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Home>Department of Physics>
Department of Physics
Alireza Valizadeh  
Assistant Professor
Room: Physics 120
Tel: 4152120
Fax: 4152104
Personal Homepage

Research interests:
Delay induced synchronization in systems of non-identical coupled oscillators, correlation transfer in systems of non-identical coupled oscillators, synaptic plasticity and interacting effects of structure and dynamics in neuronal networks, effect of impurities on the nonlinear response of the regular networks, and the ambition: auditory system and neuroscience of language and music perception!

Research area:
Nonlinear phenomena in Condensed matter, Theoretical Neuroscience

1- Sadeghi , S., Valizadeh, A., "Effect of inhomogeneity on the synchronization of delayed coupled neurons", J. Comput. Neurosci, 1-12, (2013).
2- Bolhasani, E., Azizi, Y., Valizadeh, A., "Direct connections assist neurons to detect correlation in small amplitude noises ", Front. Comput. Neurosci, 7, 108-, (2013).

We address a question on the effect of common stochastic inputs on the correlation of the spike trains of two neurons when they are coupled through direct connections. We show that the change in the correlation of small amplitude stochastic inputs can be better detected when the neurons are connected by direct excitatory couplings. Depending on whether intrinsic firing rate of the neurons is identical or slightly different, symmetric or asymmetric connections can increase the sensitivity of the system to the input correlation by changing the mean slope of the correlation transfer function over a given range of input correlation. In either case, there is also an optimum value for synaptic strength which maximizes the sensitivity of the system to the changes in input correlation.
3- Bayati, M., Valizadeh, A., "Effect of synaptic plasticity on the structure and dynamics of disordered networks of coupled neurons", Phys. Rev. E, 86, 011925-1-011925-7, (2012).

In an all-to-all network of integrate-and-fire neurons in which there is a disorder in the intrinsic oscillatory frequencies of the neurons, we show that through spike-timing-dependent plasticity the synapses which have the high-frequency neurons as presynaptic tend to be potentiated while the links originated from the low-frequency neurons are weakened. The emergent effective flowof directed connections introduces the high-frequency neurons as the more influential elements in the network and facilitates synchronization by decreasing the synaptic cost for onset of synchronization.
4- Valizadeh, A., "Enhanced response of regular networks to local signals in the presence of a fast impurity", Phys. Rev. E, 86, 016101-1-016101-6, (2012).

We consider an array of inductively coupled Josephson junctions with a fast impurity (a junction with a smaller value of the critical current) and study the consequences of imposing a small amplitude periodic signal at some point in the array. We find that when the external signal is imposed at the impurity, the response of the array is boosted and a small amplitude signal can be detected throughout the array. When the signal is imposed elsewhere, minor effects are seen on the dynamics of the array. The same results have also been seen in the presence of a single fast-spiking neuron in a chain of diffusively coupled FitzHugh-Nagumo neurons.
5- Hashemi, M., Valizadeh, A., Azizi, Y., "Effect of the duration of the synaptic activity on spike rate of a Hodgkin-Huxley neuron with delayed feedback", Phys. Rev. E, 85, 021917-1-021917-10, (2012).

A recurrent loop consisting of a single Hodgkin-Huxley neuron influenced by a chemical excitatory delayed synaptic feedback is considered.We show that the behavior of the system depends on the duration of the activity of the synapse, which is determined by the activation and deactivation time constants of the synapse. For the fast synapses, those for which the effect of the synaptic activity is small compared to the period of firing, depending on the delay time, spiking with single and multiple interspike intervals is possible and the average firing rate can be smaller or larger than that of the open loop neuron. For slow synapses for which the synaptic time constants are of order of the period of the firing, the self-excitation increases the firing rate for all values of the delay time. We also show that for a chain consisting of few similar oscillators, if the synapses are chosen from different time constants, the system will follow the dynamics imposed by the fastest element, which is the oscillator that receives excitations via a slow synapse. The generalization of the results to other types of relaxation oscillators is discussed and the results are compared to those of the loops with inhibitory synapses as well as with gap junctions.
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