What is the denominator for contrast normalisation?

The perceived speed of 1 c/deg sinusoidal gratings of contrast 0.02 was measured in the presence of high contrast (0.50) 1 c/deg sinusoidal gratings (called modifiers). The modifiers drifted or were counterphase modulated at various temporal frequencies. The presence of a modifier with temporal frequencies (0 and 3 Hz) lower than the low contrast moving grating decreased its perceived speed while the presence of modifiers with higher temporal frequencies (8, 12 and 16 Hz) increased its perceived speed. A modifier of the same temporal frequency (6 Hz) as the standard grating had no effect upon the perceived speed of the low contrast gratings. Moving modifiers are more effective than counterphase flickering modifiers in biasing the perceived speed of low contrast gratings if they move in the same direction as the test grating and less effective if they move in the opposite direction. Finally, a modifier presented in an annulus surrounding the test grating is more effective than a modifier presented in a circular patch above or below the test grating in raising the perceived speed of low contrast gratings. This suggests that perceived speed depends on the ratio of low and high temporal frequency signals averaged over a significant area of the visual field.