Greatest sharpness or acuity of vision occurs when light from an illuminated object falls on the fovea, a shallow cavity about 2 millimetres across which defines the visual axis of the eyeball. The eye normally aligns the fovea with whatever is of immediate visual interest. In practice, the visual field of the fovea focuses an area about the size of a large coin held at arm's length. In order to sense every part of this page when reading, for example, the eye must make an enormous number of tiny shifts, known as saccades, which are an important aspect of visual sensing.
The retina itself is an 'extension' of the brain which has become sensitive to light. The two types of receptor cells which equip the retina are the rod and cone cells, so named because of their relative shape when observed through a microscope: the rods are thin and the cones more bulbous. The rod cells, an estimated 120 million of which are present in each retina, sense light to dark values in dim illumination (and are therefore most useful at night) but do not sense colour. The cones are sensitive to the different colours of the spectrum and seem to be wholly responsible for the faculty of the eye to sense colour in daylight. Some 7 million cones populate the retina, 100,000 of which are packed into the central fovea, which is rod-free. The fovea not only possess a concentration of cone cells but also a one-to-one connection between each of its cones and a bipolar nerve cells, the first link in the channel of signal transmission from eye to brain.
What happens when light stimulates the retinal receptors is unknown in its entirety. No single theory yet explains how visual information is encoded on the retina and carried to the brain, now how, once there, the information is decoded. Most investigators agree that, when light is sensed by a visual receptor cell, the initial reaction is photochemical. Rod cells are known to contain a light-sensitive pigment located at the tip of each sensor which, on exposure to light sets off electrical impulses in the adjoining nerve cells. On exposure to particularly bright light, the pigment decomposes and the rod cells cease to function.
Observations indicate that cone-cell pigments are of three main types, each differing from the other two in its sensitivity to coloured light: one appears to be responsible primarily for sensing the longer ('red') wavelengths of the spectrum, another for sensing those in the middle ('green'), and the third for sensing the shorter ('blue') wavelengths. A theory of 'three-component' colour vision, proposed as early as 1802 by Thomas Young (1773-1829), was revised and clarified by Hermann von Helmholtz (1821-94) in the 1860s.
The colour sensing of an estimated 8% of males is defective; in females, colour vision defects are relatively rare (less than 0.5%). In cases of extreme 'colour blindness', the visible world may appear as lighter and darker values of one colour only or of black, grey and white only. Abnormal colour vision is usually congenital but can be caused by age, disease, drug toxicity, or prolonged exposure to high levels of sound.
For the same expenditure of light energy at each wavelength, human colour vision is not equally ensitive to all spectral colours. As a rule, wavelengths in the yellow region of the spectrum look considerably brighter than those in the red, green or blue regions. This lack of strict correspondence between stimulus and response, first investigated by Pierre Bouguer (1698-1758) can be represented in a spectral sensitivity curve on which wavelength of a spectral stimulus is plotted against the amount of light the stimulus appears to convey to the eye. |
