The effect of size,retinal locus,and orientation on the visibility of a single afterimage

Unitary and fragmentary visibilities of the afterimage of a small rectangular bar were measured at three retinal loci. Unitary visibility was higher for fovea] than for parafoveal viewing, whereas fragmentation was greater the longer the bar. A model for estimating the size of receptors has been applied and gives the mean size of a “functional unit” to be 1.5 deg in the fovea and 3–4 deg parafoveally. Unitary visibility has been shown to be higher for vertical and horizontal bars than for obliquely oriented bars. Explanations for this result have been suggested in terms of the distribution, size, and selectivity of cortical orientation selective units.