Comparison of simple reaction times across colour and luminance conditions under an abrupt decrease in background luminance

In daily life, we frequently encounter an abrupt decrease in background luminance. Although most real-world objects exhibit compound colours containing both luminance and chromaticity information, visual performance for such stimuli remains insufficiently explored. Can chromaticity compensate for decreased luminance sensitivity and support visual performance under such conditions? This study quantified reaction times (RTs) to luminance (achromatic), chromaticity (isoluminant) and compound colour (combined luminance and chromaticity) stimuli following an abrupt decrease in background luminance, to clarify whether chromatic information is necessary for shortening RTs. The main findings are as follows. First, the effect of the abrupt luminance decrease was similar across all colours. This was suggested by a modified version of Piéron’s Law incorporating the sensitivity efficiency parameter, showing no statistically significant difference between luminance and chromaticity. Second, luminance consistently produced shorter RTs both immediately after the decrease and some time thereafter, suggesting that adding chromaticity does not necessarily confer an advantage. Interestingly, RTs for compound colours, though containing both luminance and chromaticity components, resembled either luminance or chromaticity alone, with high contrast producing luminance-like RTs and low contrast producing chromaticity-like RTs, indicating a dynamic, contrast-dependent shift in processing pathways.