Measuring the attentional speed-up in the motion induction effect.

Motion induction is the illusory motion within an elongated stimulus, such as a bar or a line, when it is preceded by a priming stimulus next to one of its ends. Motion is away from this primer. The presentation of two priming spots at both ends of a stimulus bar results in motion away from both spots with a collision in the center of the bar. With a sufficiently long delay between the spots, motion will be seen only as away from the second spot. Similarly, in a bar with a luminance gradient an illusory motion is perceived as away from the high-luminance end, presumably due to the known dependence of neural processing speed on luminance. In the present study, these two illusory motions were made to oppose each other. The particular luminance gradient which would just cancel the motion induction effect when motion is seen optimally as away from the second spot (cancellation gradient) was determined, resulting again in a collision near the center of the bar. Furthermore, the luminance dependence of the reaction time to stimulus detection was measured in a separate experiment. Thus for each observer, the processing time difference associated with the cancellation gradient was established. This delta t then gives the amount of time by which processing is speeded up in motion induction due to the priming spot. In a simple model of motion processing it can also be identified as the built-in delay delta t of a typical Reichardt-type motion detector. With the present conditions, it varied between 14 and 19 msec for different observers for a bar length of 5.3 deg. In this way, we show not only that the priming effect in motion induction can be understood as a speed-up of neural processing, but also provide a way of measuring the times involved. In additional experiments, we examined the effect of bar length and luminance profile. These results allow us to estimate the gradients of the attentional fields.