Visual crowding interferes with identifying letters and numerals quickly and accurately due to their interaction with the surrounding symbols. Like most motor skills and perceptual phenomenon, crowding does not take place with the same spacing for everyone and is on a continuum. Some people who are bothered by crowding also experience aliasing in which black lines on white paper will cause hallucinations; colors may appear on the page or there may be apparent movement.
Crowding was first studied for people who have amblyopia but recent research demonstrates that it is also a factor for many people with reading difficulties. In most children, reading abilities and the ability to handle increasing density of print improve in concert. The effects of crowding become critical for some children as they progress through elementary school as the reading level increases but their visual skills do not develop commensurately. This problem is also common for people who have post-concussion syndrome.
An important distinction between normally progressing readers and disabled readers is the word length effect. When children start to read there is a direct correlation between the length of a word and how long it takes them to identify it. As normal readers progress, the word length effect decreases dramatically and they can process five-letter-words almost as fast as they can process two-letter-words. They are processing the letters in the words simultaneously. When crowding disrupts reading, the reader has difficulty keeping their place and they have to resort to seeing every letter in sequence. This is slower. It causes more errors. It detracts from comprehension. It is more fatiguing and visual distortions appear such as the apparent movement of the print. These complications may be in addition to other processing problems for disabled readers.
Research has demonstrated that most normal young readers also read better when the print is less crowded but this may not be noticed because their reading is adequate. It can be informative to look at the print which children choose to read. Fortunately, there are now a number of ways to help children who are struggling with crowding which are more effective than reading through a window or using a ruler or their finger to direct their eyes. It is now easy to enlarge print with most copiers. Print size can be changed easily on electronic readers which is often used to great advantage by older adults. Vision therapy is an effective treatment to develop the efficiency of eye movement skills and visual processing. With the proper intervention, many who are struggling to read could be enjoying reading instead of being frustrated.
The effects of visual crowding are not limited to reading. Math sheets with 100 problems on a page are an excellent example. Some children are completely overwhelmed while many others make more errors. The current early emphasis on word problems puts more print on the page and places the information horizontally. This organization is often more confusing for children whose visual systems are more easily confused by crowded displays. Subitizing, as opposed to visual crowding, is the ability to see the number of objects in a small array without counting them. Skill at subitizing correlates with early success in both reading and math.