A motion detector (300) detects motion of images represented by video pixels. A first motion detector (320) is arranged to generate first motion signals (S) representing detected motion of images represented by the pixel values. A second motion detector (360) is arranged to modify the first motion signals to generate corrected motion signals (S) in response to the relationship of a pixel being processed to a change of motion of one or more of the image in the vertical direction. The second motion detector also modifies the first motion signals in response to the value of the pixel being processed.