MAVO Year 4 · Senses · Section 11.5

The retina

Structure and function of the retina

The retina has several layers. For this course, distinguish photoreceptors, detecting light, and neurons, processing and transmitting information. Light passes through inner neural layers before reaching most photoreceptors. Signals then pass through retinal circuits to ganglion cells, whose axons form the optic nerve.

Sharp central vision depends especially on the fovea, at the centre of the macula. Its dense cone population and specialised wiring allow fine detail to be distinguished. The blind spot has no photoreceptors because the optic nerve leaves the eye there.

Cones

Cones function best at higher light levels and support colour vision. Humans usually have three cone types with different wavelength sensitivities, broadly associated with short, medium and long wavelengths. Their combined activity allows a wide range of colours to be perceived. Cones are especially dense in the central retina. In the fovea, relatively little pooling of signals preserves fine detail; a one-cone-to-one-pathway model is a useful simplification for central vision, not a rule for every cone.

Rods

Rods are more sensitive in dim light and do not support normal colour discrimination. Several rods contribute to shared neural pathways, increasing sensitivity but reducing spatial detail. Rods are widespread outside the central retina and are absent from the central fovea. They do not detect only black and white as separate colours; their activity mainly provides information about light intensity.

Colour-vision differences

Colour-vision deficiency often results from differences in cone pigments or their function. The common red–green forms make certain colour differences harder to distinguish. They do not necessarily make every red or green object look identical.

The route to the brain

Retinal neurons process information about light, contrast and other features. Ganglion-cell axons pass along the inner surface and leave through the optic disc, producing the blind spot. The optic nerves then carry information towards further visual processing in the brain.