1,006 research outputs found
Signal processing in local neuronal circuits based on activity-dependent noise and competition
We study the characteristics of weak signal detection by a recurrent neuronal
network with plastic synaptic coupling. It is shown that in the presence of an
asynchronous component in synaptic transmission, the network acquires
selectivity with respect to the frequency of weak periodic stimuli. For
non-periodic frequency-modulated stimuli, the response is quantified by the
mutual information between input (signal) and output (network's activity), and
is optimized by synaptic depression. Introducing correlations in signal
structure resulted in the decrease of input-output mutual information. Our
results suggest that in neural systems with plastic connectivity, information
is not merely carried passively by the signal; rather, the information content
of the signal itself might determine the mode of its processing by a local
neuronal circuit.Comment: 15 pages, 4 pages, in press for "Chaos
Effects of Noise in a Cortical Neural Model
Recently Segev et al. (Phys. Rev. E 64,2001, Phys.Rev.Let. 88, 2002) made
long-term observations of spontaneous activity of in-vitro cortical networks,
which differ from predictions of current models in many features. In this paper
we generalize the EI cortical model introduced in a previous paper (S.Scarpetta
et al. Neural Comput. 14, 2002), including intrinsic white noise and analyzing
effects of noise on the spontaneous activity of the nonlinear system, in order
to account for the experimental results of Segev et al.. Analytically we can
distinguish different regimes of activity, depending from the model parameters.
Using analytical results as a guide line, we perform simulations of the
nonlinear stochastic model in two different regimes, B and C. The Power
Spectrum Density (PSD) of the activity and the Inter-Event-Interval (IEI)
distributions are computed, and compared with experimental results. In regime B
the network shows stochastic resonance phenomena and noise induces aperiodic
collective synchronous oscillations that mimic experimental observations at 0.5
mM Ca concentration. In regime C the model shows spontaneous synchronous
periodic activity that mimic activity observed at 1 mM Ca concentration and the
PSD shows two peaks at the 1st and 2nd harmonics in agreement with experiments
at 1 mM Ca. Moreover (due to intrinsic noise and nonlinear activation function
effects) the PSD shows a broad band peak at low frequency. This feature,
observed experimentally, does not find explanation in the previous models.
Besides we identify parametric changes (namely increase of noise or decreasing
of excitatory connections) that reproduces the fading of periodicity found
experimentally at long times, and we identify a way to discriminate between
those two possible effects measuring experimentally the low frequency PSD.Comment: 25 pages, 10 figures, to appear in Phys. Rev.
Signal Propagation in Feedforward Neuronal Networks with Unreliable Synapses
In this paper, we systematically investigate both the synfire propagation and
firing rate propagation in feedforward neuronal network coupled in an
all-to-all fashion. In contrast to most earlier work, where only reliable
synaptic connections are considered, we mainly examine the effects of
unreliable synapses on both types of neural activity propagation in this work.
We first study networks composed of purely excitatory neurons. Our results show
that both the successful transmission probability and excitatory synaptic
strength largely influence the propagation of these two types of neural
activities, and better tuning of these synaptic parameters makes the considered
network support stable signal propagation. It is also found that noise has
significant but different impacts on these two types of propagation. The
additive Gaussian white noise has the tendency to reduce the precision of the
synfire activity, whereas noise with appropriate intensity can enhance the
performance of firing rate propagation. Further simulations indicate that the
propagation dynamics of the considered neuronal network is not simply
determined by the average amount of received neurotransmitter for each neuron
in a time instant, but also largely influenced by the stochastic effect of
neurotransmitter release. Second, we compare our results with those obtained in
corresponding feedforward neuronal networks connected with reliable synapses
but in a random coupling fashion. We confirm that some differences can be
observed in these two different feedforward neuronal network models. Finally,
we study the signal propagation in feedforward neuronal networks consisting of
both excitatory and inhibitory neurons, and demonstrate that inhibition also
plays an important role in signal propagation in the considered networks.Comment: 33pages, 16 figures; Journal of Computational Neuroscience
(published
Structure of Spontaneous UP and DOWN Transitions Self-Organizing in a Cortical Network Model
Synaptic plasticity is considered to play a crucial role in the experience-dependent self-organization of local cortical networks. In the absence of sensory stimuli, cerebral cortex exhibits spontaneous membrane potential transitions between an UP and a DOWN state. To reveal how cortical networks develop spontaneous activity, or conversely, how spontaneous activity structures cortical networks, we analyze the self-organization of a recurrent network model of excitatory and inhibitory neurons, which is realistic enough to replicate UP–DOWN states, with spike-timing-dependent plasticity (STDP). The individual neurons in the self-organized network exhibit a variety of temporal patterns in the two-state transitions. In addition, the model develops a feed-forward network-like structure that produces a diverse repertoire of precise sequences of the UP state. Our model shows that the self-organized activity well resembles the spontaneous activity of cortical networks if STDP is accompanied by the pruning of weak synapses. These results suggest that the two-state membrane potential transitions play an active role in structuring local cortical circuits
Modulation of intrinsic and synaptic excitability during sleep oscillations and electrographic seizures
Le présente mémoire fournit des nouvelles évidences montrant la modulation de l’excitabilité neuronale intrinsèque et synaptique, et la conséquence de cette modulation sur l’activité neuronale durant à la fois, les oscillations lentes du sommeil, et les crises électrographiques in vivo chez des animaux anesthésiés. Nous effectuons des enregistrements intracellulaires simultanés de neurones corticaux et des potentiels de champs locaux au niveau du gyrus suprasylvien à l’intérieur du cortex associatif pariétal (aires : 5, 7 et 21). Nous suggérons que la fluctuation de la concentration extracellulaire du calcium durant les oscillations lentes du sommeil module à la fois, l’excitabilité intrinsèque et synaptique des neurones corticaux, ainsi par conséquent, elle module affecte la relation d’input-output de ces neurones. L’apparition durant les oscillations lentes du sommeil, des crises de type Lennex-Gastaut qui sont générées corticalement, nous a permet d’étudier les propriétés spatio-temporelles des ondes paroxysmiques rapides associées avec ce type de crises. Nous suggérons que les ondes paroxysmiques rapides apparaissent comme des oscillations quasi-indépendantes même dans les localisations corticales voisines, suggérant leur origine focal.The present memoir provides new evidences showing the modulation of intrinsic and synaptic excitability of cortical neurons, and the consequence of this modulation on neuronal activity during both slow sleep oscillations and electrographic seizures in vivo in anaesthetized animals. We performed simultaneous recordings of cortical neurons with local field potentials in suprasylvian gyrus within parietal associative cortex (area 5, 7 and 21). We suggest that the fluctuation of extacellular calcium concentration during slow sleep oscillations, modulates both intrinsic and synaptic excitability cortical neurons, thus by consequence modulates the input-output relationship of these neurons. The occurrence during slow-wave sleep of cortically generated Lennox-Gastaut type of seizures admits us to study the spatio-temporal properties of paroxysmal fast runs associated with this type of seizures. We suggest that fast runs appeared as quasi-independent oscillations even in neighbouring cortical locations suggesting their focal origin
Multi-Stability and Pattern-Selection in Oscillatory Networks with Fast Inhibition and Electrical Synapses
A model or hybrid network consisting of oscillatory cells interconnected by inhibitory and electrical synapses may express different stable activity patterns without any change of network topology or parameters, and switching between the patterns can be induced by specific transient signals. However, little is known of properties of such signals. In the present study, we employ numerical simulations of neural networks of different size composed of relaxation oscillators, to investigate switching between in-phase (IP) and anti-phase (AP) activity patterns. We show that the time windows of susceptibility to switching between the patterns are similar in 2-, 4- and 6-cell fully-connected networks. Moreover, in a network (N = 4, 6) expressing a given AP pattern, a stimulus with a given profile consisting of depolarizing and hyperpolarizing signals sent to different subpopulations of cells can evoke switching to another AP pattern. Interestingly, the resulting pattern encodes the profile of the switching stimulus. These results can be extended to different network architectures. Indeed, relaxation oscillators are not only models of cellular pacemakers, bursting or spiking, but are also analogous to firing-rate models of neural activity. We show that rules of switching similar to those found for relaxation oscillators apply to oscillating circuits of excitatory cells interconnected by electrical synapses and cross-inhibition. Our results suggest that incoming information, arriving in a proper time window, may be stored in an oscillatory network in the form of a specific spatio-temporal activity pattern which is expressed until new pertinent information arrives
Dynamic Control of Network Level Information Processing through Cholinergic Modulation
Acetylcholine (ACh) release is a prominent neurochemical marker of arousal state
within the brain. Changes in ACh are associated with changes in neural activity and
information processing, though its exact role and the mechanisms through which it
acts are unknown. Here I show that the dynamic changes in ACh levels that are
associated with arousal state control informational processing functions of networks
through its effects on the degree of Spike-Frequency Adaptation (SFA), an activity
dependent decrease in excitability, synchronizability, and neuronal resonance displayed
by single cells. Using numerical modeling I develop mechanistic explanations
for how control of these properties shift network activity from a stable high frequency
spiking pattern to a traveling wave of activity. This transition mimics the change
in brain dynamics seen between high ACh states, such as waking and Rapid Eye
Movement (REM) sleep, and low ACh states such as Non-REM (NREM) sleep. A
corresponding, and related, transition in network level memory recall is also occurs
as ACh modulates neuronal SFA. When ACh is at its highest levels (waking) all
memories are stably recalled, as ACh is decreased (REM) in the model weakly encoded
memories destabilize while strong memories remain stable. In levels of ACh
that match Slow Wave Sleep (SWS), no encoded memories are stably recalled. This
results from a competition between SFA and excitatory input strength and provides
a mechanism for neural networks to control the representation of underlying synaptic
information. Finally I show that during the low ACh conditions, oscillatory conditions
allow for external inputs to be properly stored in and recalled from synaptic weights. Taken together this work demonstrates that dynamic neuromodulation is
critical for the regulation of information processing tasks in neural networks. These
results suggest that ACh is capable of switching networks between two distinct information
processing modes. Rate coding of information is facilitated during high
ACh conditions and phase coding of information is facilitated during low ACh conditions.
Finally I propose that ACh levels control whether a network is in one of
three functional states: (High ACh; Active waking) optimized for encoding of new
information or the stable representation of relevant memories, (Mid ACh; resting
state or REM) optimized for encoding connections between currently stored memories
or searching the catalog of stored memories, and (Low ACh; NREM) optimized
for renormalization of synaptic strength and memory consolidation. This work provides
a mechanistic insight into the role of dynamic changes in ACh levels for the
encoding, consolidation, and maintenance of memories within the brain.PHDNeuroscienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147503/1/roachjp_1.pd
Elucidating the Interplay of Structure, Dynamics, and Function in the Brain’s Neural Networks.
Brain’s structure, dynamics, and function are deeply intertwined. To understand how the brain functions, it is crucial to uncover the links between network structure and its dynamics. Here I examine different approaches to exploring the key connecting factors between network structure, dynamics and eventually its function. I predominantly concentrate on emergence and temporal evolution of synchronization, or coincidence of neuronal spike timings, as it has been associated with many brain functions while aberrant synchrony is implicated in many neurological disorders. Specifically, in chapter II, I investigate how the interplay of cellular properties with network coupling characteristics could affect the propensity of neural networks for synchronization. Then, in chapter III, I develop a set of measures that identify hallmarks and potentially predict autonomous network transitions from asynchronous to synchronous dynamics under various conditions. The developed metrics can be calculated in real time and therefore potentially applied in clinical situations. Finally, in chapter IV, I aim to tie the correlates of neural network dynamics to the brain function. More specifically, I elucidate dynamical underpinnings of learning and memory consolidation from in vivo recordings of mice experiencing contextual fear conditioning (CFC) and show, that the introduced notion of network stability may predict future animal performance on memory retrieval.
Overall, the results presented within this dissertation underscore the importance of concurrent analysis of networks’ dynamical and structural properties. The developed approaches may prove useful beyond the specific application presented within this thesis.PhDBiophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/120768/1/mofakham_1.pd
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