Conjunction and linear non-separability effects in visual shape encoding

Abstract

Four visual search experiments are reported which used simple 2D shapes varying on the global dimensions of aspect ratio/curvature or aspect ratio/tapering. Results indicate serial self-terminating search in all conditions. Most importantly, search rates are markedly modulated by the particular forms of structural relations existing between the targets and their distractors. Thus, single-feature targets with shape properties that are linearly separable from those of their distractors yield markedly faster search rates than linearly separable targets made of a conjunction of distractor features. In addition, linearly separable single-feature targets are searched at a much faster rate than single-feature targets which are not linearly separable. Follow-up experiments demonstrate that these conjunction and linear non-separability effects cannot be attributed to pairwise target–distractor discriminability differences across conditions. The main conclusions are that the shapes used are parsed according to elementary features in visual encoding, and that a linear discrimination mechanism is available which permits fast visual search rates if a single-feature target is linearly separable from its distractors.