Surface-Related Features Figure 13. Model activities of V1 and V2 cell pool (right, first row) resulting from a real-world scene image that includes occluding opaque and transparent surfaces. A feature map was extracted from model activities, revealing position and type (colour coded) of junction configurations (right, second row). doi:10.1371/journal.pone.0005909.g013 information). Since the Harris corner detector is not able to discriminate between different junction types, our comparison is only based on the detection performance, irrespective of the junction category. To measure the performance of the different schemes we use receiver operator characteristic (ROC) curves. This method is frequently used to evaluate true positive rate or hit rate and the false positive rate of a binary classifier system as its discrimination threshold is varied. Here, we use the junction feature map as input for the ROC analysis. Figure 14 shows the resulting ROC curves extracted from junction feature map given the test image as input. It is clearly visible that the ROC curve computed from the recurrent model responses lies well above the Harris corner detector curve and the initial feedforeward model curve. This suggest for a significantly better detection performance of our recurrent model compared to feedforward processing-based junction detection schemes. Simulations with dynamic input stimuli We particularly designed this experiment to demonstrate a model prediction, namely, that feedback leads to a brief persistence of object and material appearance. This is usually unnoticed when the scene does not change at all. If, however, appearances of surface material change during recurrent interac- tion (while keeping the shape registered) the apparent surface property should stay more prolonged depending on whether it is transparent and changes into opaque or whether it is opaque and smoothly changes into transparent. Due to the action of modulatory feedback activation the registered boundary and junction activations continue to enhance the configuration signaled from previous input features for a short period of time. This appearance history (or memory in the processing architec- ture) is known as hysteresis effect. Here, we used temporal sequences of two occluding squares where the opacity of the topmost square was linearly altered between 100% opacity and 90% opacity, thus making the occluded region increasingly more visible (invisible). The input sequence consists of 10 frames static input (opaque) followed by 10 frames linear change from 100% to 90% opacity followed by 10 frames static input (transparent, 90% opacity). The sequence was presented as described above (opaque-transparent) and in reverse temporal order (transparent-opaque). To investigate the hysteresis effect of feedback we presented both sequences to the full model (with feedback connection enabled) and to a restricted version of the model with feedback connections disabled. In the full model, feedback processing time is equal to stimulus presentation time, i.e., one feedback cycle is performed per stimulus time frame. In the restricted model, we switch all feedback connection off which constrains the model to perform only feedforward processing. Throughout the simulation, model activities indicating T- and X- junctions are extracted at positions where occluding contours of both squares intersect each other (Figure 15). The results generated by the full model show that feedback leads to a sequence directional hysteresis effect by temporally locking the prediction of a junction type (T- or X-junction). Moreover, initial ambiguities induced by predictions for different junction types are resolved by top-down feedback during the first few iterations. In contrast, when the model is restricted to feedforward processing no hysteresis effect can be observed, i.e., the model activities are equal for both input sequences, namely opaque-transparent and transparent-opaque transitions. Furthermore, no disambiguation between different junction type predictions takes place. PLoS ONE | www.plosone.org 15 June 2009 | Volume 4 | Issue 6 | e5909