Optical Illusion Eyes and the Visual Sense

In spite of the fact that our eyes are now mainly used for the identification of static objects and for establishing spatial relations, their basic functioning still rests on comparison of stimuli from neighbouring cells, which was fundamental to motion detection. When observing a static scene, the eyes perform small repetitive motions called saccades that move edges past receptors. If an image is stabilized on the retina, it soon darkens and disappears, since a motion detector responds only to motion. The same thing happens if the eye is exposed to a neutral, featureless scene, the Ganzfeld. Faint stars soon disappear when stared at, but return as soon as the image moves on the retina. Blood vessels in front of the retina surely cast shadows upon it, but these shadows are never seen. Any constant stimulus is ignored. The eye bears very little resemblance to a camera, except for its power to focus an image on the retina, and none whatsoever after this. Even a digital camera records point-by-point, which the eye does not do. The eye is not a pixel device, though location is very important.

Eyes are the portals through which electromagnetic radiation from our environment enters the visual system, exciting a flood of information from the distorted, two-dimensional image cast upon the sensitive cells of the retina. Most of vision takes place in the brain, and this begins in the retina, where the signals from neighbouring receivers are compared and a coded message dispatched on the optic nerves to the occipital cortex, behind the ears, where the information is formatted and made available to the processing activities of the brain. The eye is essentially a motion detector, its original purpose when eyes began to evolve from light-sensitive pits in the pre-Cambrian. The compound eye of the flying insect is an extremely sensitive motion detector, and well-suited to its purpose, since only moving things were of any interest. Even here, additional advantage was gained from the ability to receive light, the colours of a flower and the polarization of sunlight proving useful in certain cases. Arthropods, cephalopods, and chordates independently evolved eyes that made images, as the ability to identify objects and understand spatial relations proved valuable. The variety of these evolutionary inventions shows the great advantage of having a visual spatial sense. The most extraordinary eyes are those of the copepod Copilia, which physically scans an image projected by a lens with its light-sensitive organ, or the pinhole pupil of Nautilus.

The coded signals sent to the brain down the optic nerves are used to recognize objects, and recognition is necessary, indeed fundamental, for sight. This depends on past experience and learning, so sight is not an instinctive sense. The potential of the system, and its general characteristics are innate, of course, but the complete sense must be developed by experience. This is very easy in the young individual, and is done by comparing the information from other senses, mainly touch and smell, with the corresponding visual perceptions. Even in adults, the sense is not unalterable. An experimenter who wore glasses that inverted his retinal images so they were right-side up instead of the usual upside-down, gradually found that things started to look normal after a while. It is important to realize that vision is principally a function of processing in the brain, and that images and pixels play a very small role. Many objects are recognized from borders, at which changes occur, and the rest is inferred. The orientation of lines, and the directions of movement, are the major sources of information, and are specially coded. Visual sense is a property of consciousness, and is deeply involved in the functioning of the brain.

Structure and Functioning of the Human Eye

The retina is the light-sensitive part of the eye. There are two systems of receptors in the retina, the rods and the cones. In fact, the retina is actually a dual organ, a rod network and a cone network. Birds and reptiles have only cones, and some nocturnal animals only rods. The rods are sensitive to weak light, inoperative in strong light, and have maximum sensitivity at about 507 nm. The cones are sensitive to strong light, insensitive to weak light, and have a maximum sensitivity at 555 nm. Rod vision is called scotopic (dark-seeing) and cone vision is called photopic (light-seeing). Rods are located in all parts of the retina, and are very sensitive to motion, but give no colour discrimination. Cones are located most densely in a small area called the fovea, a shallow pit 1.5 mm in diameter, to the temporal side of the optic nerve. Their density decreases as one recedes from the fovea. The fovea has no blood vessels. All acute and colour vision is due to the cones. The fovea is surrounded by a pigmented area called the macula lutea, the yellow spot, 2-3 mm in diameter, containing the yellow dye xanthophyll, which absorbs light of wavelengths shorter than 500 nm. This spot cannot be seen in an ophthalmoscope against the living red choroid behind it in normal circumstances. Nerves from the rest of the retina go around it, as do larger blood vessels. It is richly equipped with ganglion cells, which indicates it is a site of signal processing. It plays an important but not clearly understood role in vision.

Light is electromagnetic radiation to which eyes respond. In light of a single wavelength, or spectrally pure light, the extreme range is from 380 nm to 740 nm. The sensitivity of the eye falls off at the ends of this range, so that 400 nm to 700 nm is a good approximation. Infrared radiation cannot enter the eye, and only warms its surface. Long-wave ultraviolet radiation causes fluorescence in the eye (especially in the visual purple), and the fluorescent radiation can be seen. Short-wave ultraviolet again cannot penetrate, but irritates the conjunctiva. Ultraviolet is damaging to the eye, causing irreversible changes. The dark-adapted eye is also sensitive to X-rays, which are not refracted by the eye and pass freely through it. This appears to be a direct sensitivity, since there is little fluorescence, and the stimulus can be moved around on the retina. Gamma rays can also be perceived, but this is again due to fluorescence, so a diffuse glow results. Do not try this at home!

The size of the pupil changes with different levels of brightness, expanding in dim light, and contracting in bright light, or when an object is held close. This change, over a range from 4 mm to about 8 mm, changes the retinal illumination of an extended object only by a factor of 16, far smaller than the actual dynamic brightness range of the eye of perhaps a factor of one million. A more plausible reason for the change in pupil size is to restrict the entering rays to the centre of the aperture when the illumination is sufficient, reducing aberrations and increasing the depth of field, while allowing the full aperture to be used in dim light. The rim of the lens, where the muscles are attached, is particulary irregular, and may exhibit diffraction at its radial fibres, so it is used only when necessary. Light entering near the edge of the pupil is less effective per unit area in producing retinal illumination than light passing through the centre of the pupil, a fact which could simply follow from optical principles, although there appears to be some controversy. This is known as the Stiles-Crawford effect. The slit pupil of the cat can be closed more completely in bright light than can a round pupil. The spacing of the rods in the fovea is roughly equal to the size of the diffraction pattern produced by the aperture of the pupil when contracted for vision in bright light. Given either the diameter of the pupil or the spacing of the rods, the eye is proportioned so that it could not be more acute, taking diffraction into consideration.

To the nasal side of the fovea is the blind spot, or papilla, where the optic nerve enters the eye, and where there can, therefore, be no receptors. This area is simply ignored by the eye and filled in with the neighbouring field, so that it does not disturb vision. The blind spots in the two eyes do not overlap, of course. It can occasionally be seen as a dark spot when the eye is first opened. At a distance of 7 ft, the blind spot is about 8 inches across, so it is not negligible, and may cause something not to be seen. To demonstrate it, draw two small spots on a card about 60 mm apart. Close the left eye, and fixate the left-hand spot with the right eye. When the card is at the proper distance from the eye, the right-hand spot will disappear. The retinal blood supply enters through the centre of the optic nerve.

The eye is almost a sphere, of 12 mm radius, consisting of three layers. The sclera is the tough white outer hide of the eye, merging into the cornea at the boundary called the limbus. The choroid is a dark absorbing layer, richly supplied with blood vessels. Cats have a reflecting layer of unpigmented fibrous tissue, the tapetum on the choroid. The sensitive cells of the retina are transparent cylinders of higher index of refraction than their surroundings, so they guide the light by total internal reflection. A tapetum causes light to pass through them twice, increasing the sensitivity of the eye to weak light. It also causes the pupil to become luminous, returning light in the direction it arrived, a well-known feature of cat's eyes. The choroid is continuous with the iris, which forms the pupil of the eye, and has radial and circumferential muscles to control its size. The ciliary body contains muscles to control the lens, to which it is connected by fibres called the zonule. The innermost layer comprises the retina and the lens. The ora serrata is the edge of the retina. The insertions of the six extrinsic muscles that move the eye are not shown in the diagram. The very sensitive conjunctiva lines the inner side of the eyelid, and covers the exposed front of the eye, including the cornea.

The eye is almost a sphere, of 12 mm radius, consisting of three layers. The sclera is the tough white outer hide of the eye, merging into the cornea at the boundary called the limbus. The choroid is a dark absorbing layer, richly supplied with blood vessels. Cats have a reflecting layer of unpigmented fibrous tissue, the tapetum on the choroid. The sensitive cells of the retina are transparent cylinders of higher index of refraction than their surroundings, so they guide the light by total internal reflection. A tapetum causes light to pass through them twice, increasing the sensitivity of the eye to weak light. It also causes the pupil to become luminous, returning light in the direction it arrived, a well-known feature of cat's eyes. The choroid is continuous with the iris, which forms the pupil of the eye, and has radial and circumferential muscles to control its size. The ciliary body contains muscles to control the lens, to which it is connected by fibres called the zonule. The innermost layer comprises the retina and the lens. The ora serrata is the edge of the retina. The insertions of the six extrinsic muscles that move the eye are not shown in the diagram. The very sensitive conjunctiva lines the inner side of the eyelid, and covers the exposed front of the eye, including the cornea.

The visual system is tolerant of errors in the retinal image, correcting them where possible. The eye has considerable spherical and chromatic aberration, so the image produced on the retina is very poor. The mental image is much sharper, refined by the visual system. The acute vision at the fovea is used to correct the mental image, when the focus is proper. Poor focus, however, makes edges indistinct, and since the system depends on edges, the result is discomfort and lack of sharpness in perception. The most common reason for poor focus is incorrect curvature of the cornea, where the majority of the focussing power of the eye resides (because of the large change in index of refraction at this surface). If the cornea is too steeply curved, which is quite common, distant objects are focussed short of the retina, and myopia is the result. Hypermetropia is the opposite case. An eye with neither is called emmetropic. Lack of sphericity of the cornea causes astigmatism, in which no stigmatic (point) focus exists. These errors of refraction may have other causes, as discussed in Helmholtz (Ref. 1). Young found that his marked astigmatism was not due to corneal curvature, for example. The lens becomes rigid with age, so that the ciliary muscles can no longer give it the curvature required to focus on close objects, a property called accommodation, and presbyopia is the result. All such errors of refraction can be corrected by external lenses, and an approximate correction is usually quite satisfactory. Because of chromatic aberration (the indexes of refraction vary with wavelength) blue and red are normally less well-focused than greenish-yellow. Visual perception contains little hint of this chromatic aberration, another proof that perception is subjective.

The pupil of another person's eye appears black to the observer. We have noticed that cat's eyes, and those of dogs and horses, reflect light that enters them back in the same direction because of the tapetum. In flash snapshots, the pupil often shows a fuzzy redness, caused by light entering the eye through the choroid and its blood vessels, then being scattered out the pupil. It is practically impossible to observe the back of the eye, as for medical purposes, by looking in the pupil, for three reasons. First, illumination of the retina is difficult, since it must enter via the pupil, and the light is not regularly scattered, but preferentially normal to the retina. Second, the eye is a refracting instrument, and the image of the retina produced by light going the reverse direction may not be in a suitable place for observation, and moves around as the subject's eye accommodates. Thirdly, the field of view limited by the iris is very restricted when the observer's eye is far enough away to see the retina clearly. These difficulties are overcome by the ophthalmoscope, invented by Helmholtz. The retina is illuminated by a light to the side, reflected into the pupil by a mirror with a small hole through it for viewing. The observer's eye can be brought close to the pupil by using a lens to place the image at a suitable distance for viewing. There are many possible arrangements, which have resulted in the design of small, portable instruments. A minimum ophthalmoscope can be assembled from a penlight held beside the observer's eye to illuminate the subject's pupil, and a converging lens for observation. The retina is still not easy to observe, since it is a thin transparent layer seen against the dark red choroid. It is possible to examine one's own retina by using a mirror (plane or convex) with a tiny hole in the centre. Hold the hole before the pupil and look at a source of light that is not too bright. The field of view is small, but may be moved about by moving the eye.

Properly focussed eyes can resolve two sources separated by about 6' of arc in everyday life. Exceptionally good observers can resolve down to about 4' or 3', but this is not common, and depends on the object viewed. The absolute limit of acuity, under laboratory conditions with fine gratings, seems to be between 1' and 2'. The Snellen chart for assessing visual acuity uses small characters (often E's or other hook-shaped patterns in various orientations) that subtend 5' at specified distances, and the width of whose lines are 1/5 the size of the pattern. The chart contains lines of patterns that subtend 5' at, say, 10 ft, 20 ft, 30 ft and 40 ft. If the chart is placed 20 ft from an observer, and the line subtending 5' at 20' can be resolved, the visual acuity is called 20/20. If the line subtending 5' at 40' can be resolved, the visual acuity is now 20/40, and so on.