1. An apparatus for determining the response of cells to multiple levels of a stimulus from images of the cells, the apparatus comprising: an interface configured to receive the images of the cells that have been exposed to said multiple levels of a stimulus; a memory for storing, at least temporarily, some or all of the images; and one or more processors in communication with the memory and designed or configured to (i) obtain from said images a plurality of feature values, at least some of which characterize the phenotype of cells exposed to the particular level of the stimulus, to thereby produce a separate quantitative phenotype of the cells at each level of stimulus; and (ii) identify a path through the separate quantitative phenotypes of cells exposed to the stimulus.