Contrast discrimination under task-induced mental load.

Task-induced mental load can potentially degrade visual performance, which could be important during tasks such as driving. In the natural world, most objects have visual contrasts that are supra-threshold, and so the ability to reliably distinguish the borders between these objects is related to contrast discrimination, rather than absolute contrast detection performance. This study investigated the effects of auditory task-induced mental load on contrast discrimination threshold across several spatial frequencies. Binocular contrast discrimination thresholds were measured in 14 participants at spatial frequencies of 0.25, 1, 4, 8, 14 cpd across pedestal contrast levels of 10, 16, 24, 38 and 60%. The task was repeated with an auditory 0-back task and 2-back task to impose a mental load, the magnitude of which was measured through subjective ratings and heart rate recording. A concurrent 2-back task significantly increased contrast discrimination thresholds across all spatial frequencies when compared to both the baseline and 0-back conditions. There was no significant effect of mental load on the slope of the Threshold vs Contrast function once potential ceiling effects in our staircase procedure were removed. We conclude that mental load induced by concurrent cognitive tasks can increase contrast discrimination thresholds. When evaluating an individuals' performance under natural-world condition such as driving, the effects of task-induced mental load on contrast discrimination thresholds should be considered.