A method of forming a time-varying signal representative of at test variations in a value based on pixel values from a sequence of images, the signal corresponding in length to the sequence of images, includes obtaining the sequence of images. A plurality of groups of sub-sets of pixel values are formed by selecting a sub-set of at least one pixel value from each of at least two images defining an interval to form a group of associated sub-sets using at least one of a different aperture and a different interval length for groups defined on different intervals of the sequence. Groups of sub-sets are selected to form the signal to cover different intervals of the sequence of images by obtaining spatio-temporal volumes of pixel values from a sequence of images at least based on the received sequence of images. Each volume includes pixel values within a spatial aperture from each image within an interval of the sequence.