Circuit Properties Generating Gamma Oscillations in a Network Model of the Olfactory Bulb

An olfactory bulb slice-based biosensor for multi-site extracellular recording of neural networks

Multi-site recording is the important component for studies of the neural networks. In order to investigate the electrophysiological properties of the olfactory bulb neural networks, we developed a novel slice-based biosensor for synchronous measurement with multi-sites. In the present study, the horizontal olfactory bulb slices with legible layered structures were prepared as the sensing element to construct a tissue-based biosensor with the microelectrode array. This olfactory bulb slice-based biosensor was used to simultaneously record the extracellular potentials from multi-positions. Spike detection and cross-correlation analysis were applied to evaluate the electrophysiological activities. The spontaneous potentials as well as the induced responses by glutamic acid took on different electrophysiological characteristics and firing patterns at the different sites of the olfactory bulb slice. This slice-based biosensor can realize multi-site synchronous monitoring and is advantageous for searching after the firing patterns and synaptic connections in the olfactory bulb neural networks. It is also helpful for further probing into olfactory information encoding of the olfactory neural networks.

Neurophysiological and computational principles of cortical rhythms in cognition

Synchronous rhythms represent a core mechanism for sculpting temporal coordination of neural activity in the brain-wide network. This review focuses on oscillations in the cerebral cortex that occur during cognition, in alert behaving conditions. Over the last two decades, experimental and modeling work has made great strides in elucidating the detailed cellular and circuit basis of these rhythms, particularly gamma and theta rhythms. The underlying physiological mechanisms are diverse (ranging from resonance and pacemaker properties of single cells to multiple scenarios for population synchronization and wave propagation), but also exhibit unifying principles. A major conceptual advance was the realization that synaptic inhibition plays a fundamental role in rhythmogenesis, either in an interneuronal network or in a reciprocal excitatory-inhibitory loop. Computational functions of synchronous oscillations in cognition are still a matter of debate among systems neuroscientists, in part because the notion of regular oscillation seems to contradict the common observation that spiking discharges of individual neurons in the cortex are highly stochastic and far from being clocklike. However, recent findings have led to a framework that goes beyond the conventional theory of coupled oscillators and reconciles the apparent dichotomy between irregular single neuron activity and field potential oscillations. From this perspective, a plethora of studies will be reviewed on the involvement of long-distance neuronal coherence in cognitive functions such as multisensory integration, working memory, and selective attention. Finally, implications of abnormal neural synchronization are discussed as they relate to mental disorders like schizophrenia and autism.

Amplification of asynchronous inhibition-mediated synchronization by feedback in recurrent networks

Synchronization of 30–80 Hz oscillatory activity of the principle neurons in the olfactory bulb (mitral cells) is believed to be important for odor discrimination. Previous theoretical studies of these fast rhythms in other brain areas have proposed that principle neuron synchrony can be mediated by short-latency, rapidly decaying inhibition. This phasic inhibition provides a narrow time window for the principle neurons to fire, thus promoting synchrony. However, in the olfactory bulb, the inhibitory granule cells produce long lasting, small amplitude, asynchronous and aperiodic inhibitory input and thus the narrow time window that is required to synchronize spiking does not exist. Instead, it has been suggested that correlated output of the granule cells could serve to synchronize uncoupled mitral cells through a mechanism called “stochastic synchronization”, wherein the synchronization arises through correlation of inputs to two neural oscillators. Almost all work on synchrony due to correlations presumes that the correlation is imposed and fixed. Building on theory and experiments that we and others have developed, we show that increased synchrony in the mitral cells could produce an increase in granule cell activity for those granule cells that share a synchronous group of mitral cells. Common granule cell input increases the input correlation to the mitral cells and hence their synchrony by providing a positive feedback loop in correlation. Thus we demonstrate the emergence and temporal evolution of input correlation in recurrent networks with feedback. We explore several theoretical models of this idea, ranging from spiking models to an analytically tractable model.

Neurons in many parts of the brain fire spikes rhythmically and synchronously in many behaviorally and functionally relevant contexts. There are many mechanisms for producing oscillatory synchronization between populations of biological oscillators. One way to produce synchrony is that the population of oscillators receives common correlated input. In this paper, we study a population of oscillating neurons (mitral cells) that are not directly coupled to each other but receive broadband correlated input from a second population of neurons (granule cells). The granule cell population, in turn, receives inputs from the mitral cells; hence, the mitral and granule cells are reciprocally connected. Correlated input to the oscillating mitral cells produces tighter synchrony in the activity of the mitral cell population. We hypothesize that this increased mitral cell synchrony will evoke greater activity in specific groups of granule cells and that these specific granule cells, in turn, become the source of the correlated input to the mitral cells. That is, the synchronous input from the mitral cells increases the fraction of correlated feedback. Thus, we close the correlation loop. We show through analysis and simulations that this feedback mechanism can lead to the spontaneous appearance of highly synchronous activity within the mitral cells. We show that there is good experimental support for this mechanism in the circuitry of the olfactory bulb. We speculate that such mechanisms could also arise in other parts of the brain.

Biophysical model for gamma rhythms in the olfactory bulb via subthreshold oscillations

Gamma oscillations in the olfactory bulb can be produced as an interaction of subthreshold oscillations (STOs) in the mitral cells (MCs) with inhibitory granule cells (GCs). The mechanism does not require that the GCs spike, and we work in a regime in which the MCs fire at rates lower than the fast gamma rhythm they create. The frequency of the network is that of the STOs, allowing the gamma to be modulated in amplitude with only small changes in frequency. Gamma oscillations could also be obtained with spiking GCs, but only for GCs firing close to population rate. Our mechanism differs from the more standard description of the gamma oscillation, in which the the decay time of the inhibitory cells is critical to the frequency of the network.

Specific entrainment of mitral cells during gamma oscillation in the rat olfactory bulb

Local field potential (LFP) oscillations are often accompanied by synchronization of activity within a widespread cerebral area. Thus, the LFP and neuronal coherence appear to be the result of a common mechanism that underlies neuronal assembly formation. We used the olfactory bulb as a model to investigate: (1) the extent to which unitary dynamics and LFP oscillations can be correlated and (2) the precision with which a model of the hypothesized underlying mechanisms can accurately explain the experimental data. For this purpose, we analyzed simultaneous recordings of mitral cell (MC) activity and LFPs in anesthetized and freely breathing rats in response to odorant stimulation. Spike trains were found to be phase-locked to the gamma oscillation at specific firing rates and to form odor-specific temporal patterns. The use of a conductance-based MC model driven by an approximately balanced excitatory-inhibitory input conductance and a relatively small inhibitory conductance that oscillated at the gamma frequency allowed us to provide one explanation of the experimental data via a mode-locking mechanism. This work sheds light on the way network and intrinsic MC properties participate in the locking of MCs to the gamma oscillation in a realistic physiological context and may result in a particular time-locked assembly. Finally, we discuss how a self-synchronization process with such entrainment properties can explain, under experimental conditions: (1) why the gamma bursts emerge transiently with a maximal amplitude position relative to the stimulus time course; (2) why the oscillations are prominent at a specific gamma frequency; and (3) why the oscillation amplitude depends on specific stimulus properties. We also discuss information processing and functional consequences derived from this mechanism.

Olfactory function relies on a chain of neural relays that extends from the periphery to the central nervous system and implies neural activity with various timescales. A central question in neuroscience is how information is encoded by the neural activity. In the mammalian olfactory bulb, local neural activity oscillations in the 40–80 Hz range (gamma) may influence the timing of individual neuron activities such that olfactory information may be encoded in this way. In this study, we first characterize in vivo the detailed activity of individual neurons relative to the oscillation and find that, depending on their state, neurons can exhibit periodic activity patterns. We also find, at least qualitatively, a relation between this activity and a particular odor. This is reminiscent of general physical phenomena—the entrainment by an oscillation—and to verify this hypothesis, in a second phase, we build a biologically realistic model mimicking these in vivo conditions. Our model confirms quantitatively this hypothesis and reveals that entrainment is maximal in the gamma range. Taken together, our results suggest that the neuronal activity may be specifically formatted in time during the gamma oscillation in such a way that it could, at this stage, encode the odor.

Bulbar acetylcholine enhances neural and perceptual odor discrimination

Experimental and modeling data suggest that the circuitry of the main olfactory bulb (OB) plays a critical role in olfactory discrimination. Processing of such information arises from the interaction between OB output neurons local interneurons, as well as interactions between the OB network and centrifugal inputs. Cholinergic input to the OB in particular has been hypothesized to regulate mitral cell odorants receptive fields (ORFs) and behavioral discrimination of similar odorants. We recorded from individual mitral cells in the OB in anesthetized rats to determine the degree of overlap in ORFs of individual mitral cells after exposure to odorant stimuli. Increasing the efficacy of the cholinergic neurotransmission in the OB by addition of the anticholinesterase drug neostigmine (20 mM) sharpened the ORF responses of mitral cells. Furthermore, coaddition of either the nicotinic antagonist methyllycaconitine citrate hydrate (MLA) (20 mM) or muscarinic antagonist scopolamine (40 mM) together with neostigmine (20 mM) attenuated the neostigmine-dependent sharpening of ORFs. These electrophysiological findings are predictive of accompanying behavioral experiments in which cholinergic modulation was manipulated by direct infusion of neostigmine, MLA, and scopolamine into the OB during olfactory behavioral tasks. Increasing the efficacy of cholinergic action in the OB increased perceptual discrimination of odorants in these experiments, whereas blockade of nicotinic or muscarinic receptors decreased perceptual discrimination. These experiments show that behavioral discrimination is modulated in a manner predicted by the changes in mitral cell ORFs by cholinergic drugs. These results together present a first direct comparison between neural and perceptual effects of a bulbar neuromodulator.

Expression of Zif268 in the granule cell layer of the adult mouse olfactory bulb is modulated by experience

Behavioral and physiological evidence indicates that odor processing in the main olfactory bulb is influenced by olfactory experience. At the cellular level, changes in inhibitory influence exerted by granular interneurons may contribute to restructuring odor representations. To assess experience-dependent modulation in the responsiveness of granule cells, we measured the level and spatial distribution of odor-induced expression of the immediate-early gene Zif268 in the granule cell layer of adult mice submitted or not to olfactory discrimination conditioning. We first show that stimulation by the reinforced odorant in conditioned animals did not induce any increase in Zif268 expression in contrast to stimulation with an unfamiliar odorant which induced an odor-specific three-fold increase in Zif268 expression. The same lack of Zif268 induction was observed in animals exposed to odorants without learning, indicating that familiarity to the odorant with or without conditioning similarly reduced responsiveness of granule cells to odorant stimulation. Second, conditioning induced a spatial reorganization of Zif268-positive cells leading to higher contrast and significant enlargement of their distribution pattern. The latter effect was also present in animals exposed to the odorants without conditioning but was significantly weaker. Taken together, these data indicate that distinct populations of granule cells are solicited by odorant processing, depending on its familiarity or behavioral significance. Finally, we report that the expression pattern of Zif268 in the granule cell layer is constrained by anteroposterior and dorsolateral gradients in cell density, pointing to anatomical and possibly functional disparity within the layer.

Olfactory oscillations: the what, how and what for

Olfactory system oscillations play out with beautiful temporal and behavioral regularity on the oscilloscope and seem to scream 'meaning'. Always there is the fear that, although attractive, these symbols of dynamic regularity might be just seductive epiphenomena. There are now many studies that have isolated some of the neural mechanisms involved in these oscillations, and recent work argues that they are functional and even necessary at the physiological and cognitive levels. However, much remains to be done for a full understanding of their functions.

Interaction of cellular and network mechanisms in spatiotemporal pattern formation in neuronal networks

Spatiotemporal patterning of neuronal activity is considered to be an important feature of cognitive processing in the brain as well as pathological brain states, such as seizures. Here, we investigate complex interactions between intrinsic properties of neurons and network structure in the generation of network spatiotemporal patterning in the context of seizure-like synchrony. We show that membrane excitability properties have differential effects on network activity patterning for different network topologies. We consider excitatory networks consisting of neurons with excitability properties varying between type I and type II that exhibit significantly different spike frequency responses to external current stimulation, especially at firing threshold. We find that networks with type II-like neurons show higher synchronization and bursting capacity across a range of network topologies than corresponding networks with type I-like neurons. These differences in activity patterning are persistent across different network sizes, connectivity strengths, magnitudes of random external input, and the addition of inhibitory interneurons to the network, making them highly likely to be relevant to brain function. Furthermore, we show that heterogeneous networks of mixed cell types show emergent dynamical patterns even for very low mixing ratios. Specifically, the addition of a small percentage of type II-like cells into a network of type I-like cells can markedly change the patterning of network activity. These findings suggest that cellular as well as network mechanisms can go hand in hand, leading to the generation of seizure-like discharges, suggesting that a single ictogenic mechanism alone may not be responsible for seizure generation.

Gamma Oscillations in a Nonlinear Regime: A Minimal Model Approach Using Heterogeneous Integrate-and-Fire Networks

Fast oscillations and in particular gamma-band oscillation (20–80 Hz) are commonly observed during brain function and are at the center of several neural processing theories. In many cases, mathematical analysis of fast oscillations in neural networks has been focused on the transition between irregular and oscillatory firing viewed as an instability of the asynchronous activity. But in fact, brain slice experiments as well as detailed simulations of biological neural networks have produced a large corpus of results concerning the properties of fully developed oscillations that are far from this transition point. We propose here a mathematical approach to deal with nonlinear oscillations in a network of heterogeneous or noisy integrate-and-fire neurons connected by strong inhibition. This approach involves limited mathematical complexity and gives a good sense of the oscillation mechanism, making it an interesting tool to understand fast rhythmic activity in simulated or biological neural networks. A surprising result of our approach is that under some conditions, a change of the strength of inhibition only weakly influences the period of the oscillation. This is in contrast to standard theoretical and experimental models of interneuron network gamma oscillations (ING), where frequency tightly depends on inhibition strength, but it is similar to observations made in some in vitro preparations in the hippocampus and the olfactory bulb and in some detailed network models. This result is explained by the phenomenon of suppression that is known to occur in strongly coupled oscillating inhibitory networks but had not yet been related to the behavior of oscillation frequency.

Multiple functions of GABA A and GABA B receptors during pattern processing in the zebrafish olfactory bulb

gamma-Aminobutyric acid (GABA)ergic synapses are thought to play pivotal roles in the processing of activity patterns in the olfactory bulb (OB), but their functions have been difficult to study during odor responses in the intact system. We pharmacologically manipulated GABA(A) and GABA(B) receptors in the OB of zebrafish and analysed the effects on odor responses of the output neurons, the mitral cells (MCs), by electrophysiological recordings and temporally deconvolved two-photon Ca2+ imaging. The blockade of GABA(B) receptors enhanced presynaptic Ca2+ influx into afferent axon terminals, and changed the amplitude and time course of a subset of MC responses, indicating that GABA(B) receptors have a modulatory influence on OB output activity. The blockade of GABA(A) receptors induced epileptiform firing, enhanced excitatory responses and abolished fast oscillations in the local field potential. Moreover, the topological reorganization and decorrelation of MC activity patterns during the initial phase of the response was perturbed. These results indicate that GABA(A) receptor-containing circuits participate in the balance of excitation and inhibition, the regulation of total OB output activity, the synchronization of odor-dependent neuronal ensembles, and the reorganization of odor-encoding activity patterns. GABA(A) and GABA(B) receptors are therefore differentially involved in multiple functions of neuronal circuits in the OB.

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.

Olfactory system gamma oscillations: the physiological dissection of a cognitive neural system

Oscillatory phenomena have been a focus of dynamical systems research since the time of the classical studies on the pendulum by Galileo. Fast cortical oscillations also have a long and storied history in neurophysiology, and olfactory oscillations have led the way with a depth of explanation not present in the literature of most other cortical systems. From the earliest studies of odor-evoked oscillations by Adrian, many reports have focused on mechanisms and functional associations of these oscillations, in particular for the so-called gamma oscillations. As a result, much information is now available regarding the biophysical mechanisms that underlie the oscillations in the mammalian olfactory system. Recent studies have expanded on these and addressed functionality directly in mammals and in the analogous insect system. Sub-bands within the rodent gamma oscillatory band associated with specific behavioral and cognitive states have also been identified. All this makes oscillatory neuronal networks a unique interdisciplinary platform from which to study neurocognitive and dynamical phenomena in intact, freely behaving animals. We present here a summary of what has been learned about the functional role and mechanisms of gamma oscillations in the olfactory system as a guide for similar studies in other cortical systems.

Sparsely synchronized neuronal oscillations

We discuss here the properties of fast global oscillations that emerge in networks of neurons firing irregularly at a low rate. We first provide a simple introduction to these sparsely synchronized oscillations, then show how they can be studied analytically in the simple setting of rate models and leaky integrate-and-fire neurons, and finally describe how various neurophysiological features can be incorporated in this framework. We end by a comparison of experimental data and theoretical results.

Lateral dendritic shunt inhibition can regularize mitral cell spike patterning

Mitral cells, the principal output neurons of the olfactory bulb, receive direct synaptic activation from primary sensory neurons. Shunting inhibitory inputs delivered by granule cell interneurons onto mitral cell lateral dendrites, while poorly positioned to prevent spike initiation, are believed to influence spike timing and underlie coordinated field potential oscillations. We investigated this phenomenon in a reduced compartmental mitral cell model suitable for incorporation into network simulations. Lateral dendritic shunt conductances delayed spiking to a degree dependent on both their electrotonic distance and phase of onset. Moreover, when the afferent activation of mitral cells was loosely coordinated in time, recurrent inhibition significantly narrowed the distribution of mitral cell spike times, illustrating a tendency towards coordinated synchronous activity. However, if mitral cell activity was initially disorganized, recurrent inhibition actually increased the variance in spike timing. This result suggests an essential role for early mechanisms of temporal coordination in olfaction, such as sniffing and the initial synchronization of mitral cell intrinsic oscillations by periglomerular cell-mediated inhibition.

Olfactory coding with all-or-nothing glomeruli

We present a model for olfactory coding based on spatial representation of glomerular responses. In this model distinct odorants activate specific subsets of glomeruli, dependent on the odorant's chemical identity and concentration. The glomerular response specificities are understood statistically, based on experimentally measured distributions of activation thresholds. A simple version of the model, in which glomerular responses are binary (the all-or-nothing model), allows us to account quantitatively for the following results of human/rodent olfactory psychophysics: 1) just noticeable differences in the perceived concentration of a single odor (Weber ratios) are as low as dC/C approximately 0.04; 2) the number of simultaneously perceived odors can be as high as 12; and 3) extensive lesions of the olfactory bulb do not lead to significant changes in detection or discrimination thresholds. We conclude that a combinatorial code based on a binary glomerular response is sufficient to account for several important features of the discrimination capacity of the mammalian olfactory system.

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.

An argument for an olfactory thalamus

The mammalian olfactory system is unique in that sensory receptors synapse directly into the olfactory bulb of the forebrain without the thalamic relay that is common to all other sensory pathways. We argue that the olfactory bulb has an equivalent role to the thalamus, because the two regions have very similar structures and functions. Both the thalamus and the olfactory bulb are the final stage in sensory processing before reaching target cortical regions, at which there is a massive increase in neuron and synapse numbers. Thus, both structures act as a bottleneck that is a target for various modulatory inputs, and this arrangement enables efficient control of information flow before cortical processing occurs.

Information processing in the olfactory systems of insects and vertebrates

Insects and vertebrates separately evolved remarkably similar mechanisms to process olfactory information. Odors are sampled by huge numbers of receptor neurons, which converge type-wise upon a much smaller number of principal neurons within glomeruli. There, odor information is transformed by inhibitory interneuron-mediated, cross-glomerular circuit interactions that impose slow temporal structures and fast oscillations onto the firing patterns of principal neurons. The transformations appear to improve signal-to-noise characteristics, define odor categories, achieve precise odor identification, extract invariant features, and begin the process of sparsening the neural representations of odors for efficient discrimination, memorization, and recognition.

Cholinergic modulation in the olfactory bulb influences spontaneous olfactory discrimination in adult rats

Cholinergic neuromodulation in the olfactory bulb has been hypothesized to regulate mitral cell molecular receptive ranges and the behavioral discrimination of similar odorants. We tested the effects of cholinergic modulation in the olfactory bulb of cannulated rats by bilaterally infusing cholinergic agents into the olfactory bulbs and measuring the rats' performances on separate spontaneous and motivated odor-discrimination tasks. Specifically, 6 microL/bulb infusions of vehicle (0.9% saline), the muscarinic antagonist scopolamine (7.6 mM and 38 mM), the nicotinic antagonist mecamylamine hydrochloride (3.8 mM and 19 mM), a combination of both antagonists, or the acetylcholinesterase inhibitor neostigmine (8.7 mM) were made 20 min prior to testing on an olfactory cross-habituation task or a rewarded, forced-choice odor-discrimination task. Spontaneous discrimination between chemically related odorants was abolished when nicotinic receptors were blocked in the olfactory bulb, and enhanced when the efficacy of cholinergic inputs was increased with neostigmine. Blocking muscarinic receptors reduced but did not abolish odor discrimination. Interestingly, no behavioral effects of modulating either nicotinic or muscarinic receptors were observed when rats were trained on a reward-motivated odor-discrimination task. Computational modeling of glomerular circuitry demonstrates that known nicotinic cholinergic effects on bulbar neurons suffice to explain these results.

Learning-induced oscillatory activities correlated to odour recognition: a network activity

In trained behaving rats, the expression of a prominent beta oscillatory activity in the olfactory system was previously identified as a correlate of odour recognition. The aim of the present study was to assess the putative role of a functional coupling between the olfactory bulb (OB) and higher structures in this activity. We performed a unilateral inactivation of the medial part of the olfactory peduncle by lidocaine infusion. Inactivation deprived the OB from most of its centrifugal afferences, including feedback connections from the piriform cortex (PC) while sparing the ascending fibres from the OB to higher cortical structures. This treatment reduced the amplitude of odour-induced oscillatory beta responses both in OB and PC. In parallel, gamma activity classically observed in these two structures during spontaneous activity displayed a strong enhancement. Results suggest that odour-induced oscillatory response could be the emergent feature of an olfactory functional network set up through learning.

Adjusting neurophysiological computations in the adult olfactory bulb

The olfactory bulb receives signals from olfactory sensory neurons and conveys them to higher centers. The mapping of the sensory inputs generates a reproducible spatial pattern in the glomerular layer of the olfactory bulb for each odorant. Then, this restricted activation is transformed into highly distributed patterns by lateral interactions between relay neurons and local interneurons. Thus, odor information processing requires the spatial patterning of both sensory inputs and synaptic interactions. In other words, odor representation is highly dynamic and temporally orchestrated. Here, we describe how the local inhibitory network shapes the global oscillations and the precise synchronization of relay neurons. We discuss how local inhibitory interneurons transpose the spatial dimension into temporal patterning. Remarkably, this transposition is not fixed but highly flexible to continuously optimize olfactory information processing.