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.Object recognition means calling a chair a chair, despite variations in style, viewpoint, rendering and surrounding clutter. Crowding is a breakdown of object recognition.Let us begin by sketching a popular two-step model of object recognition: feature detection and combination. Features are components of images that are detected independently 1-4 . They are typically simple and nonoverlapping. The first step in object recognition is feature detection 4 . Each neuron in the primary visual cortex responds when a feature matches its receptive field. Only the features that drive neurons hard enough are detected 5 . In the second step, the brain combines some of the detected features to recognize the object. This combining step (including 'integration', 'binding', 'segmentation', 'pooling', 'grouping', 'contour integration' and 'selective attention') is still mysterious 3,4,6-11 . Some objects are recognized through a single combining of features over the whole object, whereas other objects require separate combining over each of several regions of the object 12-14 . These distinct regions define object parts. In an object with multiple parts, each part must be recognized before they are all joined together.The best evidence that features are indivisible elements that we detect and combine is that, even with practice, people combine information across features much less well than within a feature. Searching for a conjunction of several features is usually much harder than searching for a single feature 3 . Despite reading a billion letters over a lifetime, people still recognize letters inefficiently, by detecting and combining many simple features rather than by detecting each letter as a whole 4,15 . Crowding is inappropriate feature combination that spoils object recognition (reviewed in refs. 16,17 ).

Bouma's law of crowding predicts an uncrowded central window through which we can read and a crowded periphery through which we cannot. The old discovery that readers make several fixations per second, rather than a continuous sweep across the text, suggests that reading is limited by the number of letters that can be acquired in one fixation, without moving one's eyes. That "visual span" has been measured in various ways, but remains unexplained. Here we show (1) that the visual span is simply the number of characters that are not crowded and (2) that, at each vertical eccentricity, reading rate is proportional to the uncrowded span. We measure rapid serial visual presentation (RSVP) reading rate for text, in both original and scrambled word order, as a function of size and spacing at central and peripheral locations. As text size increases, reading rate rises abruptly from zero to maximum rate. This classic reading rate curve consists of a cliff and a plateau, characterized by two parameters, critical print size and maximum reading rate. Joining two ideas from the literature explains the whole curve. These ideas are Bouma's law of crowding and Legge's conjecture that reading rate is proportional to visual span. We show that Legge's visual span is the uncrowded span predicted by Bouma's law. This result joins Bouma and Legge to explain reading rate's dependence on letter size and spacing. Well-corrected fluent observers reading ordinary text with adequate light are limited by letter spacing (crowding), not size (acuity). More generally, it seems that this account holds true, independent of size, contrast, and luminance, provided only that text contrast is at least four times the threshold contrast for an isolated letter. For any given spacing, there is a central uncrowded span through which we read. This uncrowded span model explains the shape of the reading rate curve. We test the model in several ways. We use a "silent substitution" technique to measure the uncrowded span during reading. These substitutions spoil letter identification but are undetectable when the letters are crowded. Critical spacing is the smallest distance between letters that avoids crowding. We find that the critical spacing for letter identification predicts both the critical spacing and the span for reading. Thus, crowding predicts the parameters that characterize both the cliff and the plateau of the reading rate curve. Previous studies have found worrisome differences across observers and laboratories in the measured peripheral reading rates for ordinary text, which may reflect differences in print exposure, but we find that reading rate is much more consistent when word order is scrambled. In all conditions tested--all sizes and spacings, central and peripheral, ordered and scrambled--reading is limited by crowding. For each observer, at each vertical eccentricity, reading rate is proportional to the uncrowded span.

Research in object recognition has tried to distinguish holistic recognition from recognition by parts. One can also guess an object from its context. Words are objects, and how we recognize them is the core question of reading research. Do fast readers rely most on letter-by-letter decoding (i.e., recognition by parts), whole word shape, or sentence context? We manipulated the text to selectively knock out each source of information while sparing the others. Surprisingly, the effects of the knockouts on reading rate reveal a triple dissociation. Each reading process always contributes the same number of words per minute, regardless of whether the other processes are operating.

When reasoning about time, English-speaking adults often invoke a "mental timeline" stretching from left to right. Although the direction of the timeline varies across cultures, the tendency to represent time as a line has been argued to be ubiquitous and primitive. On this hypothesis, we might predict that children also spontaneously invoke a spatial timeline when reasoning about time. However, little is known about how and when the mental timeline develops, or to what extent it is variable and malleable in childhood. Here, we used a sticker placement task to test whether preschoolers and kindergarteners spontaneously map temporal events (breakfast, lunch, and dinner) and deictic time words (yesterday, today, tomorrow) onto lines, and to what degree their representations of time are adult-like. We found that, at age 4, preschoolers were able to arrange temporal items in lines with minimal spatial priming. However, unlike kindergarteners and adults, most preschoolers did not represent time as a line spontaneously, in the absence of priming, and did not prefer left-to-right over right-to-left lines. Furthermore, unlike most adults, children of all ages could be easily primed to adopt an unconventional vertical timeline. Our findings suggest that mappings between time and space in children are initially flexible, and become increasingly automatic and conventionalized in the early school years.