Second-order four-stroke apparent motion

<p>Purpose: Four-stroke apparent motion displays contain luminance edges that alternate in position and synchronously reverse in contrast polarity. As a result they appear to move unidirectionally, a percept that is readily accommodated by current models of motion energy analysis. Second-order motion displays contain texture-defined edges, and there is some evidence that they excite second-order energy detectors that are fed by 'texture-grabbing' transforms. However, different second-order textures require different transforms, and it is not clear how many transforms are available to second-order energy analysis. We used the effectiveness of second-order four-stroke apparent motion as a diagnostic test for the availability of various tranforms to motion energy analysis. Methods: Seven four-stroke displays were created, containing contours defined by modulation of - luminance; contrast; flicker; disparity; size; motion parallax; and orientation. In each case we measured the effectiveness of both the whole cycle, and of fractions of a cycle, at generating unidirectional apparent motion. Results: All except three of the displays proved effective in both part- and whole- cycle presentations. The three failures involved flicker modulation, disparity modulation, and motion parallax. Conclusions: The successful displays contained contours that can be revealed by transformations incorporating spatial filtering and rectification; the unsuccessful displays all require more complex transformations (spatiotemporal filtering or binocular analysis). The former transformations thus appear to feed into second-order energy analysis, but the latter transformations do not.</p>