The Vernier stimuli were chosen for several reasons. First, the Vernier stimuli are similar to letter strings in size, shape and in the spacing between neighboring items. Second, adding flankers to a target Vernier induces crowding. Namely, it increases the participant's threshold, that is, the offset distance needed for accurate judgments of the offset direction. Similarly, it has been proposed that reading is based on identifying symbols in a crowded scene, and therefore large spacing, which reduces crowding, may facilitate reading. Importantly, Vernier stimuli show crowding effects both when presented foveally (Malania, Herzog, & Westheimer,
2007; Sayim, Westheimer, & Herzog,
2008) and when presented peripherally (Manassi, Sayim, & Herzog,
2012,
2013). Crowding at the fovea is important in analogy to reading, since in reading, letters are identified at the fovea, where visual acuity is best. Adequate readers tend to make the initial fixation on each word about halfway between the beginning and the middle of a word, where word recognition is maximized (Nazir, Heller, & Sussmann,
1992). Therefore, foveal visual abilities are most relevant for natural reading (Rayner & Bertera,
1979). Though para-foveal and peripheral visual processing also affect reading rate (e.g., Deutsch, Frost, Pollatsek, & Rayner,
2005), their impact is secondary (Rayner,
1975). Hence, while there might be a correlation between paragraph reading rate and elevated peripheral crowding among people with dyslexia (Cassim et al.,
2014; Moores et al.,
2011), excessive peripheral crowding cannot explain the main deficit in single word reading for people with dyslexia. Third, Vernier judgments are impaired by brief presentations, allowing parametric evaluation of backward masking. Fourth, Vernier judgments are spatial and, hence, do not rely on any temporal comparison. Sequential comparisons rely on working memory mechanisms, which were previously shown to be impaired in dyslexia even for visual stimuli (Ben-Yehudah & Ahissar,
2004; Ben-Yehudah, Sackett, Malchi-Ginzberg, & Ahissar,
2001). Finally, both crowding and backward masking with Vernier stimuli were previously used in psychiatric populations (Chkonia et al.,
2010; Chkonia et al.,
2012; Roinishvili et al.,
2015), indicating that this task can be reliably assessed in very broad populations.