Before revision:  To understand how such performance is achieved, we can probe a trained deep neural network by studying neuron activations, i.e., combinations of neuron firings, at any layer of the network in response to a particular input.
After revision:  To understand how such performance is achieved, we probe a trained deep neural network by studying neuron activations, i.e., combinations of neuron firings, at any layer of the network in response to a particular input.