Filling-in
Classical demonstrations of perceptual filling-in involve filling in at the blind spot in monocular vision, and images stabilized on the retina either by means of special lenses, or under certain conditions of steady fixation.
There is general agreement that edges play a central role in determining the apparent colour and lightness of surfaces through similar filling-in mechanisms.
These signals are strong because receptive fields are exposed to contrast, and reliable because the border produces continuous light modulation even during fixation, due to small residual eye movements.
Together these data suggested that mechanisms for the filling-in of colours, motion and texture can be dissociated and may correspond to processes in higher-order areas that are specialized for these attributes.
Striking evidence implying a spreading of neural activity like the one postulated by isomorphic filling-in theory is given by experiments of backward masking after brief presentations of uniform surfaces or textures.
The working hypothesis of these experiments is that if a response initially biased toward the boundaries fills-in to represent the interiors of uniform surfaces, it may be possible to interfere with the filling-in process and leave the percept at an incomplete stage.
This experiment is grounded on the assumption that filling-in consists of a spreading of neural activity from the boundaries of luminance and through the surfaces, that is stopped when another luminance-contrast border is reached (this is proposed by many models of brightness perception, see for example Walls 1954, Gerrits and Vendrik 1970, Cohen and Grossberg 1984), and that the process takes some time to be completed.
Moreover, the minimum target-mask delay at which the masking was effective increased with target size, suggesting that there would be a spreading phenomenon and that the farther the features delimiting a region, the more time is necessary for the filling-in to be completed.
Electro-physiological recordings in retinal ganglion cells, LGN and primary visual cortex showed that neurons of these areas responded to luminance modulation within the receptive field even in the absence of contrast borders.
The behaviour of primary visual cortex neurons seems to be in agreement with the one hypothesized by an isomorphic filling-in theory in that they both respond to luminance of the surfaces also in the absence of borders, and their activity is modulated by that of edges far outside the receptive field.
Moreover, when the temporal frequency of luminance modulation in the surrounding patches exceeded a threshold value, the induced response disappeared, suggesting that it was the result of a spreading of activity, taking a finite time to happen, likely explainable in the context of isomorphic filling-in.
They argued that such an idea would be the result of the false belief that in our brain there is a spectator, a sort of homunculus similar to ourselves, needing a filled-in image representation.
The symbolic filling-in theory postulates that such a "homunculus" need not exist, and that image information is transformed at the cortical level into an oriented feature representation.
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.
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 neuronal activity in different brain areas can be recorded in humans through non-invasive techniques, like fMRI (functional magnetic resonance imaging).
