Temporal Discharge Patterns Research Articles

Assessing the impact of climate change on spring discharge using hydrological modelling in Musanze District, Rwanda

AbstractClimate change has far-reaching impacts on water availability globally, with changing precipitation patterns and global warming contributing to increasing scarcity and unreliability of spring water in many regions. Despite this understanding, the implication of climate change on the hydrological system remains limited in certain areas, including the district of Musanze, Rwanda. This study investigates the effects of climate change on the discharge of 14 springs in Musanze using the hydrological model V2Karst. CORDEX data from two global climate models of CMIP5 are used to simulate the future spring discharges over the period 2021–2100. The results reveal significantly higher annual discharges in the RCP2.6 scenario compared to the RCP8.5 for all springs from 2021 to 2100. Nevertheless, no significant long-term trend in spring discharge is observed between the early (2021–2050) and late (2071–2100) periods. However, the intra-annual temporal discharge patterns are changing with a significant increase in the seasonality index for the RCP8.5 scenario towards the end of the twenty-first century. Additionally, for both RCPs, there is a notable increase in the number of days with discharges below 40% of the mean of annual discharges during the baseline period. The overall findings of this study suggest that climate change significantly impacts the future evolution of spring discharges in Musanze, indicating potential risks to the future availability of water in the region.

Hybrid Physically Based and Deep Learning Modeling of a Snow Dominated, Mountainous, Karst Watershed

AbstractSnow dominated mountainous karst watersheds are the primary source of water supply in many areas in the western U.S. and worldwide. These watersheds are typically characterized by complex terrain, spatiotemporally varying snow accumulation and melt processes, and duality of flow and storage dynamics because of the juxtaposition of matrix (micropores and small fissures) and karst conduits. As a result, predicting streamflow from meteorological inputs has been challenging due to the inability of physically based or conceptual hydrologic models to represent these unique characteristics. We present a hybrid modeling approach that integrates a physically based, spatially distributed, snow model with a deep learning karst model. More specifically, the high‐resolution snow model captures spatiotemporal variability in snowmelt, and the deep learning model simulates the corresponding response of streamflow as influenced by complex surface and subsurface properties. The deep learning model is based on the Convolutional Long Short‐Term Memory (ConvLSTM) architecture capable of handling spatiotemporal recharge patterns and watershed storage dynamics. The hybrid modeling approach is tested on a watershed in northern Utah with seasonal snow cover and variably karstified carbonate bedrock. The hybrid models were able to simulate streamflow at the watershed outlet with high accuracy. The spatial and temporal recharge and discharge patterns learned by the ConvLSTM model were then examined and compared with known hydrogeologic information. Results suggest that ConvLSTM simulates streamflow with higher accuracy than reference models for the study area and provides insight into spatially influenced hydrologic responses that are unavailable within lumped modeling approaches.

How Do Neurons Signal Itch?

Mechanistic theories of itch are based on neuronal specificity, stimulus intensity, and temporal or spatial discharge patterns. Traditionally, these theories are conceptualized as mutually exclusive, assuming that finding evidence for one theory would exclude the others and could sufficiently explain itch. Current experimental data primarily support the specificity or pattern theory of itch. However, in contrast to an assumed inherent exclusivity, recent results have shown that even within itch-specific pathways in the spinal cord, temporal discharge patterns are important as sustained pruriceptor is required to allow successful transsynaptic signal progression. Also, optogenetic activation of pruriceptors suggest that the combination of neuronal specificity and temporal pattern determines the sensory effect: tonic activation of pruriceptors is required to induce scratching behavior whereas short-lasting stimulation rather causes withdrawal. In addition to the mere duration of discharge, also the temporal pattern or spatial aspects could critically contribute to elicit pruritus instead of pain. Basic neurophysiological studies trying to validate neuronal theories for pruritus in their pure form provide unitary concepts leading from neuronal discharge to the itch sensation. However, the crucial clinical questions have the opposite perspective: which mechanisms explain the chronic itch in a given patient or a given disease? In trying to solve these clinical problems we should not feel bound to the mutual exclusive nature of itch theories, but rather appreciate blending several theories and also accept combinations of itch and pain. Thus, blended versions of itch theories might better suffice for an explanation of chronic itch in patients and will improve the basis for mechanistic treatment options.

Temporal Encoding is Required for Categorization, But Not Discrimination.

Core auditory cortex (AC) neurons encode slow fluctuations of acoustic stimuli with temporally patterned activity. However, whether temporal encoding is necessary to explain auditory perceptual skills remains uncertain. Here, we recorded from gerbil AC neurons while they discriminated between a 4-Hz amplitude modulation (AM) broadband noise and AM rates >4Hz. We found a proportion of neurons possessed neural thresholds based on spike pattern or spike count that were better than the recorded session's behavioral threshold, suggesting that spike count could provide sufficient information for this perceptual task. A population decoder that relied on temporal information outperformed a decoder that relied on spike count alone, but the spike count decoder still remained sufficient to explain average behavioral performance. This leaves open the possibility that more demanding perceptual judgments require temporal information. Thus, we asked whether accurate classification of different AM rates between 4 and 12Hz required the information contained in AC temporal discharge patterns. Indeed, accurate classification of these AM stimuli depended on the inclusion of temporal information rather than spike count alone. Overall, our results compare two different representations of time-varying acoustic features that can be accessed by downstream circuits required for perceptual judgments.

Modeling Larval American Shad Recruitment in a Large River

AbstractClimate change is altering the spatial and temporal patterns of temperature and discharge in rivers, which is expected to have implications for the life stages of anadromous fish using those rivers. We developed an individual‐based model to track American Shad Alosa sapidissima offspring within a coarse template of spatially and temporally variable habitat conditions defined by a combination of temperature, river velocity, and prey availability models. We simulated spawning at each river kilometer along a 142‐km reach of the Connecticut River on each day (April 1–August 31) to understand how spawning date and location drive larval recruitment differentially across years and decades (1993–2002 and 2007–2016). For both temperature and flow, interannual variation was large in comparison to interdecadal differences. Variation in simulated recruitment was best explained by a combination of season‐specific spawning temperature and location along the course of the river. The greatest potential recruitment occurred during years in which June temperatures were relatively high. In years when June and July were warmer than average, maximum recruitment resulted from spawning taking place at the upstream portion of the modeled reach. Model scenarios (stationary or passive‐drift larvae; and dams or no dams) had predictable effects. We assumed that the pools above dams had negative impacts on eggs and yolk‐sac larvae that may have been deposited there. Allowing eggs and larvae to drift passively with the current reduced spatial differences in recruitment success among spawning sites relative to stationary eggs and larvae. Our results demonstrate the importance of spatiotemporal environmental heterogeneity for producing positive recruitment over the long term. In addition, our results suggest the importance of successful passage of spawners to historical spawning sites in the Connecticut River upstream of Vernon Dam, especially as conditions shift with climate change.

Enhancing SWAT+ simulation of groundwater flow and groundwater-surface water interactions using MODFLOW routines

This paper presents a version of SWAT + that uses MODFLOW to simulate groundwater flow and groundwater-surface interactions within a watershed system. The modeling code is applied to the 470 km2 Middle Bosque River Watershed (Texas, USA) to demonstrate accuracy and differences with SWAT+. The model is tested against field-measured stream discharge, groundwater levels and fluctuation patterns, runoff, baseflow, recharge, watershed boundary groundwater flow, groundwater-surface water interactions, and evapotranspiration. The model captures the dominant baseflow patterns throughout the watershed and the key patterns of water table dynamics and relation to nearby streams. Whereas annual rates of runoff, lateral flow, and annual percolation are unchanged from a stand-alone SWAT + simulation, annual groundwater discharge rates increase by 50% (37 mm–54 mm). In particular, temporal patterns of groundwater discharge are governed by the physically-based rise and fall of groundwater near streams, rather than storm-induced recharge pulses as in the SWAT + simulation.

Neuropathic itch.

Neuropathic itch is clinically important but has received much less attention as compared to neuropathic pain. In the past decade, itch-specific pathways have been characterized on a cellular and molecular level, but their exact role in the pathophysiology of neuropathic itch is still unclear. Traditionally, mutually exclusive theories for itch such as labeled line, temporal/spatial pattern, or intensity theory have been proposed, and experimental studies in mice mainly favor the specificity theory of itch. By contrast, results in humans also suggest a role for spatial and temporal patterns in neuropathic itch. Rarefication of skin innervation in neuropathy could provide a "spatial contrast" discharge pattern, and axotomy could induce de novo expression of the itch-specific spinal neuropeptide, gastrin-releasing peptide, in primary afferent nociceptors, thereby modulating itch processing in the dorsal horn. Thus, clinical neuropathy may generate itch by changes in the spatial and temporal discharge patterns of nociceptors, hijacking the labeled line processing of itch and abandoning the canonical scheme of mutual exclusive itch theories. Moreover, the overlap between itch and pain symptoms in neuropathy patients complicates direct translation from animal experiments and, on a clinical level, necessitates collaboration between medical specialities, such as dermatologists, anesthesiologists, and neurologists.

Stream metabolism increases with drainage area and peaks asynchronously across a stream network

Quantifying the spatial and temporal dynamics of stream metabolism across stream networks is key to understanding carbon cycling and stream food web ecology. To better understand intra-annual temporal patterns of gross primary production (GPP) and ecosystem respiration (ER) and their variability across space, we continuously measured dissolved oxygen and modeled stream metabolism for an entire year at ten sites across a temperate river network in Washington State, USA. We expected GPP and ER to increase with stream size and peak during summer and autumn months due to warmer temperatures and higher light availability. We found that GPP and ER increased with drainage area and that only four sites adhered to our expectations of summer peaks in GPP and autumn peaks in ER while the rest either peaked in winter, spring or remained relatively constant. Our results suggest the spatial arrangement and temporal patterns of discharge, temperature, light and nutrients within watersheds may result in asynchronies in GPP and ER, despite similar regional climatic conditions. These findings shed light on how temporal dynamics of stream metabolism can shift across a river network, which likely influence the dynamics of carbon cycling and stream food webs at larger scales.

Quantifying Neuronal Information Flow in Response to Frequency and Intensity Changes in the Auditory Cortex.

Studies increasingly show that behavioral relevance alters the population representation of sensory stimuli in the sensory cortices. However, the mechanisms underlying this behavior are incompletely understood. Here, we record neuronal responses in the auditory cortex while a highly trained, awake, normal-hearing gerbil listens passively to target tones of high versus low behavioral relevance. Using an information theoretic framework, we model the overall transmission chain from acoustic input stimulus to recorded cortical response as a communication channel. To quantify how much information core auditory cortex carries about high versus low relevance sound, we then compute the mutual information of the multi-unit neuronal responses. Results show that the output over the stimulus-to-response channel can be modeled as a Poisson mixture. We derive a closed-form fast approximation for the entropy of a mixture of univariate Poisson random variables. A purely rate-code based model reveals reduced information transfer for high relevance compared to low relevance tones, hinting that changes in temporal discharge pattern may encode behavioral relevance.

Evidence for mutual allocation of social attention through interactive signaling in a mormyrid weakly electric fish

Mormyrid weakly electric fish produce electric organ discharges (EODs) for active electrolocation and electrocommunication. These pulses are emitted with variable interdischarge intervals (IDIs) resulting in temporal discharge patterns and interactive signaling episodes with nearby conspecifics. However, unequivocal assignment of interactive signaling to a specific behavioral context has proven to be challenging. Using an ethorobotical approach, we confronted single individuals of weakly electric Mormyrus rume proboscirostris with a mobile fish robot capable of interacting both physically, on arbitrary trajectories, as well as electrically, by generating echo responses through playback of species-specific EODs, thus synchronizing signals with the fish. Interactive signaling by the fish was more pronounced in response to a dynamic echo playback generated by the robot than in response to playback of static random IDI sequences. Such synchronizations were particularly strong at a distance corresponding to the outer limit of active electrolocation, and when fish oriented toward the fish replica. We therefore argue that interactive signaling through echoing of a conspecific's EODs provides a simple mechanism by which weakly electric fish can specifically address nearby individuals during electrocommunication. Echoing may thus enable mormyrids to mutually allocate social attention and constitute a foundation for complex social behavior and relatively advanced cognitive abilities in a basal vertebrate lineage.

Evaluating long-term patterns of decreasing groundwater discharge through a lake-bottom permeable reactive barrier

Identifying and quantifying groundwater exchange is critical when considering contaminant fate and transport at the groundwater/surface-water interface. In this paper, areally distributed temperature and point seepage measurements are used to efficiently assess spatial and temporal groundwater discharge patterns through a glacial-kettle lakebed area containing a zero-valent iron permeable reactive barrier (PRB). Concern was that the PRB was becoming less permeable with time owing to biogeochemical processes within the PRB. Patterns of groundwater discharge over an 8-year period were examined using fiber-optic distributed temperature sensing (FO-DTS) and snapshot-in-time point measurements of temperature. The resulting thermal maps show complex and uneven distributions of temperatures across the lakebed and highlight zones of rapid seepage near the shoreline and along the outer boundaries of the PRB. Repeated thermal mapping indicates an increase in lakebed temperatures over time at periods of similar stage and surface-water temperature. Flux rates in six seepage meters permanently installed on the lakebed in the PRB area decreased on average by 0.021 md−1 (or about 4.5 percent) annually between 2004 and 2015. Modeling of diurnal temperature signals from shallow vertical profiles yielded mean flux values ranging from 0.39 to 1.15 md−1, with stronger fluxes generally related to colder lakebed temperatures. The combination of an increase in lakebed temperatures, declines in direct seepage, and observations of increased cementation of the lakebed surface provide in situ evidence that the permeability of the PRB is declining. The presence of temporally persistent rapid seepage zones is also discussed.

Hyperpolarization-activated channels shape temporal patterns of ectopic spontaneous discharge in C-nociceptors after peripheral nerve injury.

Neuropathic pain is thought to be mediated by aberrant impulses from sensitized primary afferents, and the temporal summation of the discharges might also influence nociceptive processing. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels (Ih current) generate rhythmic activity in neurons within the central nervous system and contribute to nociceptors excitability in neuropathic pain. We searched for single fibres with ectopic spontaneous discharges from an invitro preparation in mice containing a neuroma formed in a peripheral branch of the saphenous nerve together with the undamaged branches. Both damaged (axotomized) and undamaged fibres (putative intact) developed ectopic spontaneous activity with different temporal spike trains: Clock-like, Irregular or Bursts. The Ih current blocker, ZD7288, significantly suppressed ectopic spontaneous discharges in nociceptive fibres (3/5 Aδ- and 24/31 C-units and 1 nonclassified) by 64%. Additionally, ZD7288 changed the spike patterns of 5/7 Clock-like and 3/4 Burst units to Irregular. Exogenous cAMP produced a significant ~65% increase in the ectopic firing in 5 Irregular fibres, which was restored by ZD7288. In six additional fibres (three Clock-like and three Irregular), exogenous cAMP had no further effect, but co-application with ZD7288 decreased their discharge by half. These units showed significant higher levels of discharges than the cAMP-sensitive ones. Our data suggest that HCN channels modulate ectopic spontaneous firing in C-nociceptors and shape their temporal patterns of discharge which will, ultimately, modify the nociceptive message received and processed by second-order neurons. We show an involvement of HCN channels in the modulation of ectopic spontaneous discharges from C-nociceptors. This finding exposes a mechanism of nociceptive transmission enhancement and highlights the clinical relevance of peripheral HCN blockade for spontaneous pain relief during neuropathy.

Assessment of the vulnerability of a coastal freshwater system to climatic and non-climatic changes: A system dynamics approach

Abstract Water resources management faces many challenges in coastal areas of developing countries, where climate change coupled with high rates of population growth and urbanization have the potential to cause severe water scarcity. Of particular concern, are sea level rise and altered precipitation regimes that will influence spatial and temporal patterns of river discharge, water levels and saltwater penetration in estuaries. A sound understanding of factors affecting the vulnerability of coastal freshwater systems is therefore needed to mitigate the potential impacts of climatic and non-climatic changes. In this study, a system dynamics modeling approach was employed to explore the vulnerability of the coastal freshwater system in Da Do Basin, Vietnam to projected sea level rise, upstream flow decline and socio-economic development. This system includes the Da Do River and irrigation channels that receive freshwater through sluice gates from the Van Uc and Lach Tray rivers. The model was developed as a learning tool for decision-makers to improve their understanding of the spatial and temporal dynamic behaviors of the system and to inform adaptation decision-making by allowing exploration of plausible future scenarios. The model was developed, calibrated and validated using both historical data and expert knowledge elucidated via stakeholder consultation. Model results indicate that under current conditions, freshwater availability is sufficient to meet existing domestic, industrial and agricultural demands. However, the coastal freshwater system changes significantly and collapses under several plausible future scenarios. Future projections suggest that declining upstream flows will be the strongest threat to the system's vulnerability. System dynamics models enable consideration of the interactive effects of a range of climatic and non-climatic drivers on water resources availability thereby facilitating improved planning for collective and proactive adaptation actions to efficiently secure freshwater resources to support socio-economic development of coastal basins in the face of climate change.

Advances in research on combining the opotogenetics with bionic olfaction sensor

There are plenty of amazing wonders existing in nature, and the crisis-crossing connections between them are performing their best duties. For example, biological olfactory system is an ingenious biosensor, since the olfactory sensory neurons in peripheral nervous respond to the external stimuli, then the signals are transmitted to the olfactory bulb to be integrated and processed, the axonal connection is precisely organized signals from about 1000 different types of odorant receptors, then sorted out in 1800 glomeruli in the mouse olfactory bulb, after delivered to the most advanced central nervous system, the information can be distinguished eventually. Cortical regions underlying vision, audition, and somatosens- ation receive sensory information from the thalamus and also make corticothalamic feedback projections that influence thalamic sensory processing. This process is responding with particularly high sentivity, specificity and rapidity. The ability to detect odorants is crucial for survival as it informs an organism with its environment: food, approaching predators, mates nearby, etc. During recently year, researchers have employed optogenetics technology to explore deeper in the mechanism of olfactory system. Microbial proteins that can be rapidly activated by light have been adapted for research in neuroscience, including ChR2 and NpHR, which permit millisecond-precision optical control of genetically defined cell types in intact neural tissue. This technology allows the use of light to alter neural processing at the level of single spikes and synaptic events, yielding a widely applicable tool for neuroscientists and biomedical engineers. An important question in the study of the olfactory system is the role of spatial factors in olfactory processing, but traditional methods have been relatively unsuccessful in elucidating their theories, odor stimulation evokes complex spatiotemporal activity in the olfactory bulb, suggesting that both the identity of activated neurons and the timing of their activity convey information about odors, the emergency of optogenetics technology can solve this problem efficiently. Biological olfactory systems can recognize and discriminate thousands of distinct odors with very high sensitivity and specificity, with the progress of research on olfactory mechanisms and the advancements of olfaction, many types of biomimetic olfactory-based biosensors have been developed by the combination of olfactory functional components with various secondary sensors. The bioinspired olfaction sensor is an innovative chemical sensing bionic system with far-reaching significance, it contains various biological components as the sensitive elements including olfactory receptors, olfactory cells and olfactory tissues, which has great applicable prospects in many fields for detecting specific odorants with high sensitivity, also by chronically coupling multiple microelectrodes to olfactory bulb of behaving rats, we extract an array of mitral/tufted cells which could generate odor-specific temporal patterns of neural discharge. They have great potential commercial prospects and promising applications in such fields as biomedicine, environmental monitoring, pharmaceutical screening and the quality control of food and water. This review interpreted the transferring mechanism of biological olfactory system and the applications of optogenetics in researches concerning the foundamental research of olfactory system. In addition, a combination of the work of our laboratory with other domestic and international researchers were presented here to predict the future scenario and the promising applications in disease diagnosis, environmental monitoring, military and food quality control.

The influence of bathymetry changes on low water level of Lake Poyang

基于2010年鄱阳湖最新地形，构建精细的鄱阳湖二维水动力数学模型，相同网格下构建1998年地形，分别模拟不同地形条件下2006年枯水年水位、流量时空分布，分析地形变化对水位、流量的影响，阐释地形影响的时空差异. 结果表明：相比1998年，2010年地形由于北部入江通道的下切，相同的2006年水文条件下，水位普遍降低；水位越低，上下游水面坡降越大，受地形影响越明显；低水位最大降幅1~2 m，而高水位最大不超过0.4 m，分别对应湖口9 m以下、15 m以上水位；地形对水位的影响程度都昌 >星子 >棠荫 >康山；都昌至湖口段水头差降低了2 m，水面坡度变缓，棠荫至都昌段水面坡度变陡，康山至湖口水头差基本不变；全年出湖总流量增加了6%；地形变化影响最显著为河道区，影响范围可波及大部分湖区，局部地形的变化使得子湖水面积也存在一定差异. 本研究首次基于水动力模拟量化了鄱阳湖地形变化对水位的影响程度和范围，结果可为水资源管理、江湖关系演变分析、湿地生态环境保护等提供科学参考.;A fine 2D hydrodynamic model of Lake Poyang was used to simulate the hydrological effect of the bathymetry changes between 1998 and 2010. Spatio temporal patterns of water levels and discharge in 2006 of two DEM were simulated to assess the bathymetry effect, considering 2006 as a typical dry year. Results showed that the north water way was deeper in 2010 than that in 1998, resulting in lower water levels. In low water level period, the water surface gradient was larger, and the effect of DEM changes was more significant than that in high level period.The low water levels decreased by 1-2 m corresponding to water level lower than 9 m at Hukou. The reductions of high water levels were lower than 0.4 m when water level at Hukou higher than 15 m. The influenced magnitude was largest at Duchang, followed by Xingzi, Tangyin and Kangshan.The water head from Duchang to Hukou decreased by 2 m in low level period. The water surface gradient decreased from Duchang to Xingzi, and increased from Tangyin to Duchang. There was no distinct gradient change from Kangshan to Hukou. The total discharge into the Yangtze River increased by 6%. The channel areas were most affected, however the influence of bathymetry changes spread across the most of the lake area.Water areas of some sub-lakes varied due to the local morphological changes. This study quantified magnitude and domain of the effect of bathymetry changes on Lake Poyang level from a hydrodynamic perspective. The outcomes may provide scientific support for water resource and ecological environment management, evolution and analysis of river-lake relationship.

Analysis of Nociceptive Information Encoded in the Temporal Discharge Patterns of Cutaneous C-Fibers.

The generation of pain signals from primary afferent neurons is explained by a labeled-line code. However, this notion cannot apply in a simple way to cutaneous C-fibers, which carry signals from a variety of receptors that respond to various stimuli including agonist chemicals. To represent the discharge patterns of C-fibers according to different agonist chemicals, we have developed a quantitative approach using three consecutive spikes. By using this method, the generation of pain in response to chemical stimuli is shown to be dependent on the temporal aspect of the spike trains. Furthermore, under pathological conditions, gamma-aminobutyric acid resulted in pain behavior without change of spike number but with an altered discharge pattern. Our results suggest that information about the agonist chemicals may be encoded in specific temporal patterns of signals in C-fibers, and nociceptive sensation may be influenced by the extent of temporal summation originating from the temporal patterns.

Wideband phase locking to modulated whisker vibration point to a temporal code for texture in the rat's barrel cortex.

Rats probe objects with their whiskers and make decisions about sizes, shapes, textures and distances within a few tens of milliseconds. This perceptual analysis requires the processing of tactile high-frequency object components reflecting surface roughness. We have shown that neurons in the barrel cortex of rats encode high-frequency sinusoidal vibrations of whiskers for sustained periods when presented with constant amplitudes and frequencies. In a natural situation, however, stimulus parameters change rapidly when whiskers are brushing across objects. In this study, we therefore analysed cortical responses to vibratory movements of single whiskers with rapidly changing amplitudes and frequencies. The results show that different neural codes are employed for a processing of stimulus parameters. The frequency of whisker vibration is encoded by the temporal pattern of spike discharges, i.e., the phase-locked responses of barrel cortex neurons. In addition, oscillatory gamma band activity was induced during high-frequency stimulation. The pivotal descriptor of the amplitude of whisker displacement, the velocity, is reflected in the rate of spike discharges. While phase-locked discharges occurred over the entire range of frequencies tested (10-600 Hz), the discharge rate increased with stimulus velocity only up to about 60 µm/ms, saturating at a mean rate of ~117 spikes/s. In addition, the results show that whisker movements of more than 500 Hz bandwidth may be encoded by phase-locked responses of small groups of cortical neurons. Thus, even single whiskers may transmit information about wide ranges of textural components owing to their set of different types of hair follicle mechanoreceptors.

Seasonal plasticity of precise spike timing in the avian auditory system.

Vertebrate audition is a dynamic process, capable of exhibiting both short- and long-term adaptations to varying listening conditions. Precise spike timing has long been known to play an important role in auditory encoding, but its role in sensory plasticity remains largely unexplored. We addressed this issue in Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii), a songbird that shows pronounced seasonal fluctuations in circulating levels of sex-steroid hormones, which are known to be potent neuromodulators of auditory function. We recorded extracellular single-unit activity in the auditory forebrain of males and females under different breeding conditions and used a computational approach to explore two potential strategies for the neural discrimination of sound level: one based on spike counts and one based on spike timing reliability. We report that breeding condition has robust sex-specific effects on spike timing. Specifically, in females, breeding condition increases the proportion of cells that rely solely on spike timing information and increases the temporal resolution required for optimal intensity encoding. Furthermore, in a functionally distinct subset of cells that are particularly well suited for amplitude encoding, female breeding condition enhances spike timing-based discrimination accuracy. No effects of breeding condition were observed in males. Our results suggest that high-resolution temporal discharge patterns may provide a plastic neural substrate for sensory coding.

Reverberation impairs brainstem temporal representations of voiced vowel sounds: challenging "periodicity-tagged" segregation of competing speech in rooms.

The auditory system typically processes information from concurrently active sound sources (e.g., two voices speaking at once), in the presence of multiple delayed, attenuated and distorted sound-wave reflections (reverberation). Brainstem circuits help segregate these complex acoustic mixtures into “auditory objects.” Psychophysical studies demonstrate a strong interaction between reverberation and fundamental-frequency (F0) modulation, leading to impaired segregation of competing vowels when segregation is on the basis of F0 differences. Neurophysiological studies of complex-sound segregation have concentrated on sounds with steady F0s, in anechoic environments. However, F0 modulation and reverberation are quasi-ubiquitous. We examine the ability of 129 single units in the ventral cochlear nucleus (VCN) of the anesthetized guinea pig to segregate the concurrent synthetic vowel sounds /a/ and /i/, based on temporal discharge patterns under closed-field conditions. We address the effects of added real-room reverberation, F0 modulation, and the interaction of these two factors, on brainstem neural segregation of voiced speech sounds. A firing-rate representation of single-vowels' spectral envelopes is robust to the combination of F0 modulation and reverberation: local firing-rate maxima and minima across the tonotopic array code vowel-formant structure. However, single-vowel F0-related periodicity information in shuffled inter-spike interval distributions is significantly degraded in the combined presence of reverberation and F0 modulation. Hence, segregation of double-vowels' spectral energy into two streams (corresponding to the two vowels), on the basis of temporal discharge patterns, is impaired by reverberation; specifically when F0 is modulated. All unit types (primary-like, chopper, onset) are similarly affected. These results offer neurophysiological insights to perceptual organization of complex acoustic scenes under realistically challenging listening conditions.

Synaptic mechanisms for generating temporal diversity of auditory representation in the dorsal cochlear nucleus.

In central auditory pathways, neurons exhibit a great diversity of temporal discharge patterns, which may contribute to the parallel processing of auditory signals. How such response diversity emerges in the central auditory circuits remains unclear. Here, we investigated whether synaptic mechanisms can contribute to the generation of the temporal response diversity at the first stage along the central auditory neuraxis. By in vivo whole-cell voltage-clamp recording in the dorsal cochlear nucleus of rats, we revealed excitatory and inhibitory synaptic inputs underlying three different firing patterns of fusiform/pyramidal neurons in response to auditory stimuli: "primary-like," "pauser," and "buildup" patterns. We found that primary-like neurons received strong, fast-rising excitation, whereas pauser and buildup neurons received accumulating excitation with a relatively weak fast-rising phase, followed by a slow-rising phase. Pauser neurons received stronger fast-rising excitation than buildup cells. On the other hand, inhibitory inputs to the three types of cells exhibited similar temporal patterns, all with a strong fast-rising phase. Dynamic-clamp recordings demonstrated that the differential temporal patterns of excitation could primarily account for the different discharge patterns. In addition, discharge pattern in a single neuron varied in a stimulus-dependent manner, which could be attributed to the modulation of excitation/inhibition balance by different stimuli. Further examination of excitatory inputs to vertical/tuberculoventral and cartwheel cells suggested that fast-rising and accumulating excitation might be conveyed by auditory nerve and parallel fibers, respectively. A differential summation of excitatory inputs from the two sources may thus contribute to the generation of response diversity.