Optical Illusion Nervous Aspects of the Visual Sense

Although the study of vision has always been pursued mainly in an empiric tradition, especially by physicists, during the 19th century a school of intuitionists gained considerable following, especially among psychologists, of which significant traces remain. The intuitionist postulates an inherent sense of space and movement, in which retinal points were mapped onto physical space. In their view, the visual system only correlates external objects with pre-existing concepts. This view gave rise to elaborate mathematical theories of the correlation of the retinal images in binocular vision, as one example. The observation that when sight was restored to individuals blind from birth, they could not make any correlation between the knowledge of objects gained by touch and their new visual apperceptions argues strongly against intuition, as do many other observations showing that mental processes and learning are important. The original intuitivist concept seemed to be that there existed a conscious, nonphysical ego, for which the body was only a vehicle, and the senses were a means of communication with it. The external universe was conceived as a material expression of a similar ego, so the correlation would naturally be close. A modern echo of intuitionism is the discovery that some basic properties of vision, such as edge perception, are to be found in the arrangement and structure of the sensory nervous system, independently of higher brain functions, and do not need to be learned or developed.

The signals from neighboring receptors in the retina are grouped by the horizontal cells to form a receptive field of opposing responses in the centre and the periphery, so that a uniform illumination of the field results in no net stimulus, but a difference in illumination of the center and the periphery does. Some receptive fields use colour differences, such as red-green or yellow-blue, so the differencing of stimuli applies to colour as well as to brightness. There is further grouping of receptive field responses in the lateral geniculate bodies and the visual cortex for directional edge detection and eye dominance. This is low-level processing preceding the high-level interpretation whose mechanisms are unclear. Nevertheless, it demonstrates the important role of differencing in the senses, which lies at the root of contrast phenomena. If the retina is illuminated evenly in brightness and colour, very little nerve activity occurs.

There are 6 to 7 million cones, and 110 to 130 million rods, in the average human retina, but only some 800,000 fibres in the optic nerve. The connections cannot, therefore, be simple, and the amount of information sent to the brain for interpretation is huge. By contrast, the auditory nerve has only about 15,000 fibres. The optic nerves cross at the optic chiasma, where all signals from the right sides of the two retinas are sent to the right half of the brain, and all signals from the left, to the left half of the brain. Each half of the brain gets half a picture. This ensures that loss of an eye does not disable the visual system. The optical nerves end at the lateral geniculate bodies, halfway back through the brain, and the signals are distributed to the occipital (visual) cortex from there. The visual cortex still has the topology of the retina, and is merely the first stage in perception, where information is made available. Visual regions in the two cerebral hemispheres are connected in the corpus callosum, which unites the halves of the visual field. The connections of the visual cortex are complete at birth, and are highly ordered, not random, but surprisingly uniform. A study of all this wiring reveals remarkably little about visual perception, and it is not known how it all works.

The presence of three photosensitive proteins in the eye (the cones are of three types) is the reason that colours can be matched by mixtures of three monochromatic lights. Unlike the ear, the eye does not perform a Fourier analysis of its stimulus, but merely interprets the relative stimulation of cones that possess the three different proteins. Light of many different compositions can produce the same perception of colour. That there were three coordinates in colour was first recognized by Thomas Young (1801), reiterated and studied by James Clerk Maxwell, and placed on a firm foundation by Hermann von Helmholtz. It became the basis of colour measurement and specification, because it was empirically validated. There were dissenting views, remarkably by Goethe, and more recently by Edwin Land (who, incidentally, had anomalous colour vision). The final discovery of the three optical proteins firmly established the trichromatic theory (1959).

Rhodopsin, or visual purple, was the first light-sensitive visual protein to be recognized, in the rods. The absorption of a quantum or two of light causes a structural change that makes the protein uncomfortable in its usual position, and in shifting to relieve this triggers a nerve impulse. The protein returns to its normal sensitive structure with a certain half-life, of the order of 15 minutes. When the light is too strong, all the rhodopsin is in the modified, or bleached, form and the rod cell is insensitive to light. The search for similar proteins in cones was protracted and difficult, but three relatives of rhodopsin with different spectral absorption were eventually identified. When bleached, these return to normal in about 7 minutes. The dark adaptation of eyes is mainly about restoring the sensitive forms of these proteins. Purkinje (Pur-KIN-yee), the remarkable Czech observer, reported in 1825 that the relative intensity of the red and blue in some signs he saw was different under high and low illumination. Blues can become quite vivid in dim light, while reds become black. There is no Purkinje effect in foveal vision, where there are no rods; the area simply becomes insensitive at low light levels.

When the optic nerves of young mammals were surgically rerouted from the optical cortex to the auditory cortex, sight was preserved, at least partially. This clearly shows the adaptability of the brain, and that a large part of its functioning results from learning, not from genetically imposed organization. It also shows that the higher visual functions are not localized in the visual cortex, which must only be a kind of switchboard supplying data. In people blind from birth, it has been found that the visual cortex assumes other functions.

Once, when I developed a peripheral floater in one eye, the annoyance was compensated by the opportunity to study my eye motions while performing visual tasks. A floater is a piece of opaque substance in front of the retina, usually a small blood clot escaped from a capillary. While reading a book, vision fixated on one side then the other. The leftward motion was obvious when the floater suddenly jerked into view before the visual sense cancelled it. There was one scan for every one or two lines. It was not line by line, and was not a continuous scan. When reading the narrower columns of a newspaper, there was no such scan, and one fixation did the job. This clearly shows the superiority of narrow columns in facilitating reading. When the fixation jumped from one point to another, the perceived view did not change in the least, not even blinking. This clearly shows that the perceived view is not the picture projected on the retinas. The retinas merely collect information that is used to create the view.

Colour

Stereopsis and colour sense are interesting and fundamental aspects of vision that are subject to experimental study and understanding. Let us first consider colour. Burnham, Haynes and Bartleson (Ref. 3) defined colour in the following terms:

"Colour is the attribute of visual experience that can be described as having quantitatively specifiable dimensions of hue, saturation, and brightness."

The CIE (Commission Internationale d'Éclairage) system of colour specification uses primaries at 700 nm, 546.1 nm (Hg), and 435.8 nm (H), which are related by a mathematical transformation to three standard stimuli. The corresponding tristimulus values X, Y and Z are weighted averages of the radiant flux, weighted by standard functions of wavelength that represent the standard observer. The normalized values or chromaticity coordinates are x = X / (X + Y + Z), y = Y/(X + Y + Z), and z = Z/(X + Y + Z). Y represents the luminance of the colour, since the weighting factor is simply the spectral sensitivity of the eye. These primaries are essentially red, green and blue, the same colours used in a colour video screen.

A recent discussion involving the roles of Goethe and Land in colour theory well demonstrates the muddle that arises when the objective and subjective aspects of colour are mixed (See References). We understand the physical basis of the spectral composition of light, which Newton studied, very well indeed. He showed that colour was not an acquired condition of light, but was dependent on its physical nature. The physiological aspects of vision, such as visual pigments, are now mainly understood. The sensory aspects are poorly known, except for superficial properties, like all mental processes. It should always be remembered that equal visual stimuli do not produce uniform and equal results, although this is true to a certain approximation, as evidenced by the success of the CIE colour specification. Land's experiments show clearly the limitations of a dogmatic physical view, but vision is physiologically a tristimulus affair after all. Goethe's experiments, however intelligent and interesting, led nowhere, either in the physical basis of colour or in the subjective. Newton's theory of colours is, indeed, quite defective, but it is the ancestor of our modern understanding.

As one perambulates the locus of spectral colours, the colour names are blue, green, yellow, orange, red, and then purple on the line between red and blue. Some authors say that the extreme blue has a slight reddish tinge that makes the traditional name of violet appropriate. When people are asked about their colour preferences, there is a great disparity of opinion, but weighted averages of the responses rank the colours in the order blue, red, green, violet (purple), orange, and yellow, for both sexes. Youth prefers warmer colours, maturity cooler ones. Yellow was the Chinese imperial colour, and violet the colour of Japanese royalty. Black was the colour of life and warmth in China, white the colour of death and cold. In Europe, it was the opposite. Saffron and black are the colours of hell in Pakistan, but saffron is the colour of Buddhist monks' robes. Ideas on colour harmony vary greatly. There are four psychological primaries that probably have no fundamental significance, other than retaining a hue invariant under changes in intensity. The yellow is produced by monochromatic 577 nm, green by 513 nm, blue by 473 nm. The red is a mixture of red and blue complementary to 495 nm. The spectral colours are called simple or homogeneous because they consist of a single wavelength. They have no special significance in colour perception, except for providing the most saturated possible hues. The same colour sensation can be produced by a wide variety of stimuli; no one-to-one relationship exists, as there is between frequency and pitch in hearing. There is no harmonic relation between colours, as in music, though many investigators have sought for such relations, beginning with Newton. In fact, colour is a secondary character of light, like timbre in music. The primary character of light is its spatial distribution, as the primary character of sound is its temporal distribution.

Mixing of lights was first effectively demonstrated by Maxwell's colour top (before 1855), in which segments of coloured paper provided quantitative amounts of colour that were mixed in the eye by spinning the top. It is very easy to make a Maxwell top for yourself. Such experiments cleared up the confusion about additive and subtractive colour mixing that had confused painters for a long time. There are no true primary colours that can be mixed to produce all colours, but three properly chosen lights can be mixed to produce a very wide range of colours, as we have already mentioned above. Maxwell's colour top was also used to study and diagnose colour blindness.

Danger (Ref. 2) gives a great deal of the lore of colour applications. He says that colour means pigments to an artist, an internal perception to a psychologist, the response of a neural system to a physiologist, an aspect of radiant energy to a physicist, and a property of objects and lights to the man in the street. We have mentioned all of these meanings so far except the first. The mixing of colours had been difficult to explain since ancient times. When pigments are mixed, the usual result is an obscure dark colour, not what one might expect (confirming Aristotle). Mixing pigments is subtractive mixing. Here, magenta subtracts green, cyan subtracts red, and yellow subtracts blue. My watercolours in primary school were red, yellow, and blue, which some stolid educationalist believed were 'primary.' Mixing these, you get black or any number of obscure browns, not the colours you need for painting. These paints were an ignorant attempt to demonstrate long-discredited ideas on colour. The yellow and blue contained enough green, and the red and yellow enough orange, to show the resulting colour to some degree, but the whole concept was very confused. The idea that yellow and blue lights make green refuses to die. It appears in a recent article in Nature (V. 404, p. 457). Colour slides have suitable subtractive primaries, video screens suitable additive primaries. Both produce very effective images that are interpreted by the visual sense to contain a full range of colour.

Curious is the association of colours with shapes, shown on the left, as given in Danger (Ref. 2). Danger also points out that if a room is illuminated by 100 light bulbs, and then 90 are extinguished, objects in the room will appear very much the same. This constancy of colour perception is remarkable. Land showed that patches sending light of exactly the same spectral character to the eye could appear as very different colours, depending on the general illumination of the scene. This demonstrates that perceived colour depends on colour contrasts, explaining in part the remarkable colour constancy of visual perception. The existence of metallic colours with lustre, such as copper and gold, as well as browns and other hues that cannot be excited by a pure spectral light is further evidence of the complex nature of colour. Brown is, in fact, a sensation produced by contrast in a yellow colour surrounded by a brighter field. Lustre or gloss is attributed to surfaces that reflect regularly, making the images in the two retinas different. It automatically arises in stereoscopic viewing of images in contrasting brightnesses or colours. It is seen in monocular vision when a surface is recognized as one that normally shows lustre in binocular vision, or gives traces of variable appearance. It is not a direct effect of binocular vision, but of interpretation.

Something called chromostereopsis has been reported, where it is said that red appears closer to the eye, blue more remote. A popular science magazine (Discover, Nov. 2000) had a page on this that claimed the effect was due to chromatic aberration, which somehow put red and blue images in different positions on the retina as a result of the change of index of refraction with wavelength. Not only does this explanation rely on an erroneous conception of stereopsis as due to retinal position, but shows a lack of understanding of optical imaging. The diagram accompanying the article is totally erroneous, showing rays refracted as if at a prism, and implying that images are carried with the rays. Chromatic aberration causes images to be at different axial distances, and at different sizes, but does not separate red and blue images when the images are stigmatic, as a prism would do. The diagrams purporting to show the effect are concentric rings of blue and red, the blue, incidentally, of much less luminosity than the red. When I view them, the blue actually appears nearer than the red, and the effect is not changed by closing one eye. The article further claims that lack of seeing the effect implies some defect of the observer's stereopsis, which is absurd. There may be a true subjective effect here, but there is probably no physical cause, as is claimed. The theory is doubtless due to some psychologist with little knowledge of optics. This is not to say that chromatic aberration might not produce some such effect in certain cases, only that this article has not presented the argument believably.

Notes and References

Note:
I have carried out most of the simple experiments described above to verify that they work as stated. The remaining ones I shall perform when I again have access to the necessary properties. I believe it is quite important to do things like this whenever possible, and not to rely on mere plausibility. It is a great deal of fun to detect a pompous swindle. The experiments are both enlightening and entertaining.

References:
H. v. Helmholtz, Handbuch der physiologischen Optik, 3rd ed. (Hamburg and Leipzig: Voss, 1909-11). English translation edited by J. P. C. Southall, Optical Society of America, 1924. Republished by Dover Publ. Co., 1962. Complete and authoritative treatment of the physical aspects of vision, with notes and additions. Helmholtz was a champion of empiricism, which many of us find the only rational standpoint in the study of the universe.
E. P. Danger, The Colour Handbook (Brookfield, VT: Gower Technical Press, 1987). This book is oustanding in the uses of colour. There is no index, but the book is well-organized.
R. W. Burnham, R. M. Haynes and C. J. Bartleson, Color: A Guide to Basic Facts and Concepts (New York: John Wiley & Sons, 1963).
Handbook of Chemistry and Physics, 71st Edition (Chemical Rubber Publishing Co., 1991), Section 10, especially pages 274-275 with photometric units and definitions.
Yves LeGrand, Light, Colour and Vision (London: Chapman and Hall, 1957). Translated from French. This book covers much of the same ground as this paper.
D. H. Hubel, Eye, Brain, and Vision (New York: W. H. Freeman and Company, 1988). An account of the neurophysiology of vision and the recent understanding of nerve functions and pathways in the visual system as revealed by the microelectrode. This knowledge is physical, like that presented in Helmholtz, but treats nerves instead of dioptrics, and does not answer any of the important questions. Lack of historical perspective, and the limitations of specialization seen in much modern work, are evident.
M. Minnaert, Light and Colour in the Open Air (New York: Dover, 1954). Discusses many subjective observations, and their regularities.
N. Ribe and F. Steinle, Exploratory Experimentation: Goethe, Land and Color Theory, in Physics Today 55, 43 (July 2002). The authors say "Newton chose to model his later Opticks after Euclid's Elements, with each of the central propositions followed by a 'Proof by Experiments'." Of course, Euclid began with Postulates and based all his propositions on them, a very different thing, deductive rather than inductive. See Minnaert for good discussions of coloured shadows and contrast phenomena, including references to Goethe. Ribe and Steinle say Goethe's observation of the blue shadow cast by evening sunlight, "for which no account is given in Newton's theory," is somehow significant, rather than a commonplace that has nothing to do with the physical nature of light.
R. Glazebrook, A Dictionary of Applied Physics (London: Macmillan, 1923). See Vol. IV, p. 72f for chromatic parallax.