Redundant target effect and the processing of colour and luminance

The redundant target effect is the observation that people typically respond faster to double targets (two targets presented simultaneously) than to either of the targets presented alone. This difference in latency is termed the redundancy gain (RG). Chromatic targets may be accompanied with luminance changes at their onset and offset. We have used a dynamic random luminance modulation technique to mask out luminance components of chromatic signals. Here we report on the presence of a significant RG for visual targets defined by their combined luminance and chromatic components as well as their chromatic content in isolation. Reaction times were measured to the onset of three classes of stimuli, namely, Long- and Short-wavelength cone sensitive (L- and S-cone) targets matched for saliency as well as luminance-defined targets. Analysis of the cumulative distributions of reaction time data showed that a neural coactivation model could fit the experimental data for chromatic targets only. When a luminance component is present, the reaction time data can be explained by a probability summation account also known as the race model.