Rectified Linear Postsynaptic Potential Function for Backpropagation in Deep Spiking Neural Networks

Spiking Neural Networks (SNNs) use spatiotemporal spike patterns to represent and transmit information, which are not only biologically realistic but also suitable for ultralow-power event-driven neuromorphic implementation. Just like other deep learning techniques, Deep Spiking Neural Networks (DeepSNNs) benefit from the deep architecture. However, the training of DeepSNNs is not straightforward because the wellstudied error back-propagation (BP) algorithm is not directly applicable. In this paper, we first establish an understanding as to why error back-propagation does not work well in DeepSNNs.We then propose a simple yet efficient Rectified Linear Postsynaptic Potential function (ReL-PSP) for spiking neurons and a Spike-Timing-Dependent Back-Propagation (STDBP) learning algorithm for DeepSNNs where the timing of individual spikes is used to convey information (temporal coding), and learning (back-propagation) is performed based on spike timing in an event-driven manner. We show that DeepSNNs trained with the proposed single spike time-based learning algorithm can achieve state-of-the-art classification accuracy. Furthermore, by utilizing the trained model parameters obtained from the proposed STDBP learning algorithm, we demonstrate ultra-low power inference operations on a recently proposed neuromorphic inference accelerator. The experimental results also show that the neuromorphic hardware consumes 0.751 mW of the total power consumption and achieves a low latency of 47.71 ms to classify an image from the MNIST dataset. Overall, this work investigates the contribution of spike timing dynamics for information encoding, synaptic plasticity and decision making, providing a new perspective to the design of future DeepSNNs and neuromorphic hardware

Self-healing phenomena of graphene: potential and applications

The present study investigates the self healing behavior of both pristine and defected single layer graphene using a molecular dynamic simulation. Single layer graphene containing various defects such as preexisting vacancies and differently oriented pre-existing cracks were subjected to uniaxial tensile loading till fracture occurred. Once the load was relaxed, the graphene was found to undergo self healing. It was observed that this self healing behaviour of cracks holds irrespective of the nature of pre-existing defects in the graphene sheet. Cracks of any length were found to heal provided the critical crack opening distance lies within 0.3-0.5 nm for a pristine sheet and also for a sheet with pre-existing defects. Detailed bond length analysis of the graphene sheet was done to understand the mechanism of self healing of graphene. The paper also discusses the immense potential of the self healing phenomena of graphene in the field of graphene based sub-nano sensors for crack sensing

Self healing nature of bilayer graphene

The phenomenon of self healing of cracks in bilayer graphene sheet has been studied using molecular dynamics simulations. The bilayer graphene sheet was subjected to uniaxial tensile load resulting in initiation and propagation of cracks on exceeding the ultimate tensile strength. Subsequently, all forces acting on the sheet were removed and sheet was relaxed. The cracks formed in the graphene sheet healed without any external aid within 0.4 ps The phenomenon of self healing of the cracks in graphene sheet was found to be independent of the length of the crack, but occurred for critical crack opening distance less than 5 Å for AA stacked sheet and 13 Å for AB stacked bilayer graphene sheet. Self healing was observed for both AB (mixed stacking of armchair and zigzag graphene sheet) and AA (both sheets of similar orientation i.e. either armchair-armchair or zigzag-zigzag) stacking of bilayer graphene sheet

Graphene heals thy cracks

Molecular Dynamics simulations revealed the phenomena of self healing of cracks which were generated in graphene on application of tensile load exceeding its ultimate tensile strength. The phenomenon of self healing was observed when the system was studied at a very slow rate of 0.05 ps. Cracks initiated in the graphene sheet was allowed to propagate till it reached a critical length following which the load was removed and the sheet was relaxed. The study revealed that self healing of cracks took place within a critical crack opening displacement range of 0.3–0.5 nm in absence of any external stimulus. However, the self healing phenomenon was found to be independent of crack length. This self healing phenomenon occurred not only in pristine graphene sheet, but also in presence of pre-existing vacancies. The mechanism of self healing has been explained by detailed bond length/angle distribution analysis

Precise tuning of cationic cyclophanes toward highly selective fluorogenic recognition of specific biophosphate anions

Cationic cyclophanes with bridging and spacer groups possess well-organized semirigid cavities and are able to encapsulate and stabilize anionic species through diverse molecular interactions. We highlight the precise tuning of functionalized cyclophanes toward selective recognition of AMP, GTP, and pyrophosphate (PPi) using fluorescence, NMR spectroscopy, and density functional theory (DFT).close0

Investigating Superoxide Transfer through a mu-1,2-O-2 Bridge between Nonheme Ni-III-Peroxo and Mn-II Species by DFT Methods to Bridge Theoretical and Experimental Views

Previously, a fast unprecedented O-2(center dot-) transfer reaction has been observed experimentally when adding a Mn-II complex into a solution containing a Ni-III-peroxo complex. Due to the fast reaction rate, no intermediates were observed. We have investigated this reaction with density functional theory (DFT) and show that DFT is unusually problematic in reproducing the correct spin state for the investigated Ni-III-peroxo complex, something which calls for examination of all previous Ni-dioxygen studies. Surprisingly, the BP86 functional is shown to yield energies more in agreement with known experiments than B3LYP. The calculations reveal for the first time an intermediate structure in a complete O-2(center dot-) transfer reaction, shown here to be a short-lived bridging Ni-(mu-1,2-O-2)-Mn structure

Rational Design of Superoxide Dismutase (SOD) Mimics: The Evaluation of the Therapeutic Potential of New Cationic Mn Porphyrins with Linear and Cyclic Substituents

Our goal herein has been to gain further insight into the parameters which control porphyrin therapeutic potential. Mn porphyrins (MnTnOct-2-PyP⁵⁺, MnTnHexOE-2-PyP⁵⁺, MnTE-2-PyPhP⁵⁺, and MnTPhE-2-PyP⁵⁺) that bear the same positive charge and same number of carbon atoms at <i>meso</i> positions of porphyrin core were explored. The carbon atoms of their <i>meso</i> substituents are organized to form either linear or cyclic structures of vastly different redox properties, bulkiness, and lipophilicities. These Mn porphyrins were compared to frequently studied compounds, MnTE-2-PyP⁵⁺, MnTE-3-PyP⁵⁺, and MnTBAP^3–. All Mn­(III) porphyrins (MnPs) have metal-centered reduction potential, <i>E</i>_1/2 for Mn^IIIP/Mn^IIP redox couple, ranging from −194 to +340 mV versus NHE, log <i>k</i>_cat(O₂^•–) from 3.16 to 7.92, and log <i>k</i>_red(ONOO^–) from 5.02 to 7.53. The lipophilicity, expressed as partition between n-octanol and water, log <i>P</i>_OW, was in the range −1.67 to −7.67. The therapeutic potential of MnPs was assessed via: (i) <i>in vitro</i> ability to prevent spontaneous lipid peroxidation in rat brain homogenate as assessed by malondialdehyde levels; (ii) <i>in vivo</i> O₂^•– specific assay to measure the efficacy in protecting the aerobic growth of SOD-deficient <i>Saccharomyces cerevisiae</i>; and (iii) aqueous solution chemistry to measure the reactivity toward major <i>in vivo</i> endogenous antioxidant, ascorbate. Under the conditions of lipid peroxidation assay, the transport across the cellular membranes, and in turn shape and size of molecule, played no significant role. Those MnPs of <i>E</i>_1/2 ∼ +300 mV were the most efficacious, significantly inhibiting lipid peroxidation in 0.5–10 μM range. At up to 200 μM, MnTBAP^3– (<i>E</i>_1/2 = −194 mV vs NHE) failed to inhibit lipid peroxidation, while MnTE-2-PyPhP⁵⁺ with 129 mV more positive <i>E</i>_1/2 (−65 mV vs NHE) was fully efficacious at 50 μM. The <i>E</i>_1/2 of Mn^IIIP/Mn^IIP redox couple is proportional to the log <i>k</i>_cat(O₂^•–), <i>i.e</i>., the SOD-like activity of MnPs. It is further proportional to <i>k</i>_<i>r</i>ed(ONOO^–) and the ability of MnPs to prevent lipid peroxidation. In turn, the inhibition of lipid peroxidation by MnPs is also proportional to their SOD-like activity. In an <i>in vivo S. cerevisiae</i> assay, however, while <i>E</i>_1/2 predominates, lipophilicity significantly affects the efficacy of MnPs. MnPs of similar log <i>P</i>_OW and <i>E</i>_1/2, that have linear alkyl or alkoxyalkyl pyridyl substituents, distribute more easily within a cell and in turn provide higher protection to <i>S. cerevisiae</i> in comparison to MnP with bulky cyclic substituents. The bell-shape curve, with MnTE-2-PyP⁵⁺ exhibiting the highest ability to catalyze ascorbate oxidation, has been established and discussed. Our data support the notion that the SOD-like activity of MnPs parallels their therapeutic potential, though species other than O₂^•–, such as peroxynitrite, H₂O₂, lipid reactive species, and cellular reductants, may be involved in their mode(s) of action(s)