The effect of exposure duration on visual character identification in single, whole and partial report

The psychometric function of single letter identification is typically described as a function of stimulus intensity. However, the effect of stimulus exposure duration on letter identification remains poorly described. This is surprising because the effect of exposure duration has played a central role in modelling performance in whole and partial report tasks in which multiple simultaneously presented letters are to be reported (Shibuya & Bundesen, 1988). Therefore, we investigated visual letter identification as a function of exposure duration. On each trial, a single randomly chosen letter (A-Z) was presented at the centre of the screen. Exposure duration was varied from 5 to 210 milliseconds. The letter was followed by a pattern mask. Three subjects each completed 54,080 trials in a 26-Alternative Forced Choice procedure. We compared the exponential, the gamma and the Weibull psychometric functions, all of these having a temporal offset included, as well as the ex-Gaussian, the log-logistic and finally the squared-logistic, which is a psychometric function we believe have not been described before. The log-logistic and the squared-logistic psychometric function fit well to experimental data in both the present study and in a previous study of single-letter identification accuracy. Also, we conducted an experiment to test the ability of the psychometric functions to fit single-letter identification data, at different stimulus contrast levels; also here the same psychometric function prevailed. Finally, after insertion into Bundesen’s Theory of Visual Attention (Bundesen, 1990), the same psychometric functions enable closer fits to data from a previous whole and partial report experiment.