Absence of express saccades to texture or motion defined targets

Subjects were asked to foveate luminance defined, texture (orientation) defined, or motion defined targets while the reaction times of their saccadic eye movements were measured. With luminance defined targets, latencies followed the known bimodal distribution of fast ("express") and normal saccades, but in five highly trained subjects neither orientation nor motion defined targets evoked express saccades. Interestingly, even luminance targets no longer generated express saccades when the contrast was adjusted so as to match the salience of orientation or motion defined targets. This suggests that the neuronal mechanism underlying the generation of express saccades is driven by localized, high-contrast stimuli at an early processing stage of the visual system. Together with the neurophysiological findings in the monkey that both orientation and motion contrast produce increased activity at early cortical levels, the negative finding of this study indicates only little, if any, direct contribution of V1 activity to the generation of express saccades.