What crowds in crowding?

In crowding, the perception of a target deteriorates in the presence of neighboring elements. Crowding is not a new research area but has fascinated researchers for centuries, dating back to 1738, as outlined in Strasburger and Wade (2015). The classic example of crowding is reading where the letters of a word mutually crowd each other. Thus, not surprisingly, crowding research has started off as reading research. In the meantime, crowding has become a primary tool to investigate vision. One main reason is that objects, as letters, are rarely met in isolation in normal life. Thus, crowding is the predominant situation for object recognition for humans and animals, and indeed, the characteristics of crowding are similar in humans and monkeys (Crowder & Olson, 2015). The interest in crowding and, accordingly, the number of publications on crowding have immensely grown, as evidenced by more than 25 publications in this issue and more than 100 in the past 10 years. The field has not only grown but also undergone significant paradigm shifts. For example, for half a century, a hallmark of crowding was Bouma’s law (Bouma, 1970; Pelli & Tillman, 2008). Bouma’s law proposes that flankers interfere with a target only when they are presented within a distance of about e/2 from the target (e denoting the eccentricity of target presentation). However, recent results have shown that elements presented outside Bouma’s window can increase (Chanceaux, Mathôt, & Grainger, 2014; Rosen, Chakravarthi, & Pelli, 2014; Vickery, Shim, Chakravarthi, Jiang, & Luedeman, 2009) or even decrease crowding strength (Manassi, Hermens, Francis, & Herzog, 2015; Sayim, Greenwood, & Cavanagh, 2014; for review, see Herzog, Sayim, Chicherov, & Manassi, 2015). Accordingly, modifications of Bouma’s law were proposed (Rosen et al., 2014) or Bouma’s law was even questioned (Herzog et al., 2015). Bouma’s law is about the spatial aspects of crowding, and crowding research has been mainly a branch of spatial vision research. Just recent studies have revealed complex and intriguing temporal characteristics of crowding (Yeshurun, Rashal, & Tkacz-Domb, 2015). For example, elements presented before a crowded target can decrease, rather than increase, crowding strength (Manassi et al., 2015; Sayim et al., 2014). Crowding can also be reduced by sharp onset transients of the target (Greenwood, Sayim, & Cavanagh, 2014), and, moreover, the spatial extent of crowding depends on stimulus duration (Tripathy, Cavanagh, & Bedell, 2014). In addition, reaction times correlate well with crowding strength (Hermens & Bell, 2014). It seems that crowding and masking share common spatial and temporal characteristics, at least under certain conditions (Lev & Polat, 2015). Taken together, these results strongly suggest that crowding is a spatiotemporal phenomenon. Next to temporal aspects, crowding has been linked to many other fields, beyond spatial vision,