Second-order processing of four-stroke apparent motion

In four-stroke apparent motion displays, pattern elements oscillate between two adjacent positions and synchronously reverse in contrast, but appear t o move unidirectionally. For example, if rightward shifts preserve contrast but leftward shifts reverse contrast, consistent rightward motion is seen. In conventional first-order displays, elements reverse in luminance contra st (e.g. light elements become dark, and vice-versa). The resulting percept ion can be explained by responses in elementary motion detectors tuned to s patio-temporal orientation. Second-order motion displays contain texture-de fined elements, and there is some evidence that they excite second-order mo tion detectors that extract spatio-temporal orientation following the appli cation of a non-linear 'texture-grabbing' transform by the visual system. W e generated a variety of second-order four-stroke displays, containing text ure-contrast reversals instead of luminance contrast reversals, and used th eir effectiveness as a diagnostic test for the presence of various forms of non-linear transform in the second-order motion system. Displays containin g only forward or only reversed phi motion sequences were also tested. Disp lays defined by variation in luminance, contrast, orientation, and size wer e effective. Displays defined by variation in motion, dynamism, and stereo were partially or wholly ineffective. Results obtained with contrast-revers ing and four-stroke displays indicate that only relatively simple non-linea r transforms (involving spatial filtering and rectification) are available during second-order energy-based motion analysis. (C) 1999 Elsevier Science Ltd. All rights reserved.