Detection of multidimensional targets in visual search.

Search performance for targets defined along multiple dimensions was investigated with an accuracy visual search task. Initially, threshold was measured for targets that differed from homogeneous distractors along a single dimension (e.g., a reddish target among achromatic distractors, or a right-tilted target among vertically oriented distractors). Threshold was then measured for a multidimensional target (a redundant target) that differed from homogeneous distractors along two dimensions (e.g., a reddish AND right-tilted target among achromatic, vertically oriented distractors). Search performance for multidimensional target combinations of chromaticity and luminance, chromaticity and orientation, and chromaticity and spatial frequency was tested. Measurements were evaluated within several summation models, allowing for a test of the mechanisms mediating the detection of multidimensional targets in search. Measurements were generally consistent with probability summation suggesting the particular combinations of stimulus dimensions tested were coded along independent, noisy, neural mechanisms.