The existence of a small blind region in the normal human eye was predicted in the seventeenth century by the French scientist Edmé Mariotte. While dissecting a human eye, Mariotte noticed the ‘optic disc’ — a hole in the back of the eyeball through which all the nerve fibres that make up the optic nerve leave the eye. He realized that, unlike the rest of the retina that surrounds the hole, this optic disc is devoid of light-sensitive photoreceptors. Applying his knowledge of optics and of the anatomy of the eye, he deduced that every eye should be blind in a corresponding small portion of the visual field. The optic disc lies about 15 degrees to the nasal side of the fovea (the part of the retina that we point towards things when we fixate them). Since the image on the retina is inverted by the optics of the eye, the ‘blind spot’ in the visual field lies to the right of the point of fixation for the right eye and to the left for the left eye.
Fig. 1
You can confirm Mariotte's observation and find your own blind spot by viewing Fig. 1. Close your right eye, hold the book about a foot away from your face and look fixedly at the little black dot on the page. Keep looking at the dot as you very slowly move the page towards you. At some critical distance, the round, hatched patch will fall on your blind spot and disappear completely. However, notice that when the patch disappears you do not experience a black hole or void in its place. You simply see this region as being filled with the same light grey as the background — a phenomenon called ‘filling-in’ or
perceptual interpolation.
The Victorian physicist Sir David Brewster was so impressed with this filling-in that he attributed it to God. In 1832 he wrote, ‘We should expect, whether we use one eye or both eyes, to see a black or dark spot on every landscape within fifteen degrees of the point which most particularly attracts our notice. The divine artificer, however, has not left his work thus imperfect … the spot, in place of being black, has always the same colour as the ground’. Curiously, Sir David was not troubled by the question of why the ‘Divine Artificer’ should have created an imperfect eye to begin with.
Since the left eye's blind spot is 15 degrees to the left of fixation and that of the right is 15 degrees to the right, they do not coincide with each other in the visual field. The region of space that falls on the blind area of one eye falls on seeing retina in the other eye. So, if we have both eyes open, we should certainly not expect (as Brewster did) that we should be aware of the blind spot. However, the filling-in of the blind spot that occurs even when the other eye is closed can be explained only by some compensatory process in the brain.
You can explore the limits of the filling-in process by viewing Fig. 2. Notice that when the disc disappears you do not see a gap in the line — you see it as continuous, right through the blind region. It is as if neurons in the visual part of your brain make a statistical estimate: they realize that it is highly unlikely that two separate line segments are precisely lined up on either side of the blind spot simply by chance. So they signal to ‘higher’ centres in the brain that this is a single continuous line — and that is what you see. On the other hand, if you ‘aim’ your blind spot at the corner of a line drawing of a square, the corner does get perceptually ‘chopped off’. Your visual system does not complete the missing corner. There are clear limits to what you can fill in.
It is unlikely that filling-in is just a quirk of the visual system that has evolved for the sole purpose of dealing with the blind spot. Rather, it appears to be a manifestation of a very general ability to construct surfaces and bridge gaps that might be otherwise distracting in an image — the same ability, in fact, that allows you to see as a complete object anything that is partly hidden from view — for instance, a rabbit behind a picket fence looks like a complete rabbit, not a sliced-up one. In our natural blind spot we have an especially obvious example of filling-in — one that provides us with an experimental opportunity to investigate the ‘laws’ that govern the process and their underlying physiology.
The physiological organization in the
brain corresponding to the blind spot (and possibly to filling-in) has recently been explored in monkeys. In both monkeys and people the retinas of both eyes are mapped systematically on to the primary visual cortex of the cerebral hemispheres (called
area 17 or
striate cortex). The maps of the visual field seen by the two eyes are in fairly precise registration — an anatomical convergence that provides the basis for
binocular fusion (singleness of vision) as well as stereoscopic depth perception. But, in each hemisphere (which receives information about the opposite half of the visual world), there is a region of the map corresponding to the blind spot of the opposite eye. Obviously, this region of the visual cortex receives no nerve fibres from the blind region of one eye. So what happens to the map in this region? Does the hole in the map get ‘sewn up’, or is there a big gap corresponding to the blind spot? The answer is simple. There is indeed a ‘gap’ in the map in the visual cortex, corresponding to the region that should receive signals from the blind portion of the opposite eye. But this patch of cortex does get signals from the region of the other eye that is looking at the same portion of the visual field. So, in terms of the field, the map in the brain is continuous, even though the input from the opposite eye is missing in the region corresponding to the blind spot.
Fig. 2
Intriguingly, within the part of the cortical map that represents the region of visual field that can be seen by only one eye, nerve cells receive input from the region of retina immediately surrounding the blind spot. This aberrant input might help explain the filling-in phenomenon. A cortical nerve cell in this region will fire impulses if a long line straddles the blind spot, or even if two halves of the line are displayed on either side of the blind spot, but not if only half the line alone is displayed. This implies that there is complicated (non-linear)
summation of signals converging on to the cell from the retina surrounding the blind spot. This process might explain filling-in.
Filling-in can occur for parts of the retina other than the optic disc. For instance, if a part of the
peripheral visual field (far from the fixation point) is blanked out by an opaque occluder, lines that run across the occluded region appear complete, just as for the blind spot itself. This suggests that the process that underlies filling-in is not confined to the cortical representation of the blind spot and may play a very general role in the interpolation of contours and surfaces across occluded regions of the visual field.
The blind spot is also important clinically because the optic disc becomes enlarged in certain conditions. An example is
papilloedema — a swelling of the optic disc that occurs when the pressure inside the skull is increased, for instance because of a brain tumour. Examination of the retina through an ophthalmoscope can reveal such an enlarged disc directly, but it can also be revealed indirectly by mapping out the area of blindness of the blind spot with a device called a
perimeter, in which tiny spots of light are flashed in various positions across the visual field and the patient is simply asked to say whether they see them.
— V. S. Ramachandran
Bibliography
- Barlow, H. B., Blakemore, C. and Pettigrew, J. D. (1967). The neural mechanism of binocular depth discrimination. Journal of Physiology.
- Gatass, R., Fiorani, M., Rosa, M. P. G., Pinon, M. C. F., Sousa, A. P. B. and Soares, J. G. M. (1992). Visual responses outside the classical receptive field, a possible correlate of perceptual completion. In The visual system from genesis to maturity, (ed. R. Lent), pp. 233-44. Birkhauser, Boston, MA.
- Ramachandran, V. S. (1992). Blind spots. Scientific American, 266, 85-91
See also eyes; vision.
