98 2 Perception, the Eye and Assistive Technology Issues

both a strength and an axis, as it has a different impact in different directions. The axis specification of the prescription gives the orientation of the axis of the cylindrical correction, which can vary from 1 to 180◦ . The power of a cylindrical lens varies from zero on its main axis to its maximal value on the axis at right angles to the main axis. The total power of the lens is the sum of the spherical and cylindrical components and therefore equal to the spherical correction on the cylindrical axis and the sum of the spherical and cylindrical corrections on the perpendicular axis.

2.6 Chapter Summary

The chapter has provided an overview of the main structures and functions of the visual system and the different types of visual impairment. In particular, it should be noted that the eye is divided into two chambers by the iris, which regulates the size of the pupil, and consequently the amount of light admitted to the eye. The eyeball has three outer layers, of which the inner sensory layer or retina is particularly important. It contains the photoreceptors: cones, which are responsible for daylight and colour vision, and rods that are used for twilight and night vision. The excitation of the rods and cones and the processing of the resulting signal in the visual cortex of the brain generates an image of the external environment in the brain.

Due to the importance of the photoreceptors in the retina, light from an object entering the eye needs to be focused on the retina. There are two mechanisms involved, eye movements using the three pairs of extraocular muscles to track objects and the optical media of the eye, which refract the incident light rays. The optical media of the eye include the front and back surfaces of the cornea and the biconvex lens, with most of the refraction coming from the front surface of the cornea. Contraction of the ciliary body from which the lens is suspended makes the elastic lens thicker and allows the eye to focus on close up objects. This is called accommodation. The amount of accommodation a person can achieve reduces with age, leading to presbyopia.

Analysis of the spatial form of an object is dependent on the receptive fields of the neurones (nerve cells) in the cortex. Three-dimensional or stereoscopic vision is best obtained from instantaneous comparison of the slightly different images seen by the two eyes, but an impression of depth and perspective can be obtained from monocular vision. Motion detection is performed by complex cells with a directional preference in area V5 of the visual cortex. Colour vision is based on the three different types of cones in the retina, which have different peak spectral absorbencies.

Most assistive technology for blind and visually impaired people is designed for people who are functionally totally blind, though only about 3% of blind people are in this category. The incidence of blindness increases with age and many types of blindness, such as macular degeneration and cataract, are increasingly common in older people. The impact of the different types of visual impairment, including loss of visual acuity and reduced field of view were illustrated by showing how people with these types of visual impairments would perceive a busy road scene.