Ergin Yilmaz

Autapse-induced multiple coherence resonance in single neurons and neuronal networks

We study the effects of electrical and chemical autapse on the temporal coherence or firing regularity of single stochastic Hodgkin-Huxley neurons and scale-free neuronal networks. Also, we study the effects of chemical autapse on the occurrence of spatial synchronization in scale-free neuronal networks. Irrespective of the type of autapse, we observe autaptic time delay induced multiple coherence resonance for appropriately tuned autaptic conductance levels in single neurons. More precisely, we show that in the presence of an electrical autapse, there is an optimal intensity of channel noise inducing the multiple coherence resonance, whereas in the presence of chemical autapse the occurrence of multiple coherence resonance is less sensitive to the channel noise intensity. At the network level, we find autaptic time delay induced multiple coherence resonance and synchronization transitions, occurring at approximately the same delay lengths. We show that these two phenomena can arise only at a specific range of the coupling strength, and that they can be observed independently of the average degree of the network.

Comparisons of various pulse shapes for DS-UWB signals over the UWB channel

In this paper, we first investigate the power spectral densities (PSD) of bipolar modulated Direct Sequence Ultra Wide Band (DS-UWB) signals using various pulse shapes under the FCC UWB emission mask. Considered pulse shapes are the first five derivatives of the Gauss pulse (p1, p2, p3, p4 and p5), the first four orthogonal modified Hermite waveforms, and Daubechies wavelets (db-q). It is observed in the PSD results that p4 and p5 Gauss pulses, the Daubechies (db-q) for q>4 comply with the FCC UWB rule by selecting proper values for the pulse duration. Then, we derive the pulse shape dependent probability of error expression for bipolar DS-UWB signals over the standard UWB channel. The five pulse shapes (p4, p5, db-5, db-6 and db-7) complying with the FCC emission mask are numerically compared by using the derived probability of error expression over the CM1 model of the Standard UWB channel. Results reveal that the Daubechies have better performance than those of the two Gauss pulses.

Performances of DS-UWB signals over the CM1 channel model

In this paper, we investigate the probability of error performance of Direct Sequence Ultra Wide Band (DS-UWB) signals using various pulse shapes over the CM1 model of the Standard UWB channel. The considered pulse shapes are the first and second derivatives of Gauss waveform, Rayleigh waveform, and first four orthogonal modified Hermite waveforms. Results reveals that the probability of errors for all available pulse shapes over the considered channel are very similar for few users. However, by increasing the number of users, the performance differences start to appear in favor of Hermite pulses.

Impact of hybrid autapse on firing regularity in a Hodgkin-Huxley neuron

In this paper, the effects of autapse (a kind of synapse formed between the axon or soma of a neuron and its own dendrites) on the firing regularity and the firing rate in single Hodgkin-Huxley neuron are investigated. We consider that Hodgkin-Huxley neuron has two autapses. It is supposed that one of them is electrical synapse and the other is chemical synapse. Obtained results indicate that autapse with proper parameter values (delay time and autaptic conductance) prominently increases the firing regularity and the firing rate of a Hodgkin-Huxley neuron.

Blocking of weak signal propagation via autaptic transmission in scale-free neuronal networks

In this paper, the effects of autapse (a kind of synapse formed between the axon or soma of a neuron and its own dendrite) on the transmission of weak signal are investigated in scale-free neuronal networks. In the study, we consider that each neuron in the network has an autapse modelled as chemical synapse. Besides, a subthreshold signal are injected to all neurons in the network. When obtained results are analyzed, it is seen that the autaptic conductance has a threshold value. According to this threshold, the effects of autapse on the transmission of weak signal are evaluated within two categories. When conductance of autapse is less than this threshold value, autapses with proper autaptic time delay values slightly increase the transmission of weak signal. When autaptic conductance is bigger than the threshold value, autapses with proper autaptic time deley values prominently block the transmission of weak signal.

Vibrational resonance in a Hodgkin-Huxley neuron

In this paper, the effects of high-frequency driving on the weak signal detection capacity of Hodgkin-Huxley neuron are investigated. It is seen that the response of Hodgkin-Huxley neuron shows vibrational resonance with respect to the amplitude of high-frequency driving, and the best detection is obtained at an optimal amplitude of high-frequency driving. Besides, the effects of the frequency of high-frequency driving on the vibrational resonance are investigated. We arrived at the result that resonant response of the neuron can also be obtained for different frequencies of high-frequency signal but larger amplitudes are required for greater frequency of high frequency signal.

Effects of autapse on the transmission of localized rhythmic activity in small-world neuronal networks

In this paper, the effects of autapse (a kind of synapse formed between the axon or soma of a neuron and its own dendrites) on the transmission of weak localized rhythmic activity are investigated depending on ion channel noise in a Newman-Watts small-word networks of stochastic Hodgkin-huxley neurons. In this study, we consider that only a neuron, called pacemaker, to which a weak periodic stimuli is injected has an autapse. Obtained results indicate that autapse with proper parameter values (delay time and coupling strength) prominently increases the transmission of weak localized rhythmic activity across the network as compared to the case in which pacemaker neuron does not have the autapse. Besides, we find that autapse can significantly enhances the transmission of weak local activity for different network topologies having different p values.

Impact of time-periodic coupling strength on the firing regularity of a scale-free network

In this paper, the effects of time-periodic coupling on the firing regularity of a scale-free network (SF), consisting of stochastic Hodgkin-Huxley neurons, have been investigated depending on ion channel noise. The effects of both the frequency and the amplitude of periodic coupling on the firing regularity have been tackled, separately. It is seen from the obtained results that the firing (spiking) regularity shows resonance like behavior depending on ion channel noise when the frequency of the periodic coupling equals integer multiple of the sub threshold oscillation frequency of H-H neurons. Additionally, it is determined that this resonance is maximal at an optimal value of the amplitude of the periodic coupling strength.

Subthreshold stimulus encoding on a stochastic scale-free neuronal network

Random networks with complex topology arise in many different fields of science. Recently, it has been shown that existing network models fail to incorporate two common features of real networks in nature: First, real networks are open and continuously grow by addition of new elements, and second, a new element connects preferentially to an element that already has a large number of connections. Therefore, a new network model, called a scale-free (SF) network, has been proposed based on these two features. In this study, we study the subthreshold periodic stimulus encoding on a stochastic SF neuronal network based on the collective firing regularity. The network consists of identical Hodgkin-Huxley (HH) neurons. We show that the collective firing (spiking) regularity becomes maximal at a given stimulus frequency, corresponding to the frequency of the subthreshold oscillations of HH neurons. We also show that this best regularity can be obtained if the coupling strength and average degree of connectivity have their optimal values.

Performance of Direct Sequence Ultra Wide Band Signals over Log-normal Fading Channels

In this paper, the probabilities of error performances of bipolar Direct Sequence Ultra Wide Band (DS-UWB) signals are investigated over a log-normal fading channel. Since the bandwidth of UWB signals is in the level of giga Hertz, the number of the resolvable paths in the channel is large. Consequently, in the receiver, a Rake receiver in conjunction with a maximum ratio combiner is used. By using the derived probability of error expression, the numerical values of the performances are calculated for the first derivative of Gaussian, the second derivative of Gaussian, Rayleigh and Daubechies-5 (db-5) pulse signals. Results show that the db-5 pulse based UWB system gives the best performance.