Optical superposition compound eyes are one way of increasing the effective lens diameter. This structure works by using several separate elements to image incident rays onto a single point, such as a single photoreceptor. This method is used in insects such as fireflies. Another way, is to use ‘neural superposition’.
Simple eyes on the other hand, use a single lens, and have evolved entirely separately in the cephalopods and vertebrates. The octopus is a good example because the optical design of its eyes is remarkably similar to the vertebrate eye. The one major difference, however is that in octopus eyes, as in all fish eyes, the lens is much more powerful because it has to compensate for the loss in refractive power from the cornea, due to being underwater.
Simple eyes have a much larger lens diameter which means that the spatial resolution achievable is much higher than is ever possible with a compound eye. Experiments by Kirschfeld have suggested that in order to obtain the same spatial resolution as a human eye, a compound eye would need to be about 1m across.
The eye is a complex structure which has caused many arguments between evolutionary biologists and theologists. Theologists believe that the eye is so perfectly designed to harvest light, and that no intermediary design would be effective, that it must have been created by a divine designer. Biologists believe that it was created step-by-step through natural selection, and that any one step is always an improvement on the last, and thus the eye did not have to be this complex to be of benefit. This argument can be backed up by the evidence of optical diseases in which humans are handicapped in their sight, yet to them, the vision they have is better than none at all, as was a primitive form of an eye.
Myopia (short-sightedness) and hypermetropia (long-sightedness) occur when the optical image of a point at infinity (i.e. the far point) falls respectively in front of or behind the retina. These errors commonly occur when the eyeball is too long, or too short, and is easily corrected with a concave or convex lens. These diseases are commonly found amongst the elderly who no longer have such deformable lens and so have difficulty varying the curvature of the lens, and thus focussing on objects at different distances effectively. This is an accommodation problem.
Accommodation (discovered by Helmholz) is brought about by the ciliary body that acts on the zonular fibres that support the lens. It contains circumferential (circular) muscle fibres that allow it to act as a sphincter. The lens can be dilated by relaxing the circumferential fibres, and is said to be unaccommodated. The radial fibres of the ciliary body also act by pulling outwards on the zonular fibres that support the lens, thereby thinning it. The lens is accommodated when the circumferential fibres contract, and the lens is forced to shrink. The tension in the zonular fibres is reduced, and the lens is allowed to relax into a thicker, more curved shape, with greater refractive power.
A related neuronal mechanism produces binocular convergence, in which the left and right eyes are positioned by the ocular muscles so that the images received by the two eyes fall on analogous parts of the two retinas, regardless of the distance between the object and the eyes. When an object is close, each of the two eyes must rotate towards the middle of the nose; when an object is far away, the two eyes rotate outward from the midline.
Photoreceptors transduce photons of electromagnetic radiation from the visible light spectrum, into electrical signals that can be interpreted by the nervous system. The energy of the electromagnetic radiation varies inversely with its wavelength, and we perceive this variation in energy as variation in colour. The outermost layer of the vertebrate retina includes two classes of photoreceptor in vertebrates: rods and cones. There are about 100 million rods in the eye and 5 million cones. A small central area called the fovea, is densely packed with cones, but in the periphery, rods outnumber cones by >20:1. Cones function best in bright light and provide high resolution, whereas rods function best in dim light but provide much less resolution. In humans, cones mediate colour vision, and rods mediate achromatic vision. These different properties are used to expand the visual capabilities of animals living in certain conditions. For example, animals that live in flat, open environments such as rabbits, usually have horizontal regions within the retina that contain a high density of cones. This concentration of cones is called the visual streak. This region corresponds to the horizon in the visual world and is thought to confer maximal resolution in this part of the scene, allowing the animal to interpret shapes on the horizon with great precision.
A receptor current exists for all sensory receptors which are usually modulated by the stimulus. In the case of vertebrate photoreceptors, the light stimulus actually reduces the circulating current, by causing the closure of ion channels. Although both invertebrates and vertebrates have eyes containing photoreceptors, they differ in their structure. Vertebrate receptor cells contain a segment with an internal structure similar to that of a cilium. This cilium connects the outer segment, which contains the photoreceptive membranes to the inner segment, which includes the nucleus and mitochondria. The photoreceptors of many invertebrates lack the ciliary structure that connects the inner and outer segments of vertebrate rods and cones and the lamellae and or stacks of disks containing visual pigment. Instead, the visual pigment is located in the microvilli formed by the plasma membrane, and these pigment-containing microvilli are organised into rhabdomeres.
Visual pigments consist of 2 major components: a protein (opsin) and a light-absorbing molecule (either retinal or 3-dehydroretinal). Opsins are protein visual pigment molecules consisting of 7 transmembrane α-helix domains. Opsins are coupled to photopigment molecules that are structurally altered by the absorption of photons, and in turn modify the opsin protein. The retinal molecule assumes two sterically distinct states in the retina. In the absence of light, the opsin and retinal are linked covalently and retinal is in an 11-cis formation. The covalent bond allows this light-absorbing molecule to act as a powerful antagonist. On capturing a photon, the retinal isomerises into the all-trans configuration, initiating a series of changes in the visual pigment, as the molecule is rendered enzymatically active. When light hits the photopigment, an intermediate metharhodopsin II forms that activates the G-protein transducin. Transducin activity closes Na+ channels and the receptor cell hyperpolarizes. Activated rhodopsin is hydrolysed spontaneously to retinal and rhodopsin which is hydrolysed spontaneously to retinal and opsin which are both used repeatedly.
Studies made on the horseshoe crab, Limulus polyphemus have revealed a lot about vision. The crab has paired lateral compound eyes as well as five simple eyes: medial and lateral pairs on the dorsal surface and a single unpaired simple eye on the ventral surface. The compound eyes are typical compound eyes whereas the simple eyes are similar in structure to the simplest eye known, which consists of a shallow open pit lined with photoreceptor cells that are backed by screening pigment.
Each ommatidium of a compound eye contains several photoreceptor cells. The photoreceptor cells of the Limulus compound eye are located at the base of each ommatidium. Each ommatidium lies beneath a hexagonal section of an outer transparent layer, the corneal lens. Visual transduction takes place in 12 retinular, or photoreceptor, cells. Each retinular cell has a rhabdomere, a part of the cell in which the plasma membrane is thrown into densely packed microvilli, making this the part of the cell that captures light energy. The microvilli greatly increase the surface area of the plasma membrane, which increases the probability that incident light will be captured by the rhodopsin molecules embedded in the membrane. Together, the 12 rhabdomeres of the retinular cells make up a rhabdome, which surrounds the dendrite of an afferent neuron, the eccentric cell.
Depolarisations of the plasma membrane can be recorded in the retinular cells when the eye is exposed to very dim light. These ‘quantum bumps’ increase in frequency as the light intensity increases (i.e. as more photons impinge on the receptors). The bumps are electrical signals generated as a result of the absorption of individual quanta of light. How can capture of a single photon lead to rapid release of so much energy? In this case, through a cascade of chemical reactions inside the cell that includes G-protein activation. Activation of the G-protein cascade occurs by diffusional contact between activated rhodopsin and molecules of G-protein, which are activated sequentially, as explained above; the activated g-protein in turn activates an effector enzyme, the PDE (phosphodiesterase); this is a 1:1 step, i.e. it has no amplification. A second stage of amplification occurs because the activated PDE is an enzyme which catalyses the destruction of cGMP. The net effect is to open ion channels, allowing cations to enter the cell. In Limulus, the receptor current through the light activated channels is carried by Na+, K+ and some Ca2+. This current causes a depolarising receptor potential. When the light goes off, the channels close again, and the membrane repolarises. The sensitivity of individual photoreceptors drops with exposure to light. This light adaptation is thought to be mediated by Ca+ ions, which enter the cells when light causes ion channels to open and which by some mechanism then reduce the current through light-activated channels.
Although the Limulus eye is simple compared to that of invertebrates, the visual system is capable of generating electrical activity that parallels some of the more sophisticated features of human visual perception. The crab does however lack the degree of colour perception seen by the human eye, because it lacks the short and long wave colour pigments in its cones.
It is interesting to see how two totally different mechanisms of visual perception can be so different yet interrelated in many ways, and that they have involved independently to perform the same function.
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