A main goal of many studies of crowding or contour interaction is to quantify precisely the lateral extent and magnitude and to explain the basis of these effects. The extent of contour interaction does not scale with the target size if the location of the target remains unchanged (for contour interaction see, e.g., Danilova & Bondarko,
2007; Siderov, Waugh, & Bedell,
2013; Simunovic & Calver,
2004; for crowding, e.g., Pelli, Palomares, & Majaj,
2004, Tripathy & Cavanagh,
2002). In peripheral viewing, the contrast of the stimulus does not change the extent of crowding, provided the contrast of the target and distractors are matched (Kooi, Toet, Tripathy, & Levi,
1994; Rashal & Yeshurun,
2014). Similar observations were reported for foveal contour interaction (Siderov et al.,
2013). With an increasingly peripheral eccentricity of the target, the extent of contour interaction or crowding increases. Bouma (
1970) reported that the extent of crowding is roughly 0.5 times the target eccentricity (for a detailed review see Pelli & Tillman,
2008). Recent work indicates that the presence of flankers outside Bouma's “window” can markedly influence the effect of flankers within the window (e.g., Herzog & Manassi,
2015). However, when peripheral crowding and contour interaction are compared, contour interaction has a more limited extent (e.g., compare Levi, Hariharan, & Klein,
2002, and Wolford & Chambers,
1984, for contour interaction vs. Chung, Levi & Legge,
2001, Pelli et al.,
2004; and Tripathy & Cavanagh,
2002 for crowding). A direct comparison between peripheral crowding and contour interaction also shows a smaller magnitude of contour interaction (Marten-Ellis & Bedell,
2015).