Visibility of an afterimage in the presence of a second afterimage

Visibility of an afterimage has been measured for a single rectangular bar on its own and in the presence of a second bar. Visibility of each bar is increased if the two bars are in the same orientation and closer than 1 deg (this effect being called “facilitation”) and is decreased if the bars are in different orientations (this effect being called “inhibition”). These interactions also depend on the relative size of the bars. If a pattern of two bars is viewed dichoptically, rivalry between the bars occurs causing a mutual decrease in visibility. The rivalry effect and the orientation-specific effects discussed above are additive in the dichoptic viewing condition. The orientation-specific effects have been likened to those found in other psychophysical studies of angle detection.