62 2 Perception, the Eye and Assistive Technology Issues

the centre of iris. The diameter of the pupil is controlled by the iris muscles, the sphincter iridis, and the dilator pupillae. It can vary from less than 1 mm to more than 9 mm. To achieve this considerable change in size the iris is able to reduce to about 13% of its maximum length, a much greater degree of contractibility than any other muscles. The iris is constantly in motion, even when there are no changes in illumination and accommodation (focusing of the lens). Its movements are largest in moderately bright light and the frequency of oscillation increases with the light intensity. The resulting instability of the pupil is most easily seen in young people. About one-fifth of the population have small differences of about 0.4 mm in the size of the pupils of the two eyes without any associated visual impairment or disease of the eye.

Stimulation of one eye with light leads to a contraction of the pupil in the other eye due to the division of the optic nerve fibres at the optic chiasm, so that the axons from both the left and right half fields are connected to the right and left hemispheres, respectively. The pupil also takes part in the near reflex, which involves the three functions of convergence of the two eyes, accommodation of each eye and constriction of both pupils when viewing a near object, even when the illumination does not change. As well as reducing retinal illumination, pupil constriction reduces blurring of the image. The near reflex results from the sphincter pupillae, ciliary and medial recti muscles all working together to improve the image, its focus on the retina and the depth of field.

2.2.4 Intraocular Pressure

The eye is an inflated approximately spherical shell. Since the cornea and sclera are thin connective tissues with little rigidity, the shape and optical functions of the eye are maintained by the intraocular pressure or pressure in the eye of about 15 mm Hg. This pressure is determined by the amount of aqueous humour in the eye and counterbalanced by the tension in the outer tunic of the eye. The main source of the aqueous humour is the blood flowing through the ciliary arterial system. The processes by which the clear transparent colourless aqueous humour is produced from the blood are not fully understood and suggestions include filtration (to remove cellular components), dialysis (to separate smaller from larger particles in solution by diffusion through a membrane) and secretion (with cells using metabolic energy to modify material). The rates of production and drainage of aqueous humour need to be the same to maintain the constant size of the eye.

The aqueous humour drains from the eye into the veins via a circular channel, called the canal of Schlemm through the trabecular network of connective tissue. The intraocular pressure is determined by the resistance to flow of these tissues combined with the rate of production of the aqueous humour. This is a channel in the corneal stroma around the eye, which is located at the filtration angle between the cornea and the eye. Blockage of the filtration angle and consequent closure of Schlemm’s canal, reduces the outflow rate of the aqueous humour, resulting in a rapid and painful increase in intraocular pressure, which often leads to angleclosure glaucoma. A slow increase in the production rate of aqueous humour or the

liquid flow resistance in the outflow pathway will lead to a slow rise in intraocular pressure and possibly also to open-angle glaucoma.

Temporary increases in intraocular pressure may result from stress or anxiety, wearing a tight collar or holding one’s breath. There is a variation of about 4 mm Hg from the early morning high to the evening low and a variation of about 2 mm Hg from 15 to 17 mm Hg with age. The aqueous humour has a greater concentration of dissolved substances than the blood, leading to an osmotic flow of water across the blood aqueous barrier. Medicaments can be used in acute-angle glaucoma to manipulate this flow rate to lower the intraocular pressure quickly to avoid damage to the eye.

2.2.5 Extraocular Muscles

Description of the movement of the eyes can most easily be done using a frame of reference that defines the axes of rotation of the eyeball or globe. Its centre of rotation depends on the position of the eyeball in the orbit. Torsional movements take place about the visual axis. In intorsion and extorsion the upper part of the eye rotates towards and away from the nose. Horizontal rotations take place about a vertical axis and vertical rotations about a horizontal axis. Elevation and depression involve upward and downward movements of the eye, whereas adduction and abduction involve horizontal rotations towards and away from the nose.

There are three pairs of extraocular muscles (see Figure 2.2):

1.The inferior and superior rectus, whose main actions are elevation and depression of the eye, with secondary actions of adduction and torsion.

2.The superior and inferior oblique, whose main actions are intorsion and extorsion. Their secondary actions involve horizontal and vertical displacements with increasing adduction.

3.The lateral and medial rectus, whose main actions are adduction and abduction, namely, horizontal rotation towards or away from the nose. They have minimal secondary actions.

Controlling eye movements requires sensory feedback about the current eye position and the ability to move the eyes either quickly or slowly, as well as to keep them relatively stationary. The extraocular muscles are controlled by three pairs of cranial nerves:

1.The oculomotor nerve (cranial nerve III)

2.The trochlear nerve (cranial nerve IV)

3.The abducens nerve (cranial nerve VI)

The oculomotor range is about 90◦, i.e. a quarter of a circle; this is the angle through which the direction of the gaze can be moved by moving the eyes without moving the head. When the visual axes are directed straight ahead, the eyes are in the primary position. Secondary and tertiary positions are obtained by purely