Homepage › Articles › Science › Neuroanatomy II: The Visual System

Neuroanatomy II: The Visual System

There are several things, before we begin, of which one should be aware. This covers only the natural eye and does not extend onto blindness, partial sightedness or the visual system of creatures non-human. However, this study is an area of neurological/psychological interest when investigating the reasons as to why people go blind, are blind and indeed perceive things in a way visually different.

Of course, many reasons exist as to why we can see, all such reasons are logical as it does seem that vision is a thing so useful to our evolutionary survival that one can easily conclude that if as a species, humans could not see – or even did not have the average visual acuity which we hold – then the species, I daresay would have made not a half of the evolutionary leaps which it has. So what exactly is involved in vision? For of course, it is safe to say that when one views an object it is not simply seen, but a vast process is taken to recognise and store the item in the brains ‘internal filing system’, or memory.

Two of the obvious components to build up the visual system are obvious; the eye and the brain. These are two interconnecting organs which are essential in the visual system. Yet process, unseen, and far more complex are going on when one views a stimulus.

The eye is a complex organ built up of many structures. We have all seen an image of an eye, and all know that an eye, externally, has certain characteristics. The outside of the eye is white (this white layer is known as the Sclera) inside of this one can see the Iris, this is where we can see the colour of ones eyes and central to that is the pupil; the dark and size changing circle at the centre of the eye. Yet that is, quite needless to say, not all there is to the eye. For example, a simple move from that is to talk of the clear dome which one can see over the eye (known as the cornea), this transparent layer, unlike the sclera will allow light to enter the eye and thus, the visual process begins. (for all we see is dependent on the presence of light, though this is explained in far more details below) The dark circular centre of the eye is referred to as the pupil. We shall call, for explanation, the pupil a light regulator. The size of the pupil will change dependent on the level of light available, this then changes ones vision as it allows more/less light through dependent on whether the pupil is dilated (bigger, allowing in more light) or constricted (smaller, allowing in less light) .

External implications: Linking to my previous article, Neuroanatomy I, this is controlled by the autonomic nervous system, specifically the sympathetic nervous system which will react to stressor situations.

To assist this process further, the eye has a lens (this can be edited by glasses), this is a section of the eye built up a layers which change their formation – essentially – to change one’s visual level e.g. allowing one to see an object at a short or long distance. (consequently it is damage to this lens i.e. cataracts, glaucoma, which then can limit ones visual acuity with regard to depth perception and short/long sightedness. The visual information has not yet reached the end of its journey, it has simply passed through the gates.

Following the exposure to the front portion of the eye (and it is now I recommend one to look at a diagram of an eye – these are easily accessible on google.com), it will pass through the main portion of the eye. An area filled with clear liquid and to the retina where the main processing begins, for it is here in the retina where communication with the brain (and thus processing) starts. It is in the retina – located at the posterior of the eye – where the neural structures (e.g. rods and cones) are located. These are consequently known as photoreceptors. (Carlson 2013)

It is rods and cones, by their nature, that are the main reason for visual processing. They have different roles and specialities when it comes to visual information. It is cones which hold the more important job. They provide the most (and more detailed) visual information, they are responsible for our seeing in good light conditions. –When you are sitting with the light on, it is the cones which are working more than the rods, which are likely activated into working when you are sitting with the light off in your room and streetlights, candles or the TV is lighting your view. They are greatly responsible also for ones visual acuity and colour perception. Thus is the reason for the location of such neural structures (or shall we call them sensory receptors – speciality neurons) to be located in a part of the eye called the forvea. The forvea is a thinner location in the retina designed to allow more light in and is the very reason for our visual acuity (it is likely damage or fault here, if not a cataract related problem in the lens, which will cause a lower visual acuity in individuals with sight difficulties). The forvea, unlike the rest of the retina, contains only cones and no rods are present (Carlson 2013). Rods cannot detect colour, this is why, when in dark, we are poor at detecting the colour of an object – as the cones are far less light sensitive than the rods.

The optic disk is here located also, this is formed by formations of axons (see neurons) which then lead to the optic nerve – thus connect directly with the brain. It is then here that the most important steps are taken. It is in this space where the neural conduction (or transduction of light information, Carlson 2013) happens. The photoreceptors – cones and rods – will form at a synapse (read neurotransmission) of bipolar cells, communication then travels through these cells to ganglia cells (making up the optic nerve with their axons) and so carried through the brain to the occipital lobe, located at the posterior most region of the brain.

After the trajectory through the optic erve, these visual signals reach something referred to as the ‘dorsal lateral geniculate nucleus’ (LGN) in the thalamus. The LGN is home to six layers of neurons, each receiving information from one eye. (The layers each have different thicknesses and build ups for the reason of their different roles in visual processing,, for more information on this one might seek the reference of other materials, google or the text mentioned previously – seen in references.). It is through this that the neurons send their visual message, via a pathway known as the ‘optic radiations’ to the primary visual cortex.

With regard to the trajectory of the image through this system, it is a common misconception that what is seen by the right eye is processed by the left hemisphere of the brain and that seen by the left processed by the right hemisphere (due to the cross over via the optic chiasm). This is not solely correct, though does run along the right lines and so one can be forgiven for their belief in it. Though this is mainly the system some (granted limited) visual information from the right eye is passed the right hemisphere for processing, the same applies also to the left.

What exactly can we see? Of course we have learned from this so far, that light in all cases is needed for visual systems to work, the level of light needed varies between species, it varies also within species (in cases where the person has a lower/higher visual acuity than is average). To avoid complications, shall one say simply that humans have a light visual range from 300 (specifically 380) to 700 (780) nm (thus standing for ‘nanometre’) anything above or below this we as a species are blind to. (Carlson 2013).

It then may be of interest for you to know that it is by light on which colour judgements are made, for it is the level of light to which we are exposed as to our detection of the colour. (it is suggested that 400nm is purple in colour and 700 red in colour, these are just examples and further research should be done by oneself if interest is peaked in this.) Light colour has several levels on which It is assessed:

Hue
Saturation
Brightness

When one refers to the wave length of light, we refer of course to the figues such as 300nm, nanometer, the term used above, refers to a measurement of exactly one-billionth of a meter in size (Carlson 2013). This then refers to the speed of light, or accurately the speed at which light travels. Now one may be aware that light is a speedy thing, travelling at speeds which we as humans can not dream of understanding. For simplicities sakes I will inform you that dependent on the wave length size (300 – 700, the relevant areas for human visual perception) is dependent on the speed. Carlson (2013) gives a great explanation of this on P.166, Chapter six and for those interested, I’d highly recommend reading this text. Essentially, the smaller the wave length the quicker the speed it travels, this is also the determinate for the hue of the light detected. Following this pattern for the list above, the brightness is dependent upon the intensity of the light and the saturation to the lights apparent purity.

Interesting Points:

The eye makes three types of movements, each controlled by the extraocular muscles which are attached to the sclera:

Vergence - the movement(s) which are cooperative and will keep the eyes focused on the same target/stimulus. I would suggest that it is then, due to problems/weakness with the extraocular muscles that the inordinate attention of one eye, different to that of the other happens when one had a ‘lazy eye’.
Saccadic - the regular and fast time movements that occur when one is scanning an area, scene or stimulus before them.
Pursuit – A slower movement when following a finger and not reading a book.

The speed at which your eye moves when performing a task is something completely out of ones control, the movements often seem so quick that one would not believe their eye to be moving at all as the image looks constant. This is not the case and the eye is always moving.

I was thinking of writing a follow up from this, if anyone would be interested. More about how we see things - so coding etc and why. If anyone is interested please contact me and inform me.