Crowding refers to the visual phenomenon that occurs when the recognition of individual items (such as graphemes) by the fovea meet with interference from outlying visual information, (which are referred to as "flankers"). Graphemes are the smallest language unit that makes a sound.
An example of a single letter grapheme is 'cat'. The initial sound of the word is represented by the letter ‘c’. While not a major problem for most individuals, deficits in this skill can be a symptom of neuropsychiatric disorders (like schizophrenia and autism) or individuals who experience synesthesia. Research involving crowding experiments and individuals with synesthesia has shown that when other letters 'flank' the target grapheme it is difficult for the individual to identify the target grapheme.