The human eye has a small blind spot where the optic nerve connects to the retina, and the only reason you don't constantly see a hole in your visual field is that your brain is filling it in with educated guesses — meaning every moment of your visual experience is partly fabrication, and your conscious mind is the last to be informed.

Every human eye has, on close examination, a small region of the retina where there are no light-sensing cells. The region is located approximately twelve to fifteen degrees to the side of the visual center, slightly below the horizontal, and is roughly seven and a half degrees tall and five and a half degrees wide. This region is the place where the optic nerve, which carries visual signals from the eye to the brain, has to physically pass through the retina on its way out of the eyeball. The optic nerve, in passing through, occupies space that would otherwise contain photoreceptor cells. The result is a small zone of the retina that cannot, by structural design, detect any light at all.

This zone is called the physiological blind spot, or in medical literature, the punctum caecum. The Encyclopedia Britannica’s documentation describes it as the small portion of the visual field of each eye that corresponds to the position of the optic disc within the retina. The blind spot is not, on the available evidence, a defect of any particular eye. The blind spot is a structural feature of the design of the vertebrate retina, present in every human eye that has ever existed.

The structural feature that the wider cultural register has not, on the available evidence, fully absorbed is what the brain is doing about this. The brain is doing something that, when one thinks about it carefully, is considerably stranger than the standard cultural framing of “vision” tends to allow for. The brain is, in real time, every moment one is awake, filling in the missing area with content that is not, in any literal sense, being detected by the eye. The filling-in is performed automatically. The filling-in is performed without conscious awareness. The filling-in produces, in the conscious experience of seeing, a continuous visual field that does not match what the retina is actually sending to the brain.

What the filling-in actually consists of

It is worth being precise about what the brain is doing during the filling-in, because the process has been studied in considerable detail and the findings are, on close examination, more interesting than the simplified version typically allows for.

The brain, faced with a region of the visual field for which it is receiving no input, performs a particular kind of educated extrapolation. The extrapolation is based on the visual information surrounding the blind spot. The Exploratorium’s demonstration of this effect uses a straightforward illustration. If one draws a horizontal line across a piece of paper, with a dot positioned to fall on the blind spot when the paper is held at the right distance, the dot will disappear from view as expected. The line, however, will not appear to have a gap where the dot was. The line will appear continuous. The brain, in real time, has extended the line through the blind spot, producing a visual experience of an unbroken line even though the portion of the line that would fall within the blind spot is not, in any literal sense, being detected.

The extrapolation is sophisticated. The brain does not, in most cases, simply fill in with a generic color or pattern. The brain fills in with a best guess about what is likely to be there, based on the surrounding context. If the blind spot falls on a wallpaper pattern, the brain fills in with continuation of the pattern. If the blind spot falls on a solid color, the brain fills in with the solid color. If the blind spot falls on a person’s face, the brain fills in with what is, in some sense, an extrapolation from the visible portions of the face into the not-visible portions.

The accuracy of the extrapolation is, on the available evidence, surprisingly high in most ordinary contexts. The wider environment, in most cases, contains the kind of regular and predictable patterns that the brain’s extrapolation algorithms are well-calibrated to handle. The extrapolation, accordingly, usually produces a filled-in version that matches what is actually present at the location, because the actual content at the location is, in most cases, a continuation of what is present at the surrounding locations.

Where the extrapolation breaks down

The extrapolation is not, on close examination, always accurate. In the cases where the actual content at the blind spot is not a continuation of the surrounding content, the brain still fills in with a continuation of the surrounding content. The filled-in version, in these cases, does not match what is actually there. The viewer is not aware of the mismatch, because the viewer does not have access to information about what is actually at the blind spot. The viewer’s conscious experience is the filled-in version. The filled-in version is, in these cases, partially incorrect.

The most striking demonstration of this involves the disappearance of small objects. The standard demonstration involves holding a piece of paper with a dot positioned to fall on the blind spot, and observing that the dot is not, in conscious experience, replaced with a hole. The dot is, more accurately, replaced with a continuation of the surrounding paper color. The dot is, in some real way, edited out of the visual experience, and the editing is performed without any signal to the conscious mind that the editing has occurred.

This is the structurally interesting feature of the entire process. The visual experience the conscious mind is having is, in some part, fabricated. The fabrication is performed by lower-level visual processing systems. The fabrication is not, in any meaningful sense, reported to the conscious mind. The conscious mind has the experience of seeing the surrounding pattern continue through the area where the blind spot is, and has no internal signal indicating that this experience is a construction rather than a perception.

What this implies, more generally

The blind spot is, on close examination, one specific example of a wider pattern in how visual experience is constructed. The wider pattern is that the conscious experience of vision is, in considerable part, an active construction performed by the brain, rather than a passive registration of what the retina is detecting. The retina sends signals. The brain interprets the signals. The interpretation involves, in many places, filling-in, extrapolation, and best-guess reconstruction of regions where the input is incomplete, ambiguous, or missing entirely.

The blind spot is the most obvious case, because the region of missing input is large enough to be measured and the filling-in is easy to demonstrate. The less obvious cases include the various small ways in which the brain interpolates between the discrete samples the retina provides, smooths over the eye movements that occur many times per second, fills in details that the peripheral vision cannot resolve, and constructs a continuous three-dimensional spatial experience from the two slightly different two-dimensional images that the two eyes provide.

The cumulative implication is that the visual experience the conscious mind is having is not, in any literal sense, the visual information the eye is collecting. The visual experience is, more accurately, the brain’s best reconstruction of what is probably out there, given the partial information the eye is providing and the various assumptions the brain has accumulated about how the visual world is typically structured. The reconstruction is good. The reconstruction is, in most cases, accurate enough that adult human beings can navigate the world without bumping into things. The reconstruction is also, on close examination, not the same thing as direct perception of an external reality.

The acknowledgment this article wants to leave

The blind spot is the small structural feature of the vertebrate retina that gives the wider cognitive process away. The blind spot is the place where the missing input is large enough that the filling-in can be measured. The filling-in is, on the available evidence, occurring constantly, at every moment of waking experience, in every part of the visual field that lacks complete sensory input. The conscious mind has no access to the filling-in process. The conscious mind is, in some real way, the last to be informed about which parts of its experience are based on direct sensory input and which parts are based on the brain’s educated guesses.

The wider implication is that the standard cultural framing of perception as the direct seeing of an external world is, on close examination, not what is actually happening. The framing is the conscious experience the brain has constructed. The actual process is considerably more active, more constructive, and more dependent on prior assumptions than the framing acknowledges. The constructed nature of perception is, in some real way, what visual neuroscience has been documenting for the last seventy years. The wider cultural register has, on the available evidence, mostly continued to operate as if the standard framing were accurate.

The not-updating is, in some real way, a structural feature of how conscious experience presents itself to itself. The visual system gives the conscious mind no signal that the reconstruction is occurring. The conscious mind, accordingly, has no internal reason to suspect that what it is experiencing is anything other than the direct perception of the world. The internal reason has to come, in most cases, from the outside, in the form of articles like this one, which can demonstrate the filling-in by directing the conscious mind to a small piece of paper with a dot on it and asking it to notice what happens to the dot when the paper is held at the right distance. The dot disappears. The disappearance is the evidence. The evidence has been there, in every human eye, the whole time.