The retina is a layer at the back of the eye that detects light. It consists of different types of specialised cells to help generate a visual image (what is seen). One of these cells is called photoreceptors. They detect light coming into the eye, converting it into electrical signals which is then transmitted to the brain through a series of other specialised cells in the retina and the optic nerve.
Human photoreceptors are differentiated into two types, rods and cones. Rods are the most abundant photoreceptor cell in the retina, responsible for vision in dim light and peripheral vision. Cone cells help us to see colour and objects in detail under bright light. The number of cone cells in human is less compared to rods and they are mainly situated in the fovea.
Macula and fovea
The macula is the central part of the retina where incoming light is mainly focused on. It is responsible for our central vision. As part of the retina, it also has specialised light detection cells called photoreceptors.
The fovea is a pit at the centre of the macula and is best visualised with a special scan called the optical coherence tomography (OCT) scan. The fovea has the highest density of cone cells. Therefore, our vision is the sharpest when light is focused here.
(1) Photograph of the back of the eye
It shows the retina, the macula (delineated with a green circle), the fovea and the optic nerve
(2) Different cell layers of the retina
The rod and cone photoreceptors are at the top of the image. When the photoreceptors detect light, electrical signals are generated and transmitted through the horizontal, bipolar and ganglion cells to the optic nerve, which is then relayed to the brain to generate an image