1. A computer-implemented method comprising: defining a dynamic region in a plurality of frames of a digital video stream, the dynamic region containing at least part of a visible feature in the plurality of frames and being moveable between a first frame and a second frame of the plurality of frames to coincide with movement of the at least part of the visible feature; analyzing a relationship between the dynamic region and an external data set that is collected from a subject viewing the visible feature, the analyzing comprising determining a correlation between the dynamic region and the external data set; and outputting a result of the analyzing, the result being indicative of at least one of interest level of the subject in the at least part of the visible feature and an attention level of the subject to the at least part of the visible feature.