Figure 16. Dynamic stimulus where two occluding bars move in opposite direction. Tracking of L-junctions (green) leads to correct motion estimates of the two bars while tracking of T-junctions leads to erronous motion estimates. Thus, the visual system might use form information, e.g., surface-based occlusion cues to selectively discount local motion estimates for moving T-junctions. doi:10.1371/journal.pone.0005909.g016 Limitations of the model Although we have shown that our model is able to produce results that are in line with several empirical findings, there are Surface-Related Features Table 1. Modulatory effect of feedback (FB) signal on feedforward (FF) signal. FF ON ON OFF OFF FB ON OFF ON OFF new activity after FF–FB combination modulatory effect / FF?(1+FB) = FF 0 0 + 0 0 0 ‘ON’ means activity present and ‘OFF’ means no activity present. Modulation (+) of feedforward activity takes only place when both signals, feedback and feedforward are present (ON). Otherwise, feedforward activity remains unchanged (0). doi:10.1371/journal.pone.0005909.t001 also some shortcomings of the model. For instance, consider a Kanizsa figure such as illustrated in Figure 17 where the gaps between contour elements are so large that the V2 bipole filter cannot bridge the gap. In this case the model would fail to produce an illusory contour signal in V2. Nevertheless, human observes still have a weak impression of seeing an illusory triangle. We suggest that higher visual areas such as V4 also play a role in illusory contour processing. Evidence comes, e.g., from Pasupathy [79] who found responses to contour features in macaque area V4. Another restriction of the model is that it does not account for different image scales. Consequently, the model focuses on fine details and suppresses coarse structures. However, the model could be provided with a pyramid of differently scaled version of the same input image. This would correspond to simply replicating the model at multiple scales. An alternative approach would be to employ scaled versions of Gabor wavelet filters in the input stage. Finally, our model does not explain how occlusion-based junctions can be distinguished from texture-based junctions. In this model, we only used stimuli that have homogenous surface reflectance properties. Thus, contours are interpreted as surface borders by the model. In natural scenes, surfaces are often textured due to surface material properties which would also lead to junctions signals and thus to ambiguities in the interpretation. However, such ambiguities could be solved in higher visual area such as V4 [80] which are not in the scope of this paper. In addition, stereo information can also help to correctly identify occlusion-based junctions. Figure 17. Stimuli where the model fails to produce illusory contour activations. Distances between the starting points of the contours are too large to be bridged by a V2 bipole cell. doi:10.1371/journal.pone.0005909.g017 PLoS ONE | www.plosone.org 20 June 2009 | Volume 4 | Issue 6 | e5909