Visually crowded targets are targets that can be recognized when presented in singular although they cannot be recognized when flanked by other stimuli (Pelli & Tillman,
2008). Thus, a crowded target is still visible but blends with its neighboring stimuli (Louie, Bressler, & Whitney,
2007). Despite the fact that crowding has been studied extensively in recent years, there is no full understanding of the process (Whitney & Levi,
2011). A current idea is that crowding represents a “filter” that is used for object perception, eye–hand movements, visual search, and reading (Levi,
2008). It is caused by the presence of internal representations as well as surrounding objects interfering with the identification of a target (Bulakowski, Post, & Whitney,
2009). The process of crowding depends on the center-to-center distance between the target and flankers (critical spacing; Toet & Levi,
1992) and the locus of the target relative to the fixation point (eccentricity). As a result, recognition of crowded targets, such as letters or words during reading, becomes more difficult (Whitney & Levi,
2011). It has been suggested that crowding can be explained in terms of the erroneous inclusion of features to be integrated within a spatial window (Pelli, Palomares, & Majaj,
2004). These spatial windows, also known as integration fields, are small in the fovea and relatively large in the periphery. In the periphery, the critical distance between target and flanker scales with eccentricity and is independent of target and flanker size. For crowding to occur, the critical spacing needs to be roughly half the eccentricity (Bouma,
1970). Thus, crowding seems most pronounced in peripheral vision (Pelli et al.,
2004). In central vision, giving the small sizes of the integration fields, crowding may be more difficult to detect. Little to no crowding was reported at the fovea for low-contrast targets (Simmers, Gray, McGraw, & Winn,
1999), and absence of foveal crowding was shown for opposite-contrast stimuli (Hess, Dakin, & Kapoor,
2000). Others have reported that foveal crowding occurs over very small distances but that sophisticated techniques are required to overcome the optical limitations (Siderov, Waugh, & Bedell,
2013; Coates, Levi, Touch, & Sabesan,
2018).