Motion accumulates while movement disappears: spatial interactions in visual motion
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When objects move, they change position over time. However, the mechanisms in human vision that could be capable of tracking these changes in position are not well understood. I constructed stimuli that combined a first-order carrier motion with a position-defined envelope movement. When viewed in the periphery, the appearance of motion was very sensitive to changes in position of its envelope, regardless the amount of carrier motion. However, when multiple motion elements were placed close together, the appearance of the stimulus came to be dominated by its carrier motion. When elements were added, sensitivity to position-defined motion decreased at the same time as the sensitivity to first-order motion increased; visual clutter thus favors first-order motion over position-defined motion. These effects are modeled in terms of two mechanisms that each contribute to motion appearance. The first-order mechanism sums motion over larger areas of space without regard to their position; in the presence of clutter, it sums all signals together. The position-defined mechanism tracks the change in location of isolated features; it cannot pool signals over space and works best with salient, uncluttered objects. This mechanism suffers crowding when flankers are introduced. While first-order motion is understood in terms of spatiotemporal filtering, position-defined motion might be viewed as a feature integration problem, involving the comparison of successive positions over time. These systems have a subtractive interaction that may serve to locate objects that are moving differently from their background. Complementary characteristics and limitations of the two systems each play roles in the perception of moving objects.