The retinal blind spot is a small region in the visual field corresponding to the area on the retina where the receptor nerves are bundled to form the optic fiber, thus excluding the presence of receptors themselves. This hole in the visual field is simply ignored by the mind. Indeed, awareness of it would be redundant and a nuisance, since the blind spot is standard equipment in every healthy eye. However, destructive lesions in the retina or optic nerve give rise to an actual experience of a blank area in the visual field. It makes sense that the abnormal condition of lesion at this primary level must be noticeable, since it could represent an absence of light in the real world. It is the job of the retina to pass on this information to the cortex, which is able to adapt with time to such lower-level injuries. Lesions in the visual cortex itself can cause similar defects in sensitivity without ever being perceived. The patient may deduce the presence of a defect, but is unable to see it directly since, so to speak, there is no one higher up to countermand the orders of the cortex.
Neurophysiologist Karl Lashley noted that the transitory scotoma he experienced due to migraines were filled in under certain conditions. Specifically, when he gazed at checkerboard patterns, or other regular patterns of wallpaper, these appeared subjectively completed over the missing parts of his visual field.
A person dressed in a black costume, so as to be invisible in the dark room, except for small white dots strategically placed at the body's joints, is seen only as a pattern of dots until the person begins to move. Then the invisible outline of the human form appears subjectively as a moving shadowy figure. The tribulations of proofreaders also demonstrate a form of pattern completion, since there is a tendency for errors to be seen as correct in their context. And short deletions in the spoken word may also go unnoticed, the normal flow of the sentence being subjectively assumed.
Phenomena of apparent change are further examples. One of these is the characteristic of human vision responsible for the ability to see a motion picture as a continuous changing image rather than a series of distinct stills. When a spot of light is briefly flashed against a darker background, followed within a specific interval by a similar spot flashed a short distance away, the subjective effect is of one spot moving from the first position to the second. If the time interval is shorter, two simultaneous flashes are perceived; if longer, two successive flashes. This is the deceptively familiar principle of the "moving" lights of the illuminated marquee. Such effects raise an interesting question: how does the mind know where the next spot will occur, in order to fill in between the two? One investigator has speculated that the intervening apparent motion is projected backward in time. Other kinds of apparent changes include transformations of size and shape. For instance, if instead of a dot a square is flashed, followed by a triangle and a circle, what will be seen is one figure changing into another in a smooth transition from square to circle.
The projection of experience backward in time is implied by other experimental evidence as well. The paradox of habituation is demonstrated, for instance, by the familiar experience of becoming accustomed to a sound, so that it is only noticed when it stops. Neurologically, an orienting response in the pattern of brainwaves begins when the habituated signal ends. One's attention is suddenly drawn to the "deafening silence". Similarly, the phenomenon of backward masking indicates that awareness of a brief, near-threshold visual stimulation can be obliterated by another, longer stimulus occurring shortly afterward. Other experiments compare the timing of experiences due to stimulation of the skin, with that of similar experiences due to direct stimulation of the part of the cortex receiving impulses from the skin. These demonstrate that the skin stimulus is subjectively referred backward in time about half a second-- presumably to compensate the time it takes for the nerve impulse to reach the cortex from the sensory surface. Strictly speaking, it seems that sensations are always replays of past events, for which we nevertheless have the subjective sense of real-time participation.
Sensations of vision and hearing are projected outward in space as characteristics of objects in an external world. This capacity of spatial projection can be acquired by the skin surfaces as well. Such projection is an everyday tactile experience. Anyone who has had to loosen a screw in a visually inaccessible place is familiar with the extension of one's sensitivity of touch to the tip of the screwdriver. No doubt blind people experience the tapping of their canes as out there more than as sensations in the hand.
Daniel Dennett  has criticized the interpretation of a range of completion effects that are by now quite familiar. I believe his point is well-taken, that the apparent experience of completion in the visual field is in truth a (mis)judgment-- something like seeing a snake when there is actually a rope-- rather than a veridical perception of some sensory quality supplied by the mind with some kind of existence. It is the projective capacity of the mind (the ability to even make such mistakes) that I wish to underline with these examples.
The projective adaptability of the brain has been dramatically studied by a type of experiment first performed around the turn of the century and subsequently repeated in several variations. First let us note that in normal vision the optical image on the retina is upside down, since it is inverted by the lens of the eye. In other words, the brain normally adapts itself to this inversion, so that the world is subjectively experienced rightside up. What would happen if the optical image itself were re-inverted? In the experiment performed, a subject wore a special lens over each eye which inverted the retinal image-- and therefore inverted the subjectively experienced visual field. Needless to say, this resulted in a very disorienting experience. Over several days, however, the subject was able to adapt to this condition-- if not perfectly and consistently-- so as to experience the world once more as normal and right-side up! Though the results of this drastic experiment were equivocal, less extreme distortions of the visual field have proven conclusively the adaptability of the human visual cortex. The effects of lenses which systematically curve the visual field, of prisms which displace it by a few degrees to one side or the other, and of special glasses which color half the visual field red and half green, have all been compensated after a few days or even hours of getting used to them, so that normal experience is restored. In one instance, where prismatic spectacles were worn that displace the visual input by a few degrees, subjects would at first consistently misreach for objects about them. But after only one hour of actively walking about, the subjects completely and exactly compensated the displacement. When the subject was passively moved about the same environment in a wheel chair, little compensation took place. This suggests that we do not see the world as passively presented by the senses, but rather as we come to know it through active interaction. We are reminded of the homunculus who, solely through interaction with changes on his display panels, pieces together a functional knowledge of an outside world-- a knowledge which is experienced as seeing. Just as the spectacled subjects in these experiments learn to see in their retinal displays the world implied through their active explorations, the little man comes to see, in the patterns of his instrument readings, a real external realm.
Not every creature has such adaptive flexibility. If the eyes of the goldfish are surgically inverted, it never manages to compensate for this disruption, always swimming away from the true location of its food. Experiments similar to those described with human subjects were performed with monkeys suffering from various cerebral lesions. Frontal lobotomy effectively prevented the hapless creatures from compensating a displaced visual input, whereas other cortical removals did not. Apparently, the adaptivity concerned in such experiments is a high-level function of the most complex nervous systems. This suggests that the very experience of reality, of the world, is the result of the same order of high-level activity as the experience of self. Subject and object rise and fall together.