Neuronal oscillations enhance stimulus discrimination by ensuring action potential precision

GABAergic circuits control input-spike coupling in the piriform cortex

Odor coding in mammals is widely believed to involve synchronized gamma frequency (30-70 Hz) oscillations in the first processing structure, the olfactory bulb. How such inputs are read in downstream cortical structures however is not known. Here we used patch-clamp recordings in rat piriform cortex slices to examine cellular mechanisms that shape how the cortex integrates inputs from bulb mitral cells. Electrical stimulation of mitral cell axons in the lateral olfactory tract (LOT) resulted in excitation of pyramidal cells (PCs), which was followed approximately 10 ms later by inhibition that was highly reproducible between trials in its onset time. This inhibition was somatic in origin and appeared to be driven through a feedforward mechanism, wherein GABAergic interneurons were directly excited by mitral cell axons. The precise inhibition affected action potential firing in PCs in two distinct ways. First, by abruptly terminating PC excitation, it limited the PC response to each EPSP to exactly one, precisely timed action potential. In addition, inhibition limited the summation of EPSPs across time, such that PCs fired action potentials in strong preference for synchronized inputs arriving in a time window of <5 ms. Both mechanisms would help ensure that PCs respond faithfully and selectively to mitral cell inputs arriving as a synchronized gamma frequency pattern.

Oscillatory bands, neuronal synchrony and hippocampal function: implications of the effects of prenatal choline supplementation for sleep-dependent memory consolidation

Choline supplementation of the maternal diet has long-term facilitative effects on spatial and temporal memory processes in the offspring. To further delineate the impact of early nutritional status on brain and behavior, we examined effects of prenatal-choline availability on hippocampal oscillatory frequency bands in 12 month-old male and female rats. Adult offspring of time-pregnant dams that were given a deficient level of choline (DEF=0.0 g/kg), sufficient choline (CON=1.1 g/kg) or supplemental choline (SUP=3.5 g/kg) in their chow during embryonic days (ED) 12-17 were implanted with an electroencephalograph (EEG) electrode in the hippocampal dentate gyrus in combination with an electromyograph (EMG) electrode patch implanted in the nuchal muscle. Five consecutive 8-h recording sessions revealed differential patterns of EEG activity as a function of awake, slow-wave sleep (SWS) and rapid-eye movement (REM) sleep states and prenatal choline status. The main finding was that SUP rats displayed increased power levels of gamma (30-100 Hz) band oscillations during all phases of the sleep/wake cycle. These findings are discussed within the context of a general review of neuronal oscillations (e.g., delta, theta, and gamma bands) and synchronization across multiple brain regions in relation to sleep-dependent memory consolidation in the hippocampus.

Subthreshold membrane-potential resonances shape spike-train patterns in the entorhinal cortex

Many neurons exhibit subthreshold membrane-potential resonances, such that the largest voltage responses occur at preferred stimulation frequencies. Because subthreshold resonances are known to influence the rhythmic activity at the network level, it is vital to understand how they affect spike generation on the single-cell level. We therefore investigated both resonant and nonresonant neurons of rat entorhinal cortex. A minimal resonate-and-fire type model based on measured physiological parameters captures fundamental properties of neuronal firing statistics surprisingly well and helps to shed light on the mechanisms that shape spike patterns: 1) subthreshold resonance together with a spike-induced reset of subthreshold oscillations leads to spike clustering and 2) spike-induced dynamics influence the fine structure of interspike interval (ISI) distributions and are responsible for ISI correlations appearing at higher firing rates (> or =3 Hz). Both mechanisms are likely to account for the specific discharge characteristics of various cell types.

GABAergic circuits control input-spike coupling in the piriform cortex

Odor coding in mammals is widely believed to involve synchronized gamma frequency (30-70 Hz) oscillations in the first processing structure, the olfactory bulb. How such inputs are read in downstream cortical structures however is not known. Here we used patch-clamp recordings in rat piriform cortex slices to examine cellular mechanisms that shape how the cortex integrates inputs from bulb mitral cells. Electrical stimulation of mitral cell axons in the lateral olfactory tract (LOT) resulted in excitation of pyramidal cells (PCs), which was followed approximately 10 ms later by inhibition that was highly reproducible between trials in its onset time. This inhibition was somatic in origin and appeared to be driven through a feedforward mechanism, wherein GABAergic interneurons were directly excited by mitral cell axons. The precise inhibition affected action potential firing in PCs in two distinct ways. First, by abruptly terminating PC excitation, it limited the PC response to each EPSP to exactly one, precisely timed action potential. In addition, inhibition limited the summation of EPSPs across time, such that PCs fired action potentials in strong preference for synchronized inputs arriving in a time window of <5 ms. Both mechanisms would help ensure that PCs respond faithfully and selectively to mitral cell inputs arriving as a synchronized gamma frequency pattern.

A model of stimulus-specific neural assemblies in the insect antennal lobe

It has been proposed that synchronized neural assemblies in the antennal lobe of insects encode the identity of olfactory stimuli. In response to an odor, some projection neurons exhibit synchronous firing, phase-locked to the oscillations of the field potential, whereas others do not. Experimental data indicate that neural synchronization and field oscillations are induced by fast GABA(A)-type inhibition, but it remains unclear how desynchronization occurs. We hypothesize that slow inhibition plays a key role in desynchronizing projection neurons. Because synaptic noise is believed to be the dominant factor that limits neuronal reliability, we consider a computational model of the antennal lobe in which a population of oscillatory neurons interact through unreliable GABA(A) and GABA(B) inhibitory synapses. From theoretical analysis and extensive computer simulations, we show that transmission failures at slow GABA(B) synapses make the neural response unpredictable. Depending on the balance between GABA(A) and GABA(B) inputs, particular neurons may either synchronize or desynchronize. These findings suggest a wiring scheme that triggers stimulus-specific synchronized assemblies. Inhibitory connections are set by Hebbian learning and selectively activated by stimulus patterns to form a spiking associative memory whose storage capacity is comparable to that of classical binary-coded models. We conclude that fast inhibition acts in concert with slow inhibition to reformat the glomerular input into odor-specific synchronized neural assemblies.

Dynamic ensemble odor coding in the mammalian olfactory bulb: sensory information at different timescales

Neural firing discharges are often temporally patterned, but it is often ambiguous as to whether the temporal features of these patterns constitute a useful code. Here we show in the mouse olfactory bulb that ensembles of projection neurons respond with complex odor- and concentration-specific dynamic activity sequences developing below and above sniffing frequency. Based on this activity, almost optimal discrimination of presented odors was possible during single sniffs, consistent with reported behavioral data. Within a sniff cycle, slower features of the dynamics alone (>100 ms resolution, including mean firing rate) were sufficient for maximal discrimination. A smaller amount of information was also observed in faster features down to 20-40 ms resolution. Therefore, mitral cell ensemble activity contains information at different timescales that could be separately or complementarily exploited by downstream brain centers to make odor discriminations. Our results also support suggestive analogies in the dynamics of odor representations between insects and mammals.

Long-term potentiation in rat hippocampal neurons is accompanied by spatially widespread changes in intrinsic oscillatory dynamics and excitability

Oscillations in neural activity are a prominent feature of many brain states. Individual hippocampal neurons exhibit intrinsic membrane potential oscillations and intrinsic resonance in the theta frequency range. We found that the subthreshold resonance frequency of CA1 pyramidal neurons was location dependent, varying more than 3-fold between the soma and the distal dendrites. Furthermore, activity- and NMDA-receptor-dependent long-term plasticity increased this resonance frequency through changes in h channel properties. The increase in resonance frequency and an associated reduction in excitability were nearly identical in the soma and the first 300 mum of the apical dendrites. These spatially widespread changes accompanying long-term synaptic potentiation also reduced the neuron's ability to elicit spikes evoked through a nonpotentiated synaptic pathway. Our results suggest that the frequency response of these neurons depends on the dendritic location of their inputs and that activity can regulate their response dynamics within an oscillating neural network.

The effect of middle temporal spike phase on sensory encoding and correlates with behavior during a motion-detection task

Previous studies have shown that sensory neurons that are the most informative of the stimulus tend to be the best correlated with the subject's perceptual decision. We wanted to know whether this relationship might also apply to short time segments of a neuron's response. We asked whether spikes that conveyed more information about a motion stimulus were also more tightly linked to the perceptual behavior. We examined single-neuron activity in middle temporal (MT) area while monkeys performed a motion-detection task. Because of a slow stimulus update (every 27 ms), activity in many MT neurons was entrained and phase-locked to the stimulus. These stimulus-entrained neuronal oscillations allowed us to separate spikes based on phase. We observed a large amount of variability in how spikes at different phases of the oscillation encoded the stimulus, as revealed by the spike-triggered average of the motion. Spikes during certain phases of the cycle were much more informative about the presence of coherent motion than others. Importantly, we found that the phases that were the most informative about the motion stimulus were also more correlated with the behavioral performance and reaction time of the animal. Our results suggest that the relationship between a neuron's spikes, the stimulus, and behavior can vary on a time scale of tens of milliseconds.

Glutamatergic transmission and plasticity between olfactory bulb mitral cells

In the olfactory bulb the sets of mitral cells that project their apical dendrite to the same glomerulus represent unique functional networks. While it is known that mitral cells release vesicular glutamate from their apical tuft it is believed that the resultant self-excitation (SE), transmitted via dendritic gap junctions, is the main form of lateral transmission within the mitral cell assembly. In this study we used simultaneous whole-cell recordings from mitral cell pairs to show that a direct form of chemical lateral excitation (LE) provides a means of mitral cell-mitral cell communication. In contrast to the ubiquitous expression and robust nature of SE, the efficacy of glutamatergic LE between mitral cells is highly variable and mediated by calcium-impermeable AMPA receptors. We also find that the strength of LE is bi-directionally modulated, in a homeostatic manner, by sniffing-like patterns of presynaptic activity. Since these changes last many minutes we suggest that such mitral cell-mitral cell interactions provide the glomerular network with a locus for olfactory plasticity and a potential mechanism for receptive field modulation.

The oscillation score: an efficient method for estimating oscillation strength in neuronal activity

We present a method that estimates the strength of neuronal oscillations at the cellular level, relying on autocorrelation histograms computed on spike trains. The method delivers a number, termed oscillation score, that estimates the degree to which a neuron is oscillating in a given frequency band. Moreover, it can also reliably identify the oscillation frequency and strength in the given band, independently of the oscillation in other frequency bands, and thus it can handle superimposed oscillations on multiple scales (theta, alpha, beta, gamma, etc.). The method is relatively simple and fast. It can cope with a low number of spikes, converging exponentially fast with the number of spikes, to a stable estimation of the oscillation strength. It thus lends itself to the analysis of spike-sorted single-unit activity from electrophysiological recordings. We show that the method performs well on experimental data recorded from cat visual cortex and also compares favorably to other methods. In addition, we provide a measure, termed confidence score, that determines the stability of the oscillation score estimate over trials.

Gamma oscillations of spiking neural populations enhance signal discrimination

Selective attention is an important filter for complex environments where distractions compete with signals. Attention increases both the gamma-band power of cortical local field potentials and the spike-field coherence within the receptive field of an attended object. However, the mechanisms by which gamma-band activity enhances, if at all, the encoding of input signals are not well understood. We propose that gamma oscillations induce binomial-like spike-count statistics across noisy neural populations. Using simplified models of spiking neurons, we show how the discrimination of static signals based on the population spike-count response is improved with gamma induced binomial statistics. These results give an important mechanistic link between the neural correlates of attention and the discrimination tasks where attention is known to enhance performance. Further, they show how a rhythmicity of spike responses can enhance coding schemes that are not temporally sensitive.

Rhythmic brain activity is observed in many neural structures and is an inferred critical component of neural processing. In particular, stimulus induced oscillations in the gamma-frequency band (30–80 Hz) are common in several cortical networks. Many experimental and theoretical studies have established the neural mechanisms by which a population of neurons produce and control gamma-band activity. However, the beneficial role, if any, of gamma activity in neural processing is rarely discussed. It is increasingly apparent that gamma oscillatory power increases with subject attention to a sensory scene. Attention is associated with enhanced performance of discrimination tasks, where relevant stimuli compete with distracters. In this study we explore how gamma-band activity serves to enhance the discrimination of stimuli. We use computational models to show that the gamma rhythmicity in a population of spiking neurons drastically reduces the response variability when a preferred stimulus is present. This drop in response variability enhances stimulus discrimination and increases the overall information throughput in sensory cortex. Our results provide a much-needed link between the dynamics of neural populations and the coding tasks they perform, as well as give insight on why—rather than how—attention mediates gamma activity.

Spatiotemporal dynamics of word processing in the human brain

We examined the spatiotemporal dynamics of word processing by recording the electrocorticogram (ECoG) from the lateral frontotemporal cortex of neurosurgical patients chronically implanted with subdural electrode grids. Subjects engaged in a target detection task where proper names served as infrequent targets embedded in a stream of task-irrelevant verbs and nonwords. Verbs described actions related to the hand (e.g, throw) or mouth (e.g., blow), while unintelligible nonwords were sounds which matched the verbs in duration, intensity, temporal modulation, and power spectrum. Complex oscillatory dynamics were observed in the delta, theta, alpha, beta, low, and high gamma (HG) bands in response to presentation of all stimulus types. HG activity (80-200 Hz) in the ECoG tracked the spatiotemporal dynamics of word processing and identified a network of cortical structures involved in early word processing. HG was used to determine the relative onset, peak, and offset times of local cortical activation during word processing. Listening to verbs compared to nonwords sequentially activates first the posterior superior temporal gyrus (post-STG), then the middle superior temporal gyrus (mid-STG), followed by the superior temporal sulcus (STS). We also observed strong phase-locking between pairs of electrodes in the theta band, with weaker phase-locking occurring in the delta, alpha, and beta frequency ranges. These results provide details on the first few hundred milliseconds of the spatiotemporal evolution of cortical activity during word processing and provide evidence consistent with the hypothesis that an oscillatory hierarchy coordinates the flow of information between distinct cortical regions during goal-directed behavior.

INHIBITORY INTERNEURONS IN THE PIRIFORM CORTEX

1. The piriform cortex (PC) is the largest subdivision of the olfactory cortex and the first cortical destination of olfactory information. Despite the relatively simple anatomy of the PC and its obvious appeal as a model system for the study of cortical sensory processing, there are many outstanding questions about its basic cell physiology. In the present article, we review what is known about GABAergic inhibitory interneurons in the PC. 2. The GABA-containing neurons in the PC are morphologically diverse, ranging from small neurogliaform cells to large multipolar forms. Some of these classes are distributed across all three main layers of the PC, whereas others have a more restricted laminar expression. 3. Distinct and overlapping populations of GABAergic basket cells in Layers II and III of the PC express different combinations of calcium-binding proteins and neuropeptides. Few Layer I interneurons express any of the molecular markers so far examined. 4. The intrinsic firing properties of one or two types of putative PC interneurons have been measured and inhibitory post-synaptic responses have been recorded in PC pyramidal cells following extracellular stimulation. However, little is known about the physiology of the subtypes of interneurons identified. 5. In view of the likely importance of PC interneurons in olfactory learning, olfactory coding and epileptogenesis, further investigation of their properties is likely to be highly informative.

Muscarinic blockade weakens interaction of gamma with theta rhythms in mouse hippocampus

theta (4-12 Hz) and gamma (40-90) oscillations are prominent rhythms in the mammalian brain. A striking feature of these rhythms, possibly vital to memory encoding, is their specific coordination in a manner that has been termed 'nesting', i.e. the preferred occurrence of bouts of gamma activity during specific phases of theta. Both rhythms are shaped by the neuromodulator acetylcholine, but it is unknown to what degree their coordination is influenced by cholinergic neuromodulation. Here, we investigated the effects of a blockade of muscarinic acetylcholine receptors by atropine on theta and gamma oscillations, and their interaction, in mouse hippocampus in vivo. Multi-site recordings from area CA1 of freely moving mice showed that under control conditions gamma activity was amplitude-modulated at theta frequencies. This coordination of theta and gamma oscillations, as assessed by cross-correlation of theta with the gamma envelope, was prominent in basal and apical dendritic laminae but not in intermediate laminae. It was stronger during active exploration than during awake immobility. Atropine (50 mg/kg intraperitoneal) altered several aspects of the individual and nested rhythms. It rendered theta activity irregular, decreased theta oscillation frequency and reduced gamma power. Atropine also reduced the amplitude-modulation of gamma oscillations at theta frequencies, in part by perturbing the coordination of the rhythms on a short time scale. Thus, our findings demonstrate that phase locking of the amplitude of gamma oscillations to theta in hippocampal area CA1 is partially governed by neuronal elements harbouring muscarinic receptors.

An olfacto-hippocampal network is dynamically involved in odor-discrimination learning

Several studies have shown that memory consolidation relies partly on interactions between sensory and limbic areas. The functional loop formed by the olfactory system and the hippocampus represents an experimentally tractable model that can provide insight into this question. It had been shown previously that odor-learning associated beta band oscillations (15-30 Hz) of the local field potential in the rat olfactory system are enhanced with criterion performance, but it was unknown if these involve networks beyond the olfactory system. We recorded local field potentials from the olfactory bulb (OB) and dorsal and ventral hippocampus during acquisition of odor discriminations in a go/no-go task. These regions showed increased beta oscillation power during odor sampling, accompanied by a coherence increase in this frequency band between the OB and both hippocampal subfields. This coherence between the OB and the hippocampus increased with the onset of the first rule transfer to a new odor set and remained high for all learning phases and subsequent odor sets. However, coherence between the two hippocampal fields reset to baseline levels with each new odor set and increased again with criterion performance. These data support hippocampal involvement in the network underlying odor-discrimination learning and also suggest that cooperation between the dorsal and ventral hippocampus varies with learning progress. Oscillatory activity in the beta range may thus provide a mechanism by which these areas are linked during memory consolidation, similar to proposed roles of beta oscillations in other systems with long-range connections.

Chemotopic odorant coding in a mammalian olfactory system

Systematic mapping studies involving 365 odorant chemicals have shown that glomerular responses in the rat olfactory bulb are organized spatially in patterns that are related to the chemistry of the odorant stimuli. This organization involves the spatial clustering of principal responses to numerous odorants that share key aspects of chemistry such as functional groups, hydrocarbon structural elements, and/or overall molecular properties related to water solubility. In several of the clusters, responses shift progressively in position according to odorant carbon chain length. These response domains appear to be constructed from orderly projections of sensory neurons in the olfactory epithelium and may also involve chromatography across the nasal mucosa. The spatial clustering of glomerular responses may serve to "tune" the principal responses of bulbar projection neurons by way of inhibitory interneuronal networks, allowing the projection neurons to respond to a narrower range of stimuli than their associated sensory neurons. When glomerular activity patterns are viewed relative to the overall level of glomerular activation, the patterns accurately predict the perception of odor quality, thereby supporting the notion that spatial patterns of activity are the key factors underlying that aspect of the olfactory code. A critical analysis suggests that alternative coding mechanisms for odor quality, such as those based on temporal patterns of responses, enjoy little experimental support.

GABAergic inhibition at dendrodendritic synapses tunes gamma oscillations in the olfactory bulb

In the olfactory bulb (OB), odorants induce oscillations in the gamma range (20-80 Hz) that play an important role in the processing of sensory information. Synaptic transmission between dendrites is a major contributor to this processing. Glutamate released from mitral cell dendrites excites the dendrites of granule cells, which in turn mediate GABAergic inhibition back onto mitral cells. Although this reciprocal synapse is thought to be a key element supporting oscillatory activity, the mechanisms by which dendrodendritic inhibition induces and maintains gamma oscillations remain unknown. Here, we assessed the role of the dendrodendritic inhibition, using mice lacking the GABA(A) receptor alpha1-subunit, which is specifically expressed in mitral cells but not in granule cells. The spontaneous inhibitory postsynaptic current frequency in these mutants was low and was consistent with the reduction of GABA(A) receptor clusters detected by immunohistochemistry. The remaining GABA(A) receptors in mitral cells contained the alpha3-subunit and supported slower decaying currents of unchanged amplitude. Overall, inhibitory-mediated interactions between mitral cells were smaller and slower in mutant than in WT mice, although the strength of sensory afferent inputs remained unchanged. Consequently, both experimental and theoretical approaches revealed slower gamma oscillations in the OB network of mutant mice. We conclude, therefore, that fast oscillations in the OB circuit are strongly constrained by the precise location, subunit composition and kinetics of GABA(A) receptors expressed in mitral cells.

Spike timing amplifies the effect of electric fields on neurons: implications for endogenous field effects

Despite compelling phenomenological evidence that small electric fields (<5 mV/mm) can affect brain function, a quantitative and experimentally verified theory is currently lacking. Here we demonstrate a novel mechanism by which the nonlinear properties of single neurons "amplify" the effect of small electric fields: when concurrent to suprathreshold synaptic input, small electric fields can have significant effects on spike timing. For low-frequency fields, our theory predicts a linear dependency of spike timing changes on field strength. For high-frequency fields (relative to the synaptic input), the theory predicts coherent firing, with mean firing phase and coherence each increasing monotonically with field strength. Importantly, in both cases, the effects of fields on spike timing are amplified with decreasing synaptic input slope and increased cell susceptibility (millivolt membrane polarization per field amplitude). We confirmed these predictions experimentally using CA1 hippocampal neurons in vitro exposed to static (direct current) and oscillating (alternating current) uniform electric fields. In addition, we develop a robust method to quantify cell susceptibility using spike timing. Our results provide a precise mechanism for a functional role of endogenous field oscillations (e.g., gamma) in brain function and introduce a framework for considering the effects of environmental fields and design of low-intensity therapeutic neurostimulation technologies.

Spatiotemporal representations in the olfactory system

A complete understanding of the mechanisms underlying any kind of sensory, motor or cognitive task requires analysis from the systems to the cellular level. In olfaction, new behavioural evidence in rodents has provided temporal limits on neural processing times that correspond to less than 150ms--the timescale of a single sniff. Recent in vivo data from the olfactory bulb indicate that, within each sniff, odour representation is not only spatially organized, but also temporally structured by odour-specific patterns of onset latencies. Thus, we propose that the spatial representation of odour is not a static one, but rather evolves across a sniff, whereby for difficult discriminations of similar odours, it is necessary for the olfactory system to "wait" for later-activated components. Based on such evidence, we have devised a working model to assess further the relevance of such spatiotemporal processes in odour representation.