In crowding, perception of a target deteriorates in the presence of neighboring elements. For example, it is easy to identify a letter when it is presented alone. However, identification strongly deteriorates when the same letter is flanked by other letters (Andriessen & Bouma,
1976; Levi,
2008; Pelli, Palomares, & Majaj,
2004; Pelli & Tillman,
2008). According to pooling models, crowding occurs when neurons from higher visual areas with larger receptive fields pool information from lower level neurons with smaller receptive fields, which leads to averaging of target and flanker signals (Balas, Nakano, & Rosenholtz,
2009; Dakin, Cass, Greenwood, & Bex,
2010; Greenwood, Bex, & Dakin,
2009,
2010; Parkes, Lund, Angelucci, Solomon, & Morgan,
2001; Pelli et al.,
2004; Põder,
2012; van den Berg, Roerdink, & Cornelissen,
2010; Wilkinson, Wilson, & Ellemberg,
1997). According to substitution models, crowding occurs because features of the target are confused with features of the flankers (Hanus & Vul,
2013; Huckauf & Heller,
2002; Strasburger,
2005; Strasburger, Harvey, & Rentschler,
1991). Both basic pooling and substitution models predict that when the number or size of flankers increases, crowding increases because more irrelevant information is pooled or more features are confused.