Do Difficulties in Stimulus Discrimination Affect Luminance Processing?

Simple reaction time is known to decay as a hyperbolic function of luminance (Piéron's function). An identical relationship has also been demonstrated recently (Pins and Bonnet, 1996 Perception & Psychophysics in press) with different choice-reaction-time tasks. Although mean choice reaction time increased with the complexity of the task, the exponents of the functions relating reaction time (RT) to luminance were found to be equal in each experiment. These results suggest that the task specific time required by the different tasks only adds to the time necessary for luminance processing. In these experiments, the different stimuli presented were easily discriminable. In the present study, we examined the effect of variation in luminance on a more difficult discrimination task involving variation in orientation. Five different luminance levels covering the entire mesopic range were used. In two conditions, tilted lines at nine different angles were used, at a spacing of 2°. In the first condition, the orientations were chosen on both sides of the vertical (the subject responded “left” or “right”); in the second condition, the orientations were on both sides of a line oriented at −40° to the vertical (the subject responded “high” or “low”). The results were compared to those of a second experiment in which only two easily discriminable orientations were used. The results show that RT is greater in the experiments in which nine orientations are used, while the effect of intensity on RT is lower. This effect does not depend on orientation.