File:KND + N.jpg

English: Crowding makes a target that is easily recognizable in isolation, unrecognizable in clutter. When looking directly at the ‘+’ the letter ‘N’ presented in the right is more easily recognized than the N in the clutter ‘KND’

The field of crowding has developed over recent years.This has helped to form an operational definition about crowding thus explaining what crowding is and how it is different from other similar effects such as masking, lateral interaction and surround suppression, other effects that make the target more challenging to see as well.

There are different criteria used to differentiate crowding from such similar effects. Firstly, crowding makes it difficult to identify an object but not detecting it among the clutter[3,4,5]. Crowded objects are collectively perceived to have high contrast, but they remain indistinct. The eccentricity of the target and the distance between the target and flankers influence crowding. As the distance between the target and the flankers' increases at a given eccentricity the ability to detect the target also improves as the eccentricity of a target is increased the more it pops out from the flankers and the more easily it is identified [6]. Crowding is anisotropic, which means it has different values when measured in different directions. Radially positioned flankers make it harder to identify the target than tangentially positioned ones.[7] crowding is stronger in the upper field of the four quadrants than the lower ones.[8].A recent study tells us that crowding is intense where the distractors and the target are in the same visual field than when they are in separate visual fields despite equal retinal distance[9]. Crowding is also asymmetrical meaning that a single flanker at an eccentric locus higher than the target makes it harder to identify the target than the single flanker at an eccentric locus closer to the fovea[6]. Crowding is not just a spatial phenomenon it happens over time as well, when a target is moving it is found to be more crowded when the flankers are leading than when they follow the target[11].