The uncrowded window of object recognition

Abstract

It is now emerging that vision is usually limited by object spacing rather than size. The visual system recognizes an object by detecting and then combining its features. 'Crowding' occurs when objects are too close together and features from several objects are combined into a jumbled percept. Here, we review the explosion of studies on crowding--in grating discrimination, letter and face recognition, visual search, selective attention, and reading--and find a universal principle, the Bouma law. The critical spacing required to prevent crowding is equal for all objects, although the effect is weaker between dissimilar objects. Furthermore, critical spacing at the cortex is independent of object position, and critical spacing at the visual field is proportional to object distance from fixation. The region where object spacing exceeds critical spacing is the 'uncrowded window'. Observers cannot recognize objects outside of this window and its size limits the speed of reading and search.

Figure 1

An A in chaff. The bars represent elementary visual features. Fixating close to the bars, at the green plus, makes it easy to recognize the letter A. If you fixate far away, on the red minus, you can still see the features, but you cannot identify the letter. Your visual system is combining over too large an area, including all the features from both the A and the surrounding chaff, which results in a jumbled percept. This is crowding. You can rule out acuity (letter size) as an explanation (for your inability to identify the A) by confirming that you can see the A while fixating the minus if your fingers hide the chaff (for a review, see ref. 17).

Figure 2

Effects of crowding. While fixating the red minus, can you tell that the clusters differ in letter identity, number and position? Crowding impairs your ability to judge these object properties,. Using your finger to cover all but the leftmost letter, you can confirm that even this most distant letter is well within your acuity (reprinted from ref. 21).

Figure 3

Crowding in a word. While fixating the red minus, it is easy to identify the isolated letter on the left, but try to identify the middle letter on the right. It is hard. Fixate the green plus and try again. Now it is easy.

Figure 4

Faces are like words. Arnold Schwarzenegger and Elvis Presley are famous, and their faces may be familiar. Fixate on the red minus between them. Can you still recognize the governor and the King? How close to each face do you have to fixate to identify it? As you fixate closer and closer to the face, you will find that you remain unable to recognize it until you are near the cheek. As with words, the parts (eyes, nose and mouth) of faces must be isolated (separated by the observer’s critical spacing) for the whole to be recognized.

Figure 5

Critical spacing is independent of object and size. Fixating on the red minus, you will be unable to identify the middle object in the first nine rows unless you isolate it by hiding the flanking objects with your fingers (or two pencils). In the last two rows, you will be unable to recognize the single object while fixating on the red minus. Grating patches, similar to those in the top two rows, are often taken to be one-feature objects. In the first row, is the middle grating vertical or tilted? The ± is our estimate of the fixation point where you can just barely identify the target. You can assess the accuracy of this threshold estimate by noting that the task is easy when you fixate to the right of the ± and hard when you fixate to the left. Critical spacing depends solely on position (and direction) in the visual field, which does not vary among rows in this demonstration. Note that halving object size has no effect on critical spacing. Critical spacing is independent of spatial frequency (see Supplementary Sources online).

Figure 6

Critical spacing is proportional to eccentricity. The observer fixated on the point indicated by a plus in the upper right and identified the orientation of a target T (right-side up or upside down?) presented (in blocks) at one of the nine locations indicated by the dots. Two flanking Ts were shown symmetrically displaced from the target in opposite directions, −45°, 0°, 45° or 90° relative to horizontal. Each vertex in the roughly elliptical contours represents the measured critical spacing of the pair of flanking letters for 75% correct identification of target orientation. Note that the critical spacing contours are not circles; the direction from target to flanker matters. These were measured with one letter size at each eccentricity. Changing letter size has no effect on the results (figure adapted from ref. 47).

Figure 7

What is your uncrowded span? Fixate on the o in the center of the word. Your uncrowded span is 3 if you can read ‘row’, 4 for ‘crow’, 5 for ‘crowd’ and a whopping 9 for ‘uncrowded’, which many observers achieve. The variation in the uncrowded span reflects the substantial individual differences in critical spacing reported previously. The Bouma law says that critical spacing is invariant across objects, not subjects (for reviews of uncrowded and visual spans, see refs. 28,32,39). Image reprinted from ref. and adapted from ref. .

Figure 8

The uncrowded window. This figure simulates crowding in reading by substituting letters in the peripheral field. Crowding spoils letter recognition, making reading impossible outside of the uncrowded window. Note that the substitutions are undetectable when you fixate on the center of the circle. As you read this caption, the words are clear and legible near your chosen point of fixation and illegibly crowded beyond that clear region. That central uncrowded field is a window through which we read (figure adapted from ref. 28).

Figure 9

Reading speed versus span. Data are from five studies of normal (black filled symbols) and dyslexic (red empty symbols) readers,,–. The normal readers were of various ages, from 1^st grade (age 6) through adult. Reading speed rose monotonically with age. The dyslexic readers were all in the 6^th or 7^th grades. The vertical scale is reading speed (1 word min⁻¹ = 0.1 character s⁻¹, assuming an average of five letters and a space for each word). The horizontal scale is letter span, estimated in various ways. Span is the width (in characters) of the uncrowded window. A reader making ρ eye movements per second, advancing an average of u characters per eye movement, reads at a rate r = ρu character s⁻¹. The diagonal line plots this proportionality, assuming 4 eye movements per second (ρ = 4 Hz), showing that this simple 4 Hz rule gives a fairly good account of all the data from normal readers (see Supplementary Methods online).

Figure 10

The Rey Complex Figure Test. The original diagram is on the left. The drawings on the right were made by normally sighted graduate students who were asked to copy, from left to right, while fixating on the central + (ignore the left-right reversal, which was the result of ambiguity of the copying instructions). A neurologist who examined these drawings found them to be typical of those produced with unrestricted viewing by patients with apperceptive agnosia. Despite the amateur drawing skill of the students, you can verify that these are reasonably good copies for your peripheral vision by fixating on the central +. Courtesy of M. Martelli (Università di Roma “La Sapienza”).