Surface-Related Features Figure 9. Model responses based on artificial input stimulus of overlapping squares. The stimulus (A) includes two different occlusion conditions: two overlapping opaque surfaces that produce T-junctions and two overlapping transparent surfaces which generate X-junctions. Furthermore, several L-junctions are visible at the corners of the squares. Model activities are summed over the orientation domain for all stages of our model. We show initial V1 simple and complex cell responses (B, C), combined V1 bipole/end-stop cell responses (D), end-stop activity (E), and V2 bipole cell activity (F) after 4 recurrent feedforward-feedback cycles. For clarification, the labels (A)–(F) correspond also to labels (A) – (F) in Figure 3. Note that end-stop cells do not respond to X-junction configurations and that V2 bipole cells do not respond at L-junctions (cp. Table 1). Note also, that responses in (D) – (F) are invariant against image contrast. doi:10.1371/journal.pone.0005909.g009 activity. In order to prevent multiple features to be active at the same position, all signals undergo a competition stage where multiple signals in a local neighborhood compete with each other. The output of this stage is presented in Figure 10c which is a combined map that represents different features, signaled by color. Finally, from this map, position and type of junctions are extracted by local maximum selection (Figure 10e). Processing of illusory contours In a third simulation, we show the capabilities of the model to uncover illusory contours in scenes. As input we used two different versions of the Kanizsa square [40](Figure 5). The first image leads to the impression of an illusory square where the corners occlude black circles. The second image gives the impression of a white square in front of concentric circles. In both cases, only parts of the square are formed by luminance contrast. However, human observers mentally see the square as a coherent object. Our model results demonstrate that the invisible contour parts are uncovered by V2 bipole cell responses. Moreover, we show that subsequent recurrent feedforward-feedback cycles help to close large gaps between like-oriented contour elements or along linearly arrange contour endings (Figure 11, Figure 12). Importantly, this has a strong influence on the junction type that is signaled by the model responses. In Figure 11, initial model responses suggest for L-junctions, which is in accordance with the physical luminance contrasts. But, after some recurrent iterations, V2 bipole cells close the gaps between contour elements which leads to a different, more global interpretation: the L-junctions PLoS ONE | www.plosone.org 11 June 2009 | Volume 4 | Issue 6 | e5909