I am trying to understand what is visual masking (or more specificaly forward masking). I have read the explanation here, but still..

I understand that a background can interfere with a frontal visual stimulus. Anyone knows that. If I use whithe letters on white page I will see nothing. But, I assume the definition has more then that.. for example, in here, in figure 1b, are they describing an experiment? If yes' then what is the experiment? I mean, what is the stimulus, what is the masking, what is the experiment question, and what is the result (I assume that there are known results to what the figure is describing...)

Thank you!!