The Relation between Target Localisation in Strings of Letters, Contrast, and Refresh Rate

Abstract

Jacobs (1987, Vision Research27 1953 – 1966) obtained psychometric curves to investigate the relationship between target localisability and the latency of primary saccades. These psychometric curves show correct scores as a function of string length, target location, and target-background similarity. They were obtained with white text on a black background. We repeated this experiment, with a few extensions. First, we did not only use white text on a black background, but we used black text on a white background as well. The stimuli were also presented with the use of isoluminant colours. Second, we varied the refresh rate of the screen (from 60 Hz up to 150 Hz). Third, we used two different instructions for the subjects: in one the subject was instructed to indicate the position of the target relative to the beginning of the string; in the other the position of the target relative to the fixation point. When the string was presented to the right of the fixation point, the results of these two instructions were predicted to be similar; in the other case the results might be different. The results of our experiments show a clear effect of the amount of contrast between text and background, and an influence of the screen refresh rate. We also found that the results differ depending on the instruction used. Sadly, we were not able to replicate all of the results of the original experiment by Jacobs.