Filling-in - Electrophysiology Recordings From Monkey Primary Visual Cortex

Electrophysiology Recordings From Monkey Primary Visual Cortex

When experiencing perceptual filling-in, the colour of an adapted surface is gradually replaced by the colour or texture of outside the surface. Friedman et al. (Friedman 1998; Friedman, Zhou et al. 1999) performed an experiment aimed at determining if surface activity of cells in monkey primary visual cortex changed in accordance with perceptual change or simply followed the modulation of the colour presented to the retina. Stimuli consisted of a disk-ring configuration similar to that illustrating the Troxler effect, but where the inner and outer part of the annulus have two physically different colours. After a few seconds of (peripheral) fixation, the disk tends to disappear, whereas the outer contour of the ring is perceived much longer, and the area of the disk is filled-in with the colour of the ring (Krauskopf 1967). These stimuli where intermixed with control stimuli, in which the physical colour of the disk was gradually changed to that of the ring. The animals were instructed to signal a colour change, and their responses to control stimuli and to test stimuli were compared in order to determine if monkeys perceive colour filling-in under steady fixation like humans.

The authors recorded the activity of surface- and edge-cells (cells whose receptive fields pointed either to the filled-in surface or to the border between the disk and the ring) in the visual cortices V1 and V2 while the monkey was performing the filling-in task. The activity of surface-cells correlated with the physical stimulus change in both areas V1 and V2, but not with the perceived colour change induced by filling-in. The activity of edge-cells followed the stimulus contrast when the disk colour changed physically; when the colours were constant, the edge signals also decayed, but more slowly. Together, these data are incompatible with the isomorphic filling-in theory, which assumes that colour signals spread from the borders into uniform regions.