Duration of visual afterimages on modulated backgrounds: postreceptoral processes

A foveal afterimage produced by a small photoflash increases in duration when the luminance of a 5.8 degrees diameter background on which it is seen is temporally modulated. At a modulation frequency of 1 Hz and a depth of modulation of 52%, the duration of the afterimage is prolonged by 335% compared to the duration obtained on a steady background. This increase has been attributed to the functional border resulting from the difference in excitability between bleached and unbleached photoreceptors. Afterimage duration is also prolonged, although only by 20%, when the luminance of the background is kept constant, while the luminance of an annular surround is modulated. This finding suggests a weak effect of neural lateral interaction (via area contrast). If the background luminance is modulated only in the contralateral eye (dichoptic presentation), afterimage duration increases by as much as 54% compared to the unmodulated state. This result indicates that afterimages, in part, are sustained by processes mediated by the visual cortex.