the principle of backprop can be accomplished in structural changes to the synapse. Although that would be analogous to changing the activation function in each

the principle of backprop can be accomplished in structural changes to the synapse. Although that would be analogous to changing the activation function in each 'neuron' of a layer in a convnet-we have just 'factored it out' into a sequence of weights used in the conv layers. Up regulating and down regulating the number of receptors can change how each synapse contributes to a biological net.