Filling-in is a remarkable perceptual phenomenon in which visual features such as colour, brightness, texture and motion of the surrounding area are perceived in a certain part of the visual field even though these features are not physically present.
One extreme possibility is that our visual system simply ignores the lack of visual input and that filling-in is a passive outcome of this. However, various psychophysical experiments suggest that some active processes are involved in the occurrence of filling-in, and that some neural computation occurs in the brain when filling-in occurs.
In the past decade, several single-unit recording experiments in monkeys and functional MRI experiments in humans have examined neural activities related to filling-in in early visual cortical areas. Many of these studies found that neurons are activated in the region of the retinotopic map of the early visual areas that represents the interior of the surface where filling-in occurs.
Neural mechanisms of filling-in investigated in the above-mentioned studies must be distinguished from topographic remapping induced by binocular retinal scotoma. When retinal lesions are made at corresponding positions in both eyes (binocular retinal scotoma), reorganization of the retinotopic map of the visual cortex occurs, but this differs from situations in which other types of filling-in occur.
Traditionally, 'symbolic' and 'isomorphic' theory have been proposed as neural mechanisms of filling-in. Symbolic theory assumes that early visual areas extract only the contrast information, but this contradicts the results of most of the recent neurophysiological and neuroimaging experiments.
Isomorphic theory assumes that when perceptual filling-in occurs, a two-dimensional array of neurons with a point-by-point representation of visual features is activated in the early visual cortex. Although neurons are activated in early visual areas during filling-in, neural responses recorded during filling-in at the blind spot differ from those predicted by isomorphic theory in several important ways.
Selective activation of neurons in deep layers of the visual cortex that represent a particular spatial scale and that are selective for particular features might be involved in the process that mediates perceptual filling-in.
This research is still at an early stage, and many questions remain to be answered about the neural mechanisms of filling-in. Understanding the details of these mechanisms is important, because it might provide answers as to where and how subjective visual experience emerges, a fundamental question about visual perception.
Filling-in is a perceptual phenomenon in which a visual attribute such as colour, brightness, texture or motion is perceived in a region of the visual field even though such an attribute exists only in the surround. Filling-in dramatically reveals the dissociation between the retinal input and the percept, and raises fundamental questions about how these two relate to each other. Filling-in is observed in various situations, and is an essential part of our normal surface perception. Here, I review recent experiments examining brain activities associated with filling-in, and discuss possible neural mechanisms underlying this remarkable perceptual phenomenon. The evidence shows that neuronal activities in early visual cortical areas are involved in filling-in, providing new insights into visual cortical functions.
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This work was supported by grants from the Ministry of Education, Science, Sports and Culture, the Ministry of Public Management, Home Affairs, Posts and Telecommunications, the Japan Society for the Promotion of Science and the Human Frontier Science Program (HFSP).
The author declares no competing financial interests.
- Surface interpolation
The ability of our visual system to reconstruct a continuous surface from incomplete visual inputs using surrounding information.
- Contrast border
A boundary of a surface generated by spatially discontinuous change (contrast) in brightness or colour. Many neurons in the retina and early visual pathway are sensitive to contrast border.
The intensity of light emitted from a light source or reflected from a surface that can be measured objectively.
- Achromatic grating
A visual stimulus consisting of alternating light and dark bars in shades of grey.
- Motion aftereffect
Also known as the waterfall illusion. Prolonged observation of a moving stimulus leads to an aftereffect in which stationary objects appear to move in the opposite direction.
If a two-dimensional array of neurons in a given area corresponds topographically (in spatial arrangement) to those on the retina, this area is said to have retinotopic organization. Early visual areas have retinotopic organization with different degrees of precision.
- Primary visual cortex
(Also known as V1 or the striate cortex). The cortical area that is the main recipient of visual information from the retinae (by way of the lateral geniculate nucleus).
- Receptive field
The area of sensory space in which stimulus presentation leads to the response of a particular sensory neuron.
The second tier of visual cortical areas, which is adjacent to V1. V2 consists of three compartments that can be visualized by cytochrome oxidase staining; thick stripe, thin stripe and interstripe.
- V2 thin stripe
One of three compartments in V2 where many neurons have sensitivity to the colour or brightness of a visual stimulus.
The third tier of visual cortical areas, which receives its main visual input from V1 and V2. V3 can be divided into two areas because dorsal and ventral parts of V3 have different connections and cell properties.
- Extrastriate areas
A belt of visually responsive areas of cortex surrounding the primary visual cortex.
- Binocular rivalry
A phenomenon that occurs when each of a subject's eyes is shown a different image. This results in a bistable visual experience. For example, perception of horizontal or vertical bars spontaneously alternates when the two bar types are viewed through different eyes simultaneously.
- Macular degeneration
A disease of the retina in which the macula, the central part of the retina, degenerates.
- Pointwise representation
One way to represent surface attributes such as colour is to activate colour-selective neurons that have a small receptive field at each point on a surface. This is an example of pointwise representation.
- Spatial frequency
A variable determined by the width of stripes on a grating. A grating with low spatial frequency has thick stripes, whereas a grating with high spatial frequency has narrow stripes.
- Pulvinar nucleus
The pulvinar is a complex of several nuclei in the thalamus that have strong connections with many visual cortical areas.
- Lateral geniculate nucleus
(LGN). The LGN is a nucleus in the thalamus that acts as a major relay station for visual signals from the retina to area V1. The LGN also receives a massive feedback projection from V1.
Qualities of conscious perceptual experience, or the 'raw' feel of sensation, such as the 'redness' of the colour red.
- Depth assignment
When a retinal image contains multiple surfaces, the depth order of the surfaces is derived by using various monocular as well as binocular visual cues in the image.
A set of two images — one of which is presented to the right eye and the other to the left — that contain binocular stereo-disparity. Different parts of the stereo-image appear to be at different depths.
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Komatsu, H. The neural mechanisms of perceptual filling-in. Nat Rev Neurosci 7, 220–231 (2006). https://doi.org/10.1038/nrn1869
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