Sarah Rosen

The Bouma law of crowding, revised: critical spacing is equal across parts, not objects.

Crowding is the inability to identify an object among flankers in the periphery. It is due to inappropriate incorporation of features from flanking objects in perception of the target. Crowding is characterized by measuring critical spacing, the minimum distance needed between a target and flankers to allow recognition. The existing Bouma law states that, at a given point and direction in the visual field, critical spacing, measured from the center of a target object to the center of a similar flanking object, is the same for all objects (Pelli & Tillman, 2008). Because flipping an object about its center preserves its center-to-center spacing to other objects, according to the Bouma law, crowding should be unaffected. However, because crowding is a result of feature combination, the location of features within an object might matter. In a series of experiments, we find that critical spacing is affected by the location of features within the flanker. For some flankers, a flip greatly reduces crowding even though it maintains target-flanker spacing and similarity. Our results suggest that the existing Bouma law applies to simple one-part objects, such as a single roman letter or a Gabor patch. Many objects consist of multiple parts; for example, a word is composed of multiple letters that crowd each other. To cope with such complex objects, we revise the Bouma law to say that critical spacing is equal across parts, rather than objects. This accounts for old and new findings.

Crowding by a repeating pattern

Theinability to recognize a peripheral target among flankers is called crowding. For a foveal target, crowding can be distinguished from overlap masking by its sparing of detection, linear scaling with eccentricity, and invariance with target size.Crowding depends on the proximity and similarity of the flankers to the target. Flankers that are far from or dissimilar to the target do not crowd it. On a gray page, text whose neighboring letters have different colors, alternately black and white, has enough dissimilarity that it might escape crowding. Since reading speed is normally limited by crowding, escape from crowding should allow faster reading. Yet reading speed is unchanged (Chung & Mansfield, 2009). Why? A recent vernier study found that using alternating-color flankers produces strong crowding (Manassi, Sayim, & Herzog, 2012). Might that effect occur with letters and reading? Critical spacing is the minimum center-to-center target-flanker spacing needed to correctly identify the target. We measure it for a target letter surrounded by several equidistant flanker letters of the same polarity, opposite polarity, or mixed polarity: alternately white and black. We find strong crowding in the alternating condition, even though each flanker letter is beyond its own critical spacing (as measured in a separate condition). Thus a periodic repeating pattern can produce crowding even when the individual elements do not. Further, in all conditions we find that, once a periodic pattern repeats (two cycles), further repetition does not affect critical spacing of the innermost flanker.

Supposing that crowding is compulsory grouping suggests a remarkably simple model for object recognition.

Grouping and crowding have each received much study, but, because they can be avoided, they have seemed marginal to the mystery of how people recognize objects. We begin by showing that the same image parameters that promote grouping also promote crowding. Joining grouping and crowding theories suggests a unified account in which grouping and crowding are two aspects of the classification underlying all object recognition. In this unified account, recognition of a simple object, like a letter, is mediated by a single combining field driving a simple classifier that can only recognize one thing at a time. It follows that the human observer has a vast array of combining fields, one for each size and retinal location, and is usually free to use whichever best matches the target. For simplicity, we suppose they are all similar, differing only in position and size. As eccentricity increases, the smallest available combining field size increases, presumably because fewer neurons are available. Crowding arises when the smallest available combining field is too large to isolate the target object from adjacent objects, as a simple classifier cannot recognize two things at once. To test the notion that the same classifiers underlie both crowding and grouping, we show that the efficiency of identification by a single isolated combining field in the periphery is practically the same as the efficiency of unrestricted central vision of the same target. We conclude that the same unit for object recognition underlies both (optional) grouping and crowding, which is compulsory grouping. Critical spacing marks the boundary between optional and compulsory. Meeting abstract presented at VSS 2015.

A new technique for measuring the critical spacing of crowding

S T U V W X Y Z Crowding is the inability to recognize a peripheral target when surrounded by ankers. Critical spacing is the minimal center-to-center distance needed between a target and anker in order to escape crowding. This is usually measured using a target identi cation task, which relies on the observer’s report. This method cannot be used with infants and other special populations. Here we develop a new technique which may be applicable to these populations. Change detection Target identi cation