Washington, Sept 26 : It's not the size of the letters, but spacing between them, which makes it hard for us to read a book from a distance, according to New York University neuroscientists.

The same applies to objects, including letters, animals, and furniture, which can only be recognized only if they are separated by enough space, the "critical spacing." Objects closer than that spacing are "crowded" and cannot be identified.

The critical spacing is a key parameter in the brain's cortical architecture underlying object recognition, said authors, NYU Professor of Psychology and Neural Science Denis Pelli and Katharine Tillman, an undergraduate researcher in NYU's College of Arts and Science.

"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). But this process is hampered when, in seeking to identify an object in clutter, your 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).

"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. When an object is not isolated, and therefore crowded, we cannot recognize it," the researchers added.

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.

Such 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.

The study is appearing in the latest issue of the journal Nature Neuroscience.