Sensitivity for structure gradient in texture discrimination tasks.

Recent experiments indicate that the segregation of visual structures ("texture discrimination") depends not only on the form of texture elements but also on their spacing. Structures with discriminable elements in close proximity can be segregated more easily than patterns in which the same texture elements are more widely spaced. In dot arrays with areas of different dot luminance, segregation was found to depend on both the luminance difference and dot spacing; discrimination of texture areas in coarse dot rasters required greater differences in luminance than in fine rasters. Also, in regular arrays of iso-luminant line patterns, the maximal spacing between neighbouring lines for which different texture areas could still be discriminated was found to be influenced by the degree of dissimilarity between elements. For lines of a given length, texture areas with small differences in orientation became indiscriminable at smaller spacings than texture areas with orthogonal line orientations. Line length additionally had a strong effect on texture discrimination; increasing the line length for a given spacing provided easier segregation of texture areas. However, over a range of raster widths, discrimination of texture areas with a given difference in line orientation varied not with absolute values of line length but with the ratio of line length to interline spacing. Overall, the data suggest that texture discrimination in man is based on the evaluation of variation in structure over space (defined as the "texture gradient"). If local variation of structure is too small, texture areas cannot be discriminated, though differences between texture elements themselves may be apparent. As far as the dependence on variation over space is concerned, discrimination of iso-luminant textures resembles the limited sensitivity of the visual system for differences in texture luminance.