Lines and dots: characteristics of the motion integration process

Local motion detectors can only provide the velocity component perpendicula r to a moving line that crosses their receptive field, leading to an ambigu ity known as the 'aperture problem'. This problem is solved exactly for rig id objects translating in the screen plane via the intersection of constrai nts (IOC. In natural scenes, however, object motions are not restricted to fronto-parallel translations, and several objects with distinct motions may be present in the visual space. Under these conditions the usual IOC const ruction is no longer valid, which raises questions as its use as a basis fo r spatial integration and selection of motion signals in uniform and non-un iform velocity fields. The influence of the motion of random dots on the pe rceived direction of a horizontal line grating was measured, when dots and lines are seen through different apertures. The random dots were mapped on a plane that translates in a fronto-parallel plane (uniform 2D translation) or in depth (3D, corresponding to a non-uniform projected velocity field, either expanding or contracting). The grating was either moving rigidly wit h the dots or in the opposite direction. Subjects' responses show that the direction of line grating movement was reliably influenced only in conditio ns consistent with rigid motion; where there was a reliable influence, the perceived direction was consistent with the dot motion pattern. This findin g points to the existence of a motion-based selection mechanism that operat es prior to the disambiguation of the line movement direction. Disambiguati on could occur for both uniform and non-uniform velocity fields, even thoug h in the last case none of the individual dots indicated the proper directi on in 2D velocity space. Finally, the capture by non-uniform motion pattern s was less robust than that by uniform 2D translations, and could be disrup ted by manipulations of the shape and size of the apertures. (C) 2001 Elsev ier Science Ltd. All rights reserved.