Relative contributions of 2D and 3D cues in a texture segmentation task, implications for the roles of striate and extrastriate cortex in attentional selection

Experimental evidence has given strong support to the theory that the primary visual cortex (V1) realizes a bottom-up saliency map (A. R. Koene & L. Zhaoping, 2007; Z. Li, 2002; L. Zhaoping, 2008a; L. Zhaoping & K. A. May, 2007). Unlike the conventional models of texture segmentation, this theory predicted that segmenting two textures in an image I(rel) comprising obliquely oriented bars would become much more difficult when a task-irrelevant texture I(ir) of spatially alternating horizontal and vertical bars is superposed on the original texture I(rel). The irrelevant texture I(ir) interferes with I(rel)'s ability to direct attention. This predicted interference was confirmed (L. Zhaoping & K. A. May, 2007) in the form of a prolonged task reaction time (RT). In this study, we investigate whether and how 3D depth perception, believed to be processed mostly beyond V1 and starting in V2 (J. S. Bakin, K. Nakayama, & C. D. Gilbert, 2000; B. G. Cumming & A. J. Parker, 2000; F. T. Qiu & R. von der Heydt, 2005; R. von der Heydt, H. Zhou, & H. S. Friedman, 2000), contribute additionally to direct attention. We measured the reduction of the interference or the RT when the position of the texture grid for I(ir) was offset horizontally from that for I(rel), forming an offset, 2D, stimulus. This reduction was compared with that when this positional offset was only present in the input image to one eye, or when it was in the opposite directions in the images for the two eyes, creating a 3D stimulus with a depth separation between I(ir) and I(rel). The contribution by 3D processes to attentional guidance would be manifested by any extra RT reduction associated with the 3D stimulus over the offset 2D stimulus. This 3D contribution was not present unless the task was so difficult that RT (by button press) based on 2D cues alone was longer than about 1 second. Our findings suggest that, without other top-down factors, V1 plays a dominant role in attentional guidance during an initial window of processing, while cortical areas beyond V1 play an increasing role in later processing. Subject-dependent variations in the manifestations of the 3D effects also suggest that this later, 3D, contribution to attentional guidance can be easily influenced by top-down control.