Assessing the effects of physical and perceived luminance contrast on RT and TMS-induced percepts

Simple reaction times (RTs) are inversely related to the luminance of a visual region, with RT increasing as luminance decreases, and decreasing as luminance increases. A potential discrepancy in the link between luminance and RT, however, stems from the perception of luminance itself. Here, we tested whether RT is modulated by a measureable amount of light (luminance) or perceptual amount of light (brightness), as two test regions having the same luminance can be perceived as having different brightness. The current study investigates the effects of brightness using probes and artificial percepts, i.e., transcranial magnetic stimulation (TMS)-induced light and dark percepts. In Experiment 1, participants performed a RT task to light and dark probes presented over two backgrounds, one exhibiting a physical luminance and the other exhibiting perceptual brightness. Experiment 2 tested whether perceptual brightness could influence RT and frequency of artificial percepts. We found that while brightness contrast modulated RT to the dark probes, the frequency of artificial percepts was susceptible to both physical and perceived changes in luminance. These data suggest that some behavioral responses can be influenced by an illusion of brightness, wherein there is no actual change in luminance, as well as the perception of TMS-induced percepts.