Sensitivity to second-order motion as a function of temporal frequency and eccentricity.

There is considerable evidence that second-order motion, such as motion consisting of a drifting contrast modulation, is detected separately from first-order motion. Some previous studies have shown that the rate at which sensitivity declines as either drift speed or eccentricity increases is the same for both types of motion. However, these studies have used second-order motion stimuli based on static noise carriers, which we have shown (Smith & Ledgeway, 1997) may be inappropriate because they can give rise to local first-order artifacts. By using dynamic noise carriers, we isolate the second-order motion mechanism and show that its temporal response is much worse than that of the first-order system but that its rate of sensitivity loss with increasing stimulus eccentricity is indeed similar to that of the first-order motion system.