An apparatus for assessing the visibility of differences between two input image sequences. The apparatus comprises a pair of pre-filtered processors, a pair of pyramid constructors and a JND processor. Each pre-filter processor receives a separate input image sequence and performs temporal filtering to produce a low-pass temporal response and a bandpass temporal response. In turn, the pyramid constructors receive the temporal responses and generate contrast pyramids for the temporal responses. The JND processor evaluates the contrast pyramids to produce an image metric which is used to assess the visibility of differences between the two input image sequences.