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The eye and forming the image

What is the eye for?

In this chapter we will review the purpose of the eye and how the complex optical and neural machinery within it functions to perform this task. The basic function of the eye is to catch and focus light on to a thin layer of specially adapted sensory receptor cells that line the back of the eye. The eyeball is connected to an elaborate arrangement of muscles that allow it to move to follow target stimuli in the environment. The lens within the eye, which helps focus light, is also connected to muscles that can alter the lens shape and thus its focal length. This allows target stimuli at different distances to be focused on the back of the eye. At the back of the eye, light energy is transformed into a neural signal by specialised receptor cells. This signal is modified in the retina, to emphasise changes and discontinuities in illumination, before the signal travels onto the brain via the optic nerve. In the sections that follow we will examine these procedures in detail.

Light

Light has a dual nature, being considered both an electromagnetic wave, which can vary in frequency and wavelength, and also a series of discrete packets of energy, called photons. Both forms of description are used in explaining how the visual system responds to light. In determining the sensitivity of the visual system to light, such as the minimum threshold of light detection, it is usual to refer to light in terms of photons. However, when discussing colour perception, it is normal to refer to light in terms of its wavelength, measured in nanometres (nm). One nanometre is 10 9m. For example, blue light is of comparatively short wavelength (around 430–460 nm), whereas red light is of comparatively long wavelength (around 560–580 nm).

Only electromagnetic radiation with a wavelength between 380 and 760 nm is visible to the human eye (Figure 2.1). The width of the

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Figure 2:3: (See also colour plate section.) Cross-sectional diagram of the human eye (reproduced by permission from Burr, 2005. Copyright (2005) Elsevier).

the nature and structure of objects in a person’s surroundings (Goodale & Milner, 1992), and is also kept informed about eyeblinks (Harl et al., 1994). It is believed that this activity in the posterior parietal cortex is important for maintaining the illusion of a continuous image of the environment during each blink, perhaps by filling in the blink with visual sensation from working memory.

A mucous membrane, called the conjunctiva, lines the eyelid and folds back to attach to the eye (Figure 2.3). The eye itself is a roughly spherical ball, about 2.5 cm in diameter. The sclera of the eye is made up of closely interwoven fibres, which appear white in colour. However, at the front of the eye, where the surface bulges out to form the cornea, the fibres of the sclera are arranged in a regular fashion. This part of the sclera is transparent and allows the entry of light. The part of the sclera surrounding the cornea is called the white of the eye. Behind the cornea is a ring of muscles called the iris. In the centre of the ring is an opening called the pupil, and the amount of light entering the eye is controlled by the pupil’s diameter. The iris contains two bands of muscles, the dilator (whose contraction enlarges the pupil) and the sphincter (whose contraction reduces it). The sphincter is enervated by the parasympathetic nervous system, which uses the neurotransmitter acetylcholine. When we are interested in something, or someone, there is an unconscious expansion of the pupils. This is an important positive social signal. To mimic this response, and make themselves appear more attractive, women once added drops to their eyes containing the alkaloid atropine. This blocks the action of acetylcholine, causing dilation of the pupil by relaxing the sphincter of the iris. This preparation was made from deadly nightshade and gave this plant its species name of Belladonna, meaning beautiful women.

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form synapses with bipolar cells, which in turn synapse onto ganglion cells, the axons of which travel through the optic nerve to the brain. These axons come together and pass through the bipolar and receptor cell layer and leave the eye at a point called the optic disc. The optic disc produces a blind spot, as no photoreceptors can be located there. We are not consciously aware of the blindspot, due to a phenomenon called ‘filling in’ (Ramachandran, 1992). Based on the visual stimulus surrounding the blindspot, the visual system fills in the ‘hole’ in the visual image to give the complete picture of the world we are used to. This process seems to be mediated by cortical neurons in areas V2 and V3 (De Weerd et al., 1995). The retina also includes the outer plexiform layer, containing horizontal cells, and the inner plexiform layer, containing amacrine cells. These cells transmit information in a direction parallel to the surface of the retina and so combine or subtract messages from adjacent photoreceptors (Figure 2.4).

Behind the retina is the pigment epithelial layer, and the ends of the photoreceptors are embedded in this layer (Figure 2.5). The specialised photoreceptors are unable to fulfil all their metabolic requirements and many of these functions, including visual pigment regeneration are carried out by the pigment epithelial layer. Behind this layer is the choroid layer, which is rich in blood vessels. Both these layers contain the black pigment melanin, and this light-absorbing pigment prevents the reflection of stray light within the globe of the eyeball. Without this pigment, light rays would be reflected in all directions within the eye and would cause diffuse illumination of the retina, rather than the contrast between light and dark spots required for the formation of precise images. Albinos lack melanin throughout their body, and so have very poor visual acuity. Visual acuity is usually measured using an eye chart, such as the Snellen eye chart, which you are likely to see at any optician’s. The measurement of acuity is scaled to a viewing distance of 20 feet between the observer and the eye chart. Normal visual acuity is defined as 20/20. Someone with worse than normal visual acuity, for example 20/40, must view a display from a distance of 20 feet to see what a person with normal acuity can see at 40 feet. Someone with better than normal visual acuity, for example 20/10, can see from a distance of 20 feet what a normal person must view from 10 feet. Even with the best of optical correction, albinos rarely have better visual acuity than 20/100 or 20/200. For nocturnal or semi-nocturnal animals, like the cat, the opposite strategy is employed. Instead of a light-absorbent coating at the back of the eye, they have a shiny surface called a tapetum. This reflects light back into the eye and, although it degrades the resolution of the image, it increases the probability of a photon being absorbed by a photoreceptor. In low light intensity environments, this increases visual sensitivity and, for a semi-nocturnal hunter, this makes good sense. It also explains why the eye of a cat seems to glow when it catches the beam of a torch or any other light source.