In the real world, we commonly receive information simultaneously through two or more senses, with the brain fusing this data to produce a single-coherent message. Lip-reading is one example of this phenomenon. Laboratory studies, on the other hand often measure the response to a stimulus by a single sense and extrapolate these results to predict real-world behaviour. In this paper, we show that semantics have a significant impact on viewers' sensitivity to the quality of a video sequence for spatially separated parts of the sequence-and, more importantly, that this difference in sensitivity can be changed by the presence of an audio signal. This result is important for any testing of subjects' responses to visual material. One example is the subjective assessment of the quality of video-in an audio-visual communications system (such as television or video conferencing).