13. In a digital medium environment supportive of image search, a system comprising: a neural network implemented at least partially using processing hardware, the neural network having a plurality of layers; an aggregation module implemented at least partially using processing hardware to cause the neural network to aggregate patterns of neurons by progressing through a sequence of layers, the patterns classifying an image as relating to a semantic class; a back propagation module implemented at least partially using processing hardware to cause the neural network to communicate relevancy of the patterns of the neurons as a positive and not negative relevancy to the semantic class by progressing backwards through the sequence of layers; a digital content generation module implemented at least partially using processing hardware to generate digital content that localizes the semantic class within the image based at least in part on the aggregated patterns of neurons and the relevancy of the patterns of the neurons to the semantic class; and an image search module implemented at least partially using processing hardware to perform a search using a plurality of items of said digital content to locate respective said images as corresponding to text of a search query.
13. In a digital medium environment supportive of image search, a system comprising: a neural network implemented at least partially using processing hardware, the neural network having a plurality of layers; an aggregation module implemented at least partially using processing hardware to cause the neural network to aggregate patterns of neurons by progressing through a sequence of layers, the patterns classifying an image as relating to a semantic class; a back propagation module implemented at least partially using processing hardware to cause the neural network to communicate relevancy of the patterns of the neurons as a positive and not negative relevancy to the semantic class by progressing backwards through the sequence of layers; a digital content generation module implemented at least partially using processing hardware to generate digital content that localizes the semantic class within the image based at least in part on the aggregated patterns of neurons and the relevancy of the patterns of the neurons to the semantic class; and an image search module implemented at least partially using processing hardware to perform a search using a plurality of items of said digital content to locate respective said images as corresponding to text of a search query. 14. The system as described in claim 13 , wherein the back propagation module is configured to communicate the relevancy from one said neuron in a first said layer through connections to a plurality of said neurons in a second said layer based on whether respective said connections between respective said neurons are: excitatory as pertaining to the aggregated pattern of the semantic class; or inhibitory as not pertaining to the aggregated pattern of the semantic class.