Individualized Closed-Loop Acoustic Stimulation Suggests an Alpha Phase Dependence of Sound Evoked and Induced Brain Activity Measured with EEG Recordings.

Following repetitive visual stimulation, post hoc phase analysis finds that visually evoked response magnitudes vary with the cortical alpha oscillation phase that temporally coincides with sensory stimulus. This approach has not successfully revealed an alpha phase dependence for auditory evoked or induced responses. Here, we test the feasibility of tracking alpha with scalp electroencephalogram (EEG) recordings and play sounds phase-locked to individualized alpha phases in real-time using a novel end-point corrected Hilbert transform (ecHT) algorithm implemented on a research device. Based on prior work, we hypothesize that sound-evoked and induced responses vary with the alpha phase at sound onset and the alpha phase that coincides with the early sound-evoked response potential (ERP) measured with EEG. Thus, we use each subject's individualized alpha frequency (IAF) and individual auditory ERP latency to define target trough and peak alpha phases that allow an early component of the auditory ERP to align to the estimated poststimulus peak and trough phases, respectively. With this closed-loop and individualized approach, we find opposing alpha phase-dependent effects on the auditory ERP and alpha oscillations that follow stimulus onset. Trough and peak phase-locked sounds result in distinct evoked and induced post-stimulus alpha level and frequency modulations. Though additional studies are needed to localize the sources underlying these phase-dependent effects, these results suggest a general principle for alpha phase-dependence of sensory processing that includes the auditory system. Moreover, this study demonstrates the feasibility of using individualized neurophysiological indices to deliver automated, closed-loop, phase-locked auditory stimulation.

Health Care Professionals' Experiences With Using Information and Communication Technologies in Patient Care During the COVID-19 Pandemic: Qualitative Study.

BACKGROUND: The COVID-19 pandemic acted as a catalyst for the use of information and communication technology (ICT) in inpatient and outpatient health care settings. Digital tools were used to connect patients, families, and providers amid visitor restrictions, while web-based platforms were used to continue care amid COVID-19 lockdowns. What we have yet to learn is the experiences of health care providers (HCPs) regarding the use of ICT that supported changes to clinical care during the COVID-19 pandemic. OBJECTIVE: The aim of this paper was to describe the experiences of HCPs in using ICT to support clinical care changes during the COVID-19 pandemic. This paper is reporting on a subset of a larger body of data that examined changes to models of care during the pandemic. METHODS: This study used a qualitative, descriptive study design. In total, 30 HCPs were recruited from 3 hospitals in Canada. One-on-one semistructured interviews were conducted between December 2022 and June 2023. Qualitative data were analyzed using an inductive thematic approach to identify themes across participants. RESULTS: A total of 30 interviews with HCPs revealed 3 themes related to their experiences using ICT to support changes to clinical care during the COVID-19 pandemic. These included the use of ICT (1) to support in-person communication with patients, (2) to facilitate connection between provider to patient and patient to family, and (3) to provide continuity of care. CONCLUSIONS: HCP narratives revealed the benefits of digital tools to support in-person communication between patient and provider, the need for thoughtful consideration for the use of ICT at end-of-life care, and the decision-making that is needed when choosing service delivery modality (eg, web based or in person). Moving forward, organizations are encouraged to provide education and training on how to support patient-provider communication, find ways to meet patient and family wishes at end-of-life care, and continue to give autonomy to HCPs in their clinical decision-making regarding service delivery modality.

A wearable EEG system for closed-loop neuromodulation of sleep-related oscillations.

Objective.Healthy sleep plays a critical role in general well-being. Enhancement of slow-wave sleep by targeting acoustic stimuli to particular phases of delta (0.5-2 Hz) waves has shown promise as a non-invasive approach to improve sleep quality. Closed-loop stimulation during other sleep phases targeting oscillations at higher frequencies such as theta (4-7 Hz) or alpha (8-12 Hz) could be another approach to realize additional health benefits. However, systems to track and deliver stimulation relative to the instantaneous phase of electroencephalogram (EEG) signals at these higher frequencies have yet to be demonstrated outside of controlled laboratory settings.Approach.Here we examine the feasibility of using an endpoint-corrected version of the Hilbert transform (ecHT) algorithm implemented on a headband wearable device to measure alpha phase and deliver phase-locked auditory stimulation during the transition from wakefulness to sleep, during which alpha power is greatest. First, the ecHT algorithm is implementedin silicoto evaluate the performance characteristics of this algorithm across a range of sleep-related oscillatory frequencies. Secondly, a pilot sleep study tests feasibility to use the wearable device by users in the home setting for measurement of EEG activity during sleep and delivery of real-time phase-locked stimulation.Main results.The ecHT is capable of computing the instantaneous phase of oscillating signals with high precision, allowing auditory stimulation to be delivered at the intended phases of neural oscillations with low phase error. The wearable system was capable of measuring sleep-related neural activity with sufficient fidelity for sleep stage scoring during the at-home study, and phase-tracking performance matched simulated results. Users were able to successfully operate the system independently using the companion smartphone app to collect data and administer stimulation, and presentation of auditory stimuli during sleep initiation did not negatively impact sleep onset.Significance.This study demonstrates the feasibility of closed-loop real-time tracking and neuromodulation of a range of sleep-related oscillations using a wearable EEG device. Preliminary results suggest that this approach could be used to deliver non-invasive neuromodulation across all phases of sleep.

Providers' perspectives on implementing resilience coaching for healthcare workers during the COVID-19 pandemic.

BACKGROUND: The COVID-19 pandemic severely exacerbated workplace stress for healthcare workers (HCWs) worldwide. The pandemic also magnified the need for mechanisms to support the psychological wellbeing of HCWs. This study is a qualitative inquiry into the implementation of a HCW support program called Resilience Coaching at a general hospital. Resilience Coaching was delivered by an interdisciplinary team, including: psychiatrists, mental health nurses allied health and a senior bioethicist. The study focuses specifically on the experiences of those who provided the intervention. METHODS: Resilience Coaching was implemented at, an academic hospital in Toronto, Canada in April 2020 and is ongoing. As part of a larger qualitative evaluation, 13 Resilience Coaches were interviewed about their experiences providing psychosocial support to colleagues. Interviews were recorded, transcribed, and analyzed for themes by the research team. Interviews were conducted between February and June 2021. RESULTS: Coaches were motivated by opportunities to support colleagues and contribute to the overall health system response to COVID-19. Challenges included finding time within busy work schedules, balancing role tensions and working while experiencing burnout. CONCLUSIONS: Hospital-based mental health professionals are well-positioned to support colleagues' wellness during acute crises and can find this work meaningful, but note important challenges to the role. Paired-coaches and peer support among the coaching group may mitigate some of these challenges. Perspectives from those providing support to HCWs are an important consideration in developing support programs that leverage internal teams.

Resilience coaching for healthcare workers: Experiences of receiving collegial support during the COVID-19 pandemic.

OBJECTIVE: To explore experiences of receiving collegial support from the department of psychiatry at an acute care hospital during the COVID-19 pandemic. METHOD: The Resilience Coaching program launched in April 2020, with the aim of offering a timely response to supporting psychosocial needs of healthcare workers (HCWs), leveraging collegial relationships and mental health training to offer support. Twenty-four HCWs were interviewed about their experiences receiving support from resilience coaches. RESULTS: Participants reported that Resilience Coaching offered hospital staff opportunities for connection, encouragement to attend to personal wellness, and avenues to learn practical skills to assist with coping. Coaching also assisted HCWs in accessing clinical mental health support when that was requested by staff. CONCLUSIONS: Resilience Coaching is a model for supporting colleagues in an acute care hospital during a pandemic. It is generally regarded positively by participants. Further study is warranted to determine how best to engage some occupational subcultures within the hospital, and whether the model is feasible for other healthcare contexts.

Distinct neural ensemble response statistics are associated with recognition and discrimination of natural sound textures.

The perception of sound textures, a class of natural sounds defined by statistical sound structure such as fire, wind, and rain, has been proposed to arise through the integration of time-averaged summary statistics. Where and how the auditory system might encode these summary statistics to create internal representations of these stationary sounds, however, is unknown. Here, using natural textures and synthetic variants with reduced statistics, we show that summary statistics modulate the correlations between frequency organized neuron ensembles in the awake rabbit inferior colliculus (IC). These neural ensemble correlation statistics capture high-order sound structure and allow for accurate neural decoding in a single trial recognition task with evidence accumulation times approaching 1 s. In contrast, the average activity across the neural ensemble (neural spectrum) provides a fast (tens of milliseconds) and salient signal that contributes primarily to texture discrimination. Intriguingly, perceptual studies in human listeners reveal analogous trends: the sound spectrum is integrated quickly and serves as a salient discrimination cue while high-order sound statistics are integrated slowly and contribute substantially more toward recognition. The findings suggest statistical sound cues such as the sound spectrum and correlation structure are represented by distinct response statistics in auditory midbrain ensembles, and that these neural response statistics may have dissociable roles and time scales for the recognition and discrimination of natural sounds.

Optimal Multichannel Artifact Prediction and Removal for Neural Stimulation and Brain Machine Interfaces.

Neural implants that deliver multi-site electrical stimulation to the nervous systems are no longer the last resort but routine treatment options for various neurological disorders. Multi-site electrical stimulation is also widely used to study nervous system function and neural circuit transformations. These technologies increasingly demand dynamic electrical stimulation and closed-loop feedback control for real-time assessment of neural function, which is technically challenging since stimulus-evoked artifacts overwhelm the small neural signals of interest. We report a novel and versatile artifact removal method that can be applied in a variety of settings, from single- to multi-site stimulation and recording and for current waveforms of arbitrary shape and size. The method capitalizes on linear electrical coupling between stimulating currents and recording artifacts, which allows us to estimate a multi-channel linear Wiener filter to predict and subsequently remove artifacts via subtraction. We confirm and verify the linearity assumption and demonstrate feasibility in a variety of recording modalities, including in vitro sciatic nerve stimulation, bilateral cochlear implant stimulation, and multi-channel stimulation and recording between the auditory midbrain and cortex. We demonstrate a vast enhancement in the recording quality with a typical artifact reduction of 25-40 dB. The method is efficient and can be scaled to arbitrary number of stimulus and recording sites, making it ideal for applications in large-scale arrays, closed-loop implants, and high-resolution multi-channel brain-machine interfaces.

Modeling stimulus-dependent variability improves decoding of population neural responses.

OBJECTIVE: Neural responses to repeated presentations of an identical stimulus often show substantial trial-to-trial variability. How the mean firing rate varies in response to different stimuli or during different movements (tuning curves) has been extensively modeled in a wide variety of neural systems. However, the variability of neural responses can also have clear tuning independent of the tuning in the mean firing rate. This suggests that the variability could contain information regarding the stimulus/movement beyond what is encoded in the mean firing rate. Here we demonstrate how taking variability into account can improve neural decoding. APPROACH: In a typical neural coding model spike counts are assumed to be Poisson with the mean response depending on an external variable, such as a stimulus or movement. Bayesian decoding methods then use the probabilities under these Poisson tuning models (the likelihood) to estimate the probability of each stimulus given the spikes on a given trial (the posterior). However, under the Poisson model, spike count variability is always exactly equal to the mean (Fano factor = 1). Here we use two alternative models-the Conway-Maxwell-Poisson (CMP) model and negative binomial (NB) model-to more flexibly characterize how neural variability depends on external stimuli. These models both contain the Poisson distribution as a special case but have an additional parameter that allows the variance to be greater than the mean (Fano factor > 1) or, for the CMP model, less than the mean (Fano factor < 1). MAIN RESULTS: We find that neural responses in primary motor (M1), visual (V1), and auditory (A1) cortices have diverse tuning in both their mean firing rates and response variability. Across cortical areas, we find that Bayesian decoders using the CMP or NB models improve stimulus/movement estimation accuracy by 4%-12% compared to the Poisson model. SIGNIFICANCE: Moreover, the uncertainty of the non-Poisson decoders more accurately reflects the magnitude of estimation errors. In addition to tuning curves that reflect average neural responses, stimulus-dependent response variability may be an important aspect of the neural code. Modeling this structure could, potentially, lead to improvements in brain machine interfaces.

A temporal integration mechanism enhances frequency selectivity of broadband inputs to inferior colliculus.

Accurately resolving frequency components in sounds is essential for sound recognition, yet there is little direct evidence for how frequency selectivity is preserved or newly created across auditory structures. We demonstrate that prepotentials (PPs) with physiological properties resembling presynaptic potentials from broadly tuned brainstem inputs can be recorded concurrently with postsynaptic action potentials in inferior colliculus (IC). These putative brainstem inputs (PBIs) are broadly tuned and exhibit delayed and spectrally interleaved excitation and inhibition not present in the simultaneously recorded IC neurons (ICNs). A sharpening of tuning is accomplished locally at the expense of spike-timing precision through nonlinear temporal integration of broadband inputs. A neuron model replicates the finding and demonstrates that temporal integration alone can degrade timing precision while enhancing frequency tuning through interference of spectrally in- and out-of-phase inputs. These findings suggest that, in contrast to current models that require local inhibition, frequency selectivity can be sharpened through temporal integration, thus supporting an alternative computational strategy to quickly refine frequency selectivity.

A Hierarchy of Time Scales for Discriminating and Classifying the Temporal Shape of Sound in Three Auditory Cortical Fields.

Auditory cortex is essential for mammals, including rodents, to detect temporal "shape" cues in the sound envelope but it remains unclear how different cortical fields may contribute to this ability (Lomber and Malhotra, 2008; Threlkeld et al., 2008). Previously, we found that precise spiking patterns provide a potential neural code for temporal shape cues in the sound envelope in the primary auditory (A1), and ventral auditory field (VAF) and caudal suprarhinal auditory field (cSRAF) of the rat (Lee et al., 2016). Here, we extend these findings and characterize the time course of the temporally precise output of auditory cortical neurons in male rats. A pairwise sound discrimination index and a Naive Bayesian classifier are used to determine how these spiking patterns could provide brain signals for behavioral discrimination and classification of sounds. We find response durations and optimal time constants for discriminating sound envelope shape increase in rank order with: A1 < VAF < cSRAF. Accordingly, sustained spiking is more prominent and results in more robust sound discrimination in non-primary cortex versus A1. Spike-timing patterns classify 10 different sound envelope shape sequences and there is a twofold increase in maximal performance when pooling output across the neuron population indicating a robust distributed neural code in all three cortical fields. Together, these results support the idea that temporally precise spiking patterns from primary and non-primary auditory cortical fields provide the necessary signals for animals to discriminate and classify a large range of temporal shapes in the sound envelope.SIGNIFICANCE STATEMENT Functional hierarchies in the visual cortices support the concept that classification of visual objects requires successive cortical stages of processing including a progressive increase in classical receptive field size. The present study is significant as it supports the idea that a similar progression exists in auditory cortices in the time domain. We demonstrate for the first time that three cortices provide temporal spiking patterns for robust temporal envelope shape discrimination but only the ventral non-primary cortices do so on long time scales. This study raises the possibility that primary and non-primary cortices provide unique temporal spiking patterns and time scales for perception of sound envelope shape.

Origins of scale invariance in vocalization sequences and speech.

To communicate effectively animals need to detect temporal vocalization cues that vary over several orders of magnitude in their amplitude and frequency content. This large range of temporal cues is evident in the power-law scale-invariant relationship between the power of temporal fluctuations in sounds and the sound modulation frequency (f). Though various forms of scale invariance have been described for natural sounds, the origins and implications of scale invariant phenomenon remain unknown. Using animal vocalization sequences, including continuous human speech, and a stochastic model of temporal amplitude fluctuations we demonstrate that temporal acoustic edges are the primary acoustic cue accounting for the scale invariant phenomenon. The modulation spectrum of vocalization sequences and the model both exhibit a dual regime lowpass structure with a flat region at low modulation frequencies and scale invariant 1/f2 trend for high modulation frequencies. Moreover, we find a time-frequency tradeoff between the average vocalization duration of each vocalization sequence and the cutoff frequency beyond which scale invariant behavior is observed. These results indicate that temporal edges are universal features responsible for scale invariance in vocalized sounds. This is significant since temporal acoustic edges are salient perceptually and the auditory system could exploit such statistical regularities to minimize redundancies and generate compact neural representations of vocalized sounds.

An Unbalanced Rearrangement of Chromosomes 4:20 is Associated with Childhood Osteoporosis and Reduced Caspase-3 Levels.

The purpose of this study was to investigate the association of a chromosome 4:20 imbalance with osteoporosis in three related children. Bone biochemistry, bone turnover markers, and dual-energy X-ray absorptiometry (DXA) scanning were performed in all three cases and bone biopsy and histomorphometry in one. The chromosome imbalance was delineated by array comparative genomic hybridization (aCGH) and analyzed for candidate genes. A potential candidate gene within the deleted region is caspase-3, previously linked to low bone mineral density (BMD) in heterozygous mice thus caspase-3 activity was measured in cases and controls. Routine bone biochemistry and markers of bone turnover did not reveal any abnormality. DXA showed reduced total and lumbar spine bone mineral content. aCGH showed an 8 megabase (Mb) deletion of terminal chromosome 4q incorporating a region previously linked to low BMD and a 15 Mb duplication of terminal chromosome 20p. Bone biopsy showed a high bone turnover state, trabecularisation of cortical bone and numerous small osteoclasts coupled with normal bone formation. Basal serum caspase-3 activity was lower in cases compared with controls. We conclude that the early-onset osteoporosis with low basal levels of caspase-3 and abnormal osteoclasts is a feature of this chromosomal translocation. Further investigation of the role of the deleted and duplicated genes and especially caspase-3 is required.

Neural spike-timing patterns vary with sound shape and periodicity in three auditory cortical fields.

Mammals perceive a wide range of temporal cues in natural sounds, and the auditory cortex is essential for their detection and discrimination. The rat primary (A1), ventral (VAF), and caudal suprarhinal (cSRAF) auditory cortical fields have separate thalamocortical pathways that may support unique temporal cue sensitivities. To explore this, we record responses of single neurons in the three fields to variations in envelope shape and modulation frequency of periodic noise sequences. Spike rate, relative synchrony, and first-spike latency metrics have previously been used to quantify neural sensitivities to temporal sound cues; however, such metrics do not measure absolute spike timing of sustained responses to sound shape. To address this, in this study we quantify two forms of spike-timing precision, jitter, and reliability. In all three fields, we find that jitter decreases logarithmically with increase in the basis spline (B-spline) cutoff frequency used to shape the sound envelope. In contrast, reliability decreases logarithmically with increase in sound envelope modulation frequency. In A1, jitter and reliability vary independently, whereas in ventral cortical fields, jitter and reliability covary. Jitter time scales increase (A1 < VAF < cSRAF) and modulation frequency upper cutoffs decrease (A1 > VAF > cSRAF) with ventral progression from A1. These results suggest a transition from independent encoding of shape and periodicity sound cues on short time scales in A1 to a joint encoding of these same cues on longer time scales in ventral nonprimary cortices.

How Critical is Patient Positioning in Radiographic Assessment of the Hip in Cerebral Palsy When Measuring Migration Percentage?

BACKGROUND: Migration percentage (MP) is an accepted method of assessing lateral displacement of the femoral head in children with cerebral palsy (CP). Difficulty in positioning of patients for pelvic radiography remains a concern for the reliability of the MP. METHODS: This 2-part quantitative study examined 100 anteroposterior pelvic radiographs for children with CP. Fifty were from a region that had a positioning protocol for hip surveillance of children with CP and 50 images were from a region without. Images were assessed for acceptability of position in relation to hip abduction/adduction and/or pelvic rotation.Ten images deemed Acceptable or Borderline from the region with no protocol were then randomly selected. MP was measured on 2 separate occasions by 5 children's orthopaedic surgeons and statistically analyzed for intrarater and interrater reliability. RESULTS: There was no statistically significant difference in the acceptability of images between the 2 regions with 60% to 66% of the images meeting the criteria outright. When allowances were made for slight variation of abduction/adduction within 5 degrees, 74% to 80% of the images were acceptable.Reliability was variable with limits of agreement between 4.96% and 15.15%. Observers more familiar with the software measuring package had higher reliability within and between occasions. Variability within and between observers decreased as MP increased. CONCLUSIONS: Poor positioning did not appear to be the main reason for the variation in reliability of MP. Repeat measurements were reliable although standardized technique, training, and familiarity with software measuring programmes did influence outcomes.

A high-density, high-channel count, multiplexed µECoG array for auditory-cortex recordings.

Our understanding of the large-scale population dynamics of neural activity is limited, in part, by our inability to record simultaneously from large regions of the cortex. Here, we validated the use of a large-scale active microelectrode array that simultaneously records 196 multiplexed micro-electrocortigraphical (µECoG) signals from the cortical surface at a very high density (1,600 electrodes/cm(2)). We compared µECoG measurements in auditory cortex using a custom "active" electrode array to those recorded using a conventional "passive" µECoG array. Both of these array responses were also compared with data recorded via intrinsic optical imaging, which is a standard methodology for recording sound-evoked cortical activity. Custom active µECoG arrays generated more veridical representations of the tonotopic organization of the auditory cortex than current commercially available passive µECoG arrays. Furthermore, the cortical representation could be measured efficiently with the active arrays, requiring as little as 13.5 s of neural data acquisition. Next, we generated spectrotemporal receptive fields from the recorded neural activity on the active µECoG array and identified functional organizational principles comparable to those observed using intrinsic metabolic imaging and single-neuron recordings. This new electrode array technology has the potential for large-scale, temporally precise monitoring and mapping of the cortex, without the use of invasive penetrating electrodes.

Novel acoustic stimuli can alter locomotor speed to hippocampal theta relationship.

Hippocampal theta (6-12 Hz) plays a critical role in synchronizing the discharge of action potentials, ultimately orchestrating individual neurons into large-scale ensembles. Alterations in theta dynamics may reflect variations in sensorimotor integration, the flow of sensory input, and/or cognitive processing. Previously we have investigated septotemporal variation in the locomotor speed to theta amplitude relationship as well as how that relationship is systematically altered as a function of novel, physical space. In the present study, we ask, beyond physical space, whether persistent and passive sound delivery can alter septal theta local field potential rhythm dynamics. Results indicate pronounced alterations in the slope of the speed to theta amplitude relationship as a function of sound presentation and location. Further, this reduction in slope habituates across days. The current findings highlight that moment-to-moment alterations in theta amplitude is a rich dynamic index that is quantitatively related to both alterations in motor behavior and sensory experience. The implications of these phenomena are discussed with respect to emergent cognitive functions subserved by hippocampal circuits.

Gene expression identifies distinct ascending glutamatergic pathways to frequency-organized auditory cortex in the rat brain.

A conserved feature of sound processing across species is the presence of multiple auditory cortical fields with topographically organized responses to sound frequency. Current organizational schemes propose that the ventral division of the medial geniculate body (MGBv) is a single functionally homogenous structure that provides the primary source of input to all neighboring frequency-organized cortical fields. These schemes fail to account for the contribution of MGBv to functional diversity between frequency-organized cortical fields. Here, we report response property differences for two auditory fields in the rat, and find they have nonoverlapping sources of thalamic input from the MGBv that are distinguished by the gene expression for type 1 vesicular glutamate transporter. These data challenge widely accepted organizational schemes and demonstrate a genetic plurality in the ascending glutamatergic pathways to frequency-organized auditory cortex.

Spectrotemporal sound preferences of neighboring inferior colliculus neurons: implications for local circuitry and processing.

How do local circuits in the inferior colliculus (IC) process and transform spectral and temporal sound information? Using a four-tetrode array we examined the functional properties of the IC and metrics of its micro circuitry by recording neural activity from neighboring single neurons in the cat. Spectral and temporal response preferences were compared for neurons found on the same and adjacent tetrodes (ATs), as well as across distant recording sites. We found that neighboring neurons had similar preferences while neurons recorded across distant sites were less similar. Best frequency (BF) was the most correlated parameter between neighboring neurons and BF differences exhibited unique clustering at ~0.3 octave intervals, indicative of the frequency band lamina. Other spectral and temporal parameters of the receptive fields were more similar for neighboring neurons than for those at distant sites and the receptive field similarity was larger for neurons with small differences in BF. Furthermore, correlated firing was stronger for neighboring neuron pairs and increased with proximity and decreasing BF difference. Thus, although response selectivities are quite diverse in the IC, spectral, and temporal preference within a local microcircuit are functionally quite similar. This suggests a scheme where local circuits are organized into zones that are specialized for processing distinct spectrotemporal cues.

Precise feature based time scales and frequency decorrelation lead to a sparse auditory code.

Sparse redundancy reducing codes have been proposed as efficient strategies for representing sensory stimuli. A prevailing hypothesis suggests that sensory representations shift from dense redundant codes in the periphery to selective sparse codes in cortex. We propose an alternative framework where sparseness and redundancy depend on sensory integration time scales and demonstrate that the central nucleus of the inferior colliculus (ICC) of cats encodes sound features by precise sparse spike trains. Direct comparisons with auditory cortical neurons demonstrate that ICC responses were sparse and uncorrelated as long as the spike train time scales were matched to the sensory integration time scales relevant to ICC neurons. Intriguingly, correlated spiking in the ICC was substantially lower than predicted by linear or nonlinear models and strictly observed for neurons with best frequencies within a "critical band," the hallmark of perceptual frequency resolution in mammals. This is consistent with a sparse asynchronous code throughout much of the ICC and a complementary correlation code within a critical band that may allow grouping of perceptually relevant cues.

Thalamocortical pathway specialization for sound frequency resolution.

Core auditory cortices are organized in parallel pathways that process incoming sensory information differently. In the rat, sound filtering properties of the primary (A1) and ventral (VAF) auditory fields are markedly different, yet both are core regions that by definition receive most of their thalamic input from the ventral nucleus (MGBv) of the medial geniculate body (MGB). For example, spike rate responses to sound intensity and frequency are more narrowly resolved in VAF vs. A1. Here we question whether there are anatomic correlates of the marked differences in response properties in these two core auditory fields. Combined Fourier optical imaging and multiunit recording methods were used to map tone frequency responses with high spatial resolution in A1, VAF, and neighboring cortices. The cortical distance representing a given octave was similar, yet response frequency resolution was about twice as large in VAF as in A1. Retrograde tracers were injected into low- and high-isofrequency contours of both regions to compare MGBv label patterns. The distance between clusters of MGBv neurons projecting to low- and high-isofrequency contours in the cortex was twice as large in caudal as in rostral MGB. This suggests that differences in A1 and VAF frequency resolution are related to the anatomic spatial resolution of frequency laminae in the thalamus, supporting a growing consensus that antecedents of cortical specialization can be attributed in part to the structural and functional characteristics of thalamocortical inputs.