Express saccades and superior colliculus responses are sensitive to short-wavelength cone contrast

Significance It is widely believed that the superior colliculus (SC) is unable to use input from short-wavelength–sensitive cones (S-cones) in the retina. If this idea were true, the role of the SC in nearly any visual-oculomotor behavior could be tested by using an S-cone stimulus to eliminate involvement of the SC. Contrary to this assumption, we show that express saccades, an SC-dependent behavior, are sensitive to S-cone contrast. This finding demonstrates that the SC is able to use input from S-cones to guide behavior, and that S-cone stimuli cannot be used to infer SC contributions to behavior. A key structure for directing saccadic eye movements is the superior colliculus (SC). The visual pathways that project to the SC have been reported to carry only luminance information and not color information. Short-wavelength–sensitive cones (S-cones) in the retina make little or no contribution to luminance signals, leading to the conclusion that S-cone stimuli should be invisible to SC neurons. The premise that S-cone stimuli are invisible to the SC has been used in numerous clinical and human psychophysical studies. The assumption that the SC cannot use S-cone stimuli to guide behavior has never been tested. We show here that express saccades, which depend on the SC, can be driven by S-cone input. Further, express saccade reaction times and changes in SC activity depend on the amount of S-cone contrast. These results demonstrate that the SC can use S-cone stimuli to guide behavior. We conclude that the use of S-cone stimuli is insufficient to isolate SC function in psychophysical and clinical studies of human subjects.