1. A computer-implemented method of testing a neural network, comprising a set of processing nodes interconnected by a plurality of interconnecting links in a sequence of layers from an input layer to an output layer, each processing node receiving an input signal over each interconnecting link connected thereto and generating a nodal output signal in response thereto and further in response to a respective connection weight value, nodal output signals from processing nodes in the output layer comprising a neural network output, in relation to a reference represented by a reference image, the method comprising the steps of: stimulating at least one processing node of the input layer of the neural network with an input vector comprising at least one input signal to obtain output signals from processing nodes at the output layer of the neural network; obtaining a plurality of samples of the output signals, wherein at least one of the plurality of samples is delayed in time from another one of the samples; generating an image from the plurality of samples; and comparing the image to the reference image to determine an operational characteristic of the neural network.