Letter recognition at low contrast levels: effects of letter size

Contrast variation was used to measure recognition thresholds for lowercase letters, with the aim of obtaining a better understanding of the role that early stages of visual processing play in letter recognition. Frequency-of-recognition curves were measured for alphabets of different letter size. Since variation of the adaptational state of the eye changes the characteristics of primary visual processing in a quantifiable way, recognition thresholds were measured both at a high (150 cd m-2) and at a low (0.9 cd m-2) adaptation level. Thresholds decreased as letter size increased, in a way comparable with data on visual acuity. At the lower adaptation level, recognition thresholds became higher, which is also in accordance with visual acuity data. Furthermore, the slopes of the frequency-of-recognition curves for alphabets as a function of log contrast decreased with decreasing letter size. It is argued that this is mainly caused by an increasing dispersion of internal representations of individual letters on the internal psychological scale as letter size decreases.