The Effects of Task Difficulty on Pupil Diameter in a Choice Reaction Time Task

The effects of task difficulty on pupil diameter in a choice reaction time (CRT) task were investigated. Pupil diameter data were collected from 24 subjects using the NASA Langley Research Center oculometer system. Task difficulty was defined by the number of paired stimulus-response alternatives and stimulus-response compatibility. A unique S-R ensemble which required an oculometeric (eye-movement) response and graphic symbols as stimuli was employed in the CRT task. The procedures used replicated Hick's Law for the first time on an oculometer. The S-R compatibility effect was not found to influence this linear trend. The results indicated that pupil diameters for the 2 alternative condition were significantly different from the 4 and 8 alternative conditions; however, pupil diameters for these two were not significantly different from each other. These findings support the hypothesis that increases in task difficulty are accompanied by increases in pupil diameter.