Zöllner Illusion
Figure 1: You can change the orientation of the diagonal lines by clicking on the picture and dragging the mouse up and down.
In this illusion, the vertical bands appear to diverge and converge. This is predicted by the bias model.
The following figure illustrates the computations.
Figure 2: You can change the orientation of the diagonal lines by clicking on the picture and dragging the mouse up and down.
The estimation of the intersection points between the edges
of the oblique and the straight lines is biased. As a result tilted edgels are estimated (shown in blue). These
edgels are the input to the next higher computational processes which fits long line to the edgels.
The input to the fittong process is information about the position and orientation of the edgels (shown in red) and this leads to tilted lines.
The theory also predicts the findings of parametric studies.
First, there is a stronger illusory perception for more tilted obliques (as can be experienced from the
above displays), because the bias increases with a decrease in the acute angle. Second, there is a stronger illusory effect when the pattern is rotated by 45 degrees.
It has beeen found in neurophysiological studies [20] that our cortex responds more to lines in horizontal and vertical than oblique orientations. Thus, if the pattern is rotated, there will be less response (that is in our model less edgels)
from the main lines. The result is more bias perpendicular to these lines and an increase in the perceived tilt.