The psychometric curve is  obtained while recording the 
threshold of balance with the constant stimuli method. The 
data refer to a fixed value of the luminance of the 
reference, LBC.

In some cases this curve is simply sigmoid-like, and to the 
left of the threshold value, c, the weight of the reference 
is superior to that of the sample to the right, the weight 
of the test is superior. 

In  cases where the psychometric curve is compound, the 
balance occurs between a and b. To the left of a, and to 
the right of b, there is unbalance in op-posite directions.

The aperture problem arises as a consequence of the 
ambiguity of one-dimensional motion of a simple striped 
pattern viewed through an aperture. Consider one image that 
can be used to construct apparent motion through an 
aperture. The striped background is masked by an occluding 
porthole. If we move the stripes upwards the pattern of 
lines in the aperture shifts. If we move the stripes 
leftwards the pattern within the aperture shifts in the 
same way.