Utilizing synthetic training data for the supervised classification of rat ultrasonic vocalizations.

Murine rodents generate ultrasonic vocalizations (USVs) with frequencies that extend to around 120 kHz. These calls are important in social behaviour, and so their analysis can provide insights into the function of vocal communication, and its dysfunction. The manual identification of USVs, and subsequent classification into different subcategories is time consuming. Although machine learning approaches for identification and classification can lead to enormous efficiency gains, the time and effort required to generate training data can be high, and the accuracy of current approaches can be problematic. Here, we compare the detection and classification performance of a trained human against two convolutional neural networks (CNNs), DeepSqueak (DS) and VocalMat (VM), on audio containing rat USVs. Furthermore, we test the effect of inserting synthetic USVs into the training data of the VM CNN as a means of reducing the workload associated with generating a training set. Our results indicate that VM outperformed the DS CNN on measures of call identification, and classification. Additionally, we found that the augmentation of training data with synthetic images resulted in a further improvement in accuracy, such that it was sufficiently close to human performance to allow for the use of this software in laboratory conditions.

Aberrant Phase Precession of Lateral Septal Cells in a Maternal Immune Activation Model of Schizophrenia Risk May Disrupt the Integration of Location with Reward.

Spatial memory and reward processing are known to be disrupted in schizophrenia. Since the lateral septum (LS) may play an important role in the integration of location and reward, we examined the effect of maternal immune activation (MIA), a known schizophrenia risk factor, on spatial representation in the rat LS. In support of a previous study, we found that spatial location is represented as a phase code in the rostral LS of adult male rats, so that LS cell spiking shifts systematically against the phase of the hippocampal, theta-frequency, local field potential as an animal moves along a track toward a reward (phase precession). Whereas shallow precession slopes were observed in control group cells, they were steeper in the MIA animals, such that firing frequently precessed across several theta cycles as the animal moved along the length of the apparatus, with subsequent ambiguity in the phase representation of location. Furthermore, an analysis of the phase trajectories of the control group cells revealed that the population tended to converge toward a common firing phase as the animal approached the reward location. This suggested that phase coding in these cells might signal both reward location and the distance to reward. By comparison, the degree of phase convergence in the MIA-group cells was weak, and the region of peak convergence was distal to the reward location. These findings suggest that a schizophrenia risk factor disrupts the phase-based encoding of location-reward relationships in the LS, potentially smearing reward representations across space.SIGNIFICANCE STATEMENT It is unclear how spatial or contextual information generated by hippocampal cells is converted to a code that can be used to signal reward location in regions, such as the VTA. Here we provide evidence that the firing phase of cells in the lateral septum, a region that links the two areas, may code reward location in the firing phase of cells. This phase coding is disrupted in a maternal immune activation model of schizophrenia risk such that representations of reward may be smeared across space in maternal immune activation animals. This could potentially underlie erroneous reward processing and misattribution of salience in schizophrenia.

Disrupted hippocampal synchrony following maternal immune activation in a rat model.

Maternal immune activation (MIA) is a risk factor for schizophrenia and other neurodevelopmental disorders. MIA in rats models a number of the brain and behavioral changes that are observed in schizophrenia, including impaired memory. Recent studies in the MIA model have shown that the firing of the hippocampal place cells that are involved in memory processes appear relatively normal, but with abnormalities in the temporal ordering of firing. In this study, we re-analyzed data from prior hippocampal electrophysiological recordings of MIA and control animals to determine whether temporal dysfunction was evident. We find that there is a decreased ratio of slow to fast gamma power, resulting from an increase in fast gamma power and a tendency toward reduced slow gamma power in MIA rats. Moreover, we observe a robust reduction in spectral coherence between hippocampal theta and both fast and slow gamma rhythms, as well as changes in the phase of theta at which fast gamma occurs. We also find the phasic organization of place cell phase precession on the theta wave to be abnormal in MIA rats. Lastly, we observe that the local field potential of MIA rats contains more frequent sharp-wave ripple events, and that place cells were more likely to fire spikes during ripples in these animals than control. These findings provide further evidence of desynchrony in MIA animals and may point to circuit-level changes that underlie failures to integrate and encode information in schizophrenia.

Hippocampal Sequencing Mechanisms Are Disrupted in a Maternal Immune Activation Model of Schizophrenia Risk.

Episodic memory requires information to be stored and recalled in sequential order, and these processes are disrupted in schizophrenia. Hippocampal phase precession and theta sequences are thought to provide a biological mechanism for sequential ordering of experience at timescales suitable for plasticity. These phenomena have not previously been examined in any models of schizophrenia risk. Here, we examine these phenomena in a maternal immune activation (MIA) rodent model. We show that while individual pyramidal cells in the CA1 region continue to precess normally in MIA animals, the starting phase of precession as an animal enters a new place field is considerably more variable in MIA animals than in controls. A critical consequence of this change is a disorganization of the ordered representation of experience via theta sequences. These results provide the first evidence of a biological-level mechanism that, if it occurs in schizophrenia, may explain aspects of disorganized sequential processing that contribute to the cognitive symptoms of the disorder.SIGNIFICANCE STATEMENT Hippocampal phase precession and theta sequences have been proposed as biophysical mechanisms by which the sequential structure of cognition might be ordered. Disturbances of sequential processing have frequently been observed in schizophrenia. Here, we show for the first time that phase precession and theta sequences are disrupted in a maternal immune activation (MIA) model of schizophrenia risk. This is a result of greater variability in the starting phase of precession, indicating that the mechanisms that coordinate precession at the assembly level are disrupted. We propose that this disturbance in phase precession underlies some of the disorganized cognitive symptoms that occur in schizophrenia. These findings could have important preclinical significance for the identification and treatment of schizophrenia risk factors.

Anterior cingulate cortex encoding of effortful behavior.

An animal's ability to assess the value of their behaviors to minimize energy use while maximizing goal achievement is critical to its survival. The anterior cingulate cortex (ACC) has been previously shown to play a critical role in this behavioral optimization process, especially when animals are faced with effortful behaviors. In the present study, we designed a novel task to investigate the role of the ACC in evaluating behaviors that varied in effort but all resulted in the same outcome. We recorded single unit activity from the ACC as rats ran back and forth in a shuttle box that could be tilted to different tilt angles (0, 15, and 25°) to manipulate effort. Overall, a majority of ACC neurons showed selective firing to specific effort conditions. During effort expenditure, ACC units showed a consistent firing rate bias toward the downhill route compared with the more difficult uphill route, regardless of the tilt angle of the apparatus. Once rats completed a run and received their fixed reward, ACC units also showed a clear firing rate preference for the single condition with the highest relative value (25° downhill). To assess effort preferences, we used a choice version of our task and confirmed that rats prefer downhill routes to uphill routes when given the choice. Overall, these results help to elucidate the functional role of the ACC in monitoring and evaluating effortful behaviors that may then bias decision-making toward behaviors with the highest utility. NEW & NOTEWORTHY We developed a novel effort paradigm to investigate how the anterior cingulate cortex (ACC) responds to behaviors with varied degrees of physical effort and how changes in effort influence the ACC's evaluation of behavioral outcomes. Our results provide evidence for a wider role of the ACC in its ability to motivate effortful behaviors and evaluate the outcome of multiple behaviors within an environment.

Hippocampal place cell encoding of sloping terrain.

Effective navigation relies on knowledge of one's environment. A challenge to effective navigation is accounting for the time and energy costs of routes. Irregular terrain in ecological environments poses a difficult navigational problem as organisms ought to avoid effortful slopes to minimize travel costs. Route planning and navigation have previously been shown to involve hippocampal place cells and their ability to encode and store information about an organism's environment. However, little is known about how place cells may encode the slope of space and associated energy costs as experiments are traditionally carried out in flat, horizontal environments. We set out to investigate how dorsal-CA1 place cells in rats encode systematic changes to the slope of an environment by tilting a shuttle box from flat to 15 ° and 25 ° while minimizing external cue change. Overall, place cell encoding of tilted space was as robust as their encoding of flat ground as measured by traditional place cell metrics such as firing rates, spatial information, coherence, and field size. A large majority of place cells did, however, respond to slope by undergoing partial, complex remapping when the environment was shifted from one tilt angle to another. The propensity for place cells to remap did not, however, depend on the vertical distance the field shifted. Changes in slope also altered the temporal coding of information as measured by the rate of theta phase precession of place cell spikes, which decreased with increasing tilt angles. Together these observations indicate that place cells are sensitive to relatively small changes in terrain slope and that terrain slope may be an important source of information for organizing place cell ensembles. The terrain slope information encoded by place cells could be utilized by efferent regions to determine energetically advantageous routes to goal locations.

Maternal immune activation leads to increased nNOS immunoreactivity in the brain of postnatal day 2 rat offspring.

Neuronal nitric oxide synthase (nNOS) is a key arginine metabolising enzyme in the brain, and nNOS-derived nitric oxide (NO) plays an important role in regulating glutamatergic neurotransmission. NO and its related molecules are involved in the pathogenesis of schizophrenia, and human genetic studies have identified schizophrenia risk genes encoding nNOS. This study systematically investigated how maternal immune activation (MIA; a risk factor for schizophrenia) induced by polyinosinic:polycytidylic acid affected nNOS-immunoreactivity in the brain of the resulting male and female offspring at the age of postnatal day (PND) 2. Immunohistochemistry revealed a markedly increased intensity of nNOS-positive cells in the CA3 and dentate gyrus subregions of the hippocampus, the somatosensory cortex, and the striatum, but not the frontal cortex and hippocampal CA1 region, in the MIA offspring when compared to control group animals. There were no sex differences in the effect. Given the role of nNOS in glutamatergic neurotransmission and its functional relationship with glutamate NMDA receptors, increased nNOS immunoreactivity may indicate the up-regulation of NMDA receptor function in MIA rat offspring at an early postnatal age. Future research is required to determine whether these changes contribute to the neuronal and behavioral dysfunction observed in both juvenile and adult MIA rat offspring.

Maternal immune activation altered microglial immunoreactivity in the brain of postnatal day 2 rat offspring.

Microglia, the resident immune cells of the central nervous system, play critical roles in neurodevelopment, synaptic pruning, and neuronal wiring. Early in development, microglia migrate via the tangential and radial migration pathways to their final destinations and mature gradually, a process that includes morphological changes. Recent research has implicated microglial abnormality in the etiology of schizophrenia. Since prenatal exposure to viral or bacterial infections due to maternal immune activation (MIA) leads to increased risk of schizophrenia in the offspring during adulthood, the present study systematically investigated how MIA induced by polyinosinic:polycytidylic acid (a mimic of viral double-stranded RNA) affected microglial immunoreactivity along the migration and maturation trajectories in the brains of male and female rat offspring on postnatal day (PND) 2. The immunohistochemistry revealed significant changes in the density of IBA-1 immunoreactive cells in the corpus callosum, somatosensory cortex, striatum, and the subregions of the hippocampus of the MIA offspring. The male and female MIA offspring displayed markedly altered microglial immunoreactivity in both the tangential and radial migration, as well as maturation, pathways when compared to their sex- and age-matched controls as evidenced by morphology-based cell counting. Given the important roles of microglia in synaptic pruning and neuronal wiring and survival, these changes may lead to structural and functional neurodevelopmental abnormalities, and so contribute to the functional deficits observed in juvenile and adult MIA offspring. Future research is required to systematically determine how MIA affects microglial migration and maturation in rat offspring.

Exposure to complex environments results in more sparse representations of space in the hippocampus.

The neural circuitry mediating sensory and motor representations is adaptively tuned by an animal's interaction with its environment. Similarly, higher order representations such as spatial memories can be modified by exposure to a complex environment (CE), but in this case the changes in brain circuitry that mediate the effect are less well understood. Here, we show that prolonged CE exposure was associated with increased selectivity of CA1 "place cells" to a particular recording arena compared to a social control (SC) group. Furthermore, fewer CA1 and DG neurons in the CE group expressed high levels of Arc protein, a marker of recent activation, following brief exposure to a completely novel environment. The reduced Arc expression was not attributable to overall changes in cell density or number. These data indicate that one effect of CE exposure is to modify high-level spatial representations in the brain by increasing the sparsity of population coding within networks of neurons. Greater sparsity could result in a more efficient and compact coding system that might alter behavioural performance on spatial tasks. The results from a behavioural experiment were consistent with this hypothesis, as CE-treated animals habituated more rapidly to a novel environment despite showing equivalent initial responding.

Maternal immune activation alters sensitivity to action-outcome contingency in adult rat offspring.

Epidemiological studies have provided convincing evidence for a role of maternal immune activation in the pathogenesis of neurodevelopmental disorders such as autism and schizophrenia. In recent years, several research groups have capitalised on this discovery and developed animal models such as the maternal immune activation (MIA) model that emulates many phenotypes characteristic of disorders such as schizophrenia. In the present series of experiments we used the MIA model to examine motivation, a core component of the negative symptomatology in schizophrenia. Contrary to what we expected, in the progressive ratio task, which assesses an animals' willingness to work for a reward under increasing effort requirements, we found that MIA rats appeared more motivated than controls. Subsequent tests showed that this seemingly enhanced motivation was not due to an overall increase in responding, nor due to enhanced attribution of incentive salience to reward associated responses. Instead, we found that the increased willingness to work exhibited by MIA animals was due to an inability to detect changes in the contingency between their behaviour and the resulting rewarding outcome. With regard to motivation, the experiments reported here are the first to subject the MIA model to a rigorous experimental analysis of behaviour by parsing underlying processes that give rise to the overt symptoms in psychiatric disease.

Prenatal immune activation alters hippocampal place cell firing characteristics in adult animals.

Prenatal maternal immune activation (MIA) is a risk factor for several developmental neuropsychiatric disorders, including autism, bipolar disorder and schizophrenia. Adults with these disorders display alterations in memory function that may result from changes in the structure and function of the hippocampus. In the present study we use an animal model to investigate the effect that a transient prenatal maternal immune activation episode has on the spatially-modulated firing activity of hippocampal neurons in adult animals. MIA was induced in pregnant rat dams with a single injection of the synthetic cytokine inducer polyinosinic:polycytidylic acid (poly I:C) on gestational day 15. Control dams were given a saline equivalent. Firing activity and local field potentials (LFPs) were recorded from the CA1 region of the adult male offspring of these dams as they moved freely in an open arena. Most neurons displayed characteristic spatially-modulated 'place cell' firing activity and while there was no between-group difference in mean firing rate between groups, place cells had smaller place fields in MIA-exposed animals when compared to control-group cells. Cells recorded in MIA-group animals also displayed an altered firing-phase synchrony relationship to simultaneously recorded LFPs. When the floor of the arena was rotated, the place fields of MIA-group cells were more likely to shift in the same direction as the floor rotation, suggesting that local cues may have been more salient for these animals. In contrast, place fields in control group cells were more likely to shift firing position to novel spatial locations suggesting an altered response to contextual cues. These findings show that a single MIA intervention is sufficient to change several important characteristics of hippocampal place cell activity in adult offspring. These changes could contribute to the memory dysfunction that is associated with MIA, by altering the encoding of spatial context and by disrupting plasticity mechanisms that are dependent on spike timing synchrony.

Spectrum of short- and long-term brain pathology and long-term behavioral deficits in male repeated hypoxic rats closely resembling human extreme prematurity.

Brain injury in the premature infant is associated with a high risk of neurodevelopmental disability. Previous small-animal models of brain injury attributable to extreme prematurity typically fail to generate a spectrum of pathology and behavior that closely resembles that observed in humans, although they provide initial answers to numerous cellular, molecular, and therapeutic questions. We tested the hypothesis that exposure of rats to repeated hypoxia from postnatal day 1 (P1) to P3 models the characteristic white matter neuropathological injury, gray matter volume loss, and memory deficits seen in children born extremely prematurely. Male Sprague Dawley rats were exposed to repeated hypoxia or repeated normoxia from P1 to P3. The absolute number of pre-oligodendrocytes and mature oligodendrocytes, the surface area and g-ratio of myelin, the absolute volume of cerebral white and gray matter, and the absolute number of cerebral neurons were quantified stereologically. Spatial memory was investigated on a radial arm maze. Rats exposed to repeated hypoxia had a significant loss of (1) pre-oligodendrocytes at P4, (2) cerebral white matter volume and myelin at P14, (3) cerebral cortical and striatal gray matter volume without neuronal loss at P14, and (4) cerebral myelin and memory deficits in adulthood. Decreased myelin was correlated with increased attention deficit hyperactivity disorder-like hyperactivity. This new small-animal model of extreme prematurity generates a spectrum of short- and long-term pathology and behavior that closely resembles that observed in humans. This new rat model provides a clinically relevant tool to investigate numerous cellular, molecular, and therapeutic questions on brain injury attributable to extreme prematurity.

Basal forebrain and prelimbic cortex connectivity is related to behavioral response in an attention task.

The basal forebrain (BF) is critical for the motivational recruitment of attention in response to reward-related cues. This finding is consistent with a role for the BF in encoding and transmitting motivational salience and readying prefrontal circuits for further attentional processing. We recorded local field potentials to determine connectivity between prelimbic cortex (PrL) and BF during the modulation of attention by reward-related cues. We find that theta and gamma power are robustly associated with behavior. Power in both bands is significantly lower during trials in which an incorrect behavioral response is made. We find strong coherence during responses that are significantly stronger when a correct response is made. We show that information flow is largely monodirectional from BF to and is strongest when correct responses are made. These experiments demonstrate that connectivity between BF and the PrL increases during periods of increased motivational recruitment of attentional resources.

Enhanced hippocampal neuronal excitability and LTP persistence associated with reduced behavioral flexibility in the maternal immune activation model of schizophrenia.

Individuals with schizophrenia display a number of structural and cytoarchitectural alterations in the hippocampus, suggesting that other functions such as synaptic plasticity may also be modified. Altered hippocampal plasticity is likely to affect memory processing, and therefore any such pathology may contribute to the cognitive symptoms of schizophrenia, which includes prominent memory impairment. The current study tested whether prenatal exposure to infection, an environmental risk factor that has previously been associated with schizophrenia produced changes in hippocampal synaptic transmission or plasticity, using the maternal immune activation (MIA) animal model. We also assessed performance in hippocampus-dependent memory tasks to determine whether altered plasticity is associated with memory dysfunction. MIA did not alter basal synaptic transmission in either the dentate gyrus or CA1 of freely moving adult rats. It did, however, result in increased paired-pulse facilitation of the dentate gyrus population spike and an enhanced persistence of dentate long-term potentiation. MIA animals displayed slower learning of a reversed platform location in the water maze, and a similarly slowed learning during reversal in a spatial plus maze task. Together these findings are indicative of reduced behavioral flexibility in response to changes in task requirements. The results are consistent with the hypothesis that hippocampal plasticity is altered in schizophrenia, and that this change in plasticity mechanisms may underlie some aspects of cognitive dysfunction in this disorder.

Persisting through subjective effort: a key role for the anterior cingulate cortex?

One shortcoming of Kurzban et al.'s model is that it is not clear how animals persist through subjectively effortful tasks, particularly over a long time course. We suggest that the anterior cingulate cortex plays a critical role by encoding the utility of an action, and signalling where efforts should be best directed based on previous and prospected experience.

Maladaptive explore/exploit trade-offs in schizophrenia.

Schizophrenia is a complex disorder that remains poorly understood, particularly at the systems level. In this opinion article we argue that the explore/exploit trade-off concept provides a holistic and ecologically valid framework to resolve some of the apparent paradoxes that have emerged within schizophrenia research. We review recent evidence suggesting that fundamental explore/exploit behaviors may be maladaptive in schizophrenia during physical, visual, and cognitive foraging. We also describe how theories from the broader optimal foraging literature, such as the marginal value theorem (MVT), could provide valuable insight into how aberrant processing of reward, context, and cost/effort evaluations interact to produce maladaptive responses.

No evidence that acute clozapine administration alters CA1 phase precession in rats.

Hippocampal phase precession, wherein there is a systematic shift in the phase of neural firing against the underlying theta activity, is proposed to play an important role in the sequencing of information in memory. Previous research shows that the starting phase of precession is more variable in rats following maternal immune activation (MIA), a known risk factor for schizophrenia. Since starting phase variability has the potential to disorganize the construction of sequences of information, we tested whether the atypical antipsychotic clozapine, which ameliorates some cognitive deficits in schizophrenia, alters this aspect of phase precession. Either saline or clozapine (5 mg/kg) was administered to rats and then CA1 place cell activity was recorded from the CA1 region of the hippocampus as the animals ran around a rectangular track for food reward. When compared to saline trials, acute administration of clozapine did not affect any place cell properties, including those related to phase precession, in either control or MIA animals. Clozapine did, however, produce a reduction in locomotion speed, indicating that its presence had some effect on behaviour. These results help to constrain explanations of phase precession mechanisms and their potential role in sequence learning deficits.

Hippocampal phase precession is preserved under ketamine, but the range of precession across a theta cycle is reduced.

BACKGROUND: Hippocampal phase precession, which depends on the precise spike timing of place cells relative to local theta oscillations, has been proposed to underlie sequential memory. N-methyl-D-asparate (NMDA) receptor antagonists such as ketamine disrupt memory and also reproduce several schizophrenia-like symptoms, including spatial memory impairments and disorganized cognition. It is possible that these impairments result from disruptions to phase precession. AIMS/METHODS: We used an ABA design to test whether an acute, subanesthetic dose (7.5 mg/kg) of ketamine disrupted phase precession in CA1 of male rats as they navigated around a rectangular track for a food reward. RESULTS/OUTCOMES: Ketamine did not affect the ability of CA1 place cells to precess despite changes to place cell firing rates, local field potential properties and locomotor speed. However, ketamine reduced the range of phase precession that occurred across a theta cycle. CONCLUSION: Phase precession is largely robust to acute NMDA receptor antagonism by ketamine, but the reduced range of precession could have important implications for learning and memory.

Place, space, and taste: combining context and spatial information in a hippocampal navigation system.

The hippocampus appears to play an important role in episodic-like, or "what, where, when" memory that may be used for goal finding. We have previously presented a model of the hippocampus describing how navigation to a distal goal location could be achieved through gradient ascent processes based on place field density. Here we extend that model to show that information about both where a goal is, and the attributes of that goal, can be incorporated with relatively simple modifications. In this model both the spatial and attribute information would be available to the animal at any location within the environment although differential recall requires two different firing modes. "Where" information is available when cells in the model are firing in "place field" firing mode. The recall of "what" information requires, however, near simultaneous firing of a large number of place cells. We discuss how this "what" firing mode could potentially depend upon sharp-wave ripple (SPW-R) events. This conception has implications for how we interpret SPW-R related firing. In particular, we suggest that the near simultaneous firing that occurs in large groups of cells during SPW-Rs may be sufficient for "what" recall and that the forward and reverse "replay" of place cell sequences that are often observed during SPW-Rs may be an epiphenomenon of this process.

The firing rate of hippocampal CA1 place cells is modulated with a circadian period.

The accurate recall of an event is usually dependent on a memory trace that encodes three pieces of information; what happened, when the event happened, and where. The established phenomenology of hippocampal CA1 pyramidal neurons could reflect mechanisms via which some of this information (where and what) is encoded; but so far there has been little evidence for a mechanism by which these cells might represent "when." It was therefore of interest to examine the activity of CA1 neurons over a substantial temporal duration. Forty-eight CA1 neurons were recorded once an hour during long (24-48 h) exposures to a single, stable environment where minimal time-of-day cues were available. Only data from the first 25 h of recording was analyzed quantitatively. We found that the mean ensemble firing rate of these cells changed predictably such that it was closely correlated (r = 0.707) to a reference sine wave with a 25-h period and a positive peak at recording start. This relationship was not explained by changes in the animal's running speed or amount of the recording environment covered in each recording session. When data were referenced to the onset or offset of the normal light-on period, the correlation with the sinusoid was abolished. At an individual cell level, the majority of neurons (n = 31) had significant correlations (P < 0.05) with the reference sine. We conclude that the firing rate of a large proportion of cells in area CA1 of the hippocampus are modulated over a circadian period but that this modulation is not entrained to light. Rather, entry into the environment and the associated food availability appear to be the entraining factors. We hypothesize that these neurons may be part of the putative food-entrainable oscillator. Such a system could enable an animal to discriminate between spatial representations on a temporal dimension with reference to the time of food availability.