Conjunction and linear non-separability effects in visual shape encoding

Four visual search experiments are reported which used simple 2D shapes var ying on the global dimensions of aspect ratio/curvature or aspect ratio/tap ering. Results indicate serial self-terminating search in all conditions. M ost importantly, search rates are markedly modulated by the particular form s of structural relations existing between the targets and their distracter s. Thus, single-feature targets with shape properties that are linearly sep arable from those of their distracters yield markedly faster search rates t han linearly separable targets made of a conjunction of distracter features . In addition, linearly separable single-feature targets are searched at a much faster rate than single-feature targets which are not linearly separab le. Follow-up experiments demonstrate that these conjunction and linear non -separability effects cannot be attributed to pairwise target-distracter di scriminability differences across conditions. The main conclusions are that the shapes used are parsed according to elementary features in visual enco ding, and that a linear discrimination mechanism is available which permits fast visual search rates if a single-feature target is linearly separable from its distracters. (C) 2000 Elsevier Science Ltd. All rights reserved.