Filling-In

Filling-in refers to a family of perceptual phenomena in which the visual system completes missing visual information across regions of the visual field where no actual information is being received, such as the eye's physiological blind spot or a scotoma, a blind area caused by damage to part of the visual system; rather than perceiving a hole or gap in the visual field at these locations, a person perceives content that has effectively been filled in based on information from the surrounding parts of the visual field. One well documented example is blind spot completion, in which a textured pattern that spans across the physiological blind spot, when viewed with only one eye, is nonetheless perceived as a single continuous texture rather than as a pattern with a gap in it. A second example is Troxler's fading, in which steadily fixing one's gaze on a central point causes an unmoving stimulus in the peripheral visual field to gradually fade from awareness and be replaced by the color or texture of the surrounding background; a closely related effect occurs with any image that is artificially stabilized on the retina so that it does not move at all relative to the eye, since such an image's color and brightness likewise fade over time and the affected area fills in with the properties of its surroundings. Researchers have proposed two broad competing accounts of how filling-in works: isomorphic filling-in, in which a color or texture signal is thought to physically spread across a map of neurons representing the visual field until it is stopped by a contour, and symbolic filling-in, in which information about the missing region is instead represented abstractly, as a feature or a label, at a higher, cortical level of processing without any signal actually spreading across space; despite extensive research, which of these accounts, or what combination of them, best explains filling-in remains debated among neuroscientists.