It is well known that contrast sensitivity has a two-part dependence on mean luminance (van Nes & Bouman,
1967). Initially, as luminance is reduced, contrast sensitivity remains invariant (the Weber region), after which it displays a square-root dependence (the Rose–de Vries region; van Nes & Bouman,
1967). The luminance at which the behavior switches from Weber to Rose–de Vries (termed the transition luminance) depends on the spatial frequency of the stimulus; the lower the spatial frequency, the lower the transition luminance and hence the more resistant the sensitivity is to changes in mean luminance. Thus, the effect of reducing mean luminance is strongly scale-dependent. This is also the case for temporally varying spatial stimuli (van Nes, Koenderink, Nas, & Bouman,
1967). This is an important issue because the use of spatial frequency broadband elements, such as dots (Billino et al.,
2008; McCollum et al.,
2000) and broadband noise (Lankheet et al.,
2000,
2002), in studies of global motion has been ubiquitous and may be one of the main reasons why previous studies have come to such conflicting conclusions of the effects of luminance on global motion sensitivity. For example, the results using broadband dots have suggested that scotopic motion sensitivity exhibits velocity (Billino et al.,
2008) as well as contrast dependence (McCollum et al.,
2000). The role that spatial scale plays in these conclusions needs to be determined. A more complete understanding of the effects of mean luminance on global motion sensitivity requires controlling the spatial scale of the motion stimuli so that their detectability can be taken into account at different light levels. Furthermore, the role of eccentricity also needs to be assessed since there is evidence that the fovea and periphery use different motion detection strategies at photopic and scotopic luminances (Takeuchi & De Valois,
2009). They found that as retinal illuminance decreases, the relative contribution of a feature-tracking mechanism in the central retina becomes larger, while motion perception in the peripheral retina continues to depend on a biphasic, first-order motion mechanism.