Crowding is unlike ordinary masking: Distinguishing feature detection and integration

A letter in the peripheral visual field is much harder to identify in the presence of nearby letters. This is called “crowding”. In general, masking is a procedure: introducing any “mask” pattern that affects discriminability of the signal. Crowding conforms to the masking paradigm, but the crowding effect is unlike ordinary masking. Here we characterize crowding, and present diagnostic tests that distinguish it from ordinary masking. In ordinary masking, the signal disappears. In crowding, it remains visible, but is ambiguous, confounded with its neighbors. Masks are usually effective only if they overlap the signal, but the crowding effect extends over a large region. The width of that region is proportional to signal eccentricity from the fovea and independent of signal size, mask size, signal and mask font, and number of masks. At 4 deg eccentricity, the threshold contrast for identification of a 0.32 deg signal letter is elevated (up to six-fold) by mask letters anywhere in a 2.3 deg region, seven times wider than the signal. In ordinary masking, threshold contrast rises as a power function of mask contrast, with a shallow log-log slope of 0.5 to 1, while in crowding, threshold is a sigmoidal function of mask contrast, with a steep log-log slope of 2 at close spacing. Most remarkably, although the threshold elevation decreases exponentially with spacing, the threshold and saturation contrasts of crowding are independent of spacing. Finally, ordinary masking is similar for detection and identification, but crowding occurs only for identification, not detection. More precisely, crowding occurs only in tasks that cannot be done based on a single detection by coarsely coded feature detectors. These results (and observers’ introspections) suggest that ordinary masking blocks feature detection, so the signal disappears, while crowding (like “illusory conjunction”) is feature integration gone awry — detected features are integrated over an inappropriately large area because smaller integration fields are absent — so the signal is ambiguous, confounded with the mask.