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 nex t to one of its ends, Motion is away from this primer, The presentatio n of two priming spots at both ends of a stimulus bar results in motio n 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 gr adient an illusory motion is perceived as away from the high-luminance end, presumably due to the known dependence of neural processing spee d on luminance. In the present study, these two illusory motions were made to oppose each other. The particular luminance gradient which wou ld just cancel the motion induction effect when motion is seen optimal ly as away from the second spot (cancellation gradient) was determined , resulting again in a collision near the center of the bar. Furthermo re, the luminance dependence of the reaction time to stimulus detectio n was measured in a separate experiment, Thus for each observer, the p rocessing time difference associated with the cancellation gradient wa s established. This Delta t then gives the amount of time by which pro cessing 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 b uilt-in delay Delta t of a typical Reichardt-type motion detector, Wit h the present conditions, it varied between 14 and 19 msec for differe nt observers for a bar length of 5.3 deg, In this way, we show not onl y that the priming effect in motion induction can be understood as a s peed-up of neural processing, but also provide a way of measuring the times involved, In additional experiments, we examined the effect of b ar length and luminance profile. These results allow us to estimate th e gradients of the attentional fields. Copyright (C) 1996 Elsevier Sci ence Ltd.