FIGURE 4.

Presynaptic firing frequency dependence of VDSP and STDP. Subfigure (A) shows the effect of scaling the presynaptic neuron input current on the VDSP update window for fixed weight w = 0.5 in a two neurons configuration. As the input current changes, the presynaptic neuron fires at various frequencies indicated by the line color. Higher presynaptic spiking frequencies result in smaller time windows. The plateau between Δt ∈ [0,2] ms is an artifact of the refractory period of 2 ms, where the membrane potential is kept at a reset value throughout. In (B), similar scaling is applied to the values of the pixels being fed to the presynaptic neurons during the MNIST classification task using the WTA architecture. Each point in (B) results from running the task 5 times with different random seeds using 10 output neurons, with standard deviation shown with the light-colored area under the curve. No adaptation mechanism was used for (B) to provide an unbiased comparison between classical STDP and VDSP in different spiking frequency regimes. No frequency-specific optimization was done during these experiments.