These are the refrences:
1-Unsupervised Learning of Digit Recognition Using Spike-Timing-Dependent Plasticity, Peter U. Diehl, Matthew Cook
2-Unsupervised learning of visual features through spike timing dependent plasticity, Timothée Masquelier, Simon J Thorpe
3-STDP-based spiking deep neural networks for object recognition,Saeed Reza Kheradpisheh, Mohammad Ganjtabesh, Simon J Thorpe, Timothée Masquelier
4-Hardware Implementation of Convolutional STDP for On-line Visual Feature Learning, A. Yousefzadeh, T. Masquelier, T. Serrano-Gotarredona, and B. Linares-Barranco (this paper has not published yet and it is attached)
5-Video of Real-time demo in FPGA for unsupervised learning: https://youtu.be/VRz1uLXa4KA
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