**4.5 Stigmergy assisted learning**

Stigmergy is a methodology where several independent agents produce an emergent behavior through indirect interaction among themselves. This is facilitated with the help of asynchronous communication through traces left in the environment by individual agents. Stigmergy has been observed in nature and widely researched upon especially in insect colonies, these principles have been applied towards solving various engineering problems. Recent advances in neuroscience have shown evidence of another type of cells called astrocytes working in tandem with neurons to regulate the behavior of the central nervous system [46]. Astrocytes are star shaped cells with several branches called as processes. The end of these branches called as end feet interface with a synapse by wrapping around it creating a region around the synaptic cleft called as microdomain as shown in **Figure 2**. Astrocytes also interface the neurons apart from the synapse providing a closed loop feedback mechanism. They also interface with other astrocytes like a synapse, instead this is called as a gap junction. Gap junction facilitates communication between astrocyte cells only through chemical means. Astrocytes are functionally very diverse and play a very important role, only a high-level concept with limited detail is introduced for understanding of relevant discussion. With the help of calcium ions as a signaling mechanism along with the help of neurotransmitters the astrocytes help regulate the efficiency of synaptic transmission. These cells play a critical role in maintaining homeostasis, modulating LTP, LTD and structural plasticity in the brain.

Spiking activity results in release of neurotransmitters and change in concentration among different ions in the microdomain and extra cellular space. These

#### **Figure 9.**

*Stigmergic interactions between astrocytes and neurons (modified from [47]).*

#### *Brain-Inspired Spiking Neural Networks DOI: http://dx.doi.org/10.5772/intechopen.93435*

A target neuron would generate a spike *zi*ðÞ¼ *t* 1 and non-target neurons would remain silent *zi*ðÞ¼ *t* 0. Based on the expected output spike train target neuron should fire within the short STDP window ½ � *t* � *ϵ*, *t* . Based on the presynaptic activity usually zero or one spike in the STDP window, the synaptic weights are increased proportionally. The presynaptic activity is the count of spikes in the

upon firing undergo weight depression in the same way. The difference between the desired spike pattern and output spike pattern is used as the guide for identifying target neurons and non-target neurons as the backpropagation rule. Same methodology is used for each layer while back propagating. Among the several learning methods inspired by ANN algorithms a few use strategies where the ANN is trained in its native form and tuned based on a shadow SNN and finally use those

Stigmergy is a methodology where several independent agents produce an emergent behavior through indirect interaction among themselves. This is facilitated with the help of asynchronous communication through traces left in the environment by individual agents. Stigmergy has been observed in nature and widely researched upon especially in insect colonies, these principles have been applied towards solving various engineering problems. Recent advances in neuroscience have shown evidence of another type of cells called astrocytes working in tandem with neurons to regulate the behavior of the central nervous system [46]. Astrocytes are star shaped cells with several branches called as processes. The end of these branches called as end feet interface with a synapse by wrapping around it creating a region around the synaptic cleft called as microdomain as shown in **Figure 2**. Astrocytes also interface the neurons apart from the synapse providing a closed loop feedback mechanism. They also interface with other astrocytes like a synapse, instead this is called as a gap junction. Gap junction facilitates communication between astrocyte cells only through chemical means. Astrocytes are functionally very diverse and play a very important role, only a high-level concept with limited detail is introduced for understanding of relevant discussion. With the help of calcium ions as a signaling mechanism along with the help of neurotransmitters the astrocytes help regulate the efficiency of synaptic transmission. These cells play a critical role in maintaining homeostasis, modulating LTP, LTD and structural

Spiking activity results in release of neurotransmitters and change in concentra-

tion among different ions in the microdomain and extra cellular space. These

*Stigmergic interactions between astrocytes and neurons (modified from [47]).*

<sup>0</sup> ð Þ. On the other hand, the non-target neurons

*t*0 <sup>¼</sup>*t*�*<sup>ϵ</sup>sh <sup>t</sup>*

½ � *<sup>t</sup>* � *<sup>ϵ</sup>*, *<sup>t</sup>* interval denoted as <sup>P</sup>*<sup>t</sup>*

*Biomimetics*

adapted weights on SNN for inference.

**4.5 Stigmergy assisted learning**

plasticity in the brain.

**Figure 9.**

**88**

changes are monitored as traces for indirect communication by astrocytes. Astrocytes themselves behaving like an environment with calcium ion concentration gradients within the cell acting as a medium for other neuron agents to indirectly infer these changes. This interaction creates a feedback mechanism in an asynchronous and distributed manner [47]. **Figure 9** shows the emergent stigmergy pattern in the brain. Short term activity and long-term activity gets communicated over a distance to other synapses over a spatial domain. Greater the distance, lower would be the influence. The details about the stigmergy based brain plasticity is presented in [47], interested readers are encouraged to explore further. This is a relatively new discovery and extensive research is underway to understand the role of astrocytes in overall brain mechanics.
