Recently, we provided evidence that undetected near-threshold visual stimuli are able to trigger motor preparation in primary motor cortex, as witnessed by a component of the event related potentials (ERPs) called lateralized readiness potential (LRP) (Minelli A. et al.; SFN abstract n. 386.19, 2003). A possible candidate for the neural site subserving this effect is the superior colliculus (SC), which might be crucially involved in implicit processing of visual stimuli (Marzi et al.; Progress in Brain Research, 2003). Given that the SC appears not to receive direct projections from short-wave sensitive cones (S cones), we used a short-wave monochromatic purple stimulus to trigger the motor response. Ten normal observers were required to respond to lateralized visual stimuli presented at two different luminance levels following a visual warning signal: i) high luminance stimuli leading to 100% detection; ii) low luminance stimuli leading to 50% detection. Therefore, the same physical stimulation could elic...