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.
In this paper, an amplitude and phase gradient-modulated surface is introduced to design a low cost and simple radar cross section (RCS) reducer metasurface. The simultaneous gradual amplitude and phase differences between adjacent unit cells achieve more degrees of freedom in the design approach, which leads to bandwidth enhancement of RCS reduction. A dual-layer stacked patch unit cell analyzed with a transmission line method is proposed to design the different required unit cells. The sinusoidal modulation applied on top and bottom layers of two stacked FR-4 substrates is used to realize the unit cells with gradual amplitude and phase variations. Finally, an ultra-wideband dual-layer stacked modulated surface composed of 26 × 26 unit cells is fabricated to demonstrate the idea. This surface achieves more than 10 dB RCS reduction from 9 GHz to 40.7 GHz (128%) for normal incident waves. Moreover, this surface has more than 118% and 88% RCS reduction bandwidths for transverse magnetic and transverse-electric obliquely polarized waves, respectively. Low profile, low cost, lightweight, and a simple assembling procedure are the main specifications of the proposed structure rather the state-of-the-art references, which candidate it as an ultra-wideband monostatic RCS reduction surface in practical applications.
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.
In this paper, the design and fabrication of modulated metasurfaces is proposed to achieve a wideband monostatic radar cross section (RCS) reduction. This structure is composed of holography and modulated surface tiles arranged in checkerboard configurations where the holography surface converts the incoming wave into the surface waves and the modulated surface redirects it into the other directions rather than the normal one. This surface reduces the RCS more than 10 dB from 11.57-26.1 GHz (77%) for normal incidence. Using only one low cost commercially available FR-4 substrate layer in this surface facilitates its applications rather than the state-of-the-art RCS reduction references.
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