The Blind Spot
The natural blind spot (scotoma) is due to lack of receptors (rods or cones) where the optic nerve and blood vessels leave the eye. There can also be artificial blind spots when something blocks light from reaching the photoreceptors, or when there is local adaptation of the retina as just after seeing a bright light. Blindness is absence of seeing. It may be experienced as blackness, or very differently it may be nothing. The sudden blindness of switching off the light is blackness (and black is a colour); nothingness is lack of visual sensation, as for the world behind one’s head.
The ''natural'' blind spot
The natural blind spot occurs where axons passing over the front of the retina converge to form the head of the optic nerve, resulting in a hole in the photoreceptor mosaic. Natural blind spots are present only in the eyes of vertebrates; cephalopods, such as octopus, more rationally have retinal axons that pass over the back of the retina, so the optic nerve does not have to pass through the photoreceptor layer to exit the eye. These differences reflect the different origins of vertebrate and cephalopod eyes: invertebrate eyes develop from the skin whereas vertebrate eyes, including human eyes, are outgrowths from the brain and apparently reversal was embryologically impossible. Human eyes are irrational as the light has to go through the blood vessels and nerve fibres before reaching the retina. The blood vessels especially can be seen by looking through a pinhole in a card and moving it back and forth. The shadows cast by the vessels appear (by motion parallax and selective adaptation) like a spider's web, but only while the card is being moved and for a very short time afterwards.
Lengthy fixation of a small light (or stimulating the eye with a bright flash), can produce a few seconds of local blindness or temporary general loss of vision. This is known as the Troxler Effect, which is not fully understood but seems to originate in the retina.
Seeing the blind spot
Each eye has a surprisingly large blind region, about 4° of visual angle, the width across your four fingers held at arm's length. Luckily, they are in different locations in each eye, the one in the left eye is about 10° (two hand widths at arm's length) to the left of central vision, and the one in the right eye, equally far out on the other side. Surprisingly, we are normally unaware of these natural blind spots. They are either filled in perceptually (a remarkable phenomenon) or they are ignored and so not seen. These are very different possibilities for explaining why the eyes’ blind spots are not generally seen or noticed, even when one eye is covered. The blind spot can be demonstrated very simply on paper with a fixation point and for example a letter such as an X, separated horizontally.
Fixate the O with only the right eye open, and move your head slowly nearer and further until at a certain distance (about 25 cm) the X will disappear, when it falls on the blind spot of the open right eye.
Active and passive theories - created or ignored
As we have said, there are two hypotheses concerning the invisibility of scotomas. Either they may be filled in by brain activity, or simply ignored. There is evidence for both, under various conditions, giving scotomata and ‘filling in’ surprising importance for understanding the physiology and psychology of vision. These two ideas have very different implications: creating implies active brain processes of memory and imagination affecting vision, ignoring is simply rejecting what is there or what is signalled. The American philosopher Daniel C. Dennett (Dennett, 1991) has suggested passive ignoring as important, including for thinking about consciousness. However there is physiological evidence (Komatsu et al, 2000) for active creation of perceptual phenomena in the absence of stimuli, suggesting there is more than ignoring.
Phenomena attributed to filling-in
The creative hypothesis is the active filling-in of blind regions by surrounding objects or patterns, including their colours, which are seen though there are no signals from this region of the eye. Filling-in is more easily seen over small areas. In the above experiment the filled-in region is seen as white, as it is surrounded by white; but filling-in works for any colour and even for complex patterns. Surprisingly, filling-in of blind spots or blank regions occurs with dynamic random visual noise such as the ‘snow’ on a television screen when not tuned to a station, looking like the wild dancing of hundreds of frenetic ants (Ramachandran and Gregory, 1991). This is very strong evidence for active creation by the brain as there is no such stimulation at that time from the eyes and this is very different from the familiar static afterimages.
Authorship of this article
This article was written by Richard Gregory shortly before his death on 17 May 2010. In order to continue with the editorial process of approving the article, I asked Patrick Cavanagh to become a co-author and, ultimately, the article's curator.
Robert P. O'Shea, Editor, Vision.
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