Spacing, Not Size, Matters in Visual Recognition, NYU Researchers Find

You might think that the farthest distance at which you can hold a book and still read it quickly is determined by the size of the letters. However, NYU neuroscientists have concluded that it’s the spacing between letters, not their size, which matters. In general, objects, such as letters, can be recognized only if they are separated by enough space—the “critical spacing.” Objects closer than that spacing are “crowded” and cannot be identified. A broad review of this crowding phenomenon, which appeared this fall in the journal Nature Neuroscience, shows that this critical spacing is the same for all objects, including letters, animals, and furniture.
    According to the authors, professor of psychology and neural science Denis Pelli and Katharine Tillman, an undergraduate researcher in the College of Arts and Science, the critical spacing is a key parameter in the brain’s cortical architecture underlying object recognition.
    “The idea that spacing limits object recognition could not be simpler, but it has been very hard to accept because it displaces a firmly held belief that visibility is limited by size, not spacing,” wrote Pelli and Tillman.
    The human visual system recognizes a simple object by detecting and then combining its features (lines or edges). However, this process is impaired when, in seeking to identify an object in clutter, the brain combines features over too large an area surrounding the object, failing to isolate the object’s features from those of the clutter. This usually happens when the cluttered object is in peripheral vision (the corner of your eye), as shown by studying the following:

R      +     ARE

    Fix your eyes on the plus. It is easy to recognize the letter “R” if it is alone, as on the left. However, if the letter “R” is among other letters, it is much more difficult to recognize. This is an example of crowding.
    “We can easily see a single bird flying in the sky because there is no crowding, but most of our visual world is cluttered, and each object that we identify must be isolated from the clutter,” the researchers added. “When an object is not isolated, and therefore crowded, we cannot recognize it.”
    The critical spacing is greater for objects that are more peripheral (farther from fixation). Objects are crowded when their spacing is less than critical and uncrowded when their spacing is more than critical. This dichotomy defines an “uncrowded window” through which we are able to read and search. The size of the uncrowded window increases through childhood and accounts for the increase in reading speed.
—James Devitt


Arnold Schwarzenegger and Elvis Presley are famous, so 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. Faces are like words; the parts (eyes, nose, mouth) must be isolated (separated by the observer’s critical spacing) in order for the whole to be recognized.

The Elvis Presley image is used by permission, Elvis Presley Enterprises, Inc. The Arnold Schwarzenegger image is publicly available from the California State Government Web site.