Surface-Related Features Figure 4. Response properties of different model cell populations for different structural configurations together with their most likely interpretation (cue type). Numbers denote the modality of the response distribution across cell pools located at the position marked with a red dot for each structure. A bar means that the cell population is not responsive for this structure. Note, that each structure has a specific neural response profile across different model cell populations which can be used to extract separate saliency maps. For a better understanding, we sketched the configuration of filters together with the underlying structure. Remember, that V2 bipole sub-fields are connected multiplicatively (signalled by a ‘‘N’’), leading to zero activity of the whole bipole cell if input from one sub-field is missing (symbolized by red crosses). On the other hand, V1 bipole sub-fields are additively connected (signalled by a ‘‘#’’) which has the effect that input from one sub-field is sufficient to create activity. doi:10.1371/journal.pone.0005909.g004 matching. To read-out T-junction signals we multiplicatively combine activities of V2 bipole cells with V1 end-stop cells. More precisely, we use a pairwise combination of orientation specific V2 bipole cell activities and orientation specific V1 end-stop cell activities. Pairs of cells that correspond to same orientations are not considered. The multiplicative operation realizes an AND- gate, implying that both cell populations have to be active in order to signal the presence of T-junctions. A saliency map that signals the presence of T-junctions is then extracted by summing over the orientation domain. In general, this map represents the priority of the gathered evidence in favor of the particular scene feature. L-Junctions. L-junctions, also termed V-junctions or corners, are formed by two contour segments which terminate in the same projected location. We extract corner signals in a similar way than T-junctions signals. Instead of combining V1 end-stop activities with V2 bipole activities we multiplicatively combine activities of differently oriented V1 end-stop cells among each other (Figure 6). A combined orientation invariant corner saliency signal is then Figure 5. Stimuli used in our experiments that show illusory contours effects. In both, the Kanizsa shape and the five-line Varin shape, observers have the impression of seeing a white square which is partly formed by illusory contours. There is strong evidence that illusory contours are induced by horizontally and vertically arrange line-endings (A) or by collinearly arranged luminance contrasts (B). doi:10.1371/journal.pone.0005909.g005 PLoS ONE | www.plosone.org 7 June 2009 | Volume 4 | Issue 6 | e5909