The influence of stimulus duration on visual illusions and simple reaction time.

Target detection is affected by stimulus intensity. For instance, participants respond faster to larger objects than to smaller objects. In order to compute an object's size, the brain integrates contextual information, for example object distance. Accordingly, the perceived size of an object can be altered via depth cues which modulate perceived object distance. Recently, it has been demonstrated that reaction times are influenced by the perceived rather than by the retinal size of an object, thus indicating that manual responses are generated after the perceptual integration of distance and retinal size. However, the timing aspects of these integration processes to date remain largely unclear. Therefore, the present study investigated the influence of stimulus duration on size-distance integration by means of a simple reaction time paradigm and the well-known Ponzo illusion. In experiment 1, participants responded faster to perceptually longer lines within an illusion-inducing background, whereas no such effect was associated with a neutral background. Experiment 2 revealed that this effect depended on stimulus duration. Stimuli were reliably perceived even with the shortest durations. However, illusion-induced modulations of response times were not observed for stimulus durations shorter than 40 ms. The findings indicate that the integration of context and object information requires visual input to last for at least 40 ms. The data furthermore show that as long as the visual system has not enough time to integrate context and object information, size perception is formed on the basis of lower-level representations.