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).