC, 6−60%; PRF, 1,500, 3,000 and 4,500 Hz), and RSUs from the POm exhibit a synchronized response. The changes of directional correlation between S1 RSUs and POm RSUs indicate the activation of feedback modulation. Delayed responses and correlation changes were further observed at ∼200 ms postsonication from the neurons in S1 and POm, indicating feedforward modulation. Our results reveal that tFUS can modulate the feedback and feedforward CTC pathways by selectively activating cortical RSUs and adjusting the tFUS parameters, particularly the PRF, to maximize cortical RSU activity could enhance the modulation of the CTC pathway.

To build in vitro tissues for therapeutic applications, it is essential to replicate the spatial distribution of cells that occurs during morphogenesis in vivo. However, it remains technically challenging to simultaneously regulate the geometric alignment and aggregation of cells during tissue fabrication. Here, we introduce the acoustofluidic bioassembly induced morphogenesis, which is the combination of precise arrangement of cells by the mechanical forces produced by acoustofluidic cues, and the morphological and functional changes of cells in the following in vitro and in vivo cultures. The acoustofluidic bioassembly can be used to create tissues with regulated nano-, micro-, and macro-structures. We demonstrate that the neuromuscular tissue fabricated with the acoustofluidic bioassembly exhibits enhanced contraction dynamics, electrophysiology, and therapeutic efficacy. The potential of the acoustofluidic bioassembly as an in situ application is demonstrated by fabricating artificial tissues at the defect sites of living tissues. The acoustofluidic bioassembly induced morphogenesis can provide a pioneering platform to fabricate tissues for biomedical applications.

The control of speech and vocal production involves the calculation of error between the intended vocal output and the resulting auditory feedback. This model has been supported by evidence that the auditory cortex (AC) is suppressed immediately before and during vocal production yet remains sensitive to differences between vocal output and altered auditory feedback. This suppression has been suggested to be the result of top-down signals about the intended vocal output, potentially originating from frontal cortical (FC) areas. However, whether FC is the source of suppressive and predictive signaling to AC during vocalization remains unknown. Here, we simultaneously recorded neural activity from both AC and FC of marmoset monkeys during self-initiated vocalizations. We found increases in neural activity in both brain areas from 1 to 0.5 s before vocal production (early pre-vocal period), specifically changes in both multi-unit activity and theta-band power. Connectivity analysis using Granger causality demonstrated that FC sends directed signaling to AC during this early pre-vocal period. Importantly, early pre-vocal activity correlated with both vocalization-induced suppression in AC as well as the structure and acoustics of subsequent calls, such as fundamental frequency. Furthermore, bidirectional auditory-frontal interactions emerged during experimentally altered vocal feedback and predicted subsequent compensatory vocal behavior. These results suggest that FC communicates with AC during vocal production, with frontal-to-auditory signals that may reflect the transmission of sensory prediction information before vocalization and bidirectional signaling during vocalization suggestive of error detection that could drive feedback-dependent vocal control.

Accurate perception of objects within the environment independent of context is essential for the survival of an organism. While neurons that respond in an invariant manner to different stimulus waveforms resulting from identitypreserving transformations of objects are thought to provide a neural correlate of context-independent perception, how such responses emerge in the brain remains poorly understood. Here, we demonstrate that burst firing in neural populations can give rise to an invariant representation of highly heterogeneous natural communication stimuli. Multi-unit recordings from central sensory neural populations showed that considering burst spike trains led to invariant representations at the population but not the single neuron level. Computational modeling further revealed that optimal invariance is achieved at burst firing levels seen experimentally. Taken together, our results demonstrate an important function for burst firing toward establishing invariant representations of sensory input in neural populations.

Epidural electrical stimulation of the spinal cord (ES) is used to restore/improve locomotor movements in patients. However, the neuronal mechanisms underlying activation of locomotor networks by ES are unknown. Here, we analyzed the effects of ES on the activity of individual spinal neurons of different functional locomotor groups. Neuronal activity was recorded in decerebrate cats before and during ES that evoked locomotion. During ES initial period (NW-period, before the locomotion onset), the activity was increased in most neurons as compared to that before ES. We found that ES-caused activation of neurons of locomotor network to the average level similar to that observed during locomotion is not sufficient for initiation of locomotion. We demonstrated that the onset of ES-evoked locomotion was associated with specific changes in the activity of individual neurons within functional locomotor groups and in their responses to epidural stimuli. With the locomotion onset, there was a trend for individual neurons with extremely high activity during NW-period to decrease their activity, while for neurons with extremely low activity to become more active. Also, in neurons contributing to generation of a specific mode of locomotion, peaks and troughs in responses to individual epidural stimuli were significantly less pronounced as compared with those observed during NW-period. We suggest that these changes allow ES to maintain the high level of excitability of the locomotor network necessary for its operation without distortion of the locomotor rhythm. The obtained results advance our understanding of the neuronal mechanisms underlying activation of locomotor network by ES.

Although control of movement involves many cortical association areas, bidirectional brain-machine interfaces (BMIs) typically decode movement intent from the motor cortex and deliver feedback information to the primary somatosensory cortex (S1). Compared to the S1, the parietal and premotor areas encode more complex information about object properties, hand pre-shaping, and reach trajectories. BMIs therefore might deliver richer information to those cortical association areas than to primary areas. Here, we investigated whether instructions for a center-out task could be delivered via intracortical microstimulation (ICMS) in the anterior intraparietal area (AIP), dorsal posterior parietal cortex (dPPC), or dorsal premotor cortex (PMd) as well as the ventral premotor cortex (PMv) and S1. Two monkeys successfully learned to use AIP, PMv, or S1-ICMS instructions, but neither learned to use dPPC- or PMd-ICMS instructions. The AIP, PMv, and S1 may thus be effective cortical territory for delivering information to the brain, whereas the dPPC or PMd may be comparatively ineffective.

Understanding how downstream brain areas decode sensory information represented by neural populations remains a central problem in neuroscience. While decoders that are optimized to extract the maximum amount of information have been extensively used in research, whether these are physiologically realistic remains at best unclear. Here we show that a physiologically realistic decoding scheme based on correlations between neural activities in the absence of stimulation can predict downstream neural responses as well as the optimal decoder. Simultaneous recordings were made from primary sensory neural populations and their downstream midbrain targets in the electrosensory system of Apteronotus leptorhynchus . We found that neural populations exhibited significant correlations in the absence of stimulation (i.e. ‘baseline’), with downstream neural activity lagging primary sensory neural activity with a short latency. We then investigated how primary sensory neural activities were combined downstream. Overall, a decoder that assigned weights to each primary sensory neuron and was trained solely on baseline correlations performed as well as the optimal decoder trained on neural responses to stimulation. Interestingly, both decoders greatly outperformed schemes for which every neuron was assigned the same weight or when the weights were shuffled, indicating that neural identity is critical. Taken together, our results suggest that the brain uses decoding strategies that perform at optimal levels but are qualitatively different from those predicted from optimal solutions. image Key points How neural signals are decoded to give rise to perception remains poorly understood. We recorded from primary sensory neural populations and their downstream targets. A physiologically realistic decoder performed as well as the optimal solution to predict downstream responses. We found important qualitative differences between how information is decoded and the optimal solution. Our results demonstrate that the brain can do as well as an optimal decoder but uses a different strategy.

The hippocampus (HPC) supports spatial working memory (SWM) through its interactions with the prefrontal cortex (PFC). However, it is not clear whether and how the dorsal (dHPC) and ventral (vHPC) poles of the HPC make distinct contributions to SWM and whether they differentially influence the PFC. To address this question, we optogenetically silenced the dHPC or the vHPC while simultaneously recording from the PFC of mice performing a SWM task. We found that whereas both HPC subregions were necessary during the encoding phase of the task, only the dHPC was necessary during the choice phase. Unexpectedly, silencing of either subregion did not affect PFC neurons’ ability to represent the animal’s position, but did alter how it was represented. In contrast, only silencing of the vHPC affected their coding of spatial goals. These results thus reveal distinct contributions of the dorsal and ventral HPC poles to SWM and the coding of behaviorally relevant spatial information by PFC neurons.