Visual crowding

What is crowding? Crowding is the curious phenomenon in which objects that can be recognized when viewed in isolation are rendered unrecognizable in clutter. Crowding is ubiquitous when ‘seeing sidelong’, as Lettvin (1976) described peripheral vision. Figure 1A provides a simple illustration: when looking directly at the plus, the isolated letter N on the right is easily read, while the same letter N in the word ‘AND’ is quite difficult. Interestingly, the viewing distance doesn’t matter much, as changing the viewing distance scales both the size (and spacing) of the letters and their eccentricity. Of course, it is well known that visual acuity is reduced in peripheral vision. The threshold size for an isolated letter in peripheral vision may be as small as ~1/50 of the eccentricity; however, for recognition to be unimpaired, the critical distance between neighboring letters may be as large as 1⁄2 of the eccentricity. Crowding is not restricted to letters — it occurs with a wide range of objects, including faces and shapes. More than 40 years ago, Herman Bouma reported that the distance over which crowding occurs (the critical distance) is proportional to the eccentricity of the target. This proportionality is sometimes referred to as Bouma’s Law. One implication of this ‘law’ is that the critical distance represents a fixed distance on the cortex because of the logarithmic conformal mapping of the visual world onto the retinotopic visual cortex. In this view, objects can only be recognized when they are sufficiently separated on retinotopically organized cortex. Crowding is probably not so simple, however, because the critical distance depends on the orientation of the target and flankers and their location in the visual field. It is larger when the target and flankers are arranged along an axis that is radial to the fixation target than one that is arranged orthogonally (Figure 1B), and, surprisingly, an outer (more peripheral) flanker is more potent than Quick guide an inner (less peripheral) flanker at the same distance from the target. Moreover, the critical distance can be modulated by attention, learning as well as details of the stimuli and tasks.