How vision works

Posted on 28 November 2013

Your brain is more complex than a computer. Take, for instance, the complex interactions between the eye and brain that make it possible for you to see. Neurologist Dr Alexander Landmann, from Mediclinic Sandton, helps explain how it works…

Like all our senses, we often take our sight for granted. We may also associate it just with our eyes, forgetting that without the brain, our vision wouldn’t be possible. ‘I’d say about 25% to a third of the brain is involved in processing sight.’ says Dr Landmann.

Yet all of this processing happens so quickly, it’s actually hard to comprehend. Sight is a combination of light wavelengths that reach us from the front through our eyes, electrical nerve signals that send this information along, and processing that takes part in various regions of the brain, primarily in the visual cortex of the occipital lobe at the very back of the brain. And all of this happens in fractions of time, all the time.  ‘A visual signal takes around 100 milliseconds, that’s 0,1 of a second, to reach the back of the brain,’ says Dr Landmann.

Here’s a brief explanation: First, light travels from an object into the eye via the clear outer layer, the cornea. The light is refracted so it can enter the eye. As it goes through the lens, light is bent further in order to focus it on the back of the eye, where the retina is.
The retina contains millions of photosensitive nerve cells – photoreceptors – which can be divided into two types: rods (about 10 million) and cones (about 7 million).

Rods allow us to see in dim light, they detect motion and what lies in our peripheral vision. Cones are more sensitive to the light spectrum and are responsible for us seeing colour and detail.

In under a second, rods and cones convert light into electrical impulses. This happens because of rhopsodin, a photosensitive chemical they contain.

These impulses go the ganglion cells before going to the optic nerve, a collection of retinal fibres that connects the eyes to the brain. At the end of the optic nerve, you have a crossover of information: There’s a crossover of information in the chiasma of the optic nerve: visual clues from the right eye goes to the left part of the brain and vice versa.

Next, via the optic tract, visual information is sent to the brain’s thalamus, where it is modulated in the Lateral Geniculate Nucleus (LGN). In LGN, information is separated according to its main properties into two streams: The top parvocellular layers work on colour and form. The magnocellular layers work on contrast, depth and motion.

Nerve fibres relay this information to the primary visual cortex, the main region where sight is interpreted, which is located in the brain’s occipital lobe.

Dr Landmann explains further: ‘Here you get the most direct representation of what we see in through our eyes; what we see is read by the brain in a relatively unchanged way. In V1, the striate cortex of the primary visual cortex, sight is made up of dots in a similar arrangement to what they are in on the eye retina. There’s a point to point map between the visual cortex and the retina.’

At this point, some visual fibres also send information to other parts of the brain in a loop that feeds back to the primary visual cortex and the eyes – things like pupil and eye muscle control and behaviour are adjusted to ascertain depth and help us identify and focus on objects.

‘There are various cues for depth perception,’ says Dr Landmann. ‘One is when something is close to you in your mid-line sight, your eyes will converge together to see the object in focus. The degree of convergence tells the brain how far away the object is. For objects on either side of the midline, each eye sees independently and depending on how each signal is perceived allows the brain to make calculations about objects you see. More than the primary part of the visual cortex is involved in this.’

For instance, the temporal area holds visual memories and helps with object recognition, while the parietal area performs spatial processing telling you how the object related to its environment. If both your eyes are working normally, information from both eyes is fused together allowing us to see the world in three-dimension.

So seeing occurs as a result of an intricate (and fast) interrelationship between our eyes, our nerves and our brains.

If you have questions on this post, or any other medical matter, please visit us on Facebook.

The information provided in this article was correct at the time of publishing. At Mediclinic we endeavour to provide our patients and readers with accurate and reliable information, which is why we continually review and update our content. However, due to the dynamic nature of clinical information and medicine, some information may from time to time become outdated prior to revision.