In this blog post, we will take a closer look at how visual information received through the eyes is processed in the brain and how we perceive the world.
We can obtain external information about objects within the range of our eyes, such as their colour, texture, thickness, and size. By recognising the objective and external characteristics of the objects around us, we obtain clues that enable us to make correct judgements and respond appropriately to the situations we find ourselves in.
We call this sense ‘vision,’ and the sense organ that receives a series of external information about objects within our range of perception is the eye. Vision is by far the most primary and instinctive of the five senses, as it collects the information necessary for humans to perceive the outside world and make comprehensive judgements about it. We live every day surrounded by a flood of visual information, so let’s take a look at how the visual information we receive through our eyes is processed in the brain.
A camera has a structure similar to that of the human eye, so much so that it is said to be modelled after the human eye. When a photograph is taken, light from a light source is reflected off an object and passes through the lens of the camera, where it is refracted and focused onto the film inside the camera’s dark box. The lens of the camera corresponds to the lens of the human eye, while the choroid and retina correspond to the dark box and film inside the camera. The thickness of the lens is adjusted by the contraction and relaxation of the ciliary muscle, which determines the degree of refraction of light depending on the distance, and the refracted light is formed into an image on the retina. The world refracted through the lens is projected onto the film called the retina.
So how is this projected image transmitted to the brain? The retina is composed of multiple layers of cells, and information flows along these layers. Visual information formed on the retina passes through three layers of photoreceptor cells, bipolar cells, and ganglion cells, and is then transmitted to the optic nerve. There are two types of photoreceptor cells: rod cells and cone cells. Rod cells recognise brightness, while cone cells recognise the three primary colours of red, green, and blue. Many photoreceptor cells form clusters called photoreceptor fields. These photoreceptor fields correspond one-to-one with each cell in the bipolar cell layer, which is located outside the photoreceptor cell layer. The collection of photoreceptor cells, called the receptive field, is circular and divided into two areas: the centre and the periphery. The activation of bipolar cells depends on which of these two areas the photoreceptor cells in the photoreceptor field are active. Bipolar cells, each with their own photoreceptor field, are connected horizontally by horizontal cells, which secrete inhibitory neurotransmitters to make the external images transmitted from the photoreceptor field clearer. The outer layer of the bipolar cells and the last layer of the retinal cell layer are the ganglion cells. Like bipolar cells, ganglion cells also have circular receptive fields with a central area and a peripheral area. Just as bipolar cells have circular receptive fields composed of photoreceptor cells, ganglion cells have receptive fields composed of a group of bipolar cells. There are three types of ganglion cells: W cells, X cells, and Y cells. The visual information received by the receptive fields connected to each ganglion cell varies depending on the type. X cells receive information from cone cells and are responsible for distinguishing the colours of external objects. Y cells are responsible for focusing on objects based on their movement and have a relatively wider receptive field than X cells. The function of W cells is still unknown, and they are the smallest of the three types of ganglion cells. Amacrine cells, which connect the ganglion cells horizontally, maintain the light sensitivity of the ganglion cells in response to changes in the background light level. The visual information that passes through these three layers of cells is then transmitted to the brain via the optic nerve.
The brain is a collection of nerve cells. How is the information received through the sensory organs, the eyes, processed in the brain? The optic nerve acts as a bridge between the eyes and the brain. The optic nerve is one of the 12 pairs of cranial nerves that extend from the brain and connects to the left and right eyes. A unique feature of these nerves is that the left and right nerves cross each other. Therefore, the information received by each eye is transmitted to the opposite side of the brain. The crossed visual information is stored in the thalamus. The thalamus is called the control centre of the senses because all senses except smell pass through it before being transmitted to the cerebral cortex, where they are sent to the corresponding sensory cortex. Each sense is transmitted to a distinct area of the thalamus, and in the case of sight, it passes through the lateral geniculate nucleus and moves to the primary visual cortex. Like the retina, the lateral geniculate nucleus consists of multiple layers of cells, with each layer processing a specific type of visual information. The visual cortex, which is responsible for sight in the brain, receives information from the left and right eyes through the thalamus. The primary visual cortex is located in the occipital lobe and consists of vertical layers. Information from a single ganglion cell is transmitted to millions of neurons that make up the visual cortex, which recognises various combinations of information such as the direction, wavelength, position, and movement of light. There are seven layers of visual cortex, and visual information that reaches the primary visual cortex is transmitted along different pathways depending on its type. In the case of colour, it passes through the fourth visual cortex to the temporal lobe, while movement and spatial information is transmitted to the right parietal lobe via the fifth visual cortex. Once visual information reaches the area of the brain where it is finally processed, we are able to fully analyse the images formed on the retina and perceive the world around us as if watching a movie.
The physical reality in front of us is transmitted to the brain and converted into electrochemical signals, and we are able to see the world based on these signals. In our daily lives, we take the scenery around us for granted, but behind the scenes, there is a constant process of signal transmission between the eyes and the brain. The information received by the eyes passes through hundreds of millions of neurons before we are able to consciously perceive it.
The sophistication and complexity of this series of visual information transmission processes can be said to be the true mystery of the human body. This visual information processing process is directly linked to our survival.
For example, in situations such as avoiding predators or searching for food in the wild, visual information enables immediate survival responses. Visual information also plays an important role in modern society. When driving, communicating through facial expressions and gestures, or finding our way in a new environment, we constantly use visual information to make decisions and adjust our behaviour. This shows that the ability to process and interpret visual information is an important factor in human intelligence and adaptability.
In addition, advances in modern technology have enabled new visual experiences such as virtual reality and augmented reality. These technologies utilise our visual information processing abilities to break down the boundaries between reality and virtuality, providing new forms of learning, entertainment, and treatment methods. In the future, visual information processing technology will continue to advance and contribute to improving our quality of life.
Therefore, the collection and processing of visual information is not a simple physiological process, but an important mechanism that expands human experience and knowledge. Understanding and utilising this process is essential not only for individual growth but also for the development of society as a whole. Through visual information, we can make better decisions, gain deeper understanding, and have richer experiences.
