Orientation processing mechanisms revealed by the plaid tilt illusion

The tilt after-effect (TAE) and tilt illusion (TI) have revealed a great de al about the nature of orientation coding of 1-dimensional (1D) lines and g ratings. Comparatively little research however has addressed the mechanisms responsible for encoding the orientation of 2-dimensional (2D) plaid stimu li. A multi-stage model of edge detection has recently been proposed [Georg eson, M. A. (1998) Image & Vision Computing, 16(6-7). 389-405] to account f or the perceived structure of a plaid stimulus that incorporates extraction of the zero-crossings (ZCs) of the plaid. Data is presented showing that t he ZCs of a plaid inducing stimulus can interact with vertical grating test stimulus to induce a standard tilt illusion. However, by considering the s econd-order structure of a plaid rather than ZCs, it was shown that the per ceived orientation of the vertical test grating results from the combinatio n of orientation illusions due to the first- and second-order components of an inducing plaid. The data suggest that the mechanisms encoding the orien tation of second-order contours are similar to, and interact directly with, those that encode first-order contours. (C) 2001 Elsevier Science Ltd. All rights reserved.