How is it that we can see the world around us? It’s quite a bit more complicated than most people think – not simply light hitting the retina and an inverted picture being turned the right way round. Here’s an overview of how we are able to see.
First of all, light enters the eye and hits the retina at the back of our eye.
The retina is made up of several layers of different cells which detect and then start to process the visual input. The cells which respond to light and colour are called photoreceptors. There are two different types of photoreceptors: rods for detecting light and dark, and cones, which detect colour. Cones are concentrated in the fovea, which is the area of the eye used for fixating on stimuli.
The diagram above shows the organisation of cells in the retina. The ganglion cells take the output of the retina to the brain via the optic nerve. The optic disk (see diagram of the eye) is where these projections leave the eye – another name for this area is the blindspot as there are no photoreceptors in this area.
The information from the retina is projected to the Lateral Geniculate Nucleus (see below). From there, it travels to the striate cortex in the occipital lobe – this area is also known as the primary visual cortex or V1.
From the primary visual cortex, the visual information travels through other areas in the occipital lobe, where aspects such as colour and motion processing occur.
After processing in the visual cortex, the information is projected to other areas of the brain. There are then two ‘streams’ in the brain which are specialised for different aspects of vision:
1. the dorsal stream projects towards the parietal lobe and is important in identifying object location.
2. the ventral stream projects towards the temporal lobe and is important in identifying and recognising objects.
These were identified by Goodale and Milner in 1992.
I hope you liked this post on the visual system, check back soon for my next posts about visual disorders and colour vision 🙂