Contrast gain mechanism or transient channel? Why the effects of a background pattern alter over time

If a brief test pulse is presented on a prolonged background pedestal, it is strongly masked if presented at the start of the pedestal, and is only weakly masked if presented 200 ms after the start. This has been suggested to occur due to contrast gain mechanisms that reduce the representation of the pedestal and therefore reduce its masking effects. We show here that the effect is due to the large transient in contrast that accompanies the onset of the pedestal. We find similar masking at pedestal offset, when the pedestal is continually flickered, or when pedestal and test have a high spatial frequency. These results were all predicted on the basis of sustained and transient channels.