Expectancy-related changes in firing of dopamine neurons depend on hippocampus

The orbitofrontal cortex (OFC) and hippocampus (HC) are both implicated in forming the cognitive or task maps that support flexible behavior. Previously, we used the dopamine neurons as a sensor or tool to measure the functional effects of OFC lesions (Takahashi et al., 2011). We recorded midbrain dopamine neurons as rats performed an odor-based choice task, in which errors in the prediction of reward were induced by manipulating the number or timing of the expected rewards across blocks of trials. We found that OFC lesions ipsilateral to the recording electrodes caused prediction errors to be degraded consistent with a loss in the resolution of the task states, particularly under conditions where hidden information was critical to sharpening the predictions. Here we have repeated this experiment, along with computational modeling of the results, in rats with ipsilateral HC lesions. The results show HC also shapes the map of our task, however unlike OFC, which provides information local to the trial, the HC appears to be necessary for estimating the upper-level hidden states based on the information that is discontinuous or separated by longer timescales. The results contrast the respective roles of the OFC and HC in cognitive mapping and add to evidence that the dopamine neurons access a rich information set from distributed regions regarding the predictive structure of the environment, potentially enabling this powerful teaching signal to support complex learning and behavior.

Firing patterns of ventral hippocampal neurons predict the exploration of anxiogenic locations

The ventral hippocampus (vH) plays a crucial role in anxiety-related behaviour and vH neurons increase their firing when animals explore anxiogenic environments. However, if and how such neuronal activity induces or restricts the exploration of an anxiogenic location remains unexplained. Here, we developed a novel behavioural paradigm to motivate rats to explore an anxiogenic area. Male rats ran along an elevated linear maze with protective sidewalls, which were subsequently removed in parts of the track to introduce an anxiogenic location. We recorded neuronal action potentials during task performance and found that vH neurons exhibited remapping of activity, overrepresenting anxiogenic locations. Direction-dependent firing was homogenised by the anxiogenic experience. We further showed that the activity of vH neurons predicted the extent of exploration of the anxiogenic location. Our data suggest that anxiety-related firing does not solely depend on the exploration of anxiogenic environments, but also on intentions to explore them.

Propofol anesthesia concentration rather than abrupt behavioral unresponsiveness linearly degrades responses in the rat primary auditory cortex

Despite extensive knowledge of its molecular and cellular effects, how anesthesia affects sensory processing remains poorly understood. In particular, it remains unclear whether anesthesia modestly or robustly degrades activity in primary sensory regions, and whether such changes are linked to anesthesia drug concentration versus behavioral unresponsiveness, which are typically confounded. Here, we used slow gradual intravenous propofol anesthesia induction together with auditory stimulation and intermittent assessment of behavioral responsiveness while recording epidural electroencephalogram, and neuronal spiking activity in primary auditory cortex (PAC) of eight rats. We found that all main components of neuronal activity including spontaneous firing rates, onset response magnitudes, onset response latencies, postonset neuronal silence duration, late-locking to 40 Hz click-trains, and offset responses, gradually changed in a dose-dependent manner with increasing anesthesia levels without showing abrupt shifts around loss of righting reflex or other time-points. Thus, the dominant factor affecting PAC responses is the anesthesia drug concentration rather than any sudden, dichotomous behavioral state changes. Our findings explain a wide array of seemingly conflicting results in the literature that, depending on the precise definition of wakefulness (vigilant vs. drowsy) and anesthesia (light vs. deep/surgical), report a spectrum of effects in primary regions ranging from minimal to dramatic differences.

Complementary Roles of Primate Dorsal Premotor and Pre-Supplementary Motor Areas to the Control of Motor Sequences

We are able to temporally organize multiple movements in a purposeful manner in everyday life. Both the dorsal premotor (PMd) area and pre-supplementary motor area (pre-SMA) are known to be involved in the performance of motor sequences. However, it is unclear how each area differentially contributes to controlling multiple motor sequences. To address this issue, we recorded single-unit activity in both areas while monkeys (one male, one female) performed sixteen motor sequences. Each sequence comprised either a series of two identical movements (repetition) or two different movements (nonrepetition). The sequence was initially instructed with visual signals but had to be remembered thereafter. Here, we showed that the activity of single neurons in both areas transitioned from reactive- to predictive encoding while motor sequences were memorized. In the memory-guided trials, in particular, the activity of PMd cells preferentially represented the second movement (2M) in the sequence leading to a reward generally regardless of the first movement (1M). Such activity frequently began even before the 1M in a prospective manner, and was enhanced in nonrepetition sequences. Behaviorally, a lack of the activity enhancement often resulted in premature execution of the 2M. In contrast, cells in pre-SMA instantiated particular sequences of actions by coordinating switching or nonswitching movements in sequence. Our findings suggest that PMd and pre-SMA play complementary roles within behavioral contexts: PMd preferentially controls the movement that leads to a reward rather than the sequence per se, whereas pre-SMA coordinates all elements in a sequence by integrating temporal orders of multiple movements.SIGNIFICANCE STATEMENT Although both dorsal premotor (PMd) area and pre-supplementary motor area (pre-SMA) are involved in the control of motor sequences, it is not clear how these two areas contribute to coordination of sequential movements differently. To address this issue, we directly compared neuronal activity in the two areas recorded while monkeys memorized and performed multiple motor sequences. Our findings suggest that PMd preferentially controls the final action that ultimately leads to a reward in a prospective manner, whereas the pre-SMA coordinates switching among multiple actions within the context of the sequence. Our findings are of significance to understand the distinct roles for motor-related areas in the planning and executing motor sequences and the pathophysiology of apraxia and/or Parkinson's diseases that disables skilled motor actions.

State transitions in the statistically stable place cell population correspond to rate of perceptual change

The hippocampus occupies a central role in mammalian navigation and memory. Yet an understanding of the rules that govern the statistics and granularity of the spatial code, as well as its interactions with perceptual stimuli, is lacking. We analyzed CA1 place cell activity recorded while rats foraged in different large-scale environments. We found that place cell activity was subject to an unexpected but precise homeostasis—the distribution of activity in the population as a whole being constant at all locations within and between environments. Using a virtual reconstruction of the largest environment, we showed that the rate of transition through this statistically stable population matches the rate of change in the animals’ visual scene. Thus, place fields near boundaries were small but numerous, while in the environment’s interior, they were larger but more dispersed. These results indicate that hippocampal spatial activity is governed by a small number of simple laws and, in particular, suggest the presence of an information-theoretic bound imposed by perception on the fidelity of the spatial memory system.

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Neural activity in rodent CA1 place cell populations is homeostatically balanced

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Hippocampal place field size and frequency are governed by proximity to boundaries

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Transition rate through place cell population matches rate of change in visual scene

Extracellular single-unit recordings from peripheral nerve axons in vitro by a novel multichannel microelectrode array

The peripheral nervous system (PNS) is an attractive target for modulation of afferent input (e.g., nociceptive input signaling tissue damage) to the central nervous system. To advance mechanistic understanding of PNS neural encoding and modulation requires single-unit recordings from individual peripheral neurons or axons. This is challenged by multiple connective tissue layers surrounding peripheral nerve fibers that prevent electrical recordings by existing electrodes or electrode arrays. In this study, we developed a novel microelectrode array (MEA) via silicon-based microfabrication that consists of 5 parallel hydrophilic gold electrodes surrounded by silanized hydrophobic surfaces. This novel hydrophilic/hydrophobic surface pattern guides the peripheral nerve filaments to self-align towards the hydrophilic electrodes, which dramatically reduces the technical challenges in conducting single-unit recordings. We validated our MEA by recording simultaneous single-unit action potentials from individual axons in mouse sciatic nerves, including both myelinated A-fibers and unmyelinated C-fibers. We confirmed that our recordings were single units from individual axons by increasing nerve trunk electrical stimulus intensity, which did not alter the spike shape or amplitude. By reducing the technical challenges, our novel MEA will likely allow peripheral single-unit recordings to be adopted by a larger research community and thus expedite our mechanistic understanding of peripheral neural encoding and modulation.

Anesthesia-induced loss of consciousness disrupts auditory responses beyond primary cortex

Despite its ubiquitous use in medicine, and extensive knowledge of its molecular and cellular effects, how anesthesia induces loss of consciousness (LOC) and affects sensory processing remains poorly understood. Specifically, it is unclear whether anesthesia primarily disrupts thalamocortical relay or intercortical signaling. Here we recorded intracranial electroencephalogram (iEEG), local field potentials (LFPs), and single-unit activity in patients during wakefulness and light anesthesia. Propofol infusion was gradually increased while auditory stimuli were presented and patients responded to a target stimulus until they became unresponsive. We found widespread iEEG responses in association cortices during wakefulness, which were attenuated and restricted to auditory regions upon LOC. Neuronal spiking and LFP responses in primary auditory cortex (PAC) persisted after LOC, while responses in higher-order auditory regions were variable, with neuronal spiking largely attenuated. Gamma power induced by word stimuli increased after LOC while its frequency profile slowed, thus differing from local spiking activity. In summary, anesthesia-induced LOC disrupts auditory processing in association cortices while relatively sparing responses in PAC, opening new avenues for future research into mechanisms of LOC and the design of anesthetic monitoring devices.

Sleep Differentially Affects Early and Late Neuronal Responses to Sounds in Auditory and Perirhinal Cortices

A fundamental feature of sleep is reduced behavioral responsiveness to external events, but the extent of processing along sensory pathways remains poorly understood. While responses are comparable across wakefulness and sleep in auditory cortex (AC), neuronal activity in downstream regions remains unknown. Here we recorded spiking activity in 435 neuronal clusters evoked by acoustic stimuli in the perirhinal cortex (PRC) and in AC of freely behaving male rats across wakefulness and sleep. Neuronal responses in AC showed modest (∼10%) differences in response gain across vigilance states, replicating previous studies. By contrast, PRC neuronal responses were robustly attenuated by 47% and 36% during NREM sleep and REM sleep, respectively. Beyond the separation according to cortical region, response latency in each neuronal cluster was correlated with the degree of NREM sleep attenuation, such that late (>40 ms) responses in all monitored regions diminished during NREM sleep. The robust attenuation of late responses prevalent in PRC represents a novel neural correlate of sensory disconnection during sleep, opening new avenues for investigating the mediating mechanisms.SIGNIFICANCE STATEMENT Reduced behavioral responsiveness to sensory stimulation is at the core of sleep's definition, but it is still unclear how the sleeping brain responds differently to sensory stimuli. In the current study, we recorded neuronal spiking responses to sounds along the cortical processing hierarchy of rats during wakefulness and natural sleep. Responses in auditory cortex only showed modest changes during sleep, whereas sleep robustly attenuated the responses of neurons in high-level perirhinal cortex. We also found that, during NREM sleep, the response latency predicts the degree of sleep attenuation in individual neurons above and beyond their anatomical location. These results provide anatomical and temporal signatures of sensory disconnection during sleep and pave the way to understanding the underlying mechanisms.

Adaptive Efficient Coding of Correlated Acoustic Properties

Natural sounds such as vocalizations often have covarying acoustic attributes, resulting in redundancy in neural coding. The efficient coding hypothesis proposes that sensory systems are able to detect such covariation and adapt to reduce redundancy, leading to more efficient neural coding. Recent psychoacoustic studies have shown the auditory system can rapidly adapt to efficiently encode two covarying dimensions as a single dimension, following passive exposure to sounds in which temporal and spectral attributes covaried in a correlated fashion. However, these studies observed a cost to this adaptation, which was a loss of sensitivity to the orthogonal dimension. Here we explore the neural basis of this psychophysical phenomenon by recording single-unit responses from the primary auditory cortex in awake ferrets exposed passively to stimuli with two correlated attributes, similar in stimulus design to the psychoacoustic experiments in humans. We found: (1) the signal-to-noise ratio of spike-rate coding of cortical responses driven by sounds with correlated attributes remained unchanged along the exposure dimension, but was reduced along the orthogonal dimension; (2) performance of a decoder trained with spike data to discriminate stimuli along the orthogonal dimension was equally reduced; (3) correlations between neurons tuned to the two covarying attributes decreased after exposure; and (4) these exposure effects still occurred if sounds were correlated along two acoustic dimensions, but varied randomly along a third dimension. These neurophysiological results are consistent with the efficient coding hypothesis and may help deepen our understanding of how the auditory system encodes and represents acoustic regularities and covariance.SIGNIFICANCE STATEMENT The efficient coding (EC) hypothesis (Attneave, 1954; Barlow, 1961) proposes that the neural code in sensory systems efficiently encodes natural stimuli by minimizing the number of spikes to transmit a sensory signal. Results of recent psychoacoustic studies in humans are consistent with the EC hypothesis in that, following passive exposure to stimuli with correlated attributes, the auditory system rapidly adapts so as to more efficiently encode the two covarying dimensions as a single dimension. In the current neurophysiological experiments, using a similar stimulus design and the experimental paradigm to the psychoacoustic studies of Stilp et al. (2010) and Stilp and Kluender (2011, 2012, 2016), we recorded responses from single neurons in the auditory cortex of the awake ferret, showing adaptive efficient neural coding of two correlated acoustic attributes.

Expectancy-Related Changes in Dopaminergic Error Signals Are Impaired by Cocaine Self-Administration

Addiction is a disorder of behavioral control and learning. While this may reflect pre-existing propensities, drug use also clearly contributes by causing changes in outcome processing in prefrontal and striatal regions. This altered processing is associated with behavioral deficits, including changes in learning. These areas provide critical input to midbrain dopamine neurons regarding expected outcomes, suggesting that effects on learning may result from changes in dopaminergic error signaling. Here, we show that dopamine neurons recorded in rats that had self-administered cocaine failed to suppress firing on omission of an expected reward and exhibited lower amplitude and imprecisely timed increases in firing to an unexpected reward. Learning also appeared to have less of an effect on reward-evoked and cue-evoked firing in the cocaine-experienced rats. Overall, the changes are consistent with reduced fidelity of input regarding the expected outcomes, such as their size, timing, and overall value, because of cocaine use.

Optical recording reveals topological distribution of functionally classified colorectal afferent neurons in intact lumbosacral DRG

Neuromodulation as a non-drug alternative for managing visceral pain in irritable bowel syndrome (IBS) may target sensitized afferents of distal colon and rectum (colorectum), especially their somata in the dorsal root ganglion (DRG). Developing selective DRG stimulation to manage visceral pain requires knowledge of the topological distribution of colorectal afferent somata which are sparsely distributed in the DRG. Here, we implemented GCaMP6f to conduct high-throughput optical recordings of colorectal afferent activities in lumbosacral DRG, that is, optical electrophysiology. Using a mouse ex vivo preparation with distal colorectum and L5-S1 DRG in continuity, we recorded 791 colorectal afferents' responses to graded colorectal distension (15, 30, 40, and 60 mmHg) and/or luminal shear flow (20-30 mL/min), then functionally classified them into four mechanosensitive classes, and determined the topological distribution of their somata in the DRG. Of the 791 colorectal afferents, 90.8% were in the L6 DRG, 8.3% in the S1 DRG, and only 0.9% in the L5 DRG. L6 afferents had all four classes: 29% mucosal, 18.4% muscular-mucosal, 34% low-threshold (LT) muscular, and 18.2% high-threshold (HT) muscular afferents. S1 afferents only had three classes: 19.7% mucosal, 34.8% LT muscular, and 45.5% HT muscular afferents. All seven L5 afferents were HT muscular. In L6 DRG, somata of HT muscular afferents were clustered in the caudal region whereas somata of the other classes did not cluster in specific regions. Outcomes of this study can directly inform the design and improvement of next-generation neuromodulation devices that target the DRG to alleviate visceral pain in IBS patients.

Ventrolateral periaqueductal gray neurons prioritize threat probability over fear output

Faced with potential harm, individuals must estimate the probability of threat and initiate an appropriate fear response. In the prevailing view, threat probability estimates are relayed to the ventrolateral periaqueductal gray (vlPAG) to organize fear output. A straightforward prediction is that vlPAG single-unit activity reflects fear output, invariant of threat probability. We recorded vlPAG single-unit activity in male, Long Evans rats undergoing fear discrimination. Three 10 s auditory cues predicted unique foot shock probabilities: danger (p=1.00), uncertainty (p=0.375) and safety (p=0.00). Fear output was measured by suppression of reward seeking over the entire cue and in one-second cue intervals. Cued fear non-linearly scaled to threat probability and cue-responsive vlPAG single-units scaled their firing on one of two timescales: at onset or ramping toward shock delivery. VlPAG onset activity reflected threat probability, invariant of fear output, while ramping activity reflected both signals with threat probability prioritized.

Physiological and pathophysiological firing properties of single postganglionic sympathetic neurons in humans

It has long been known from microneurographic recordings in human subjects that the activity of postganglionic sympathetic axons occurs as spontaneous bursts, with muscle sympathetic nerve activity (MSNA) exhibiting strong cardiac rhythmicity via the baroreflex and skin sympathetic nerve activity showing much weaker cardiac modulation. Here we review the firing properties of single sympathetic neurons, obtained using highly selective microelectrodes. Individual vasoconstrictor neurons supplying muscle or skin, or sudomotor neurons supplying sweat glands, always discharge with a low firing probability (~30%) and at very low frequencies (~0.5 Hz). Moreover, they usually fire only once per cardiac interval but can fire greater than four times within a burst. Modeling has shown that this pattern can best be explained by individual neurons being driven by, on average, two preganglionic inputs. Unitary recordings of muscle vasoconstrictor neurons have been made in several pathophysiological states, including heart failure, hypertension, obstructive sleep apnea, bronchiectasis, chronic obstructive pulmonary disease, depression, and panic disorder. The augmented MSNA in each of these diseases features an increase in firing probability and discharge frequency of individual muscle vasoconstrictor neurons above that seen in healthy subjects, yet firing rates rarely exceed 1 Hz. However, unlike patients with heart failure, all patients with respiratory disease or panic disorder, and patients with hyperhidrosis, exhibited an increase in multiple within-burst firing, which emphasizes the different modes by which the sympathetic nervous system grades its output in pathophysiological states of high sympathetic nerve activity.

Role of Hippocampal CA2 Region in Triggering Sharp-Wave Ripples

Sharp-wave ripples (SPW-Rs) in the hippocampus are implied in memory consolidation, as shown by observational and interventional experiments. However, the mechanism of their generation remains unclear. Using two-dimensional silicon probe arrays, we investigated the propagation of SPW-Rs across the hippocampal CA1, CA2, and CA3 subregions. Synchronous activation of CA2 ensembles preceded SPW-R-related population activity in CA3 and CA1 regions. Deep CA2 neurons gradually increased their activity prior to ripples and were suppressed during the population bursts of CA3-CA1 neurons (ramping cells). Activity of superficial CA2 cells preceded the activity surge in CA3-CA1 (phasic cells). The trigger role of the CA2 region in SPW-R was more pronounced during waking than sleeping. These results point to the CA2 region as an initiation zone for SPW-Rs.

Responses in Rat Core Auditory Cortex are Preserved during Sleep Spindle Oscillations

STUDY OBJECTIVES

Sleep is defined as a reversible state of reduction in sensory responsiveness and immobility. A long-standing hypothesis suggests that a high arousal threshold during non-rapid eye movement (NREM) sleep is mediated by sleep spindle oscillations, impairing thalamocortical transmission of incoming sensory stimuli. Here we set out to test this idea directly by examining sensory-evoked neuronal spiking activity during natural sleep.

METHODS

We compared neuronal (n = 269) and multiunit activity (MUA), as well as local field potentials (LFP) in rat core auditory cortex (A1) during NREM sleep, comparing responses to sounds depending on the presence or absence of sleep spindles.

RESULTS

We found that sleep spindles robustly modulated the timing of neuronal discharges in A1. However, responses to sounds were nearly identical for all measured signals including isolated neurons, MUA, and LFPs (all differences < 10%). Furthermore, in 10% of trials, auditory stimulation led to an early termination of the sleep spindle oscillation around 150-250 msec following stimulus onset. Finally, active ON states and inactive OFF periods during slow waves in NREM sleep affected the auditory response in opposite ways, depending on stimulus intensity.

CONCLUSIONS

Responses in core auditory cortex are well preserved regardless of sleep spindles recorded in that area, suggesting that thalamocortical sensory relay remains functional during sleep spindles, and that sensory disconnection in sleep is mediated by other mechanisms.

Transition from Target to Gaze Coding in Primate Frontal Eye Field during Memory Delay and Memory-Motor Transformation

The frontal eye fields (FEFs) participate in both working memory and sensorimotor transformations for saccades, but their role in integrating these functions through time remains unclear. Here, we tracked FEF spatial codes through time using a novel analytic method applied to the classic memory-delay saccade task. Three-dimensional recordings of head-unrestrained gaze shifts were made in two monkeys trained to make gaze shifts toward briefly flashed targets after a variable delay (450-1500 ms). A preliminary analysis of visual and motor response fields in 74 FEF neurons eliminated most potential models for spatial coding at the neuron population level, as in our previous study (Sajad et al., 2015). We then focused on the spatiotemporal transition from an eye-centered target code (T; preferred in the visual response) to an eye-centered intended gaze position code (G; preferred in the movement response) during the memory delay interval. We treated neural population codes as a continuous spatiotemporal variable by dividing the space spanning T and G into intermediate T–G models and dividing the task into discrete steps through time. We found that FEF delay activity, especially in visuomovement cells, progressively transitions from T through intermediate T–G codes that approach, but do not reach, G. This was followed by a final discrete transition from these intermediate T–G delay codes to a “pure” G code in movement cells without delay activity. These results demonstrate that FEF activity undergoes a series of sensory–memory–motor transformations, including a dynamically evolving spatial memory signal and an imperfect memory-to-motor transformation.

Neural Estimates of Imagined Outcomes in Basolateral Amygdala Depend on Orbitofrontal Cortex

Reciprocal connections between the orbitofrontal cortex (OFC) and the basolateral nucleus of the amygdala (BLA) provide a critical circuit for guiding normal behavior when information about expected outcomes is required. Recently, we reported that outcome signaling by OFC neurons is also necessary for learning in the face of unexpected outcomes during a Pavlovian over-expectation task. Key to learning in this task is the ability to build on prior learning to infer or estimate an amount of reward never previously received. OFC was critical to this process. Notably, in parallel work, we found that BLA was not necessary for learning in this setting. This suggested a dissociation in which the BLA might be critical for acquiring information about the outcomes but not for subsequently using it to make novel predictions. Here we evaluated this hypothesis by recording single-unit activity from BLA in rats during the same Pavlovian over-expectation task used previously. We found that spiking activity recorded in BLA in control rats did reflect novel outcome estimates derived from the integration of prior learning, however consistent with a model in which this process occurs in the OFC, these correlates were entirely abolished by ipsilateral OFC lesions. These data indicate that this information about these novel predictions is represented in the BLA, supported via direct or indirect input from the OFC, even though it does not appear to be necessary for learning.

SIGNIFICANCE STATEMENT

The basolateral nucleus of the amygdala (BLA) and the orbitofrontal cortex (OFC) are involved in behavior that depends on knowledge of impending outcomes. Recently, we found that only the OFC was necessary for using such information for learning in a Pavlovian over-expectation task. The current experiment was designed to search for neural correlates of this process in the BLA and, if present, to ask whether they would still be dependent on OFC input. We found that although spiking activity in BLA in control rats did reflect the novel outcome estimates underlying learning, these correlates were entirely abolished by OFC lesions.

Electrophysiological evidence of alterations to the nucleus accumbens and dorsolateral striatum during chronic cocaine self-administration

As drug use becomes chronic, aberrant striatal processing contributes to the development of perseverative drug-taking behaviors. Two particular portions of the striatum, the nucleus accumbens (NAc) and the dorsolateral striatum (DLS), are known to undergo neurobiological changes from acute to chronic drug use. However, little is known about the exact progression of changes in functional striatal processing as drug intake persists. We sampled single-unit activity in the NAc and DLS throughout 24 daily sessions of chronic long-access cocaine self-administration, and longitudinally tracked firing rates (FR) specifically during the operant response, an upward vertical head movement. A total of 103 neurons were held longitudinally and immunohistochemically localised to either NAc Medial Shell (n = 29), NAc Core (n = 30), or DLS (n = 54). We modeled changes representative of each category as a whole. Results demonstrated that FRs of DLS Head Movement neurons were significantly increased relative to baseline during all sessions, while FRs of DLS Uncategorised neurons were significantly reduced relative to baseline during all sessions. NAc Shell neurons' FRs were also significantly decreased relative to baseline during all sessions while FRs of NAc Core neurons were reduced relative to baseline only during training days 1-18 but were not significantly reduced on the remaining sessions (19-24). The data suggest that all striatal subregions show changes in FR during the operant response relative to baseline, but longitudinal changes in response firing patterns were observed only in the NAc Core, suggesting that this region is particularly susceptible to plastic changes induced by abused drugs.

Neural coding of sound envelope in reverberant environments

Speech reception depends critically on temporal modulations in the amplitude envelope of the speech signal. Reverberation encountered in everyday environments can substantially attenuate these modulations. To assess the effect of reverberation on the neural coding of amplitude envelope, we recorded from single units in the inferior colliculus (IC) of unanesthetized rabbit using sinusoidally amplitude modulated (AM) broadband noise stimuli presented in simulated anechoic and reverberant environments. Although reverberation degraded both rate and temporal coding of AM in IC neurons, in most neurons, the degradation in temporal coding was smaller than the AM attenuation in the stimulus. This compensation could largely be accounted for by the compressive shape of the modulation input-output function (MIOF), which describes the nonlinear transformation of modulation depth from acoustic stimuli into neural responses. Additionally, in a subset of neurons, the temporal coding of AM was better for reverberant stimuli than for anechoic stimuli having the same modulation depth at the ear. Using hybrid anechoic stimuli that selectively possess certain properties of reverberant sounds, we show that this reverberant advantage is not caused by envelope distortion, static interaural decorrelation, or spectral coloration. Overall, our results suggest that the auditory system may possess dual mechanisms that make the coding of amplitude envelope relatively robust in reverberation: one general mechanism operating for all stimuli with small modulation depths, and another mechanism dependent on very specific properties of reverberant stimuli, possibly the periodic fluctuations in interaural correlation at the modulation frequency.

Differing properties of putative fast-spiking interneurons in the striatum of two rat strains

Local circuits within the striatum of the basal ganglia include a small number of γ-aminobutyric acid (GABA)-ergic fast-spiking interneurons (FSI). The number of these cells is reduced in disorders of behavioral control, but it is unknown whether this is accompanied by altered electrophysiological properties. The genetically hypertensive (GH) rat strain exhibits impulsiveness and hyperactivity. We investigated if resting-state FSI activity is affected in this strain using extracellular recordings. We also examined the effect of systemic amphetamine (AMPH), a stimulant drug used in the treatment of these particular behavioral deficits. Putative FSI (pFSI) were encountered less often in GH rats compared to the Wistar control strain. pFSI in GH rats also exhibited a higher mean firing rate, higher intraburst firing rate, lower interburst interval, and shorter bursts compared to controls. AMPH increased the mean overall firing rate of Wistar rat pFSI but did not significantly alter the firing properties of this subtype in GH rats. These differences in the resting-state electrophysiological activity of pFSI in GH rats point to them as a cell type of particular interest in understanding striatal functioning across different strains.

Statistical learning of recurring sound patterns encodes auditory objects in songbird forebrain

Auditory neurophysiology has demonstrated how basic acoustic features are mapped in the brain, but it is still not clear how multiple sound components are integrated over time and recognized as an object. We investigated the role of statistical learning in encoding the sequential features of complex sounds by recording neuronal responses bilaterally in the auditory forebrain of awake songbirds that were passively exposed to long sound streams. These streams contained sequential regularities, and were similar to streams used in human infants to demonstrate statistical learning for speech sounds. For stimulus patterns with contiguous transitions and with nonadjacent elements, single and multiunit responses reflected neuronal discrimination of the familiar patterns from novel patterns. In addition, discrimination of nonadjacent patterns was stronger in the right hemisphere than in the left, and may reflect an effect of top-down modulation that is lateralized. Responses to recurring patterns showed stimulus-specific adaptation, a sparsening of neural activity that may contribute to encoding invariants in the sound stream and that appears to increase coding efficiency for the familiar stimuli across the population of neurons recorded. As auditory information about the world must be received serially over time, recognition of complex auditory objects may depend on this type of mnemonic process to create and differentiate representations of recently heard sounds.

Processing of harmonics in the lateral belt of macaque auditory cortex

Many speech sounds and animal vocalizations contain components, referred to as complex tones, that consist of a fundamental frequency (F0) and higher harmonics. In this study we examined single-unit activity recorded in the core (A1) and lateral belt (LB) areas of auditory cortex in two rhesus monkeys as they listened to pure tones and pitch-shifted conspecific vocalizations (“coos”). The latter consisted of complex-tone segments in which F0 was matched to a corresponding pure-tone stimulus. In both animals, neuronal latencies to pure-tone stimuli at the best frequency (BF) were ~10 to 15 ms longer in LB than in A1. This might be expected, since LB is considered to be at a hierarchically higher level than A1. On the other hand, the latency of LB responses to coos was ~10 to 20 ms shorter than to the corresponding pure-tone BF, suggesting facilitation in LB by the harmonics. This latency reduction by coos was not observed in A1, resulting in similar coo latencies in A1 and LB. Multi-peaked neurons were present in both A1 and LB; however, harmonically-related peaks were observed in LB for both early and late response components, whereas in A1 they were observed only for late components. Our results suggest that harmonic features, such as relationships between specific frequency intervals of communication calls, are processed at relatively early stages of the auditory cortical pathway, but preferentially in LB.

Neural correlates of auditory recognition memory in the primate dorsal temporal pole

Temporal pole (TP) cortex is associated with higher-order sensory perception and/or recognition memory, as human patients with damage in this region show impaired performance during some tasks requiring recognition memory (Olson et al. 2007). The underlying mechanisms of TP processing are largely based on examination of the visual nervous system in humans and monkeys, while little is known about neuronal activity patterns in the auditory portion of this region, dorsal TP (dTP; Poremba et al. 2003). The present study examines single-unit activity of dTP in rhesus monkeys performing a delayed matching-to-sample task utilizing auditory stimuli, wherein two sounds are determined to be the same or different. Neurons of dTP encode several task-relevant events during the delayed matching-to-sample task, and encoding of auditory cues in this region is associated with accurate recognition performance. Population activity in dTP shows a match suppression mechanism to identical, repeated sound stimuli similar to that observed in the visual object identification pathway located ventral to dTP (Desimone 1996; Nakamura and Kubota 1996). However, in contrast to sustained visual delay-related activity in nearby analogous regions, auditory delay-related activity in dTP is transient and limited. Neurons in dTP respond selectively to different sound stimuli and often change their sound response preferences between experimental contexts. Current findings suggest a significant role for dTP in auditory recognition memory similar in many respects to the visual nervous system, while delay memory firing patterns are not prominent, which may relate to monkeys' shorter forgetting thresholds for auditory vs. visual objects.

Odor-evoked activity in the mouse lateral entorhinal cortex

The entorhinal cortex is a brain area with multiple reciprocal connections to the hippocampus, amygdala, perirhinal cortex, olfactory bulb and piriform cortex. As such, it is thought to play a large role in the olfactory memory process. The present study is the first to compare lateral entorhinal and anterior piriform cortex odor-evoked single-unit and local field potential activity in mouse. Recordings were made in urethane-anesthetized mice that were administered a range of three pure odors and three overlapping odor mixtures. Results show that spontaneous as well as odor-evoked unit activity was lower in lateral entorhinal versus piriform cortex. In addition, units in lateral entorhinal cortex were responsive to a more restricted set of odors compared to piriform. Conversely, odor-evoked power change in local field potential activity was greater in the lateral entorhinal cortex in the theta band than in piriform. The highly odor-specific and restricted firing in lateral entorhinal cortex suggests that it may play a role in modulating odor-specific, experience- and state-dependent olfactory coding.

Firing patterns of muscle vasoconstrictor neurons in respiratory disease

Because the cardiovascular system and respiration are so intimately coupled, disturbances in respiratory control often lead to disturbances in cardiovascular control. Obstructive Sleep Apnea (OSA), Chronic Obstructive Pulmonary Disease (COPD), and Bronchiectasis (BE) are all associated with a greatly elevated muscle vasoconstrictor drive (muscle sympathetic nerve activity, MSNA). Indeed, the increase in MSNA is comparable to that seen in congestive heart failure (CHF), in which the increase in MSNA compensates for the reduced cardiac output and thereby assists in maintaining blood pressure. However, in OSA – but not COPD or BE – the increase in MSNA can lead to hypertension. Here, the features of the sympathoexcitation in OSA, COPD, and BE are reviewed in terms of the firing properties of post-ganglionic muscle vasoconstrictor neurons. Compared to healthy subjects with low levels of resting MSNA, single-unit recordings revealed that the augmented MSNA seen in OSA, BE, COPD, and CHF were each associated with an increase in firing probability and mean firing rates of individual neurons. However, unlike patients with heart failure, all patients with respiratory disease exhibited an increase in multiple within-burst firing which, it is argued, reflects an increase in central sympathetic drive. Similar patterns to those seen in OSA, COPD, and BE were seen in healthy subjects during an acute increase in muscle vasoconstrictor drive. These observations emphasize the differences by which the sympathetic nervous system grades its output in health and disease, with an increase in firing probability of active neurons and recruitment of additional neurons being the dominant mechanisms.

On the number of preganglionic neurons driving human postganglionic sympathetic neurons: a comparison of modeling and empirical data

Postganglionic sympathetic axons in awake healthy human subjects, regardless of their identity as muscle vasoconstrictor, cutaneous vasoconstrictor, or sudomotor neurons, discharge with a low firing probability (∼30%), generate low firing rates (∼0.5 Hz) and typically fire only once per cardiac interval. The purpose of the present study was to use modeling of spike trains in an attempt to define the number of preganglionic neurons that drive an individual postganglionic neuron. Artificial spike trains were generated in 1-3 preganglionic neurons converging onto a single postganglionic neuron. Each preganglionic input fired with a mean interval distribution of either 1000, 1500, 2000, 2500, or 3000 ms and the SD varied between 0.5×, 1.0×, and 2.0× the mean interval; the discharge frequency of each preganglionic neuron exhibited positive skewness and kurtosis. Of the 45 patterns examined, the mean discharge properties of the postganglionic neuron could only be explained by it being driven by, on average, two preganglionic neurons firing with a mean interspike interval of 2500 ms and SD of 5000 ms. The mean firing rate resulting from this pattern was 0.22 Hz, comparable to that of spontaneously active muscle vasoconstrictor neurons in healthy subjects (0.40 Hz). Likewise, the distribution of the number of spikes per cardiac interval was similar between the modeled and actual data: 0 spikes (69.5 vs 66.6%), 1 spike (25.6 vs 21.2%), 2 spikes (4.3 vs 6.4%), 3 spikes (0.5 vs 1.7%), and 4 spikes (0.1 vs 0.7%). Although some features of the firing patterns could be explained by the postganglionic neuron being driven by a single preganglionic neuron, none of the emulated firing patterns generated by the firing of three preganglionic neurons matched the discharge of the real neurons. These modeling data indicate that, on average, human postganglionic sympathetic neurons are driven by two preganglionic inputs.

Heterogeneous modulation of place cell firing by changes in context

Hippocampal place cells show spatially localized activity that can be modulated by both geometric information (e.g., the distances and directions of features in the environment) and nongeometric information (e.g., colors, odors, and possibly behaviors). Nongeometric information may allow the discrimination of different spatial contexts. Understanding how nongeometric (or contextual) information affects hippocampal activity is important in light of proposals that the hippocampus may play a role in constructing a representation of spatial context. We investigated the contextual modulation of place cell activity by recording hippocampal place cells while rats foraged in compound contexts comprising black or white color paired with lemon or vanilla odor. Some cells responded to the color or odor changes alone, but most responded to varying combinations of both. Thus, we demonstrate, for the first time, that there is a heterogeneous input by contextual inputs into the hippocampus. We propose a model of contextual remapping of place cells in which the geometric inputs are selectively activated by subsets of contextual stimuli. Because it appears that different place cells are affected by different subsets of contextual stimuli, the representation of the entire context would require activity at the population level, supporting a role for the hippocampus in constructing a representation of spatial context.

Context-dependent neuronal activity in the lateral amygdala represents fear memories after extinction

The context in which fear memories are extinguished has important implications for treating human fear and anxiety disorders. Extinction of Pavlovian fear conditioning is context specific; after extinction, fear responses are reduced only in the extinction context and remain elevated in every other context. Contextual modulation of spike firing in the amygdala is a putative mechanism for the context-specific expression of extinguished fear. To test this possibility, we conditioned rats to fear two auditory conditional stimuli (CSs) and then extinguished each CS in separate and distinct contexts. Thereafter, single-unit activity in the lateral nucleus of the amygdala (LA) and freezing behavior were recorded during tests in which each CS was presented in each extinction context. Hence, each CS was tested in its own extinction context and in the context of the other CS. Conditional freezing was context dependent; fear to an extinguished CS was low in its own extinction context and high in the other test context. Similarly, the majority of LA neurons exhibited context-dependent spike firing; short-latency spike firing was greater to both CSs when they were presented outside of their own extinction context. In contrast, behavioral and neuronal responses to either non-extinguished CSs or habituated auditory stimuli were not contextually modulated. Context-dependent neuronal activity in the LA may be an important mechanism for disambiguating the meaning of fear signals, thereby enabling appropriate behavioral responses to such stimuli.