The eye and the brain - How do we perceive light?

The Visual Spectrum

The eye is inherently almost blind as the visual spectrum forms such a tiny part of the whole electro-magnetic spectrum. What would happen if the eye could see all the other wavelengths of electromagnetic radiation? Would the world be bathed in a mass of new colours? Would we be able to see through solid objects? A piece of work that addresses these thoughts is skyear which detects electromagnetic radiation and displays it as cololured light in the visual spectrum.

The Mechanics of the Eye

The eyes are the mechanical component which feed coded information in the form of electrical pulses or neural activity to the brain. The structure of the eye is extremely specialised, running from the top to the bottom of the section in fig 3.1: The cornea is sustained not by blood but by the aqueous humour which providesall of it's nutrients, the same is true of the lens. The cornea and aqueous humour do much of the refractive work while the lens is held in tension by the zonula and released by the ciliary muscle to make any minor adjustments needed for focusing closer objects. The shape of the lense is only partly responsible for this refraction as the material of the lens has a different refractive index in the centre as at the outside edge. Light is bent more at the edge of the lense due to a higher refractive index. The iris works as an automatic aperture directing most of the light onto the central, and optically best part of the lens. The colour of the iris serves no function, it must only be dark enough to block out the light. This is an interesting twist on an otherwise very specific and specialised mechanism, perhaps it slipped through the net on the eye's evolutionary journey? The faceted shades of the iris are intrinsically beautiful though seemingly functionless.

The eyes are held in place by six muscles which move the eye in two types movements. When following a moving object the eye moves smoothly, following the object. If an object is stationary the eye flicks over the image in rapid jerky movements in order to sweep the image over the retina. If the retinal image is stabilised the eye become blind to it, this has been proven by attaching an object to the eye using a contact lense so the object moves with the eye. As the object moves with the eye the image remains stationary on the retina. The result is that the retina become blind to the object.

I recently came across a web page highlighting why the eye is not like a camera. Whilst it is easy to draw these parallels it is just as important to explore why the eye is different from the camera.

The process of collecting light rays is very similar in both the eye and the camera but this is where the similarities end. The differences lie in the way light rays are processed. The end point of any collection of light rays that enter the eye, from a recorded image to the real world is the brain. This is where the sense of sight is perceived. There is a complex feedback between the eye and the brain which creates 'a dynamic searching for the best interpretation available' (Gregory, 1966, pg 13), this dynamic searching is what informs us and provides us with so much information.

Colour

Having covered the basics of the eyes structure and how it moves I want to look at the way we perceive colour.

How we can differentiate colours of light is somewhat of a mystery since the frequency of light is at millions of millions of cycles per second yet a nerve can only distingish pulses at thousand cycles a second. An answer came from Thomas Young who suggested that colour was a construct of the mind and we perceived light based on three primary colours, red, green and violet. He proposed that any other colour could be created from mixtures of these three and the eye was equiped with receptors for each colour, by combinations of stimuli any colour could be created. We now know these receptors as cone cells, of which there are three types, each one responsive to a spread of wavelengths (as can be seen from this graph).

Our colour perception goes much further than just the ability to recognise colour, as I began to point out earier in 'Daylight - information from the sky' colour informs us of the time of day and year. These assumptions are based on our memories of the routines of night and day year after year but also on our understanding of the way the earth orbits the sun in an eliptical path producing changing seasons and sun angles. Colour also has the ability to change our mood, Goethe wrote: With yellow 'the eye rejoices, the heart expands, the spirit is cheered and we immediately feel warmed.' The expression to 'look at the world with rose tinted spectacles' shows the effect of looking at the world with an orande/ red tint. The research into how colour affects mood seems to be fairly inconclusive because of the subjective nature of reactions.

Enlightenment - The camera obscura and Plato's cave