We have explored pattern recognition by examining how observers identify letters. We measured the contrast required for identification of a letter briefly presented in visual noise. Comparisons across tasks and stimuli that vary in difficulty are made possible by pitting human against ideal observers, and expressing the results as efficiency, the squared ratio of the threshold contrasts of ideal and human observers. We find that efficiency for letter identification is independent of duration, overall contrast, and eccentricity, and only weakly dependent on size, suggesting that letters are identified similarly across this wide range of viewing conditions.
Efficiency is also independent of age and years of reading.
However, efficiency does vary across alphabets and type styles, with more complex forms yielding lower efficiencies, as one might expect from theories of Gestalt perception. This, and the surprisingly fixed ratio of detection and identification thresholds, suggest that letter identification is mediated by detection of 2 to 5 visual features, each with a complexity between 13 and 30.