If the eye is fixated at the central point of a co-ordinate grid, light from this central point will fall upon the fovea centralis, the centre of the visual axis - the line joining an object being looked at, with the fovea. The fovea is the area of greatest visual acuity (but quite low sensitivity) on the retina. By charting the ability to detect an object moving through this co-ordinate frame, the borders of the visual field can be mapped (perimetry), but blind areas within the visual field may also be detected. By having a computer driven target and a button which is depressed and released as the target appears and disappears respectively, this mapping can be done quite effectively by computer. The borders of the receptive field are defined superiorly by the supra-orbital ridge, medially by the nose-bridge, and is quite large laterally, limited by the angle of acceptance of the pupil relative to the location of the retina. Within the normal visual field, is a physiological blind spot, located at about 15 degrees to the temporal side of the fixation point, and slightly below the horizontal meridian. This marks the location from which light would fall upon the optic disc. The figure to the left (above) shows the visual field for the right eye.

In perimetry the eyes are evaluated one at a time, with the contralateral eye masked. If both eyes fixate the same target as they normally do, it is clear that their visual fields overlap considerably, but not completely. There is a distinct zone where binocular vision is possible, and a zone of monocular vision. The area of the blind spot in each eye is complemented by a seeing area of the contralateral eye.

Focal damage on the retina can give rise to blind spots or zones known as scotomata (singular scotoma). Damage to the visual pathway can give rise to blindness in areas of the visual field, following characteristic patterns, depending upon the location of the lesion (see below).

Damage to the right optic nerve (1) leads to blindness in the right eye (total loss of visual field). Damage in the optic chiasma (2), leads to bitemporal hemianopia - blindness in both temporal half-fields, since damage involves the crossing fibres from the nasal half of the retinae, which view inputs from the temporal fields. Damage at the chiasma due to pituitary tumor or craniopharyngioma may selectively damage the inferior fibres in the chiasma. These fibres originate from the inferior half of the retinae which receive input from the superior part of the external field. This will lead to superior bitemporal quadrantanopia. Damage in the right optic tract (3) which carries axons from the right temporal and the left nasal retinal halves, will lead to homonymous hemianopia - the loss of vision in the same, contralateral (left) half of the visual field in both eyes. Focal damage in the right optic radiations (4,5,6) which are spread out, will lead to less extensive losses, confined to the superior or inferior quadrants of the contralateral visual fields of both eyes, depending upon the location of the lesion. In these cases, note that vision tends to be spared in the region of the macula lutea (macular sparing). The exact reason for this is unclear, but may be related to (a) the extensive number of axons which originate from this small area, (b) the fact that the occipital pole to which these axons project receives a dual blood supply, and so may be protected if occlusion of one avenue of blood supply is blocked, or (c) the observation that axons emerging from the macula may project to both sides, so that some axons will be spared when there is unilateral damage.

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