Two video signals, typically an original signal and a degraded version of the same signal, are analyzed firstly to identify perceptually relevant boundaries of elements forming video images depicted therein. These boundaries are then compared to determine the extent to which the properties of the boundaries defined in one image are preserved in the other, to generate an output indicative of the perceptual difference between the first and second signals. The boundaries may be defined by edges, color, luminance or texture contrasts, disparities between frames in a moving or stereoscopic image, or other means. The presence, absence, difference in clarity or difference in means of definition of the boundaries is indicative of the perceptual importance of the differences between the signals, and therefore of the extent to which any degradation of the signal will be perceived by the human viewer of the resulting degraded image. The results may also be weighted according to the perceptual importance of the image depicted--for example the features which identify a human face, and in particular those responsible for visual speech cues.