Retinal image inversion and the visual field map
The eye is like a pinhole camera, so the image on the retina (the light-sensitive layer lining the back of the eye) is upside down and reversed.
Remember
Swap both ways: upper becomes lower and outer becomes inner. Damage to one retina produces a defect in the field of that eye only, named after the field lost, not the retina damaged.
Think of the eye as a pinhole camera. The cornea (the transparent front surface of the eye) and the lens behind it bend light, so the image landing on the retina is upside down and back to front compared with the world outside.
The visual field (the part of the world the eye sees) is flipped upside down and left to right on the retina. Light from the upper field lands on the lower (inferior) retina, and light from the lower field lands on the upper (superior) retina. Light from the temporal field (the side towards the temple) lands on the nasal retina (the half nearer the nose), and light from the nasal field lands on the temporal retina. Combined, the upper temporal field lands on the lower nasal retina.
A field defect is a loss of vision in part of the visual field. Damage to the inferior nasal retina of the right eye removes the superior temporal part of that eye's visual field. A defect is named after the field that is lost, not the retina that is damaged. Retinal damage lies in front of the optic chiasm (where the two optic nerves meet), so it can only affect the field of its own eye.
The pathway keeps this arrangement all the way to the visual cortex (the part of the brain at the back that processes vision), a property called retinotopic mapping. The brain then corrects conscious perception, so we still see the world the right way up.
Key facts
- Upside down
- light from the upper visual field lands on the lower retina, and light from the lower field lands on the upper retina.
- Left to right
- light from the temporal (outer) visual field lands on the nasal retina, the half nearest the nose.
- Both flips
- the upper temporal field lands on the lower nasal retina.
- Naming a defect
- a defect is named after the field that is lost, not the retina that is damaged.
- One eye
- retinal damage lies in front of the optic chiasm (where the two optic nerves meet), so it can only affect the field of its own eye.
- Perception
- the retinal map is kept as far as the visual cortex, but conscious perception is corrected so the world looks the right way up.
Exam traps
- The inferior temporal visual field falls on the superior nasal retina, so it is not the field lost after damage to the inferior nasal retina.
- Damage to the right retina cannot directly cause a defect in the left eye, because each retina produces a defect in its own eye.
- For light from the upper temporal field, the upper nasal retina fits the left to right flip (temporal field to nasal retina) but not the upside down flip, because an upper field lands on the lower retina.
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